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December 06, 2021, 02:25:27 pm

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#### james.358

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##### Re: VCE Chemistry Question Thread
« Reply #9210 on: October 16, 2021, 05:02:27 pm »
+5
I'm just wondering why rinsing the conical flask and volumetric flask with water in a titration has NO EFFECT on the calculated concentrations of titrant and aliquot solutions? Sources say that this is because the amount of moles are not affected. However, wouldn't having slightly more water make the solutions diluted?

Hey there,

Rinsing the conical flask with water definitely affects the concentration of the aliquot solution. However, it doesn't affect the accuracy of the titration as the moles of the reactants are unchanged, its just that the concentration is lower as you correctly stated

However, if you rinse the volumetric flask with water, it will not affect the concentration of the standard solution. Since you know how much of the primary standard you have, and you're just filling the flask up with water anyways.

I think what you were thinking of is rinsing the burette with water. That will lower the concentration of the titre, so you definitely cannot do it, or else it would become a systematic error.

Hope this helps!
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#### reverie

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##### Re: VCE Chemistry Question Thread
« Reply #9211 on: October 16, 2021, 06:19:47 pm »
0
Hey guys,

Not a very important question but why don't coefficients affect oxidation numbers?

Thanks

#### miyukiaura

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##### Re: VCE Chemistry Question Thread
« Reply #9212 on: October 25, 2021, 10:07:48 am »
+1
Hey guys,

Not a very important question but why don't coefficients affect oxidation numbers?

Thanks

I think of the coefficients as merely a way of balancing the equation, to give the correct stoichiometric ratio for the reaction. This doesn't affect the oxidation numbers of the individual atoms because the coefficients don't affect the extent to which a compound is oxidised or reduced. This is my understanding of it anyway.
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#### miyukiaura

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##### Re: VCE Chemistry Question Thread
« Reply #9213 on: October 25, 2021, 10:10:04 am »
0
Can anyone explain how the equilibrium constant Kc differs from the equilibrium position? And why is it that Kc is only affected by temperature, whereas the equilibrium position can be changed by changes in concentration, adding/removing reactants, etc.? Thanks
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#### wingdings2791

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##### Re: VCE Chemistry Question Thread
« Reply #9214 on: October 25, 2021, 11:21:02 am »
+6
Can anyone explain how the equilibrium constant Kc differs from the equilibrium position? And why is it that Kc is only affected by temperature, whereas the equilibrium position can be changed by changes in concentration, adding/removing reactants, etc.? Thanks

Hi miyukiaura,
$K_{eq}$ is defined as the ratio between the rates of $R_f$ and $R_b$ that leads to constant conditions within the system. Temperature is the only factor that affects $K_c$ because it is the only factor that can instantaneously change the rates of $R_f$ and $R_b$ differently.

When other conditions in the system are altered, eg. changes to pressure, mol of reagents, concentration etc., these changes do not instantaneously alter the rates of $R_f$ and $R_b$. For instance, assume some products were removed from a system. The consequent increase in the rate of $R_b$ is a response to the decrease in [products], aiming to re-establish equilibrium (LCP), rather than a direct impact of removing products. The decrease in [products] is the instantaneous change to the system and changes to the rates of $R_f$ and $R_b$ counter the instantaneous change. Changing conditions such as concentration doesn't affect rates or the equilibrium constant directly; $K_{eq}$ remains the same and the system is simply put out of balance and tries to restore equilibrium. Catalysts also don't affect the value of $K_{eq}$ because they increase the rates of $R_f$ and $R_b$ proportionally.

With temperature changes, the instantaneous impact on the system is the rates of $R_f$ and $R_b$. Think of collision theory: changing the average kinetic energy of particles will change the amount/proportion of successful collisions, directly impacting rate and therefore changing the value of $K_c$. I don't think it's within the scope of the study design, but temperature is proportional to the log of $K_{eq}$, the ratio of $R_f$ to $R_b$, meaning temperature necessarily affects $R_f$ and $R_b$ differently.
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#### miyukiaura

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##### Re: VCE Chemistry Question Thread
« Reply #9215 on: October 25, 2021, 01:05:18 pm »
+1
Thank you wingdings that genuinely made it very clear
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#### jasperray

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##### Re: VCE Chemistry Question Thread
« Reply #9216 on: October 28, 2021, 06:56:46 pm »
0
This is something I've been unsure about for a while. In electrolysis of an aqueous solution, water is present. However does this imply that the ions H+(aq) and OH-(aq) are also present and available to react?

#### james.358

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##### Re: VCE Chemistry Question Thread
« Reply #9217 on: October 28, 2021, 07:43:52 pm »
+6
Hey Jasper,

Although it's technically true that there are both hydronium and hydroxide ions in water, their concentrations are so low (~10-7M) that you can safely consider them negligible when considering what reactants are available when predicting reactions.

This also goes for the O2 + 2H2O + 4e- <--> 4OH- half equation in the electrochemical series. By common sense there is probably oxygen in the room while you're electrolysing something, but oxygen is not considered a reactant in aqueous solutions, so make sure to be careful!

Hope this helps,
James
VCE Class of 2021
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#### mcpunjavu

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##### Re: VCE Chemistry Question Thread
« Reply #9218 on: October 31, 2021, 11:28:41 am »
0
anyone have any Ideas on nailing experimental design questions? and where to find them?

