Post by: Joseph41 on January 30, 2019, 03:36:09 pm
QCE CHEMISTRY Q&A THREAD
What is this thread for?
If you have general questions about the QCE Chemistry course (both Units 1&2 and 3&4) or how to improve in certain areas, this is the place to ask! 👌
Who can/will answer questions?
Everyone is welcome to contribute; even if you're unsure of yourself, providing different perspectives is incredibly valuable.
Please don't be dissuaded by the fact that you haven't finished Year 12, or didn't score as highly as others, or your advice contradicts something else you've seen on this thread, or whatever; none of this disqualifies you from helping others. And if you're worried you do have some sort of misconception, put it out there and someone else can clarify and modify your understanding!
There'll be a whole bunch of other high-scoring students with their own wealths of wisdom to share with you, so you may even get multiple answers from different people offering their insights - very cool.
To ask a question or make a post, you will first need an ATAR Notes account. You probably already have one, but if you don't, it takes about four seconds to sign up - and completely free!
Post by: alphabeta on December 12, 2019, 01:23:27 pm
Can someone explain how they would go about naming this molecule according to current IUPAC nomenclature conventions?
Thanks in advance,
alphabeta
Post by: Bri MT on December 12, 2019, 07:33:45 pm
1. Count the number of carbons in the ring, there are 6 cyclohexane is the name of a cyclic alkane with 6 carbons
2. Identify the other things we have going on in the ring.
There are:
- 2 C=C in the ring
- 1 C=O
- 1 C-OH
- 1 C-CH3
Of these, it's the C=O that has highest priority so we count from there picking a direction that reduces the numbers of our other functional groups.
In the end we obtain: 3-hydroxy-4-methylcyclohexa-2,5-dien-1-one
Let me know if any part of this doesn't make sense :)
Post by: alphabeta on December 12, 2019, 08:30:51 pm
I am surprised you have been asked to name a cyclic alkane but nonetheless here's the procedure:
1. Count the number of carbons in the ring, there are 6 cyclohexane is the name of a cyclic alkane with 6 carbons
2. Identify the other things we have going on in the ring.
There are:
- 2 C=C in the ring
- 1 C=O
- 1 C-OH
- 1 C-CH3
Of these, it's the C=O that has highest priority so we count from there picking a direction that reduces the numbers of our other functional groups.
In the end we obtain: 3-hydroxy-4-methylcyclohexa-2,5-dien-1-one
Let me know if any part of this doesn't make sense :)
Thanks Bri!
I'm still a little confused about some parts. I wasn't actually aware that there was an ''order of functional groups''. Like how you mentioned the numbering of the Carbons depends on where a functional group is. Is there like an order or priority depending on what type of functional group is there? Also I don't understand why 2,5-dien us there. I'm not quite sure what dien is. And based on the structure, i'm not sure why 6 isn't in the dien part too.
Aside from that, everything makes sense to me.
Thanks heaps Bri! Really appreciate it. :)
Post by: Bri MT on December 12, 2019, 08:42:04 pm
Thanks Bri!
I'm still a little confused about some parts. I wasn't actually aware that there was an ''order of functional groups''. Like how you mentioned the numbering of the Carbons depends on where a functional group is. Is there like an order or priority depending on what type of functional group is there? Also I don't understand why 2,5-dien us there. I'm not quite sure what dien is. And based on the structure, i'm not sure why 6 isn't in the dien part too.
Aside from that, everything makes sense to me.
Thanks heaps Bri! Really appreciate it. :)
No worries!
Different functional groups have different priority, and this comes into play when you have multiple functional groups to consider. I cannot definitively state whether this is assessable at a QCE level as it's not explicitly part of the syllabus but it is part of the IUPAC naming system.
dien is indicating that there are 2 double bonds - one connecting the 2nd carbon to the 3rd carbon & one connecting the 5th carbon to the 6th carbon. Remember that you go for low numbers whenever you have a choice.
Hope this clarifies things!
Post by: alphabeta on December 12, 2019, 09:41:02 pm
No worries!
Different functional groups have different priority, and this comes into play when you have multiple functional groups to consider. I cannot definitively state whether this is assessable at a QCE level as it's not explicitly part of the syllabus but it is part of the IUPAC naming system.
dien is indicating that there are 2 double bonds - one connecting the 2nd carbon to the 3rd carbon & one connecting the 5th carbon to the 6th carbon. Remember that you go for low numbers whenever you have a choice.
Hope this clarifies things!
Thanks Bri! Can you tell me the functional groups from highest priority to lowest? Makes much more sense now! :)
Post by: Bri MT on December 12, 2019, 09:54:02 pm
Thanks Bri! Can you tell me the functional groups from highest priority to lowest? Makes much more sense now! :)
You're absolutely welcome!
Here they are:
Caboxyl (highest)
ester
amide
nitrile
aldehyde
ketone
hydroxyl
amine
alkene
alkyne
halogen (lowest)
Post by: alphabeta on January 06, 2020, 12:27:54 pm
I was recently looking at the qce chemistry syllabus (unit 3, topic 2) and I wasn't quite sure what one of the dot points were referring to. The dot point was:
• recognise the limitation associated with standard reduction potentials
Is this referring to the fact that the EMF can only be calculated accurately at standard conditions? If not, could someone please explain what it is referring to?
Thanks! :)
Post by: Bri MT on January 06, 2020, 12:45:53 pm
Hey everyone!
I was recently looking at the qce chemistry syllabus (unit 3, topic 2) and I wasn't quite sure what one of the dot points were referring to. The dot point was:
• recognise the limitation associated with standard reduction potentials
Is this referring to the fact that the EMF can only be calculated accurately at standard conditions? If not, could someone please explain what it is referring to?
Thanks! :)
This would be referring to standard conditions being met :)
Post by: alphabeta on January 08, 2020, 03:04:45 pm
I'm just checking if I did my titration calculations correctly. So for a titration of hydrochloric acid using a standard solution of sodium carbonate. 2.5 g of sodium carbonate was dissolved in 200 mL water. The equation is:
2HCl + Na2CO3 --> 2NaCl + CO2 + H2O
The attached results were collected. The unknown concentration is HCl.
If I have missed any values, please let me know.
Thanks so much!
Post by: Bri MT on January 08, 2020, 03:50:25 pm
Hello everyone! :)
I'm just checking if I did my titration calculations correctly. So for a titration of hydrochloric acid using a standard solution of sodium carbonate. 2.5 g of sodium carbonate was dissolved in 200 mL water. The equation is:
2HCl + Na2CO3 --> 2NaCl + CO2 + H2O
The attached results were collected. The unknown concentration is HCl.
If I have missed any values, please let me know.
Thanks so much!
Hey :)
Can you explain how you arrived at your average titre? It seems to be lower than each of your individual titres.
Post by: alphabeta on January 08, 2020, 04:13:46 pm
Quote
Hey :)
Can you explain how you arrived at your average titre? It seems to be lower than each of your individual titres.
So, the titre values are 28.6, 28.8, 29, 28 and 28. Adding them up gives 142.4. 142.4 divided by 5 gives 28.48.
Post by: Sine on January 08, 2020, 04:49:42 pm
So, the titre values are 28.6, 28.8, 29, 28 and 28. Adding them up gives 142.4. 142.4 divided by 5 gives 28.48.Your average titre calculation is fine.
Whether it is correct or not will depend on the concentration of HCl (which from what I can see you have not calculated yet). So I am not 100% clear on your question.
Post by: Bri MT on January 08, 2020, 05:08:12 pm
So, the titre values are 28.6, 28.8, 29, 28 and 28. Adding them up gives 142.4. 142.4 divided by 5 gives 28.48.
Sorry I completely misread your handwriting!
Like Sine, I'm not sure what your question is
Post by: alphabeta on January 08, 2020, 05:33:34 pm
Quote
Your average titre calculation is fine.
Whether it is correct or not will depend on the concentration of HCl (which from what I can see you have not calculated yet). So I am not 100% clear on your question.
Quote
Sorry I completely misread your handwriting!
Like Sine, I'm not sure what your question is
Sorry for being so vague - I was in a bit of a rush lol. So, to clear it up, the titration was using the concentration of sodium carbonate to find the concentration of hydrochloric acid. This is how I calculated its concentration:
moles of sodium carbonate = m/M = 2.5/105.99 = 0.023587 mol
moles of HCl = 2 times 0.023587 (as per molar ratio in equation)
= 0.047174 mol
The concentration of HCl = moles/volume = 0.047174/0.02848 = 1.656 M
So, my question is: is what I did right?
Thank you! :)
Post by: Sine on January 08, 2020, 05:41:55 pm
-snip-I did a rough calculation before of this and got the same so I think it is right.
However, I should note that if this was a practical experiment whether this number is accurate would depend on how well you did your titrations.
Hope this helps :)
Post by: alphabeta on January 08, 2020, 07:32:53 pm
Quote
I did a rough calculation before of this and got the same so I think it is right.
However, I should note that if this was a practical experiment whether this number is accurate would depend on how well you did your titrations.
Hope this helps :)
Thanks Sine! Really appreciate your time. :)
Post by: alphabeta on February 02, 2020, 12:16:05 pm
Just a quick question:
Is 'A being titrated with B' the same as 'A being titrated against B? In both cases, is the substance of unknown concentration the same?
Thanks! :D
Post by: Erutepa on February 02, 2020, 05:12:51 pm
Hi all - me again!I usually see it written as "A is titrated with B" (where B is the titrant/standard solution), however I am sure that saying A is titrated against B is the same and fine to say :)
Just a quick question:
Is 'A being titrated with B' the same as 'A being titrated against B? In both cases, is the substance of unknown concentration the same?
Thanks! :D
Post by: alphabeta on February 02, 2020, 07:38:38 pm
I usually see it written as "A is titrated with B" (where B is the titrant/standard solution), however I am sure that saying A is titrated against B is the same and fine to say :)Thanks heaps Erutepa! I usually see it as A titrated with B as well. :)
Also, I have stumbled across another problem. So, with titration graphs I know at the half-equivalence point the pH = pKa. However, I have also come across some more information that says the indicator changes colour when the pH = pKa, meaning that it wouldn't change at the end point. Can someone clear this up for me?
Thanks so much
Post by: Bri MT on February 02, 2020, 10:40:58 pm
The indicator changes colour when pH = pKa. In this case, we're referring to the pKa of the indicator.
At the half equivalence point, enough titrant (known concentration) has been added to the analyte (unknown concentration) that half of it is protonated and half of it is deprotonated, thus making pKa equal to pH.
