HSC Chemistry Question Thread

[KT Nyunt]

hey:)
how do I write an equation for the incomplete combustion of ethyne (c2h2) that produces a ratio of soot to CO2 of 5:1

^ I've attached this in case this doesn't work

A tip on balancing combustion equations: always balance the oxygen last
- Some schools would prefer not to have the fraction but that's an easy fix (just times everything by two)
- Also, some schools would say water is a liquid - H20 (l) - and that's acceptable as well... my school argues that at those high temperatures in combustion, H20 would be in a gaseous state.

Hope the helps!

[itssona]

(Image removed from quote.)
^ I've attached this in case this doesn't work

A tip on balancing combustion equations: always balance the oxygen last
- Some schools would prefer not to have the fraction but that's an easy fix (just times everything by two)
- Also, some schools would say water is a liquid - H20 (l) - and that's acceptable as well... my school argues that at those high temperatures in combustion, H20 would be in a gaseous state.

Hope the helps!

thank you so much!!

[Mate2425]

Hi all, i was wondering if someone had any strategies/ideas that could help to boost and maximise my marks in the HSC Chemistry Course.

[cocopops201]

Hi, I need help with this carbonated water LCP question ASAP!!
My question is,
how do you find the trend? and how do you properly answer the question so it is clear and concise.

The questions are:
a) the pH of the carbonated water would also vary with the pressure of carbon dioxide gas. Indicate this trend by sketching a graph for pH on the grid above.
b)Using equilibrium principles, account for both relationships shown on the graph.

[p.s. I had to crop the photo, bc it was too large in KB]

thank you!!!!

[KT Nyunt]

Hi all, i was wondering if someone had any strategies/ideas that could help to boost and maximise my marks in the HSC Chemistry Course.

Hey here are some tips I think help me greatly: hopefully some of these may be of use to you


Note making
I often collate information from many different places and then put them together in my notes (or just pick which site explains something best and then add that to my notes)
Places where I get most of my information:

class notes
this ATAR notes book: https://atarnotes.com/product/hsc-chemistry-notes/
https://easychem.com.au
and this Excel chemistry book: https://www.booktopia.com.au/excel-hsc-chemistry-excel/prod9781741252996.html?source=pla&gclid=CjwKCAjwi6TYBRAYEiwAOeH7GVgANB_bGYaYOWK2a42otUMTiX7QAibgc3Y05Z-bWpXW_3-52GIKwhoCApcQAvD_BwE
Other people's notes on ATAR notes: https://atarnotes.com/notes/?pag=1&state=3719&subject=4855&unit=0&key=&sort=date

I find for me, going through at least more than one explanation helps to consolidate my understanding of a concept.

Teaching as a tool for revision
I often teach my parents and I have a study group at school for biology and chemistry... essentially I use my notes as 'prompters' and teach whoever would listen. This is a really quick way to go over content, a good way to measure how well you understand something (how clearly you can explain it/ teach it = how well you understand it) and it really helps to make any content stick. Since Chem is pretty content heavy, this may be a very helpful strategy to improve your general knowledge and understanding of the topic.

Practice questions
To help maximising marks in the exam, definitely practice questions are the way to go.
These are some useful places to get some past paper questions:
http://www.quarkology.com/12-chemistry/12-chemistry-home.html
https://thsconline.github.io/s/yr12/Chemistry/
These are particularly useful for calculations. This is a big part of them exams and mastery of these questions just comes with practice and exposure to a lot of these questions.

In the exam
With Chem, I often find I finish the test with about 15-30 mins to spare because I've rushed through it. However, this is normally also very detrimental to my marks as I don't check over my answers as well I would like to be able to. (ie. I would look over at a question and be like 'yeahhh I answered that right' but when I recieved my marks, I had left out one piece of information that cost me getting full marks on a question. In saying that, I believe it's better to give a bit of time to really think about what the question is asking and what might be in the criteria in order to formulate a more cohesive answer before you even start writing. This may take more time to go through the test, but it's better to get it right the first time rather then rely on picking up on the mistake when you check over it at the end (if that makes sense).

Another trick I've learnt is for those moments when you come across a 4-5 mark question and you're not too sure of what to put down. This happened to me in my HY on a question about buffers. The question was more convoluted with a little story about acid rain and a swimming pool and what the caretaker should do to the pool and in the exam I just stared at it thinking 'BUT WHAT DO YOU WANT FROM ME?'. I then resorted to regurgitating any information that I think might be in the criteria based on what I've seen with previous buffer questions (ie. reference to Le Chatelier, how a buffer is made, an equation etc). I wrote it out in dotpoint form, hoping some of it would get me some marks and I ended up getting full marks for that question. So when in doubt, just say what you know, and it might get you some marks!


Hope this helps!

[KT Nyunt]

Hi, I need help with this carbonated water LCP question ASAP!!
My question is,
how do you find the trend? and how do you properly answer the question so it is clear and concise.

The questions are:
a) the pH of the carbonated water would also vary with the pressure of carbon dioxide gas. Indicate this trend by sketching a graph for pH on the grid above.
b)Using equilibrium principles, account for both relationships shown on the graph.

[p.s. I had to crop the photo, bc it was too large in KB]

thank you!!!!