#### jkfleur

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##### Re: VCE Chemistry Question Thread
« Reply #9219 on: October 31, 2021, 02:08:12 pm »
0
When should we use the density of water as 0.997g mL^-1, vs assuming that the density of water is just 1? For calculations involving finding Q with the specific heat capacity of water, I'm not sure as to when the given volume of water should be multiplied by 1 or by 0.997. I know in the databook it gives the density of water as 0.997g mL^-1 to be at 25 degrees C, so should I be using it in those Q calculations or not?
« Last Edit: October 31, 2021, 02:11:25 pm by jkfleur »
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#### ArtyDreams

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##### Re: VCE Chemistry Question Thread
« Reply #9220 on: October 31, 2021, 02:22:40 pm »
+4
When should we use the density of water as 0.997g mL^-1, vs assuming that the density of water is just 1? For calculations involving finding Q with the specific heat capacity of water, I'm not sure as to when the given volume of water should be multiplied by 1 or by 0.997. I know in the databook it gives the density of water as 0.997g mL^-1 to be at 25 degrees C, so should I be using it in those Q calculations or not?

Hi! It is best to use the density of water as given in the data booklet, as this is what VCAA would want to see in your working out

anyone have any Ideas on nailing experimental design questions? and where to find them?

Hi! You can find some practise experimental design questions in past VCAA exams - they're usually right at the end of the exams! As for nailing them, practise is the best, as you will find that the answers to experimental design questions start becoming repetitive after a while. Do some practise ones and get your teacher to mark and give you some feedback. However, some things you should study/look over are:

- Types of errors (systematic, random etc)
- Structures of aims, conclusions and hypothesis
- Different ways of representing data
- difference between accuracy/precision

Also, be familiar with the specific errors that can occur in certain experiments. Especially for titrations and calorimetry. What could go wrong if you wash an apparatus with water? What factors impact the energy transfer in a calorimetry experiment?

Hope this helps!

#### mcpunjavu

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##### Re: VCE Chemistry Question Thread
« Reply #9221 on: October 31, 2021, 02:32:43 pm »
0
Hi! It is best to use the density of water as given in the data booklet, as this is what VCAA would want to see in your working out

Hi! You can find some practise experimental design questions in past VCAA exams - they're usually right at the end of the exams! As for nailing them, practise is the best, as you will find that the answers to experimental design questions start becoming repetitive after a while. Do some practise ones and get your teacher to mark and give you some feedback. However, some things you should study/look over are:

- Types of errors (systematic, random etc)
- Structures of aims, conclusions and hypothesis
- Different ways of representing data
- difference between accuracy/precision

Also, be familiar with the specific errors that can occur in certain experiments. Especially for titrations and calorimetry. What could go wrong if you wash an apparatus with water? What factors impact the energy transfer in a calorimetry experiment?

Hope this helps!

awesome thanks!

#### Stormbreaker-X

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##### Re: VCE Chemistry Question Thread
« Reply #9222 on: October 31, 2021, 05:55:51 pm »
0
Which topics often appear on chemistry exams?
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#### jkfleur

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##### Re: VCE Chemistry Question Thread
« Reply #9223 on: November 01, 2021, 03:35:34 pm »
0
Hi! It is best to use the density of water as given in the data booklet, as this is what VCAA would want to see in your working out

Hi! You can find some practise experimental design questions in past VCAA exams - they're usually right at the end of the exams! As for nailing them, practise is the best, as you will find that the answers to experimental design questions start becoming repetitive after a while. Do some practise ones and get your teacher to mark and give you some feedback. However, some things you should study/look over are:

- Types of errors (systematic, random etc)
- Structures of aims, conclusions and hypothesis
- Different ways of representing data
- difference between accuracy/precision

Also, be familiar with the specific errors that can occur in certain experiments. Especially for titrations and calorimetry. What could go wrong if you wash an apparatus with water? What factors impact the energy transfer in a calorimetry experiment?

Hope this helps!
Hi, I just checked the 2020 chemistry exam report and on question 6c which involved the calculation of the amount of energy produced by combustion of CH4 to heat water, they specifically pointed out that it was an error to use the 0.997g mL^-1 density value. I guess this means it shouldn't be used? When do they expect us to use it then, if ever?
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#### wingdings2791

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##### Re: VCE Chemistry Question Thread
« Reply #9224 on: November 01, 2021, 04:46:40 pm »
+4
Hi, I just checked the 2020 chemistry exam report and on question 6c which involved the calculation of the amount of energy produced by combustion of CH4 to heat water, they specifically pointed out that it was an error to use the 0.997g mL^-1 density value. I guess this means it shouldn't be used? When do they expect us to use it then, if ever?

Hi jkfleur, this is because the mass of water is already given in the question as 350.0g, so it would be wrong to try and convert mLs to g when the data is already in g. For other questions where the amount of water is given as a volume, you certainly would use $d=0.997gmL^{-1}$. Also, keep in mind that this may vary for different questions: use a given density if provided, and if not, use $d$ even outside of SLC (as apparently it is better to use any specified data than to make something up/assume equal mass and volume, even when under non-standard conditions)
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