These are different pKa s and thus it makes sense that they occur at different pH values.
If you're unclear about ether of these please feel free to ask :)
Edited for clarity
Post by: alphabeta on February 03, 2020, 08:08:13 pm
Hey,
The indicator changes colour when pH = pKa. In this case, we're referring to the pKa of the indicator.
At the half equivalence point, enough analyte has been added that half of it is protonated and half of it is deprotonated, thus making pKa equal to pH.
These are different pKa s and thus it makes sense that they occur at different pH values.
If you're unclear about ether of these please feel free to ask :)
Thanks Bri MT!
So, just to make sure, in the first instance it is talking about the pKa of the indicator, and in the second instance, it is talking about the pKa of the unknown solution?
I really want to make sure I have a firm grip of the titration concepts, so are there any worksheets with simple and challenging titration problems I can do (preferably with worked solutions, if not I can confirm my answers with you)?
Thanks so much again Bri, appreciate it. :)
Post by: Bri MT on February 04, 2020, 10:37:09 am
Thanks Bri MT!
So, just to make sure, in the first instance it is talking about the pKa of the indicator, and in the second instance, it is talking about the pKa of the unknown solution?
I really want to make sure I have a firm grip of the titration concepts, so are there any worksheets with simple and challenging titration problems I can do (preferably with worked solutions, if not I can confirm my answers with you)?
Thanks so much again Bri, appreciate it. :)
I've edited my above post for clarity since titration jargon can be confusing but yeah.
Let's say, for example, that you have NaOH in your burette (titrant, known concentration) and you're trying to figure out what the concentration of some acetic/ethanoic acid (analyte) is. To find the concentration of the acetic acid you want to identify the volume of NaOH required to reach the equivalence point, where the stoichiometric ratio is met. To visually approximate this, an indicator is added which is a weak acid/base that will quickly change which conjugate is present in a greater quantity and, since each conjugate is a different colour, will change colour at that point. When the indicator transitions from one colour to another, there is the same amount of each conjugate present and the pKa (of the indicator) = pH (of the solution). In this example, your indicator might be something like phenolphthalein
Before the equivalence point is reached, there will be a stage where you've added sufficient titrant (in this example NaOH) that half of the analyte (in this case acetic acid) is protonated and half is deprotonated - in other words, half is the conjugate acid and half is the conjugate base. Thus, we are looking at another situation where pKa =pH but this time it's the pKa of our acetic acid since that's what has a 50/50 balance with conjugate base & conjugate acid. For half of our acetic acid to be deprotonated, the volume of NaOH required is half that needed to fully deprotonate our acetic acid in accordance with the equation NaOH + CH3COOH -> H2O + CH3OONa . I.e. the v(NaOH) is half of what v(NaOH) is at the equivalence point.
Remembering that the stoichiometric ratio is met at the equivalence point (which we approximate with the end point, where the indicator changes colour), this is why pH = pKa at the half equivalence point for the acetic acid and is at the equivalence point for the indicator.
Hope this clarifies things!
I don't think I have a worksheet like that on hand but I'll look around and see if I can find any good question sets like that for you. If you haven't already, you might find it useful to do the maths for why pH = pKa when each conjugate is present in the same amount to solidify your understanding.
Post by: A.Rose on February 18, 2020, 04:35:31 pm
I am a year 12 Chemistry student and I was wondering if you could help me with some questions I have about a Galvanic cell experiment I am doing for my student experiment.
The experiment I decided to do was to change the concentration of the copper sulphate electrolyte in a Daniell cell and measure the voltage produced. The other electrolyte; Zinc nitrate was not changed and kept constant at 0.1M. The copper sulphate concentration increased from 0.25M, 0.5M, 0.75M to 1M. The experiment was performed when the temperature was 24 degrees. The anode was Zinc and the cathode was Copper.
In order to obtain theoretical values - should I use the Nernst equation? Would you be able to tell me how to use the Nernst equation for my experiment?
Also, I understand that voltage increases with concentration, but compared to standard conditions should the voltage produced be higher or lower than the voltage produced at standard conditions and 1M for each electrolyte?
If you can help me that would be absolutely amazing and this is a really important assignment.
Thanks!!
Post by: Bri MT on February 20, 2020, 10:47:07 am
Hello
I am a year 12 Chemistry student and I was wondering if you could help me with some questions I have about a Galvanic cell experiment I am doing for my student experiment.
The experiment I decided to do was to change the concentration of the copper sulphate electrolyte in a Daniell cell and measure the voltage produced. The other electrolyte; Zinc nitrate was not changed and kept constant at 0.1M. The copper sulphate concentration increased from 0.25M, 0.5M, 0.75M to 1M. The experiment was performed when the temperature was 24 degrees. The anode was Zinc and the cathode was Copper.
In order to obtain theoretical values - should I use the Nernst equation? Would you be able to tell me how to use the Nernst equation for my experiment?
Also, I understand that voltage increases with concentration, but compared to standard conditions should the voltage produced be higher or lower than the voltage produced at standard conditions and 1M for each electrolyte?
If you can help me that would be absolutely amazing and this is a really important assignment.
Thanks!!
Hey :)
Welcome to the forums!
The Nernst equation is applicable here since you're working with non-standard conditions :). I'm guessing what might be confusing you in the calculation is Q? Q is calculated using the same formula as Kc but we use Q (reaction quotient) rather than Kc (equilibrium constant) if the system isn't at equilibrium. Please also make sure you remember to convert your temperature to Kelvin before plugging it in.
When we apply the Nernst equation to your set up, Q is what is changing between experiments so let's look at that. For Q = [Zn2+]/[Cu2+], we see that Q increases as [Cu2+] decreases. This suggests that your voltage will decrease as [Cu2+] decreases.
Hope this helps, please feel free to ask any follow up questions & best of luck with your assignment :)
Post by: A.Rose on March 20, 2020, 07:52:42 pm
I have just started Organic chemistry and I have a quick question on how to draw 4-oxo propyl hexanoate. (The answer from my textbook is attached) I understand the carboxyl group and everything just not the placement of the ketone. Isn't carbon 1 the carbon with the carboxyl group attached? In the answer, I don't know why the ketone is where it is if carbon 1 is where the carboxyl group is. The answer seems to have carbon 1 as the very end carbon...?
Post by: Bri MT on March 23, 2020, 11:55:02 am
Hello!
I have just started Organic chemistry and I have a quick question on how to draw 4-oxo propyl hexanoate. (The answer from my textbook is attached) I understand the carboxyl group and everything just not the placement of the ketone. Isn't carbon 1 the carbon with the carboxyl group attached? In the answer, I don't know why the ketone is where it is if carbon 1 is where the carboxyl group is. The answer seems to have carbon 1 as the very end carbon...?
Hey,
Counting starts from the end of a molecule and with the choice of what end being made to lower the numbers you use. I think where you may be getting confused here is reading this as [4-oxo-propyl][hexanoate] and thus counting from the hydroxyl end of the propanol, with that end of the molecule going to the ester functional group. Note in in this question you can't interpret it that way due to the limited number of carbons in propanol.
In this question you need to read it as [4-oxo] [propyl hexanoate]. I.e. the ketone functional group needs to be 4 carbons from the end. You can only do this in the way shown in the textbook answer.
Hope this makes sense!
Post by: A.Rose on April 03, 2020, 10:16:14 am
Would someone be able to check my calculation using the Nernst equation? See attached:
Thank you so much!! :D
Post by: Bri MT on April 07, 2020, 11:51:58 am
Plugging in the same values as you used my calculator gave me a slightly different value (1.12016 V) so maybe double check your calculation checking for:
- reading the decimal places carefully
- having a x symbol between the fraction and the log / not using the simplified fraction form. (Some calculators read 3 1/2 as 7/2 or 3 + 1/2 rather than as 3/2 )
Note that some of the values you used had more accuracy than is published in the QCAA formula and data book and so using them for practice questions may produce slightly different results than you would see in suggested solutions
Finally, I recommend losing some of your decimal places as they imply much more accuracy in the data than what you actually have.
Hope this helps!
Post by: A.Rose on April 09, 2020, 11:58:59 am
I have a few questions about my Chemistry Student experiment I am doing at the moment that relates to how I should conclude in regard to the validity of the experimental relationship and what relationship my experimental data is demonstrating; taking in consideration the percentage error in the gradient and possible faults in equipment.
My experiment involved a Daniell cell where I changed the concentration of copper sulphate from 0.25M, 0.5M, 0.75M and 1M and measured the voltage produced. The temperature was at 24 degrees and I used the Nernst equation to obtain the theoretical voltages. The Zinc sulphate was kept constant at 0.1M.
I just thought it would be easier to write my questions on a word doc since they are a bit long and so that I can put my graphs next to my questions.
Thank you so much! :D
Post by: Bri MT on April 11, 2020, 11:28:04 am
Hi!
I have a few questions about my Chemistry Student experiment I am doing at the moment that relates to how I should conclude in regard to the validity of the experimental relationship and what relationship my experimental data is demonstrating; taking in consideration the percentage error in the gradient and possible faults in equipment.
My experiment involved a Daniell cell where I changed the concentration of copper sulphate from 0.25M, 0.5M, 0.75M and 1M and measured the voltage produced. The temperature was at 24 degrees and I used the Nernst equation to obtain the theoretical voltages. The Zinc sulphate was kept constant at 0.1M.
I just thought it would be easier to write my questions on a word doc since they are a bit long and so that I can put my graphs next to my questions.
Thank you so much! :D
Hi!
I think you should revise your understand of some of the terminology to make sure you're using that accurately but there's definitely some great scientific thinking in there :D
Hope you find this helpful!
Post by: A.Rose on April 16, 2020, 03:09:26 pm
I really really need some help with finding the concentration uncertainty for my dilutions!! The image I attached has my calculations and info regarding equipment uncertainty.
I am not confident that I am doing it right.
Basically, I began with 1M of copper sulphate so this is C1. And I want to make the following concentrations: 0.25M, 0.5M, 0.75M and 1M so all of these are C2. Then I want all of these solutions to have the same volume of 0.25L so this is V2. And hence the V1 volumes are 0.0625L, 0.125L, 0.1875L and 0.25L respectively for each concentration stated above.
I used the following equipment with its uncertainty; volumetric pipette (±0.00075L), cylinder (±0.0005L) and a volumetric flask (±0.0003L).
V1 was measured I believe using the pipette and a cylinder. This was then all put into a 250mL volumetric flask and distilled water was added to bring the total volume (V2) to 0.25L.
