VCE Chemistry Question Thread

[Willba99]

Hey guys, you know how the density of water is 0.997g/ml (from the data book) when do you use that in calculations?

If they said something like "Find the change in temperature if 100mL of water is heated with 15kJ of energy" then you would have to convert the 100mL into 99.7g or whatever to use in Q=cmT - basically whenever they give you water in terms of volume instead of mass, treat it how you would treat calculations involving petrol or diesel and convert it to mass first. Then you can calculate mol, Q, etc

[Phenomenol]

Thanks.

Could you explain the contradiction that is occurring? I can't quite see it, you meant that the Kc value in part c is different to the one used in part e, I wrote the rounded value in part c. But then in part e, the unrounded value is used.

Two things always need to be true:
1. The equilibrium constant is unchanged (since there is no temperature change)
2. For every mol of C₂H₆ formed while equilibrating, there must be 1 mol of both C₂H₄ and H₂ consumed (stoichiometry), and vice versa.

If you try using the same equilibrium constant (as you have done) in part c. for part e. then you will end up with the change in [C₂H₄] being 0.004 M. This makes no sense because you have interpreted the change in [C₂H₆] to be 0.020 M. So the stoichiometry is not consistent.

Alternatively, you can set everything to have the same change in concentration as it approaches equilibrium again (correct stoichiometry). So you can set [C₂H₄] to be 0.040 because you have interpreted the change in [C₂H₆] to be 0.020 M. But naturally here, if you try to calculate the equilibrium constant from these values, it will be different to that in part d.

So how do we make sure both the K_c value and stoichiometry is correct? We simply cannot assume the change in [C₂H₆] is 0.020 M, i.e. we cannot assume the final [C₂H₆] is 0.18 M. You would let the final [C₂H₆] = (0.16 + x) M and final [C₂H₄] = (0.060 - x) M. You can then sub these values and [H₂] = 0.012 M (from the axis label) into the known K_c and solve for x to find all the final concentrations. This is the most rigorous approach.

The problem I have with the way the question is written is in part b. you are meant to interpret some of the dotted lines to be exactly halfway between axis labels. But with part e. you are not meant to! You are not at fault for interpreting the final [C₂H₆] to be 0.18 M because you have been conditioned by a previous part of the question to do so.

[syubi]

Quick question, for the universal gas equation, is the pressure 101.3 kPa or do we use 100 kPa now?

Thanks!

[VanillaRice]

Quick question, for the universal gas equation, is the pressure 101.3 kPa or do we use 100 kPa now?

Thanks!

The pressure will be whatever is defined by the question. If you're referring to the conversion between atmospheres and kPa, the data book lists the conversion to be 100kPa = 0.987atm, or in other words, 101.3kPa = 1atm.

Hope this helps

[Willba99]

Quick question, for the universal gas equation, is the pressure 101.3 kPa or do we use 100 kPa now?

Thanks!

If it’s at SLC then it’s 101.3kpa but if it’s STP then it’s 100kpa

[BlinkieBill]

No, the new SLC is 100kPa  ==> molar volume of ideal gas is now 24.8 L/mol.
It used to be 101.3kPa.

Check the data book to confirm

[exit]

For if this is a vague question because I barely know what I'm asking (hence why I'm asking)..

But if a question states what the property of _ (insert feature of cell here such as membrane/separated etc), we can say stuff like it has to be inert, catalystic, porous, cheap, high surface area etc. But how the different properties these must exhibit? Should be a simple question.

[MG226]

In an amino acid, if the side chain has an amide (asparagine and glutamine) and is in acidic conditions, will it accept one or two protons? Have had two different teachers give both answers do not sure which one to go with if it’s on the exam

[VanillaRice]

In an amino acid, if the side chain has an amide (asparagine and glutamine) and is in acidic conditions, will it accept one or two protons? Have had two different teachers give both answers do not sure which one to go with if it’s on the exam

Are you referring to both the side chain amine and the alpha amine group? You're probably receiving two answers because 'acidic conditions' covers a wide pH range. For reasons that aren't covered in VCE chemistry, if there are two amines in an amino acid, they generally will not accept protons at the same time (or pH). So, yes, they can both accept a proton, but whether or not they actually do depends on the pH itself. If I were to answer such a question, I would personally say that they are 'likely' to be protonated, but I would hope/assume that VCAA have written their exam questions to be a bit more specific.

