Login

Welcome, Guest. Please login or register.

March 29, 2024, 08:21:31 am

Author Topic: HSC Chemistry Question Thread  (Read 1040652 times)  Share 

0 Members and 2 Guests are viewing this topic.

Happy Physics Land

  • ATAR Notes Legend
  • Forum Obsessive
  • ***
  • Posts: 335
  • MAXIMISE your marks by MINIMISING your errors
  • Respect: +38
Re: Chemistry Question Thread
« Reply #120 on: March 07, 2016, 05:06:10 pm »
+1
Hi Jake!

We did an experiment in class where we titrated sodium carbonate and hydrochloric acid, creating a neutral salt (I think... it's a strong acid and strong base right?). Our teacher told us that methyl orange was the best indicator, but this doesn't make sense to me. Shouldn't we use bromothymol blue or phenolphthalein as the indicator as the equivalence point would be around pH 7?
I thought maybe it was to do with the presence of carbon dioxide as that's an acidic oxide, but not sure if that's right or not. Unless I've gotten something completely wrong!

Thanks!

Hey Grace:

That was a great experiment you did, and this actually is a pretty interesting question. One thing I can assure you is that the existence of CO2(g) as an outcome of this reaction definitely wouldnt affect which indicator to use, because CO2(aq) is the one that  would react with water to form H2CO3(aq) which is acidic. Of course you may wanna argue that theres an equilibrium equation of CO2(g) --> CO2(aq) and some CO2(g) may have converted into CO2(aq) due to slight changes in temperature or pressure. Even if this equilibrium reaction does take place, its usually almost negligible and it definite wouldnt influence your choice of indicators.

In regards to which indicator to use, l think that your teacher hasnt been very accurate with her words. When we test the pH of things, we normally use two indicators at the same time to ascertain the pH range of this substance that we are testing. (If you need further elaboration on that point just ask again). So we dont usually only use one indicator, say for example we do use methyl orange and the NaCl we are testing in this case is a neutral substance, then the methyl orange would turn yellow. But hey, methyl orange's pH is defined over 3.1-4.4, so the yellow colour can possibly mean its a neutral substance, but at the same time it can also mean that NaCl is weakly acidic or even basic! Thats why we needs a second indicator for the purpose of determining the salt's pH range. So personally, if you have an option over which indicators use, I would use methyl red which ranges from 4.4 -6.3 pH and phenolphthalein which ranges from 8.3 -11.0 pH. So to test this salt is neutral, you would see that methyl red shows a yellow colour, and phenolphthalein showing no colour (i.e. colourless). This means that the salt has a pH that ranges from 6.3 to 8.3, and hence you can justify that the salt is neutral or just a little bit acidic or basic.

Thats my response to your question, if Jake comes around soon he can probably give you a better response than I can. But if you have any questions, dont hesitate to ask! :)

Best Regards
Happy Physics Land
Mathematics: 96
Maths Extension 2: 93
Maths Extension 1: 97
English Advanced: 92
Physics: 95
Chemistry: 92
Engineering Studies: 90
Studies of Religion I: 98

2017 ATAR: 99.70
University of Sydney Civil Engineering and Commerce
University of Sydney Faculty of Civil Engineering Scholar
Student Representatives Council Student Housing Officer
City of Sydney Council Sydney Ambassador
University of Sydney Business School Student Mentor
Entrepreneur, Company of Year Junior Achievements Australia

neeshy900

  • Fresh Poster
  • *
  • Posts: 1
  • Respect: 0
  • School: Murray High School
  • School Grad Year: 2016
Re: Chemistry Question Thread
« Reply #121 on: March 07, 2016, 06:57:33 pm »
0
Any tips on titration prac exams?? Stressing out :( :(

Happy Physics Land

  • ATAR Notes Legend
  • Forum Obsessive
  • ***
  • Posts: 335
  • MAXIMISE your marks by MINIMISING your errors
  • Respect: +38
Re: Chemistry Question Thread
« Reply #122 on: March 07, 2016, 08:19:18 pm »
+1
Any tips on titration prac exams?? Stressing out :( :(

Hello Neeshy:

Yes I can definitely understand how those titration prac exams can be extremely concerning, but dont worry, we are all here to help you! I will provide you with some specific tips below and hope they help.