I am not sure which uncertainties each V1 and V2 actually have when I go about plugging all the values into C1V1=C2V2 to achieve each C2 value with its correct concentration uncertainty.
Particularly - do I include the uncertainty of the flask if the uncertainty of V2 is based purely on the uncertainty of V1 and the distilled water added? (In my new calculations attached I haven't included the flask).
Thank you so much!
This is a small part of my student experiment that is due in a couple of days so help would be very much appreciated!! :D :D
Post by: Bri MT on April 17, 2020, 10:35:06 pm
Hi!
I really really need some help with finding the concentration uncertainty for my dilutions!! The image I attached has my calculations and info regarding equipment uncertainty.
I am not confident that I am doing it right.
Basically, I began with 1M of copper sulphate so this is C1. And I want to make the following concentrations: 0.25M, 0.5M, 0.75M and 1M so all of these are C2. Then I want all of these solutions to have the same volume of 0.25L so this is V2. And hence the V1 volumes are 0.0625L, 0.125L, 0.1875L and 0.25L respectively for each concentration stated above.
I used the following equipment with its uncertainty; volumetric pipette (±0.00075L), cylinder (±0.0005L) and a volumetric flask (±0.0003L).
V1 was measured I believe using the pipette and a cylinder. This was then all put into a 250mL volumetric flask and distilled water was added to bring the total volume (V2) to 0.25L.
I am not sure which uncertainties each V1 and V2 actually have when I go about plugging all the values into C1V1=C2V2 to achieve each C2 value with its correct concentration uncertainty.
Particularly - do I include the uncertainty of the flask if the uncertainty of V2 is based purely on the uncertainty of V1 and the distilled water added? (In my new calculations attached I haven't included the flask).
Thank you so much!
This is a small part of my student experiment that is due in a couple of days so help would be very much appreciated!! :D :D
Ok I typed all that before the updated edit so I'm typing again now:
For V2 uncertainty - if the water was added based on the location of the line and meniscus rather than measuring out a specific quantity of water and adding that - the uncertainty should be based on the equipment rather than being derived.
Edit: looks like you fixed the v1 issue, I've made slight wording changes for clarity
Post by: aleenabinu on April 19, 2020, 10:12:49 pm
For my SEI, i am doing determination of Vitamin C concentration in Fresh Lemon and Lemon juice using Iodine solution by titration.
I did the calculations and got very small numbers 0.0302M (for fresh lemon) and 0.0181M (for lemon juice). I have a feeling i got it wrong in finding the moles of ascorbic acid section because i'm unsure of the balanced equation.
I've watched few videos on how to do the calculations but they all used Potassium iodate and thats like a different equation.
This is the equation i used and hence got 1:1 mole ratio:
C6H8O6 + I2 --> 2I- + C6H6O6 + 2H+
Post by: sweetcheeks on April 19, 2020, 10:35:36 pm
Hi,
For my SEI, i am doing determination of Vitamin C concentration in Fresh Lemon and Lemon juice using Iodine solution by titration.
I did the calculations and got very small numbers 0.0302M (for fresh lemon) and 0.0181M (for lemon juice). I have a feeling i got it wrong in finding the moles of ascorbic acid section because i'm unsure of the balanced equation.
I've watched few videos on how to do the calculations but they all used Potassium iodate and thats like a different equation.
This is the equation i used and hence got 1:1 mole ratio:
C6H8O6 + I2 --> 2I- + C6H6O6 + 2H+
I wouldn't be so sure that those results are too low. What are you comparing them to?
Think about the pH of lemons and lemon juice and relate that to the H+ concentration. Keep in mind, ascorbic acid is a weak acid, and the pH is buffered, but none-the-less you can get an approximate value.
Post by: aleenabinu on April 21, 2020, 05:14:32 pm
I wouldn't be so sure that those results are too low. What are you comparing them to?
Think about the pH of lemons and lemon juice and relate that to the H+ concentration. Keep in mind, ascorbic acid is a weak acid, and the pH is buffered, but none-the-less you can get an approximate value.
Thank you for replying!
I am comparing them to the recommended daily amount.
These are my calculations. Could you please tell me if i went wrong in any places? Thank youuu :)
Post by: Bri MT on April 22, 2020, 01:22:58 pm
Thank you for replying!
I am comparing them to the recommended daily amount.
These are my calculations. Could you please tell me if i went wrong in any places? Thank youuu :)
Hey!
There's some places where I can't check your work because, for example, I don't know what your concordant titres actually were.
One thing that did stand out is the concentrations of iodine you plugged into n=cv. Was this 0.005 M or 5 M? The reason you divide the volume by 1000 for the titres is because they're in mL and you need to convert them to L; if your concentration was 5.00 M you should not be dividing that by 1000. Aside from that, I can't see any alarm bells in your calculations, though my personal preference would be for your molar ratio (the unknown/known fraction) to be specific to the molecules you were working with.
Good luck & I hope you found this helpful :)
Post by: aleenabinu on April 25, 2020, 05:54:44 pm
Hey!
There's some places where I can't check your work because, for example, I don't know what your concordant titres actually were.
One thing that did stand out is the concentrations of iodine you plugged into n=cv. Was this 0.005 M or 5 M? The reason you divide the volume by 1000 for the titres is because they're in mL and you need to convert them to L; if your concentration was 5.00 M you should not be dividing that by 1000. Aside from that, I can't see any alarm bells in your calculations, though my personal preference would be for your molar ratio (the unknown/known fraction) to be specific to the molecules you were working with.
Good luck & I hope you found this helpful :)
Thank you!! My teacher checked it and she said it looks fine.
One more question. Im comparing my concentrations to the recommended daily amount. I just realised that they're all in mg. I know you can convert concentration to mg, but that would just be for that quantity of sample. Is there a way i could convert it and obtain more accurate results?
Post by: Bri MT on April 25, 2020, 07:33:17 pm
Thank you!! My teacher checked it and she said it looks fine.
One more question. Im comparing my concentrations to the recommended daily amount. I just realised that they're all in mg. I know you can convert concentration to mg, but that would just be for that quantity of sample. Is there a way i could convert it and obtain more accurate results?
No worries!
hmmm... I'm not sure what question you're trying to answer, but you could consider converting the RDI to mol, then use that to find the volume of lemon juice required to meet the RDI based on your concentration calculation. This wouldn't make your results more accurate but it could increase your ability to analyse the relationship in a valid way
Post by: aleenabinu on April 28, 2020, 11:16:02 pm
No worries!
hmmm... I'm not sure what question you're trying to answer, but you could consider converting the RDI to mol, then use that to find the volume of lemon juice required to meet the RDI based on your concentration calculation. This wouldn't make your results more accurate but it could increase your ability to analyse the relationship in a valid way
thank you sm!! this helped alot.
as i am just determining the concentration of ascorbic acid in fresh lemon and bottled lemon juice, would i need to graph anything? now that i think about it, it wouldve been smart to record the pH but we didn't. or could i just insert a table with intial, final readings and tire volume and still get top marks in that criteria?
is iodine a weak or strong base? i can't find an exact answer anywhere :(
also, when explaining about the relationship, what actually is the relationship in this case? as more standard solution is added, more closer to the end point ? i dont knowww :-\
Post by: Bri MT on April 29, 2020, 04:02:15 pm
thank you sm!! this helped alot.
as i am just determining the concentration of ascorbic acid in fresh lemon and bottled lemon juice, would i need to graph anything? now that i think about it, it wouldve been smart to record the pH but we didn't. or could i just insert a table with intial, final readings and tire volume and still get top marks in that criteria?
is iodine a weak or strong base? i can't find an exact answer anywhere :(
also, when explaining about the relationship, what actually is the relationship in this case? as more standard solution is added, more closer to the end point ? i dont knowww :-\
Hey,
Based on the questions you've been asking I think you need to go back to the start of the experiment and ask yourself:
- what's my hypothesis?
- what are my dependent and independent variables?
When graphing a categorical independent variable against a continuous dependent variable I recommend a bar or column graph (technically could also go with a boxplot or strip plot but you tend not to see those in highschool)
I would suggest that volume of standard solution is not one of the variables you're interested in graphing :)
A quick search is telling me that iodine is a weak base
Post by: jasmine24 on August 12, 2020, 07:46:53 am
Post by: Bri MT on August 12, 2020, 11:29:32 am
I was able to find this paper: Relationships among pH, aluminum solubility and aluminum complexation with organic matter in acid forest soils of the Northeastern United States
In it they showed a few theoretical models including:
Al(OH)3(s) + H^+ <-> Al3+ + 3H2O
RAl^(3-x) + xH^+ <-> Al^3+ + RHx (where R^x- is an organic soil site)
and a version where x was specified as 3
RAl + 3H^+ <-> Al^3+ + RH3
In each of these you can see that the H+ is on the opposite side of a reversible reaction with the aluminium ions. As pH decreases, by Le Chatlier's principle the system responds by favouring the forwards reaction thereby producing more Al3+
I hope this helps!
Post by: keltingmeith on August 12, 2020, 11:46:57 am
While Bri's answer is fantastic, you might notice that it relies on scientific literature from 2016 - that's REALLY REALLY recent. Here's a list of some of the other things you have learned/will learn this year (assuming you're in year 12?) and when they were discovered:
Equilibrium - 1803
Galvanic Cells - 1790
Bronsted-Lowry Acid-Base Model - 1923
The pH Scale - 1909
Le Chatelier's Principle - 1884
NMR - 1945 (was awarded the Nobel prize in physics in 1952, neat)
Markovnikov's Rule - 1869
...
I think you get the point. You're studying old shit (the most recent of these is over half a century old!!) - and it's a little unreasonable for you to be read up on current-day research. But aside from that, there's something to note - a lot of what you're studying in your curriculum and textbooks ISN'T informed by current-day research. Solubility is a big one of those.
The truth with solubility is that in general - we DON'T know how it works. We can make guesses, such as why are silver halides insoluble if halides are typically soluble? Well, it would be because silver cations are insoluble. But wait - then why are silver cations insoluble? Well, the answer is going to be in one of:
-Silver-water interaction energy
-Silver-halide interaction energy
-The entropy of a silver-dissolved system (everything is soluble if you get it hot enough, even if that means taking it to infinity degrees)
The question is - which one? And the answer is typically - we don't know, but we can use one of these things to figure it out:
-Equilibrium constants
-Electrochemical potentials
-Actual chemical reactions that might take place (eg, metal oxides that are typically insoluble will be dissolved if you add enough acid, because the acid actually causes the oxides to chemically turn into water)
But all of these are typically just consequences of one of the above three things - interaction energy (or enthalpy) of one of the complexes, or the entropy of the system.