Hope this helps

EDIT: Misread the question, please see my comment below.

[nazif15]

In an amino acid, if the side chain has an amide (asparagine and glutamine) and is in acidic conditions, will it accept one or two protons? Have had two different teachers give both answers do not sure which one to go with if it’s on the exam

No, just remember only the ionizations of amino functional groups are affected in low pH (i.e. gains a H), and the carboxy functional groups are affected in high pH (i.e loses a H)

[MG226]

Are you referring to both the side chain amine and the alpha amine group? You're probably receiving two answers because 'acidic conditions' covers a wide pH range. For reasons that aren't covered in VCE chemistry, if there are two amines in an amino acid, they generally will not accept protons at the same time (or pH). So, yes, they can both accept a proton, but whether or not they actually do depends on the pH itself. If I were to answer such a question, I would personally say that they are 'likely' to be protonated, but I would hope/assume that VCAA have written their exam questions to be a bit more specific.

Hope this helps

No, just remember only the ionizations of amino functional groups are affected in low pH (i.e. gains a H), and the carboxy functional groups are affected in high pH (i.e loses a H)

Yes it will accept two protons

Thank you for the responses, so just to clarify - amides (-CONH2) are basic or neutral. My question was poorly worded but was sort of what I was intending on asking. One of the teachers said that NH2 will always be basic regardless of any other bonds but the other said that the C=O in the amide changes the bonding and makes the NH2 of the amide neutral. I’ve done a bit of googling and everything I have come across has said that they are neutral and goes into stuff that is not in the course as to why they are not basic. So I guess that is technically the correct answer but now I’m more worried as to what will be marked as correct on the exam because it seems that most schools have taught that they are basic and from the past papers I’ve looked at I have been unable to find any questions which relate to these specific amino acids where I could just check what answer was given.

[VanillaRice]

Thank you for the responses, so just to clarify - amides (-CONH2) are basic or neutral. My question was poorly worded but was sort of what I was intending on asking. One of the teachers said that NH2 will always be basic regardless of any other bonds but the other said that the C=O in the amide changes the bonding and makes the NH2 of the amide neutral. I’ve done a bit of googling and everything I have come across has said that they are neutral and goes into stuff that is not in the course as to why they are not basic. So I guess that is technically the correct answer but now I’m more worried as to what will be marked as correct on the exam because it seems that most schools have taught that they are basic and from the past papers I’ve looked at I have been unable to find any questions which relate to these specific amino acids where I could just check what answer was given.

I'd like to add onto my response - it seems I misread amide as amine (apologies!). The reason why the nitrogen in amides cannot become protonated is because (not VCE) it technically isn't there anymore - it's 'resonance stabilised' over the amide (similar to how benzene isn't actually made of up distinct single and double bonds, but rather something in between - the electrons jump around). In short, no, amides do not become protonated. There is no lone pair available to form a bond with a hydrogen. I think amides are a new addition to the study design this year (more specifically, primary amides)? If so, this would be the reason for the confusion, and lack of past questions. In the previous study design, I believe the only context for amide groups was that they were what joined amino acids together.

Hope this clarifies things

[-273.15]

How many exams have people done?
And what has been your approach? Ive been doing vcaa exams but not feeling confident given new study design and limited resources :/

[Phenomenol]

How many exams have people done?
And what has been your approach? Ive been doing vcaa exams but not feeling confident given new study design and limited resources :/

You can look at exams from previous years for practice, but you need to be very conscious of what is in the current study design and what isn't. Preferably, use the trial exams from this year that are provided by your school/elsewhere or write out some revision notes from your textbook - bullet points on each chapter/formula sheet.

Best of luck!

[DailyInsanity]

Hey Guys,

2 quick questions about significant figures: In our calculations, do we carry through with sig.figs rules throughout each step of the process (as this is what VCAA does on the exam reports)?

And, if you leave an answer to a certain amount of sig.figs in a previous part of the question, and you have to use that number for further calculations, does that previously calculated number have to be taken into account when determining how many sig.figs you give for your answer - or is it always just determined off the numbers VCAA gives you?

Thank you!