Preparation of a Primary Standard Solution:
- A standard solution is a solution of known composition and concentration used for quantitative analytical procedures (in this case, titration)
- Standard solution made by dissolving a primary standard volume of water
- Solids substances are desirable for preparing standard solution because they are easier to weigh
- It is also desirable for the substances to be easily obtained in its pure form, which reduces the cost and makes it easier to perform calculations with minimal errors involving the existence of impurities
- Substances with large molar mass is desirable to allow for reduced weighing errors
- And finally, the substance must also have a high water solubility - obviously, because the substance is required to be dissolved in water for standard solution preparation

*Procedure for preparing primary standard solutions may be obtained from your textbook

Dilution Calculations

- Since we are adding only solvent, moles of solute remains unchanged
- Before we dilute (solution 1, concentrated) the equation that applies is n1 = C1V1
- After we dilute, V and C have both changed, hence the equation that applies is n2=C2V2
- But since the moles of solute is the same before and after, hence the equation that applies is C1V1 = C2V2

General Overview of Neutralisation Reaction

- HEAT is released in neutralisation reactions between strong acids and strong bases (e.g. NaOH + HCl, the most common example)
- ACID + BASE ----> SALT + WATER
- Neutralisation reaction between strong acid and strong base is essentially the reaction of a proton and OH ion to form water

Titration

- An analytical process which involves finding the volume of a solution of accurately known conc. that is required to react completely with a known volume of a solution of unknown concentration. Hence it is a volumetric analysis.
- Summary of Procedure:
       1. A known volume of a solution of unknown conc. placed in a conical flask
       2. A solution of known conc. added until all of the analyte has reacted, volume is recorded
       3. Using the mole ratio of the reaction the unknown conc. is calculated

Calculation of Concentration of an Unknown Solution:

1. Write a balanced equation
2. Calculate number of moles of reactant A (n=CV)
3. Using the moles of reactant A and the stoichiometric molar ratio in the equation, calculate the number of moles of Reactant B used
4. Use C=n/V to calculate the conc. of Reactant B

These are basically all I have to say, a bit content heavy, if Jake happens to come around he can give you more advices. Anyways good luck my friend, if you have any questions, dont hesitate to ask!

Best Regards
Happy Physics Land
Mathematics: 96
Maths Extension 2: 93
Maths Extension 1: 97
English Advanced: 92
Physics: 95
Chemistry: 92
Engineering Studies: 90
Studies of Religion I: 98

2017 ATAR: 99.70
University of Sydney Civil Engineering and Commerce
University of Sydney Faculty of Civil Engineering Scholar
Student Representatives Council Student Housing Officer
City of Sydney Council Sydney Ambassador
University of Sydney Business School Student Mentor
Entrepreneur, Company of Year Junior Achievements Australia

RuiAce

  • ATAR Notes Lecturer
  • Honorary Moderator
  • Great Wonder of ATAR Notes
  • *******
  • Posts: 8814
  • "All models are wrong, but some are useful."
  • Respect: +2575
Re: Chemistry Question Thread
« Reply #123 on: March 07, 2016, 08:25:28 pm »
0
Hi Jake!

We did an experiment in class where we titrated sodium carbonate and hydrochloric acid, creating a neutral salt (I think... it's a strong acid and strong base right?). Our teacher told us that methyl orange was the best indicator, but this doesn't make sense to me. Shouldn't we use bromothymol blue or phenolphthalein as the indicator as the equivalence point would be around pH 7?
I thought maybe it was to do with the presence of carbon dioxide as that's an acidic oxide, but not sure if that's right or not. Unless I've gotten something completely wrong!

Thanks!

Sodium carbonate is not a strong base...

Methyl orange was used because you're titrating a WEAK base with a strong acid!
_________________________

HPL mentioned to use something in it's pure form to prepare a standard solution. An example of this would be to use ANHYDROUS sodium carbonate, not hydrated sodium carbonate.
« Last Edit: March 07, 2016, 08:28:00 pm by RuiAce »

RuiAce

  • ATAR Notes Lecturer
  • Honorary Moderator
  • Great Wonder of ATAR Notes
  • *******
  • Posts: 8814
  • "All models are wrong, but some are useful."
  • Respect: +2575
Re: Chemistry Question Thread
« Reply #124 on: March 07, 2016, 08:30:33 pm »
0
Hey Grace:

That was a great experiment you did, and this actually is a pretty interesting question. One thing I can assure you is that the existence of CO2(g) as an outcome of this reaction definitely wouldnt affect which indicator to use, because CO2(aq) is the one that  would react with water to form H2CO3(aq) which is acidic. Of course you may wanna argue that theres an equilibrium equation of CO2(g) --> CO2(aq) and some CO2(g) may have converted into CO2(aq) due to slight changes in temperature or pressure. Even if this equilibrium reaction does take place, its usually almost negligible and it definite wouldnt influence your choice of indicators.