TL;DR - the answer to most solubility questions is basically, "we don't know, the answer is complicated", and hence why we need modern-day research like the article that Bri found to inform these kinds of questions. So, you shouldn't expect to be asked these questions at high school level (unless your textbook can offer you an answer) - but if you're interested for the sake of pure interest, then go nuts - I'll be the last person to discourage reading scientific literature for the fun of it.
EDIT: See below for Bri showing me up with my QCE knowledge
. So yeah, turns out NMR isn't in the curriculum, oops. And I was unaware that QCE has a research investigation - looks damn cool though, I gotta say!
Post by: Bri MT on August 12, 2020, 12:06:33 pm
Legit, in the time it took me to come to this topic, Bri beat me with an answer :') But there is something I want to add:
While Bri's answer is fantastic, you might notice that it relies on scientific literature from 2016 - that's REALLY REALLY recent. Here's a list of some of the other things you have learned/will learn this year (assuming you're in year 12?) and when they were discovered:
Equilibrium - 1803
Galvanic Cells - 1790
Bronsted-Lowry Acid-Base Model - 1923
The pH Scale - 1909
Le Chatelier's Principle - 1884
NMR - 1945 (was awarded the Nobel prize in physics in 1952, neat)
Markovnikov's Rule - 1869
...
Don't remind me about how the QCE system doesn't include NMR in the syllabus :'(
Quote from: QCAA
Analytical techniques
• explain how proteins can be analysed by chromatography and electrophoresis
• select and use data from analytical techniques, including mass spectrometry, x-ray crystallography and infrared spectroscopy, to determine the structure of organic molecules
• analyse data from spectra, including mass spectrometry and infrared spectroscopy, to communicate conceptual understanding, solve problems and make predictions.
Aside from that reminder, always good to see your additions :)
TL;DR - the answer to most solubility questions is basically, "we don't know, the answer is complicated", and hence why we need modern-day research like the article that Bri found to inform these kinds of questions. So, you shouldn't expect to be asked these questions at high school level (unless your textbook can offer you an answer) - but if you're interested for the sake of pure interest, then go nuts - I'll be the last person to discourage reading scientific literature for the fun of it.
I'm assuming Jasmine that you're looking at this for SHE and/or IA3? Absolutely if you ever got something like that question on an exam or test without them providing the required information that would be very unreasonable.
Post by: jasmine24 on August 12, 2020, 04:41:50 pm
I'm assuming Jasmine that you're looking at this for SHE and/or IA3? Absolutely if you ever got something like that question on an exam or test without them providing the required information that would be very unreasonable.
Post by: 1729 on August 12, 2020, 04:59:24 pm
It's for the research investigation :) Would I have to mention that it is a theoretical model and would this be a limitation?I'm not the best person to answer these kinds of questions as I am not familar with the QCE study design at all. But I don't think you really do need to mention that it is a theoretical model because it should be assumed. Just make sure the mention what you are assuming and that it is a "perfect world" if you aren't entirely sure. (for example, "neglecting the forces between atoms ...")
Post by: Bri MT on August 12, 2020, 05:15:08 pm
It's for the research investigation :) Would I have to mention that it is a theoretical model and would this be a limitation?
When asking that question something to ask yourself is:
- How important is the accuracy of these models to the claims you are making?
You're very probably fine to just reference them properly :)
Post by: matthew hay on August 26, 2020, 06:41:54 pm
Do you or anyone know if there is an annotated syllabus? or even annotated Units 4 syllabus?? Please reply no if not (it saves me the extravagant search!
Thanks and kind regards, Matt :D
Post by: Bri MT on August 26, 2020, 07:43:54 pm
Hi there,
Do you or anyone know if there is an annotated syllabus? or even annotated Units 4 syllabus?? Please reply no if not (it saves me the extravagant search!
Thanks and kind regards, Matt :D
Hey,
I've upload a units 3&4 syllabus annotations file here.
Please feel free to let me know if you have any feedback on this & I hope you find it useful :)
Post by: A.Rose on August 29, 2020, 10:11:59 am
I need some help understanding electrolysis! I am quite rusty since learning it last year.
I'm confused with this question (the answers in this paper are sometimes wrong so if you disagree with the answer it could be wrong but maybe not - I'm not sure). The answer is in the textbox.
I know galvanic cells and electrolytic cells are the same in terms of reduction happens at the cathode and oxidation at the anode and I know the anode of positive and cathode negative but this question has two different electrodes and I'm confused why the half questions are Zinc and Nickel - what happens at the aluminium electrode? Any explanations for how electrolysis works, in general, will be much appreciated!
Thank you!
Post by: keltingmeith on August 29, 2020, 12:35:47 pm
Hi!
I need some help understanding electrolysis! I am quite rusty since learning it last year.
I'm confused with this question (the answers in this paper are sometimes wrong so if you disagree with the answer it could be wrong but maybe not - I'm not sure). The answer is in the textbox.
I know galvanic cells and electrolytic cells are the same in terms of reduction happens at the cathode and oxidation at the anode and I know the anode of positive and cathode negative but this question has two different electrodes and I'm confused why the half questions are Zinc and Nickel - what happens at the aluminium electrode? Any explanations for how electrolysis works, in general, will be much appreciated!
Thank you!
For this question, you're going to want to look at page 10 of your data booklet. In an electrochemical cell - regardless of if it's galvanic or electrolytic - the chemicals that react are ALWAYS going to be the ones that give the BIGGEST difference (this will end up being the BIGGEST Ecell). So, first thing you need to do is figure out what chemicals you're working with. In this case, you have:
Al(s)
Ni(s)
Zn2+ (aq)
NO3- (aq)
So, look for ANY equations in the electrochemical series that contain these chemicals and ions. There's also a very sneaky one to be aware of - all of this is happening in an aqueous solution, so water could be involved, too! So you need to consider that reaction, too. In an exam, you could just highlight these - but I'm going to have to write them out, d'oh. They are:
Al (s) ---> Al3+ (aq) + 3e-, Eo=+1.68
2H2O (l) + 2e- ---> H2 (g) + 2OH- (aq), Eo=+0.83
Zn2+ (aq) + 2e- ---> Zn (s), Eo=-0.76
Ni (s) ---> Ni2+ (aq) + 2e-, Eo=-0.24
And looking further down, there are no nitrate equations - so we don't need to worry about it. Now, to make this easier to read, I'm going to display the data from most negative E to most positive E - the same way it's displayed in the data booklet:
Zn2+ (aq) + 2e- ---> Zn (s), Eo=-0.76
Ni (s) ---> Ni2+ (aq) + 2e-, Eo=-0.24
2H2O (l) + 2e- ---> H2 (g) + 2OH- (aq), Eo=+0.83
Al (s) ---> Al3+ (aq) + 3e-, Eo=+1.68
So, looking at this, what will make the biggest difference? Ionic zinc and solid aluminium! So, we calculate the Ecell and we get +0.92. Well, that's not going to work for an electrolytic cell - the Ecell needs to be negative! Okay, what if we try with water - then we get an Ecell of 0.07. Still problematic... That means, the only electrolytic cell that will work is the one with solid nickel and ionic zinc - which has an Ecell of -1.00 (huh, neat).
Basically, you can think of it as this - if the zinc reacted with the water or aluminium, then we'd be CREATING electricity. However, electrolytic cells don't create electricity - they consume it. So, the moment you start putting electricity into the system, the aluminium isn't going to want to react anymore (kinda like in equilibrium - you've changed the system by feeding it electricity, so the system won't want to produce electricity anymore).
Now, I know what you're thinking. "That's very long and tedious, I don't have the time to do this in an exam!" - well, I've got good news for you - we can cheat a little bit. Essentially, this method boils down to there's a difference between general reactivity, and electrochemical reactivity. In fact, electrochemical reactivity correlates exactly with the electrochemical series. When it's written the way it is in the QCE data booklet, the left side goes up by decreasing oxidant strength, and the right side goes up by increasing reductant strength. In any electrolytic cell, the weakest oxidant will react with the weakest reductant. In this case, the weakest oxidant (at the top of the left) is ionic zinc, and the weakest reductant (at the bottom on the right) is solid nickel.
EDIT: After doing all of this, I've realised that I read the equation for water wrong - so in fact, the answer should have nickel reacting with water. I imagine the textbook writers simply forgot they were doing everything in water, hence why they didn't include that equation - it's an easy mistake to make!
Post by: matthew hay on September 06, 2020, 04:32:16 pm
Can someone explain chromatography and electrophoresis of proteins is, or refer me to a post if there is one?
I've read my text-book, looked at videos, read the ATAR Notes course-book and still don't understand it. From what I've grasped, the textbook talks about size-exclusion chromatography and ATAR notes course-book talks about chromatography in general. I misunderstand what's actually happening, and it feels like the more it read into it, the more I'm confused about the topic. Also, do we need to understand the formula found in the attachments section below?
There's definitely new words involved in this topic which only adds to the confusion. Also how deep do we go into this topic, like what's the objective - the syllabus is kinda vague?
I get that it must be a hassle to respond, so whoever does end up responding Thank You :)
Post by: Bri MT on September 06, 2020, 05:13:43 pm
You should've done chromatography in year 11, did your school cover this?
The basic idea of chromatography is this: you have a stationary phase (stationary: not moving) and a mobile phase (mobile: moving). If something would really rather hang out with the mobile phase, it'll only lag behind the mobile phase by a little bit. On the other hand, if it's not that sure whether it would rather hang out with the mobile phase or the stationary phase, it'll move slower. This means that when you try this with multiple different things, they'll separate out based on how good they are at sticking with the mobile phase rather than the stationary phase.
Size based chromatography works on the principle of smaller proteins being more likely to fit in small spaces in the stationary phase and smaller molecules moving more slowly.
In electrophoresis molecules are attracted to one side due to their electrical charge & their mass slows them down. This means the bigger a molecule is and the lower its charge, the longer it will take to move. So, you apply the voltage (to attract your molecules) for a limited time and if everything has the same charge you know that the molecule that moved the furthest is the smallest, the one that moved 2nd furthest is 2nd smallest etc.
That formula is not in the syllabus and the syllabus dot points do not lead me to believe the equation is required either.
