HSC Chemistry Question Thread

[MisterNeo]

Hey there! I sort of guessed my way through this question but can't find any solid answers online, all there is are marking guidelines but I'm not finding them too helpful because I'm stuck. If someone could check my part a) and explain why it's those two solutions as well as an approach for q17 I'd really appreciate it. Thank you!!

Hey there! The answers are in the pic below. The first reaction is between a metal carbonate and an acid because it produces bubbles of CO2.
Solution 3-4 requires you to know your solubility rules as they form precipitates. PbCl2 and BaCO3 are insoluble.
Hope this helps

[Shadowxo]

I see now thank you for your help
I was trying to do this question. But i havent dealt with density before so I'm not sure how to start.

Hi
So in this question, it gives you the density (grams per mL) and volume (mL) so you multiply them together to get the mass (grams)
mass = 19.6*1.67 = 32.732g
The thing is, it's not pure sulphuric acid (it has an unknown concentration) so you can't continue like Neo and use n=m/M as some of that mass is water.
With the NaOH:
n(NaOH)=cV = 0.390*0.0126 = 0.004914 mol
Using the equation H₂SO₄ + 2NaOH => Na₂SO₄ + 2H₂O
n(H₂SO₄) in 25mL = 1/2 * n(NaOH) = 2 * 0.004914 = 0.002457
n(H₂SO₄) in 1L = n(H₂SO₄) in original solution = 0.002457 * 40 = 0.09828
Now convert that into mass using m=n * M = 0.09828 * 98.08 = 9.639g
% mass = m(pure sulphuric acid) / m(original solution) = 9.639 / 32.732 * 100% = 29.45% = 29.4%

I haven't taken into account sig figs and my wording may not be the best but this is the solution

[Shadowxo]

Thanks for pointing that out! Didn't know why I did that.. 

It's an easy mistake to make as sulphuric acid can be liquid (so all sulphuric acid) or aqueous (diluted). Just have to look at the question and what information they do/don't give.

[RuiAce]

As far as the course goes, you should always be assuming that sulfuric acid is "diluted" until you reach the option topic, and you may look at concentrated sulfuric acid then.

[RuiAce]

It's an easy mistake to make as sulphuric acid can be liquid (so all sulphuric acid) or aqueous (diluted). Just have to look at the question and what information they do/don't give.

Well, whilst we do take it as a liquid it isn't actually "all" acid. It's just concentrated to the point it may as well be (don't remember exactly how concentrated but it's at least 98%, which basically becomes 18M). It's better to say "almost all" acid.

[bsdfjnlkasn]

Hey there,

I was wondering if I could get some solid feedback for this 4 marker, I typed it up as it's neater and the HSC success one book I have, doesn't include papers between 2004 and 2008. I can't seem to find any concrete answers online and so would appreciate a mark estimate and areas for improvement as I'm sure I haven't included everything that I should have. Were we required to write two points for each detail asked? So two details of the method and two more for the trends?

Any advice would be greatly appreciated, thank you!!

[anotherworld2b]

Can I have some help with these 2 questions please? I'm not sure if I'm doing them correctly

[jakesilove]

Hey there,

I was wondering if I could get some solid feedback for this 4 marker, I typed it up as it's neater and the HSC success one book I have, doesn't include papers between 2004 and 2008. I can't seem to find any concrete answers online and so would appreciate a mark estimate and areas for improvement as I'm sure I haven't included everything that I should have. Were we required to write two points for each detail asked? So two details of the method and two more for the trends?

Any advice would be greatly appreciated, thank you!!

Hey! Thanks for posting up your solution, always happy to mark things like this

I would break up the marks into two sections; firstly, 'explaining' how the information was obtained (one mark) and secondly, outlining changes that have occurred (three marks).

You've clearly and easily received the first mark. No comment there except to say that it's potentially a little long (if you hand write those first three sentences, that's potentially a whole paragraph!).

Now, moving onto the more interesting part. I would have used subheadings to separate the two sections; I would recommend you get used to doing things like that!

Whilst outside of the timeframe, the natural first sentence would have been something like 'Ozone had been severely depleted above Antarctica due to the use of CFCs'. This sets you up for everything else you're about to say, and just makes logical sense to begin with.

Including the date of the Montreal Protocol is brilliant! I like how you've worked in specific detail, that's really important with questions like this. You've clearly discussed natural Ozone depletion (not how I would have done it, but totally legit), but explained that CFCs were more significant. The rest of your paragraph is great, and clearly explains changes over time.

I think there are a few areas of improvement;

See above re first sentence and logical flow

You could easily have improved the specific detail (when were CFCs banned? What were they replaced with? A throw away line on all this is plenty)

You could easily have improved the inclusion of chemical detail (Why was Ozone depleted by CFCs? Relevant equations?)

Basically, I think this could easily be a three out of four, or a four out of four. However, by including the above suggestions, it will definitely be a four out of four!

[jakesilove]

Can I have some help with these 2 questions please? I'm not sure if I'm doing them correctly

The first one is absolutely right! No redox there.

Second one is also absolutely right! No redox either.