In regards to which indicator to use, l think that your teacher hasnt been very accurate with her words. When we test the pH of things, we normally use two indicators at the same time to ascertain the pH range of this substance that we are testing. (If you need further elaboration on that point just ask again). So we dont usually only use one indicator, say for example we do use methyl orange and the NaCl we are testing in this case is a neutral substance, then the methyl orange would turn yellow. But hey, methyl orange's pH is defined over 3.1-4.4, so the yellow colour can possibly mean its a neutral substance, but at the same time it can also mean that NaCl is weakly acidic or even basic! Thats why we needs a second indicator for the purpose of determining the salt's pH range. So personally, if you have an option over which indicators use, I would use methyl red which ranges from 4.4 -6.3 pH and phenolphthalein which ranges from 8.3 -11.0 pH. So to test this salt is neutral, you would see that methyl red shows a yellow colour, and phenolphthalein showing no colour (i.e. colourless). This means that the salt has a pH that ranges from 6.3 to 8.3, and hence you can justify that the salt is neutral or just a little bit acidic or basic.

Thats my response to your question, if Jake comes around soon he can probably give you a better response than I can. But if you have any questions, dont hesitate to ask! :)

Best Regards
Happy Physics Land

Unnecessary. Bromothymol blue will be GREEN at around pH7, which is a very clearly distinguishable colour from blue and yellow. As with litmus with purple and blue/red.

The rule of thumb is:

Strong B Weak A -> Phenolphthalein
Strong B Strong A -> Litmus/Bromothymol blue
Weak B Strong A -> Methyl orange

Happy Physics Land

  • ATAR Notes Legend
  • Forum Obsessive
  • ***
  • Posts: 335
  • MAXIMISE your marks by MINIMISING your errors
  • Respect: +38
Re: Chemistry Question Thread
« Reply #125 on: March 07, 2016, 09:03:30 pm »
0
Unnecessary. Bromothymol blue will be GREEN at around pH7, which is a very clearly distinguishable colour from blue and yellow. As with litmus with purple and blue/red.

The rule of thumb is:

Strong B Weak A -> Phenolphthalein
Strong B Strong A -> Litmus/Bromothymol blue
Weak B Strong A -> Methyl orange

Umm, you reckon l should just delete my post?.....
Mathematics: 96
Maths Extension 2: 93
Maths Extension 1: 97
English Advanced: 92
Physics: 95
Chemistry: 92
Engineering Studies: 90
Studies of Religion I: 98

2017 ATAR: 99.70
University of Sydney Civil Engineering and Commerce
University of Sydney Faculty of Civil Engineering Scholar
Student Representatives Council Student Housing Officer
City of Sydney Council Sydney Ambassador
University of Sydney Business School Student Mentor
Entrepreneur, Company of Year Junior Achievements Australia

lazydreamer

  • Trailblazer
  • *
  • Posts: 30
  • Newbie needing help :P
  • Respect: +1
  • School: Tech H.S
  • School Grad Year: 2016
Re: Chemistry Question Thread
« Reply #126 on: March 07, 2016, 09:06:24 pm »
+2
Any tips on titration prac exams?? Stressing out :( :(

Great run-down from HPL! You know it's a titration, so make sure you know exactly how to carry it out, step-by-step. Play it out in your mind if you want. That will give you some confidence and reassurance going into it. But don't get complacent, 'cause anything can happen!
A few tips, not sure if it'll help but...:
- There are different ways to rinse your burette, pipette and standard flask, make sure you know them!
- If you can, label your glassware, good lab. practice and prevents confusion, which you don't need in an exam yea?
- Don't forget to close the stopcock when rinsing the burette, lol
- Rinse down the edges of the flask as you titrate, esp. nearing the end-point, in case some analyte/titrant gets stuck on the sides, which will result in error

ummm that's all i can think of right now, best of luck in your exam, i'm sure you'll do great :D

RuiAce

  • ATAR Notes Lecturer
  • Honorary Moderator
  • Great Wonder of ATAR Notes
  • *******
  • Posts: 8814
  • "All models are wrong, but some are useful."
  • Respect: +2575
Re: Chemistry Question Thread
« Reply #127 on: March 07, 2016, 09:06:58 pm »
0
Umm, you reckon l should just delete my post?.....