Please let me know what made sense and what didn't from those explanations and then I can add more/clarify as appropriate :)
Post by: matthew hay on September 07, 2020, 02:01:44 pm
Hey,
You should've done chromatography in year 11, did your school cover this?
The basic idea of chromatography is this: you have a stationary phase (stationary: not moving) and a mobile phase (mobile: moving). If something would really rather hang out with the mobile phase, it'll only lag behind the mobile phase by a little bit. On the other hand, if it's not that sure whether it would rather hang out with the mobile phase or the stationary phase, it'll move slower. This means that when you try this with multiple different things, they'll separate out based on how good they are at sticking with the mobile phase rather than the stationary phase.
Size based chromatography works on the principle of smaller proteins being more likely to fit in small spaces in the stationary phase and smaller molecules moving more slowly.
In electrophoresis molecules are attracted to one side due to their electrical charge & their mass slows them down. This means the bigger a molecule is and the lower its charge, the longer it will take to move. So, you apply the voltage (to attract your molecules) for a limited time and if everything has the same charge you know that the molecule that moved the furthest is the smallest, the one that moved 2nd furthest is 2nd smallest etc.
That formula is not in the syllabus and the syllabus dot points do not lead me to believe the equation is required either.
Please let me know what made sense and what didn't from those explanations and then I can add more/clarify as appropriate :)
Ahh, I have a clearer picture now, thank you! I have another question though, will we be expected to know how to use/understand the Retention factor equation? I haven't seen it in my textbook, but it is in the ATAR Notes coursebook.
Post by: Bri MT on September 08, 2020, 02:06:34 pm
Ahh, I have a clearer picture now, thank you! I have another question though, will we be expected to know how to use/understand the Retention factor equation? I haven't seen it in my textbook, but it is in the ATAR Notes coursebook.
No worries!
There are a couple of reasons I included it in the courseguide:
- calculating Rf values for chromatography is in the units 1&2 syllabus
- the equation can aid conceptual understanding of how chromatography works
I think it is unlikely that they will ask you to calculate Rf values but they would expect you to have learnt it in year 11.
Post by: Dumdumsim on September 08, 2020, 10:07:16 pm
Post by: Bri MT on September 09, 2020, 04:35:39 pm
Just wondering if there is the annotated solutions for QCAA chemistry sample exam.
Hey!
Closest thing I know of to that is this set of MCQ solutions I wrote up last year.
Hope this helps & if you have any questions about the answers I've suggested please feel free to ask :)
Post by: snr.mmorris4.19 on September 12, 2020, 06:59:58 pm
Post by: keltingmeith on September 12, 2020, 07:07:47 pm
how is 2-methylprop-1-ene an isomer of butene? Isn't the carbon number in the parent chain different? I'm so confused with this!
I think you're numbering or drawing one of these two wrong. See the attached - structures + names for each (ignore the (E) part if you don't know what that is - it's just another method of identifying cis/trans stereoisomers, and not something you need to know. I generated these using a computer program and wasn't sure how to turn the E off, sorry!) along with the chemical formula. As you can see, the chemical formula for each is exactly the same.
Post by: snr.mmorris4.19 on September 12, 2020, 07:38:21 pm
I think you're numbering or drawing one of these two wrong. See the attached - structures + names for each (ignore the (E) part if you don't know what that is - it's just another method of identifying cis/trans stereoisomers, and not something you need to know. I generated these using a computer program and wasn't sure how to turn the E off, sorry!) along with the chemical formula. As you can see, the chemical formula for each is exactly the same.thankyou!
Post by: snr.mmorris4.19 on September 13, 2020, 08:17:21 am
Post by: Bri MT on September 13, 2020, 10:12:06 am
What are they referring to when the question asks, calculate the concentration in the cleaning solution of a titration? What is the cleaning solution?
Could you please share the whole question?
I'm not sure how to answer this without more information.
Post by: cunglee0805 on September 19, 2020, 05:36:38 pm
Post by: fun_jirachi on September 19, 2020, 06:45:54 pm
hi!! I had a question about standard electrode potentials. In the data booklet the all the reactions are reduction reactions, so If you reverse an equation to write the oxidation reaction do you have to change the sign (+/-) of the E° value ?
Yes, you have to change the sign :)
Post by: cunglee0805 on September 20, 2020, 12:11:01 pm
Yes, you have to change the sign :)
thanks so much! I had another question when asked to explain what is occurring in a system at a molecular and atomic level as it approaches equilibrium/when equilibrium is established what points do we need to cover?
Post by: keltingmeith on September 20, 2020, 06:07:27 pm
thanks so much! I had another question when asked to explain what is occurring in a system at a molecular and atomic level as it approaches equilibrium/when equilibrium is established what points do we need to cover?
Good question! What do you think is all the points?
Post by: cunglee0805 on September 20, 2020, 09:13:36 pm
Good question! What do you think is all the points?
hmm I'm not quite sure but I know that with equilibriums bonds between reactants and products are constantly being broken and reformed. also that the rates of the forward and reverse reaction are equal but I really don't know how to distinguish between molecular level and atomic level
Post by: keltingmeith on September 20, 2020, 10:33:20 pm
hmm I'm not quite sure but I know that with equilibriums bonds between reactants and products are constantly being broken and reformed. also that the rates of the forward and reverse reaction are equal but I really don't know how to distinguish between molecular level and atomic level
Right, so your issue isn't talking about things on a molecular level, but rather to differentiate between an atomic and a molecular level. I'm gonna be honest, at a guess, I feel like this is so complex that you probably won't be asked about this - but with no previous exams to inform us of whether the QCAA will discuss this or not, we don't have the luxury of saying, "this is beyond the scope of QCE - don't worry".
So, let's talk worst-case scenario. You seem quite comfortable with the molecular description of things - the rates of the forward reaction and reverse reaction are equal. I'm gonna use a case example that you should be familiar with - the self-ionization of water:
Alright, so from this, we know that if we only have one free proton and one free hydroxide ion, we have 1,000,000,000,000 (1 quadrillion, holy fuck) molecules of water. But this is only a snapshot, and what's actually happening is that the rate at which one of those quadrillion water molecules breaks apart, the free proton and hydroxide ion are also recombining into water. This is what you described on a molecular level, but for some not-yet-decided reaction - I just picked a specific example.
What about on the atomic level? Well, if a molecular description talks about the things that are smaller than a molecule (bonds between atoms in the molecule), then an atomic level descriptions talks about the things that are smaller than atoms - protons, neutrons, and electrons. It turns out that protons and neutrons don't actually do much in chemical reactions, only the electrons do, so that's what we should talk about.
Well, on the atomic level, you have an oxygen bound to two hydrogens. Over time, the bond between one of the hydrogens is changing - the electrons that are being shared will move towards the oxygen, so that the oxygen now has 7 electrons solely in its valence shell, instead of 6. At the same time, an oxygen attached to a hydroxide ion has 7 electrons solely in its valence shell - to which a free proton will approach it, and slowly one of those electrons will enter the first energy level of the free proton, and so now the oxygen shares two electrons, not just 1, and only has 6 electrons solely in its valence shell.
Make sense? An atomic description would discuss the electrons within atoms, just as a molecular description would discuss the bonds within molecules. Tbh, in an exam, I would hope that either would be expected, and that either would be accepted, as long as you get the key point across that whatever description you use, the key point that the rate at which these events happen is happening is the same, is used. But, without prior evidence, it's a bit hard to make this statement if you want to be 100% safe - though it is worth noting that usually these assessment authorities WANT you to do well, and so normally err on the side of the reason.
EDIT: also highly welcome people's collective input on this last paragraph - maybe they've heard rumours, or have their own thoughts on whether this level of detail is too much, or whether it's reasonable to expect the QCAA to be this nitpicky or not. My thought is it's not and you don't need to be ready to answer on this level of nuance, but discussion is great for this kind of thing!
Post by: Bri MT on September 21, 2020, 12:59:09 pm
The QCAA syllabus dot points in question here are:
• appreciate that observable changes in chemical reactions and physical
changes can be described and explained at an atomic and molecular level
• symbolise equilibrium equations by using ⇌ in balanced chemical equations
• understand that, over time, physical changes and reversible chemical
reactions reach a state of dynamic equilibrium in a closed system, with the
relative concentrations of products and reactants defining the position of equilibrium
I think it's important to note here that these are two separate dot points rather than a statement about applying atomic understanding to equilibrium. Additionally, the use of "appreciate" as the verb suggests to me that a high level of detail would not be expected.
So yeah, I think keltingmeith's description is great for your understanding but I highly doubt that it would be needed.
If you really want to chase this up, I recommend looking at WACE given that they have these dot points:
observable changes in chemical reactions and physical changes can be described and explained at an
atomic and molecular level
over time, in a closed system, reversible physical and chemical changes may reach a state of dynamic
equilibrium, with the relative concentrations of products and reactants defining the position of
equilibrium
Post by: laurahhh on September 22, 2020, 02:57:45 pm
Post by: keltingmeith on September 22, 2020, 03:00:55 pm
Hello! I have been stuck on this question for about an half hour and I literally don't know how the textbook got the answer.
Okay, so what have you tried? I could just give you the answer, but it'd be a great bonus if I could also tell you where you went wrong in your approach ;)
Post by: laurahhh on September 22, 2020, 03:16:48 pm
Okay, so what have you tried? I could just give you the answer, but it'd be a great bonus if I could also tell you where you went wrong in your approach ;)
Ah well, I tried to work backwards from the answer and I'm no where close. So it's easy to say that I haven't done anything right. :-\
Post by: keltingmeith on September 22, 2020, 03:47:02 pm
Ah well, I tried to work backwards from the answer and I'm no where close. So it's easy to say that I haven't done anything right. :-\
Yeah, so like, answers a crutch - you don't really learn if relying on answers or worked solutions. Think of doing your QCE like you were running a race. Let's say you go out all the time and practice. Every day. But all you ever do is walk. Then, it comes time to do that race - your body is used to being outside, it's used to moving a lot, but the moment you start to run, it has no idea what to do, because it's too used to walking. Sure, you'll probably do better than if you didn't do all that walking, but will you do as well as you possible can? Always try just doing a question first - even if you don't know where to start, just try something. You'll learn a lot more that way ;)
As for this one, we have a lot of numbers that are potentially just there to throw us off. So let's break everything up into steps:
1. Sodium carbonate solution was made. [Na2CO3] is now known. Numbers were 5.267g of sodium carbonate in 250mL of water.
2. Sodium carbonate was titrated with unknown HCl. [HCl] is now known. Numbers were 10 mL of sodium carbonate solution with 21.3 mL of hydrochloric acid solution.