[bsdfjnlkasn]

Hey! Thanks for posting up your solution, always happy to mark things like this

I would break up the marks into two sections; firstly, 'explaining' how the information was obtained (one mark) and secondly, outlining changes that have occurred (three marks).

You've clearly and easily received the first mark. No comment there except to say that it's potentially a little long (if you hand write those first three sentences, that's potentially a whole paragraph!).

Now, moving onto the more interesting part. I would have used subheadings to separate the two sections; I would recommend you get used to doing things like that!

Whilst outside of the timeframe, the natural first sentence would have been something like 'Ozone had been severely depleted above Antarctica due to the use of CFCs'. This sets you up for everything else you're about to say, and just makes logical sense to begin with.

Including the date of the Montreal Protocol is brilliant! I like how you've worked in specific detail, that's really important with questions like this. You've clearly discussed natural Ozone depletion (not how I would have done it, but totally legit), but explained that CFCs were more significant. The rest of your paragraph is great, and clearly explains changes over time.

I think there are a few areas of improvement;

See above re first sentence and logical flow

You could easily have improved the specific detail (when were CFCs banned? What were they replaced with? A throw away line on all this is plenty)

You could easily have improved the inclusion of chemical detail (Why was Ozone depleted by CFCs? Relevant equations?)

Basically, I think this could easily be a three out of four, or a four out of four. However, by including the above suggestions, it will definitely be a four out of four!

Awesome Jake! Thanks for all this incredible detail, it's really helped guide me through questions of a similar mark allocation How would you have described ozone depletion? Would the whole process of chlorine free radicals decomposing ozone into oxygen be necessary to write equations for? Also, more of a general question, when providing equations in answers, how much should we explain what is going on because often it seems that the equations speak for themselves - and obviously you don't want to be wasting time repeating yourself

Thanks again for your help, I really appreciate it!!

[jakesilove]

Awesome Jake! Thanks for all this incredible detail, it's really helped guide me through questions of a similar mark allocation How would you have described ozone depletion? Would the whole process of chlorine free radicals decomposing ozone into oxygen be necessary to write equations for? Also, more of a general question, when providing equations in answers, how much should we explain what is going on because often it seems that the equations speak for themselves - and obviously you don't want to be wasting time repeating yourself

Thanks again for your help, I really appreciate it!!

Most of the time, the equation will speak for itself! In a question like this, I don't think you need to run through the whole CFC process, maybe just include the overall reaction? ie. Ozone + Oxygen free radical --> 2 Oxygen etc.

[Alalamc]

Hey Guys these 2 questions are really bugging me can someone EXPLAIN them to me
1)
The molar enthalpy of combustion of camphor, C10H16O, is -5898 kJ/mol. How much energy is released on combustion of 1 gram of camphor?
2) Determine the enthalpy of reaction when 1 mole of ethene reacts with oxygen to form carbon dioxide and water. Using the bond energies (attached)
Thankyou guys !!!

[jakesilove]

Hey Guys these 2 questions are really bugging me can someone EXPLAIN them to me
1)
The molar enthalpy of combustion of camphor, C10H16O, is -5898 kJ/mol. How much energy is released on combustion of 1 gram of camphor?
2) Determine the enthalpy of reaction when 1 mole of ethene reacts with oxygen to form carbon dioxide and water. Using the bond energies (attached)
Thankyou guys !!!

Hey! Here's how we work out the first one. We know that the heat of combustion is



But, we want the answer to be in kJ/g, right? To get from kJ/mol to kJ/g, we would need to do something like this



Cool! So, what do we know that has units of mol/g (or, rather, g/mol and then we can just divide it)?. The molar mass! The molar mass of Camphor is



So,

A more intuitive way of doing this is thinking about what kJ per mol actually means. It means kJ per 152.23 grams! So, just divide by the grams to get kJ per gram

Now, onto your second, question, which I really don't know about WAY OUTSIDE OF HSC

It looks like we do it like this. Our equation is



Okay. Now it gets tough. In Ethene, there is one double-bonded Carbon-Carbon bond, and four single-bonded Carbon-Hydrogen bond. From the formula sheet, the total bond energy will be



As there is only one mole, the bond energy of Ethene is 2276 kJ. In Oxygen, there is one double-bonded Oxygen-Oxygen bond.



As there are three moles, the bond energy will be 1497kJ.

Now, we need to look at the products. Carbon dioxide has two double-bonded Carbon-Oxygen bonds. From the formula sheet



Since there are two moles, the bond energy will be 3196 kJ. In Water, there are two single-bonded Hydrogen-Oxygen bonds. Thus,



As there are two moles of water, the bond energy will be 1840 kJ.

Adding up all the reactant energies, we get 3773 kJ. Adding up all the product energies, we get 5036 kJ. To find the enthalpy change, we subtract the products from the reactants, giving us



Note that I've never done this before, so I may be very wrong.

[anotherworld2b]

Hi i was wondering if I could have some help finding the oxidation numbers for this question please.

[jakesilove]

Hi i was wondering if I could have some help finding the oxidation numbers for this question please.

Hey! Not sure which question in particular you're looking for help with; could you provide some working out, so I can point you in the right direction?