No real point. Once it's posted it's there. It's still a learning experience and who knows if someone else will make a similar mistake.

amandali

  • Trendsetter
  • **
  • Posts: 122
  • Respect: 0
  • School: strathfield
  • School Grad Year: 2016
Re: Chemistry Question Thread
« Reply #128 on: March 09, 2016, 05:24:16 pm »
0
why is this unacceptable
HSO3- + H20 <--> SO3(2-) + H3O+
HSO3- + H2O <--> H2SO3 + OH-

to show that hydrogen sulfite ion is amphiprotic

RuiAce

  • ATAR Notes Lecturer
  • Honorary Moderator
  • Great Wonder of ATAR Notes
  • *******
  • Posts: 8814
  • "All models are wrong, but some are useful."
  • Respect: +2575
Re: Chemistry Question Thread
« Reply #129 on: March 09, 2016, 06:03:19 pm »
0
why is this unacceptable
HSO3- + H20 <--> SO3(2-) + H3O+
HSO3- + H2O <--> H2SO3 + OH-

to show that hydrogen sulfite ion is amphiprotic

Technically it should be. Because you are still showing how it can act as a base or an acid according to B-L theory.

Easiest thing to do, however, is to demonstrate it reacting with a different acid and a different base just to clear up ambiguity.
HSO3- + HCl(aq) -> H2SO3(aq) + Cl-
HSO3- + OH- -> SO32- + H2O(l)

If you got marked incorrect by a teacher, you will have to dispute it with them

Happy Physics Land

  • ATAR Notes Legend
  • Forum Obsessive
  • ***
  • Posts: 335
  • MAXIMISE your marks by MINIMISING your errors
  • Respect: +38
Re: Chemistry Question Thread
« Reply #130 on: March 09, 2016, 11:06:36 pm »
0
Hey guys, need a hand here if anyone dont mind!

The question is: with the aid of appropriate equations, explain why the dihydrogen phosphate ion H2PO4- is amphiprotic, yet an aqueous solution of KH2PO4 has a greater pH than 7?

Thank you very much in advance :D !!!

Mathematics: 96
Maths Extension 2: 93
Maths Extension 1: 97
English Advanced: 92
Physics: 95
Chemistry: 92
Engineering Studies: 90
Studies of Religion I: 98

2017 ATAR: 99.70
University of Sydney Civil Engineering and Commerce
University of Sydney Faculty of Civil Engineering Scholar
Student Representatives Council Student Housing Officer
City of Sydney Council Sydney Ambassador
University of Sydney Business School Student Mentor
Entrepreneur, Company of Year Junior Achievements Australia

jakesilove

  • HSC Lecturer
  • Honorary Moderator
  • Part of the furniture
  • *******
  • Posts: 1941
  • "Synergising your ATAR potential"
  • Respect: +196
Re: Chemistry Question Thread
« Reply #131 on: March 10, 2016, 07:12:35 am »
+1
Hey guys, need a hand here if anyone dont mind!

The question is: with the aid of appropriate equations, explain why the dihydrogen phosphate ion H2PO4- is amphiprotic, yet an aqueous solution of KH2PO4 has a greater pH than 7?

Thank you very much in advance :D !!!

Hey HPL!

In a nutshell, here is the way you need to answer the question in order to get full marks. There is some guess work here, but I only make fair assumptions based (lol) on the question.

First, you need to write out equations showing Dihydrogen Phosphate acting as both an acid (proton donor) and base (proton acceptor). To do this, write an equation with the Dihydrogen Phosphate and any acid, and then Dihydrogen Phosphate and any base. You need to explain what an amphiprotic substance is (ability to donate AND ability to accept protons), and then relate it to your equations.
Then, all you really need to say is that Dihydrogen Phosphate is a STRONGER base than water. As such, the water will act as an acid (as it is also amphiprotic), donating protons to the Dihydrogen Phosphate (which acts as a base). This produces hydroxide ions (due to the water losing a proton) and therefore the pH is greater than 7.

Hope this helps!

Jake
ATAR: 99.80

Mathematics Extension 2: 93
Physics: 93
Chemistry: 93
Modern History: 94
English Advanced: 95
Mathematics: 96
Mathematics Extension 1: 98

Studying a combined Advanced Science/Law degree at UNSW

Happy Physics Land

  • ATAR Notes Legend
  • Forum Obsessive
  • ***
  • Posts: 335
  • MAXIMISE your marks by MINIMISING your errors
  • Respect: +38
Re: Chemistry Question Thread
« Reply #132 on: March 10, 2016, 06:40:23 pm »
+1
Hey HPL!