3. HCl was titrated with unknown Ba(OH)2. [Ba(OH)2] is now known. Numbers were 25 mL of barium hydroxide and 27.1 mL of hydrochloric acid.
So, let's do a together. So, if we look at our steps, we need to look at step 2. The easiest way to do this is to start with an equation we KNOW we have to use, then figure out what we do and don't have in that equation, and work backwards. So, to figure out the concentration of HCl, we use the equation:
We know V, but not n. Okay, so we need to find n - we can use the fact that 10mL of the carbonate solution was used to react with the HCl. Okay, since they react by the chemical equation:
We can use the mathematical equation:
Okay, so we need the amount of mol of sodium carbonate. Well, to find that, we HAVE to use the information in step 2. Remember - if you want the mol from a step, you ALWAYS have to use the mol from the same step. So that would mean the mol from the 10mL, so we use the equation:
But we don't know the concentration for sodium carbonate!! So, the question is, can we use the same concentration from step 1 in step 2? The answer is yes - you can never use information from a later step, but you can use information from a previous step if it does not change. The concentration of sodium carbonate does not change from step 1 to step 2, so we can use it. This means we use the equation:
I'm just gonna call those n1 and V1 to make my life easier. Okay, so we know the volume is 250mL - but we don't know n! So, we have to find n. Ah, but we do know that sodium carbonate was weighed, so we can use the equation:
And finally, we have all of those numbers! So, we calculate n1:
And we can finally go through all of our previous equations:
which gives us the final molarity of HCl - 0.0467.
So I'm sure this felt like a lot of maths to do all at once, but remember - all we did is figure out what equation we had to use, and then just went backwards one at a time - which should be a little less daunting if you go through this process yourself. Why don't you now try b and c, and see if you can do those yourself?
Post by: laurahhh on September 22, 2020, 08:16:03 pm
Yeah, so like, answers a crutch - you don't really learn if relying on answers or worked solutions. Think of doing your QCE like you were running a race. Let's say you go out all the time and practice. Every day. But all you ever do is walk. Then, it comes time to do that race - your body is used to being outside, it's used to moving a lot, but the moment you start to run, it has no idea what to do, because it's too used to walking. Sure, you'll probably do better than if you didn't do all that walking, but will you do as well as you possible can? Always try just doing a question first - even if you don't know where to start, just try something. You'll learn a lot more that way ;)
Thank you! :)
I got halfway there so I guess I'm not too bad. But the answer from the textbook said the molarity of HCl was 0.5M. Which is why I'm very confused.
Post by: keltingmeith on September 22, 2020, 08:33:30 pm
Thank you! :)
I got halfway there so I guess I'm not too bad. But the answer from the textbook said the molarity of HCl was 0.5M. Which is why I'm very confused.
So, I independently went over my answer again, and realised I did make one mistake - the mole ratio of HCl and carbonate above is wrong, so it should be:
But that brings the final answer to 0.2 M (multiply my original answer by 4), not 0.5 M. I disagree with both, because the smallest amount of sig figs we're given is 3, not 1. But yeah, not sure how they get 0.5 M, likely a typo. I also looked up the question on-line and found a bunch of other people answering it, and they all agree with me. So, if you're not willing to trust some random on AN, how about some random on AN + 2 other randoms on the internet ;)
Post by: Bri MT on September 23, 2020, 07:35:24 am
Thank you! :)
I got halfway there so I guess I'm not too bad. But the answer from the textbook said the molarity of HCl was 0.5M. Which is why I'm very confused.
So, I independently went over my answer again, and realised I did make one mistake - the mole ratio of HCl and carbonate above is wrong, so it should be:
But that brings the final answer to 0.2 M (multiply my original answer by 4), not 0.5 M. I disagree with both, because the smallest amount of sig figs we're given is 3, not 1. But yeah, not sure how they get 0.5 M, likely a typo. I also looked up the question on-line and found a bunch of other people answering it, and they all agree with me. So, if you're not willing to trust some random on AN, how about some random on AN + 2 other randoms on the internet ;)
It's not rare for textbooks to have some mistakes in them.
Highschool doesn't really have the right terminology for it but I had keltingmeith as a "substitute teacher" in 1st year uni chemistry & as his signature says he's been doing a PhD in chemistry - I'd definitely recommend you trust this "random" over the textbook answer.
Post by: Declan.B on September 29, 2020, 12:09:18 pm
The QCE syllabus mentions using analytical techniques, including x-ray crystallography to determine the structure of organic molecules. I haven't done much of that in class and have had some difficulty finding questions relevant to this online. Also, the mock exam had no significant question relating to x-ray crystallography in this way.
I'm happy with my knowledge of how x-ray crystallography works, but I'm having trouble finding questions where it is used to predict the structures of organic molecules at a year 12 level.
Post by: keltingmeith on September 29, 2020, 01:00:43 pm
Hi,
The QCE syllabus mentions using analytical techniques, including x-ray crystallography to determine the structure of organic molecules. I haven't done much of that in class and have had some difficulty finding questions relevant to this online. Also, the mock exam had no significant question relating to x-ray crystallography in this way.
I'm happy with my knowledge of how x-ray crystallography works, but I'm having trouble finding questions where it is used to predict the structures of organic molecules at a year 12 level.
So, I've done x-ray crystallography before. It's hard. And not just the doing it bit, but the actual trying to use it to find a structure bit. We're talking about requiring to do millions of calculations for a 10 atom asymmetric unit. You won't be expected to predict a structure based on x-ray data - it's just not feasible. I think it's safe to assume you should only need to be able to identify XRD as a potential tool, or maybe to discuss how it works. I think the fact that it's not directly in the sample exams is quite telling of what you can expect from it.
Post by: Declan.B on September 29, 2020, 01:12:17 pm
So, I've done x-ray crystallography before. It's hard. And not just the doing it bit, but the actual trying to use it to find a structure bit. We're talking about requiring to do millions of calculations for a 10 atom asymmetric unit. You won't be expected to predict a structure based on x-ray data - it's just not feasible. I think it's safe to assume you should only need to be able to identify XRD as a potential tool, or maybe to discuss how it works. I think the fact that it's not directly in the sample exams is quite telling of what you can expect from it.Alright, thank you.
It did seem very complex from what I saw online, as is why I was wondering the year 12 applicable parts. Yet, this is what the syllabus stated on it:
"select and use data from analytical techniques, including mass spectrometry,
x-ray crystallography and infrared spectroscopy, to determine the structure of
organic molecules".
Does this seem to be an error on the QCAA's part with the x-ray crystallography component?
Post by: keltingmeith on September 29, 2020, 01:15:42 pm
Alright, thank you.
It did seem very complex from what I saw online, as is why I was wondering the year 12 applicable parts. Yet, this is what the syllabus stated on it:
"select and use data from analytical techniques, including mass spectrometry,
x-ray crystallography and infrared spectroscopy, to determine the structure of
organic molecules".
Does this seem to be an error on the QCAA's part with the x-ray crystallography component?
I don't know if I'd call it an error - I think they just didn't think it through very well lmao. Usually study designs are made before sample questions, so I think they made the study design, then sat down to write questions and just went, "... fuck, this is harder than I thought it would be".
Post by: Declan.B on September 29, 2020, 01:43:32 pm
I don't know if I'd call it an error - I think they just didn't think it through very well lmao. Usually study designs are made before sample questions, so I think they made the study design, then sat down to write questions and just went, "... fuck, this is harder than I thought it would be".
oh ok, thanks for that
Post by: babo on October 22, 2020, 02:31:45 pm
Post by: keltingmeith on October 22, 2020, 03:32:16 pm
Hello, i was wondering if we needed to know the structures of nanotubes and how they were made. Also for other molecular machines? as the syllabus is very vague about this. do we need to know about their reactions conditions?
Had to double-take when you said molecular machines - that's what my PhD project is on, hahah! So, molecular machines themselves (the thing that won the Nobel prize in 2016) are not on the syllabus, so maybe you're using the term to mean something else? I'm guessing you mean molecular manufacturing? (I know it seems like manufacturing and machines should be interchangeable - but annoyingly, they're not, sorry about that. Though presumably for the purposes of QCE, the assessors probably won't mind)
I'm not an authority on this, and I only know as much as anybody else who has read the syllabus. My understanding of that section is that you don't need to be able to go into a great deal of depth about any of this. You should understand that there are things made by very specifically positioning them in certain ways, and not just by having a bunch jumbled around in solution (eg, proteins are made by attaching amino acids to a solid support, such that the end carboxyl group in the primary structure is not available for bonding, but the end amine group is). I don't think you need to give reaction conditions.
Post by: babo on October 26, 2020, 09:01:59 am
Post by: piperfred on October 29, 2020, 11:07:15 am
Hello, i was wondering if we needed to know the structures of nanotubes and how they were made. Also for other molecular machines? as the syllabus is very vague about this. do we need to know about their reactions conditions?
Hi babo!
I'm studying chemistry too, and here's what I have in my notes about molecular machines and nanotubes if it helps! Just a disclaimer that I'm not certain if this is exactly what we need (like every other 2020 chemistry senior in the state), but I'm fairly confident this will put me in pretty good stead in terms of content :)
Be able to broadly define:
Designer chemicals
Nanoparticles
How nanoparticles are produced -- top-down approach and bottom-up approach
Stereospecificity
Host-guest systems and broad uses (used to bind to and remove hazardous substances from the environment)
The orientation effect (e.g. Ribosomes and the molecular machine Rotaxane)
By broadly define I really mean just one or two sentences to give you a rough idea. I'm sure there wouldn't be anything to complex on the exam, maybe just a multiple choice question.
Hope this helps!
Piper
Post by: keltingmeith on October 29, 2020, 05:09:56 pm
Hi babo!
I'm studying chemistry too, and here's what I have in my notes about molecular machines and nanotubes if it helps! Just a disclaimer that I'm not certain if this is exactly what we need (like every other 2020 chemistry senior in the state), but I'm fairly confident this will put me in pretty good stead in terms of content :)
Be able to broadly define:
Designer chemicals
Nanoparticles
How nanoparticles are produced -- top-down approach and bottom-up approach
Stereospecificity
Host-guest systems and broad uses (used to bind to and remove hazardous substances from the environment)
The orientation effect (e.g. Ribosomes and the molecular machine Rotaxane)
By broadly define I really mean just one or two sentences to give you a rough idea. I'm sure there wouldn't be anything to complex on the exam, maybe just a multiple choice question.