Hi
For a) I'm not too sure on how you get the values for the pH graph. I initially thought you could pick a value for pCO2, and then find it's corresponding concentration of carbonate ions and then use that concentration and plug it into the pH formula (-log[H+]). But the values I got were weird so hopefully someone else might have a better idea.

Although for b) essentially this is your average Equilibrium question with Le Chatelier
criteria for this question would most likely be something like:
- includes equation (I included a picture of this equation if you need it) - remember to include states!
- Explains how increase in pressure causes both equilibriums to shift to the right (as per Le Chatelier's principle - references moles of gases and increasing concentration of reactants)
- Explains how an increase in pressure leads to increase in carbonate ion concentration, which lowers the pH as this is an acid.

If you want, you could post your response and I'd be happy to give feedback

[cocopops201]

If you want, you could post your response and I'd be happy to give feedback

Wow!! thanks so much

[aadharmg]

What is the indicator used for a H2SO4 vs NaOH titration? Some places it says Bromothymol blue as they are strong acids and bases, but other places say phenolpthalein for reasons I don't know.

[RuiAce]

What is the indicator used for a H2SO4 vs NaOH titration? Some places it says Bromothymol blue as they are strong acids and bases, but other places say phenolpthalein for reasons I don't know.

Titration with H2SO4 is usually a nuisance, and probably why it never comes up in HSC chemistry. The fact that H2SO4 is diprotic causes a lot of problems.

H2SO4 works like this. It has 2 protons it can donate. The first proton, is that of a strong acid, so we have no problem. However, the second proton, is that of a weak acid. Because of the second proton not behaving in a same way like the first, strong one, the endpoint starts slipping into the basic range. This is why in the real world, you'd go for phenolphthalein instead.

[LaraC]

Hello!
Can anyone please help me out with reasons why the pH of our lake (local catchment area) is slightly higher than our home tap water? There isn't much agriculture near the lake to cause increases in nitrates/nitrites etc,  so I'm a little unsure as to the reason for this measure in my first-hand experiment?

Also, can anyone give reasons behind why the hardness of the tap water would be significantly higher than the hardness of the lake water?

Thanks in advance!!

[aadharmg]

Titration with H2SO4 is usually a nuisance, and probably why it never comes up in HSC chemistry. The fact that H2SO4 is diprotic causes a lot of problems.

H2SO4 works like this. It has 2 protons it can donate. The first proton, is that of a strong acid, so we have no problem. However, the second proton, is that of a weak acid. Because of the second proton not behaving in a same way like the first, strong one, the endpoint starts slipping into the basic range. This is why in the real world, you'd go for phenolphthalein instead.

That makes sense. Say I was to use Bromothymol, which theoretically corresponds to such a titration (strong acid - strong base), would it have a major difference on my results or would my results (Bromothymol/Phenolphthalein) be the same, i.e. the amounts of whatever substance required to neutralise the other?

[lalall]

Hi!

Can someone pls help?
In a titration prac,  does rinsing the glasswares correctly improve the validity or accuracy? What are the other factors affecting validity and? And is reliability just affected by the repetition and consistancy of the result?

Thanks in advance!

[varun.amin]

Hello!
Can anyone please help me out with reasons why the pH of our lake (local catchment area) is slightly higher than our home tap water? There isn't much agriculture near the lake to cause increases in nitrates/nitrites etc,  so I'm a little unsure as to the reason for this measure in my first-hand experiment?

Also, can anyone give reasons behind why the hardness of the tap water would be significantly higher than the hardness of the lake water?

Thanks in advance!!

Hi LaraC!

I believe the increased pH could be due to levels of CO2 absorption.  Basically photosynthesis uses up dissolved carbon dioxide, which acts like carbonic acid (H2CO3) in water. CO2 removal, in effect, reduces the acidity of the water and so pH increases. Perhaps if there is a large quantity of aquatic plants this could lead to a slightly higher than average pH. Not too sure on the second question

[RuiAce]

That makes sense. Say I was to use Bromothymol, which theoretically corresponds to such a titration (strong acid - strong base), would it have a major difference on my results or would my results (Bromothymol/Phenolphthalein) be the same, i.e. the amounts of whatever substance required to neutralise the other?

Someone can correct me but I feel like it would, which is concerning.

Which sources are telling you bromothymol blue and which are telling you phenolphthalein?

[varun.amin]

Hi!

Can someone pls help?
In a titration prac,  does rinsing the glasswares correctly improve the validity or accuracy? What are the other factors affecting validity and? And is reliability just affected by the repetition and consistancy of the result?

Thanks in advance!

Hi Lalall!,

1. I'm inclined to say that correct rinsing of glassware improves accuracy, as this would help you get a result closer to the actual value. But at the same time, validity is also a measure of how well your variables are controlled, the variable being the pH of the solutions. I think it impacts both.
2. Some of the other things impacting validity are :
          -Volume of unknown solution
          -Type of indicator used
          -Concentration of standard solution
          -Equipment used
          -Use of a control, that is, the aliquot with indicator untitrated.
3.  Reliability is determined through repetition of titration and then the evaluation of the concordance of results across the three titres. By repeating it multiple times, outliers can be to identified and excluded.