In a nutshell, here is the way you need to answer the question in order to get full marks. There is some guess work here, but I only make fair assumptions based (lol) on the question.

First, you need to write out equations showing Dihydrogen Phosphate acting as both an acid (proton donor) and base (proton acceptor). To do this, write an equation with the Dihydrogen Phosphate and any acid, and then Dihydrogen Phosphate and any base. You need to explain what an amphiprotic substance is (ability to donate AND ability to accept protons), and then relate it to your equations.
Then, all you really need to say is that Dihydrogen Phosphate is a STRONGER base than water. As such, the water will act as an acid (as it is also amphiprotic), donating protons to the Dihydrogen Phosphate (which acts as a base). This produces hydroxide ions (due to the water losing a proton) and therefore the pH is greater than 7.

Hope this helps!

Jake

Ahhhh ok thank you Jake! We actually had a pretty intense discussion on the most suitable equations to include and my teacher agreed with your reasoning as welll mmmm. Thank you so much Jake, greatly appreciated! :D

P.S. nice pun!
« Last Edit: March 10, 2016, 06:42:00 pm by Happy Physics Land »
Mathematics: 96
Maths Extension 2: 93
Maths Extension 1: 97
English Advanced: 92
Physics: 95
Chemistry: 92
Engineering Studies: 90
Studies of Religion I: 98

2017 ATAR: 99.70
University of Sydney Civil Engineering and Commerce
University of Sydney Faculty of Civil Engineering Scholar
Student Representatives Council Student Housing Officer
City of Sydney Council Sydney Ambassador
University of Sydney Business School Student Mentor
Entrepreneur, Company of Year Junior Achievements Australia

amandali

  • Trendsetter
  • **
  • Posts: 122
  • Respect: 0
  • School: strathfield
  • School Grad Year: 2016
Re: Chemistry Question Thread
« Reply #133 on: March 13, 2016, 03:47:20 pm »
0
isnt this reaction endothermic

Since Z increases as temperature decreases so :
X (g) + Y (g) + heat <--> Z(g)


Happy Physics Land

  • ATAR Notes Legend
  • Forum Obsessive
  • ***
  • Posts: 335
  • MAXIMISE your marks by MINIMISING your errors
  • Respect: +38
Re: Chemistry Question Thread
« Reply #134 on: March 13, 2016, 07:03:21 pm »
+2
isnt this reaction endothermic

Since Z increases as temperature decreases so :
X (g) + Y (g) + heat <--> Z(g)

Hey Amanda!

Quite a tricky question actually, took me a while to figure out.
So we know the equilibrium equation is here is a certain moles of X + a certain moles of Y =  a certain moles of Z. For convenience's sake, lets just assume first that the equation is X + Y <--> Z.
So as we can see, as temperature increases in all three trends, the yield of Z decreases. Le Chatelier's principle tells us that as temperature of system increases, the system is disturbed and hence the equilibrium shifts to the endothermic reaction in order to consume the heat in the system to minimise this disturbance. Since yield of Z decreases as temperature increases, the yield of X and Y are favoured, and according to Le Chatelier we know that the endothermic process is favoured. Therefore we can conclude that the reverse process of Z---> X + Y is endothermic. Hence the forward process of X + Y ---> Z becomes exothermic.

 We know now that the answer definitely cant be A or C because they both display an endothermic reaction. So this leaves us with B and D. Great! Now lets have a look at the pressure. As pressure increases, the yield of Z actually increases (as you can see, the graph representing 200 atm pressure is way above the two other graphs). According to La Chatelier's principle, as pressure increases, the reaction would favour the side with the least moles of gas in order to alleviate the pressure by having less gas particles occupying a reduced volume. Since in the graph the production of Z is favoured and according to Le Chatelier's principle the side with least moles of gas should be favoured, then it can be concluded that Z must have less moles of gas than the reactants X and Y.D is the only answer that suits both criteria of being exothermic and Z being the side with least moles of gas.

Hence the correct answer is D.

If you have any further questions dont hesitate to ask! :)

Best Regards
Happy Physics Land
Mathematics: 96
Maths Extension 2: 93
Maths Extension 1: 97
English Advanced: 92
Physics: 95
Chemistry: 92
Engineering Studies: 90
Studies of Religion I: 98

2017 ATAR: 99.70
University of Sydney Civil Engineering and Commerce
University of Sydney Faculty of Civil Engineering Scholar
Student Representatives Council Student Housing Officer
City of Sydney Council Sydney Ambassador
University of Sydney Business School Student Mentor
Entrepreneur, Company of Year Junior Achievements Australia