Hope this helps!
Piper
Okay, so like, I'm now really intrigued since you've specifically mentioned rotaxanes. Not saying you're wrong at all - because nobody knows what the hell is going to be on this exam - but am curious if rotaxanes were taught by your teacher or the textbook? It does make sense for the "orientation effect" (which is a fancy term that NOBODY in the field actually uses lmao, classic high school trying to make things complicated) to be assessable, and rotaxanes are an example of using the orientation effect, but still am curious
Post by: piperfred on October 29, 2020, 07:29:35 pm
Okay, so like, I'm not really intrigued since you've specifically mentioned rotaxanes. Not saying you're wrong at all - because nobody knows what the hell is going to be on this exam - but am curious if rotaxanes were taught by your teacher or the textbook? It does make sense for the "orientation effect" (which is a fancy term that NOBODY in the field actually uses lmao, classic high school trying to make things complicated) to be assessable, and rotaxanes are an example of using the orientation effect, but still am curious
Hiya!
My textbook (QCE Unit 3 and 4 Pearson Chemistry) used rotaxanes as an example of a molecular machine that uses orientation effect. It was pretty brief, only half a page I think, including information about them mimicking ribosomes and the fact that they're used to produce peptides.
It's funny that you say no one uses the phrase "orientation effect"! I remember there being a multi-choice question on it in the mock and a heap of us being surprised they went into that much detail; that's partly why I've memorised rough details about rotaxanes because I found the mock much more detail-orientated than I expected.
Post by: pamon2 on November 30, 2020, 07:01:25 pm
This is my first ever post, so apologies if I've put this in the wrong place or if I'm lacking in forum etiquette, I'm really quite new to this ^^ However, I really just wanted to ask about data analysis in the sciences. Particularly, how does one thoroughly analyse data in chem, phy and bio (and all other sciences) in order to reach those 20s? + Where can I go to teach myself how to analyse data and would anyone have any online resources they could offer pertaining to this?
I don't think my school properly equipped anyone with the required data analysis skills in Grade 10, so going into Grade 11 no one could get an A in chem :( I'm hoping to work on this before Grade 12.
Thanks in advance,
Paul :)
Post by: Bri MT on December 01, 2020, 08:24:58 am
Hi there!
This is my first ever post, so apologies if I've put this in the wrong place or if I'm lacking in forum etiquette, I'm really quite new to this ^^ However, I really just wanted to ask about data analysis in the sciences. Particularly, how does one thoroughly analyse data in chem, phy and bio (and all other sciences) in order to reach those 20s? + Where can I go to teach myself how to analyse data and would anyone have any online resources they could offer pertaining to this?
I don't think my school properly equipped anyone with the required data analysis skills in Grade 10, so going into Grade 11 no one could get an A in chem :( I'm hoping to work on this before Grade 12.
Thanks in advance,
Paul :)
Hey,
Welcome to the forums! :)
You might have seen already but I have made a series of articles talking about data test preparation which you might find helpful:
🧬 Biology: https://atarnotes.com/biology-data-test-advice-qce/
🧪 Chemistry: https://atarnotes.com/chemistry-data-test-advice-qce/
🧲 Physics: https://atarnotes.com/physics-data-test-advice-qce/
🧠 Psychology: https://atarnotes.com/psychology-data-test-advice-qce/
Great to hear that you're tackling this early!
If you look at page 25 of the chemistry syllabus you'll see that it has a section on measurement uncertainty and error. Everything in that part you want to become familiar with (and not just for chemistry). The Victorian equivalent of QCAA has a section for their science subjects where they give advice on scientific investigations and measurement in science which you might find useful. Please note that the information on assessment is different across states, but the meaning of uncertainty, accuracy, significant figures etc. is not.
Please feel free to follow up if you have any more questions, comments etc. :)
Post by: jasmine24 on January 19, 2021, 04:05:57 pm
Thanks!
Post by: Bri MT on January 19, 2021, 04:09:08 pm
Hi, I was wondering how to identify the endpoint on a titration curve. I know its when the indicator changes colour but im not sure how this could be identified by just looking at the curve
Thanks!
Hi!
You know that bit where the curve goes really steep? Like it's basically straight-up? That's what you're looking for
no worries :)
Post by: jasmine24 on January 19, 2021, 07:34:22 pm
Hi!So is it the whole steep part? I was working on a question from my textbook with a graph sort of like the one i attached and it said the endpoint was at the point i labelled and I'm confused as to how it is
You know that bit where the curve goes really steep? Like it's basically straight-up? That's what you're looking for
no worries :)
Thank you
Post by: Bri MT on January 19, 2021, 07:38:58 pm
Post by: jasmine24 on February 02, 2021, 06:29:32 pm
TIA :)
Question: https://www.qcaa.qld.edu.au/downloads/senior-qce/sciences/snr_chemistry_19_ia1_smple_ass_inst.pdf
Answers: https://www.qcaa.qld.edu.au/downloads/senior-qce/sciences/snr_chemistry_19_ia1_smple_m_scheme.pdf
Post by: Bri MT on February 04, 2021, 01:44:32 pm
Hi, would anyone be able to explain dataset 1 - item 3. Im not sure how pKa = 14 - 9.2 as i thought at the half-equivalence point pKa = pH
TIA :)
Question: https://www.qcaa.qld.edu.au/downloads/senior-qce/sciences/snr_chemistry_19_ia1_smple_ass_inst.pdf
Answers: https://www.qcaa.qld.edu.au/downloads/senior-qce/sciences/snr_chemistry_19_ia1_smple_m_scheme.pdf
Hey!
The first thing that draws my attention here is is:
Ammonia is a base and is the substance of unknown concentration (analyte)
We are adding to this our HCl, an acid and our substance of known concentration (titrant)
For all of this I'm assuming 25 C as per usual in QCE chem pH questions
Remember we started with a base and we're adding acid, at this stage we are still on the basic side. At the half-equivalence point we have that half of the base has reacted with the HCl, so pKb = pOH by the exact same logic that gets you to the more commonly discussed pH = pKa.
Remembering that: pKa + pKb = 14 = pH + pOH, we then just do a quick conversion to get the desired value.
We can think of it like this:
An ammonium ion (right hand side of the equation) has a pKa value of roughly 9.2. We want to get the reverse-direction value, ammonia's one, so we do 14 - pk[ammonium] = 14 - 9.2.
We can also think of it like this:
We have pH of 9.2. We want pOH since we're looking at the constant for a base. 14 - pH = 14 - 9.2
TLDR; QCAA used pKa even though they were asking you about a base, which makes things confusing.
Let me know if this clears things up :)
Post by: jasmine24 on February 04, 2021, 08:16:17 pm
Hey!Thank you so much, this def cleared things up! Since they asked for the pKa i just assumed it was an acid
The first thing that draws my attention here is is:
Ammonia is a base and is the substance of unknown concentration (analyte)
We are adding to this our HCl, an acid and our substance of known concentration (titrant)
For all of this I'm assuming 25 C as per usual in QCE chem pH questions
Remember we started with a base and we're adding acid, at this stage we are still on the basic side. At the half-equivalence point we have that half of the base has reacted with the HCl, so pKb = pOH by the exact same logic that gets you to the more commonly discussed pH = pKa.
Remembering that: pKa + pKb = 14 = pH + pOH, we then just do a quick conversion to get the desired value.
We can think of it like this:
An ammonium ion (right hand side of the equation) has a pKa value of roughly 9.2. We want to get the reverse-direction value, ammonia's one, so we do 14 - pk[ammonium] = 14 - 9.2.
We can also think of it like this:
We have pH of 9.2. We want pOH since we're looking at the constant for a base. 14 - pH = 14 - 9.2
TLDR; QCAA used pKa even though they were asking you about a base, which makes things confusing.
Let me know if this clears things up :)
Post by: justsomerandom21 on July 13, 2021, 05:32:51 pm
Post by: jinx_58 on August 05, 2021, 08:09:02 pm
Experiments were conducted to see the reaction between sodium thiosulphate and hydrochloric acid.
At times, the sulfur dioxide was lost to the atmosphere whereas sometimes it was trapped in the reaction.
Would this affect the reaction rate?
Many thanks,
jinx_58
Post by: jasmine24 on August 22, 2021, 01:38:40 pm
"appreciate that each class of organic compound displays characteristic chemical properties and undergoes specific reactions based on the functional group present; these reactions, including acid-base and oxidation reactions, can be used to identify the class of the organic compound"
I cant find any notes on it :(
Post by: Bri MT on August 24, 2021, 10:14:40 am
can someone help me with this syllubus objective?
"appreciate that each class of organic compound displays characteristic chemical properties and undergoes specific reactions based on the functional group present; these reactions, including acid-base and oxidation reactions, can be used to identify the class of the organic compound"
I cant find any notes on it :(
Hey!
This is is about understanding molecules will react differently depending on their functional groups. For example, alkenes can undergo addition reactions but alkanes can't since alkanes only have single carbon-carbon bonds. Therefore, if we are unsure whether a molecule is an alkane or alkene we can test this by seeing whether it is able to undergo addition reactions (i.e. using the bromine test). Similarly, we know that primary or secondary (but not tertiary) alcohols can experience oxidation and that aldehydes (but not ketones) can be oxidised to carboxylic acids.
I hope this helps :)
Post by: justsomerandom21 on March 09, 2022, 10:42:08 pm
Do any kind souls have any advice on good titrations IA2s (student experiment)? Any advice on things that you did that went well would be appreciated too :).
Thanks,
justsomerandom21
Post by: jinx_58 on March 14, 2022, 09:47:26 am
Howdy,
Do any kind souls have any advice on good titrations IA2s (student experiment)? Any advice on things that you did that went well would be appreciated too :).
Thanks,
justsomerandom21
Greetings!
I am doing the same thing right now. I went through the Deadly EEI website (http://seniorchem.com/eei.html), and found some cool pracs there.
Hope this helps,
- jinx_58
Post by: tr0206 on April 19, 2022, 12:51:07 am
Just a quick question for a Year 11 assignment; I'm investigating PEM fuel cells against Lithium-Ion Batteries, and am trying to find a meaningful way to compare their energy outputs.
As far as I can understand, a Fuel Cell rated as having a 250kW capacity would have an equivalent capacity of 250kWh assuming it had enough fuel (Hydrogen) for one hour of travel at (x) speed. I thought this because internet research told me that kWh = kW x (Time). Is this logic accurate for comparing kWh to kW between Fuel Cells and Batteries? If not, could anyone recommend another method of comparing these units.
Thanks.
Post by: priya22 on May 07, 2022, 11:04:38 am
For chemistry I did the galvanic cell prac. My group used magnesium sulphate (with aluminum electrode) and copper sulphate (copper electrode). We tested different concentrations and had so many issues with getting the voltage but, we eventually got our results.
The results are pretty all over the place (doesn't show any correlation- the number are like {it is in order}; 0.08, 0.05, 0.14, 0.53, 0.15) and now i'm stuck trying to figure out how to write for the rest of the report and the kind of analysis required to get some good analysis done.
I would like some help with what I can actually write.
Post by: Phytoplankton on May 07, 2022, 06:42:28 pm
hi people,
For chemistry I did the galvanic cell prac. My group used magnesium sulphate (with aluminum electrode) and copper sulphate (copper electrode). We tested different concentrations and had so many issues with getting the voltage but, we eventually got our results.
The results are pretty all over the place (doesn't show any correlation- the number are like {it is in order}; 0.08, 0.05, 0.14, 0.53, 0.15) and now i'm stuck trying to figure out how to write for the rest of the report and the kind of analysis required to get some good analysis done.
I would like some help with what I can actually write.
Hey priya22,
Yeah, those results do seem pretty all over the place. So, there are a few recommendations I would give you.
The first one would be depending on your due date. If you have a bit of time, I would recommend redoing the practical. My guess as to what would be wrong is the voltmeter so I would consider switching out the voltmeter for another one if you were redoing it.
If redoing the practical is not an option, then I would go and ask your teacher what you should do in this case and maybe even ask if she has sample/past-practical data you can use for the report. If she does give data to you, then hooray, you get to analyse some good data. If she doesn't, then she'll give you a way to tackle the problem and more insight on how to work with it.
If in the very unlikely case that asking your teacher doesn't work out at all, then I would analyse the data you have right now (while trying to make some sense with it in terms of log, linear, polynomial graphs etc.) and attribute the lack of valid and reliable data to the countless errors in a school laboratory such as variation in lab conditions, equipment uncertainty, experimenter errors etc.
TLDR: ask your teacher.
Anyways, hope this gave you even the slightest idea what to do but feel free to ask any more questions.
Post by: priya22 on May 09, 2022, 11:15:15 am
Hey priya22,Thank you very much!
Yeah, those results do seem pretty all over the place. So, there are a few recommendations I would give you.
The first one would be depending on your due date. If you have a bit of time, I would recommend redoing the practical. My guess as to what would be wrong is the voltmeter so I would consider switching out the voltmeter for another one if you were redoing it.
If redoing the practical is not an option, then I would go and ask your teacher what you should do in this case and maybe even ask if she has sample/past-practical data you can use for the report. If she does give data to you, then hooray, you get to analyse some good data. If she doesn't, then she'll give you a way to tackle the problem and more insight on how to work with it.
If in the very unlikely case that asking your teacher doesn't work out at all, then I would analyse the data you have right now (while trying to make some sense with it in terms of log, linear, polynomial graphs etc.) and attribute the lack of valid and reliable data to the countless errors in a school laboratory such as variation in lab conditions, equipment uncertainty, experimenter errors etc.
TLDR: ask your teacher.
Anyways, hope this gave you even the slightest idea what to do but feel free to ask any more questions.
It's too late to ask the teacher, so I'll go with the last option.
I have a few more questions: for the experiment I used a aluminium electrode in magnesium sulphate in the half-cell that was used to change the concentration (cause there was no magnesium), is this correct, what are the effects of not using a magnesium electrode (in the MgSO4), and how would I talk about this in the report?
Plus, I had trouble with getting the voltage to show up on the voltage meter, i suspect it's due to the use of aluminium instead of magnesium, is this correct?
Post by: Phytoplankton on May 10, 2022, 05:46:15 pm
Thank you very much!
It's too late to ask the teacher, so I'll go with the last option.
I have a few more questions: for the experiment I used a aluminium electrode in magnesium sulphate in the half-cell that was used to change the concentration (cause there was no magnesium), is this correct, what are the effects of not using a magnesium electrode (in the MgSO4), and how would I talk about this in the report?
Plus, I had trouble with getting the voltage to show up on the voltage meter, i suspect it's due to the use of aluminium instead of magnesium, is this correct?
Happy to help!
Yes, you are correct about the voltage. Your voltmeter won't show any voltage because your redox reaction in that half-cell simply cannot, spontaneously, occur. This is because the two species at play in that half cell here are Al solid metal and Mg2+ (which is the MgSO4 as the electrolyte). As seen in the electrochemical series, Mg is more reactive than Al (in other words, more likely to oxidise or lose electrons). So in order for your galvanic cell to work, the Mg would have to take in electrons (reduce) but it won't allow that because Mg is stronger than Al to do what it wants (so Mg2+ likes its ion state). You can also see this in the standard electrode potential table where the value for Mg reduction is much lower than Al suggesting that Mg does not like to be reduced.
To answer your first question, to make a galvanic cell work, the electrode is supposed to be either an inert metal or the metal of the solution. So, in your case, your aluminium electrode is neither inert nor (obviously) Mg (as MgSO4 is your electrolyte solution). If you place a metal (Mg) into its own metal solution (MgSO4) and keep your Cu electrode in your CuSO4 solution, then you will certainly get a voltage. This is because, now the Mg electrode will freely break down into its ions (as it likes to be oxidised) and the Cu2+ ions will deposit as copper solid onto the copper electrode (as it likes to be reduced). This movement of electrons (and ions) will produce a voltage equal to the difference of the electrode potentials in the table.
As for what you would do in the report, I recommend you talk about this theory I talked about above (with more elaboration) and thus, explain why you didn't get a voltage with your respective electrodes. The issue is that, assuming you didn't get any voltage values at all for your experiment, you wouldn't have any graphs/tables to do any analysis in which you may lose marks on the two criteria below:
- "appropriate application of algorithms, visual and graphical representations of data about chemical equilibrium systems or oxidation and reduction demonstrated by correct and relevant processing of data"
- "thorough identification of relevant trends, patterns or relationships"
In the end, you want to discuss all of these issues in your evaluation area where you talk about your errors, however, I really don't have any advice to help you in the two criteria above as you don't have any graphs/tables to process and analyse (really sorry about that). Remember, you will still have raw data and it will be all 0 as you didn't get any voltages.
I hope this helped but try the best you can to analyse and relate it to your theory/rationale. Again, feel free to ask more questions.
Post by: priya22 on May 14, 2022, 10:11:21 am
Happy to help!
Yes, you are correct about the voltage. Your voltmeter won't show any voltage because your redox reaction in that half-cell simply cannot, spontaneously, occur. This is because the two species at play in that half cell here are Al solid metal and Mg2+ (which is the MgSO4 as the electrolyte). As seen in the electrochemical series, Mg is more reactive than Al (in other words, more likely to oxidise or lose electrons). So in order for your galvanic cell to work, the Mg would have to take in electrons (reduce) but it won't allow that because Mg is stronger than Al to do what it wants (so Mg2+ likes its ion state). You can also see this in the standard electrode potential table where the value for Mg reduction is much lower than Al suggesting that Mg does not like to be reduced.
To answer your first question, to make a galvanic cell work, the electrode is supposed to be either an inert metal or the metal of the solution. So, in your case, your aluminium electrode is neither inert nor (obviously) Mg (as MgSO4 is your electrolyte solution). If you place a metal (Mg) into its own metal solution (MgSO4) and keep your Cu electrode in your CuSO4 solution, then you will certainly get a voltage. This is because, now the Mg electrode will freely break down into its ions (as it likes to be oxidised) and the Cu2+ ions will deposit as copper solid onto the copper electrode (as it likes to be reduced). This movement of electrons (and ions) will produce a voltage equal to the difference of the electrode potentials in the table.
As for what you would do in the report, I recommend you talk about this theory I talked about above (with more elaboration) and thus, explain why you didn't get a voltage with your respective electrodes. The issue is that, assuming you didn't get any voltage values at all for your experiment, you wouldn't have any graphs/tables to do any analysis in which you may lose marks on the two criteria below:
- "appropriate application of algorithms, visual and graphical representations of data about chemical equilibrium systems or oxidation and reduction demonstrated by correct and relevant processing of data"
- "thorough identification of relevant trends, patterns or relationships"
In the end, you want to discuss all of these issues in your evaluation area where you talk about your errors, however, I really don't have any advice to help you in the two criteria above as you don't have any graphs/tables to process and analyse (really sorry about that). Remember, you will still have raw data and it will be all 0 as you didn't get any voltages.
I hope this helped but try the best you can to analyse and relate it to your theory/rationale. Again, feel free to ask more questions.
Thank you. but now my teacher told me to work on another group's results. I am not sure how Zinc nitrate and Copper sulphate voltaic cells (the other group's one) works could you explain?
and their data is decreasing over an increasing concentration, I am not sure why?
Post by: Phytoplankton on May 17, 2022, 07:32:37 am
Thank you. but now my teacher told me to work on another group's results. I am not sure how Zinc nitrate and Copper sulphate voltaic cells (the other group's one) works could you explain?
and their data is decreasing over an increasing concentration, I am not sure why?
Hey priya22,
Ok that sounds a bit better now. Copper sulfate and zinc nitrate is one of the most common galvanic cells school's work on and I'm assuming its a copper electrode in the CuSO4 and a zinc electrode in Zn(NO3)2. All you have to do is apply the same theory I talked about before to this as well. So, take a look at either the electrochemical series or the reactivity series. You can see that zinc is more reactive meaning it would like to oxidise (lose electrons) - this can be seen as it is higher in the reactivity series and shows a lower Eo in the electrochemical series. This means that the zinc electrode will happily oxidise into its ions and thus, the copper ions in the electrolyte will happily deposit solid copper onto the electrode. This will produce a movement of electrons (with help from the salt bridge as well), creating electricity. Ensure you discuss the movement of ions and electrons in the solutions and the salt bridge.
As for the data you got, the voltage should be increasing as you increase the concentration and assuming you mean voltage when you say "data", that shouldn't be right. Increasing concentration means you increase the ions in the electrolyte which means it carries more of an electrical current, yielding a higher voltage. If the decrease of your voltage is small, I think you should be able to explain it through errors or otherwise, your other group may have accidentally wrote the data the wrong way so you can just fix that. That's really all I can say about that.
It was good that you got another group's data as well as half cells that make sense. Good luck and again, feel free to ask questions!