VCE Chemistry Question Thread

[Darkknight26]

Hi everyone,

this may be a bit off topic, but does anyone feel like the STAV exam is more difficult than the VCAA exam?

Thanks

[jyce]

Hi everyone,

this may be a bit off topic, but does anyone feel like the STAV exam is more difficult than the VCAA exam?

Thanks

If you're referring to either the 2014 or 2015 STAV exam, then definitely yes.

[Splash-Tackle-Flail]

Can denaturation occur due to temperature being too low? or is it just when it's too high?

Also, I get that the secondary structure of proteins is the hydrogen bonding between different peptides, regarding their COHN groups, but what effect does this have on the structure of the protein?

[jyce]

Can denaturation occur due to temperature being too low? or is it just when it's too high?

Also, I get that the secondary structure of proteins is the hydrogen bonding between different peptides, regarding their COHN groups, but what effect does this have on the structure of the protein?

Denaturation does not occur when the temperature is low. For denaturation to occur, bonds responsible for the active site need to be broken, and breaking bonds requires energy as provided by a high temperature. Instead, at a low temperature, the activity of an enzyme is low because it takes a much longer time for the enzyme to interact with the substrate. Lower temperatures mean things move more slowly, NOT that bonds break!

In regards to your second question, the hydrogen bonding between different peptide groups causes coiling and/or pleating of the protein.

Good questions, by the way 

[Splash-Tackle-Flail]

Denaturation does not occur when the temperature is low. For denaturation to occur, bonds responsible for the active site need to be broken, and breaking bonds requires energy as provided by a high temperature. Instead, at a low temperature, the activity of an enzyme is low because it takes a much longer time for the enzyme to interact with the substrate. Lower temperatures mean things move more slowly, NOT that bonds break!

In regards to your second question, the hydrogen bonding between different peptide groups causes coiling and/or pleating of the protein.

Good questions, by the way 

Thanks heaps! And thanks for taking the time to answer all these Chem questions in the lead up to exams If any of these come up Ill be thanking you and this thread in the exam lol

[RazzMeTazz]

Do we need to know the exact pH range for when the equivalence point occurs in the different types of possible titrations (e.g. strong acid + weak base, weak acid + strong base), etc?  I know that when you have a strong base titrated with a weak acid, the equivalence point occurs at an alkaline pH and vice versa for the other type but do we need to know the exact pH range?

Or is this dependent on the particular acid and base used?

Thankyou!

[jyce]

Do we need to know the exact pH range for when the equivalence point occurs in the different types of possible titrations (e.g. strong acid + weak base, weak acid + strong base), etc?  I know that when you have a strong base titrated with a weak acid, the equivalence point occurs at an alkaline pH and vice versa for the other type but do we need to know the exact pH range?

Or is this dependent on the particular acid and base used?

Thankyou!

It's dependent on the particular acid and base used and their concentrations, so no you don't have to know exact equivalence point pHs.

[IndefatigableLover]

With reference to percentage ionisation/dissociation, to work out the initial concentration of the acid if not given, we just take the concentration of the products and add them up to give us our initial concentration? (This is in reference to VCAA 2012, Q3d).

[jyce]

With reference to percentage ionisation/dissociation, to work out the initial concentration of the acid if not given, we just take the concentration of the products and add them up to give us our initial concentration? (This is in reference to VCAA 2012, Q3d).

That question is an absolute nightmare! If you are not given the initial concentration of the acid, DON'T assume the initial concentration = the equilibrium concentration of the acid. This is because we can actually calculate the difference between the two:
[acid]initial = [acid]equilibrium + [acid]reacted to reach equilibrium, where [acid]reacted = [H⁺]equilibrium OR [conjugate acid]equilibrium

So no, you wouldn't "add [the concentrations of the products] to give us our initial concentration"; rather, you would add what is left of the acid together with the equilibrium concentration of ONE of the products.

[Splash-Tackle-Flail]

What is aspirin and what do we need to know about it? We didn't do a SAC on it or anything, and all I know is there's some soluble version when it loses a hydrogen ion and ionic bonds to a sodium cation.. Like it's structure is whack-do we need to memorise this? And what about the weird double ethanoic acid?

[IndefatigableLover]

That question is an absolute nightmare! If you are not given the initial concentration of the acid, DON'T assume the initial concentration = the equilibrium concentration of the acid. This is because we can actually calculate the difference between the two:
[acid]initial = [acid]equilibrium + [acid]reacted to reach equilibrium, where [acid]reacted = [H⁺]equilibrium OR [conjugate acid]equilibrium

So no, you wouldn't "add [the concentrations of the products] to give us our initial concentration"; we would rather add what is left of the acid together with the equilibrium concentration of ONE of the products.

Haha it was a nightmare (according to the Assessor's Report, I got 2/3 since I got 99.6% which was obviously wrong since it was so high for a weak acid but I didn't know what else to do at the time when I did the exam...)
But I'll remember that formula if anything comes up for this year's exam!

And out of curiosity, what do you think will be on this year's exam jyce?
My prediction is they'll test the bulk on Electrolysis and Biofuels with Spectroscopy as a smokey (the calculation of the ppm question last year had a low percentage so I assume it'll be tested more rigorously this year but that's about it)!

[jyce]

What is aspirin and what do we need to know about it? We didn't do a SAC on it or anything, and all I know is there's some soluble version when it loses a hydrogen ion and ionic bonds to a sodium cation.. Like it's structure is whack-do we need to memorise this? And what about the weird double ethanoic acid?

You should be familiar with how salicylic acid reacts with ethanoic anhydride (or "the weird double ethanoic acid", as you put it) in a condensation reaction to form aspirin and ethanoic acid. Basically, ethanoic anhydride is formed from the condensation reaction between two ethanoic acid molecules, and this compound serves to replace the hydroxyl group on salicylic acid with a methyl ester group.

[jyce]

Haha it was a nightmare (according to the Assessor's Report, I got 2/3 since I got 99.6% which was obviously wrong since it was so high for a weak acid but I didn't know what else to do at the time when I did the exam...)
But I'll remember that formula if anything comes up for this year's exam!

And out of curiosity, what do you think will be on this year's exam jyce?
My prediction is they'll test the bulk on Electrolysis and Biofuels with Spectroscopy as a smokey (the calculation of the ppm question last year had a low percentage so I assume it'll be tested more rigorously this year but that's about it)!

It's okay. I have 'fond' memories of getting that question wrong too!

As for what I predict will feature more heavily on this year's exam:
- acidity constants (this was barely covered last year)
- electrochemistry, particularly in terms of applying your knowledge of electrochemistry to unfamiliar scenarios (the three electrochemistry questions on last year's exam were done horribly)
- calorimetry (not specifically covered last year)
- chromatography, in Section B (only featured in Section A last year, and there was only 2 marks of chromatography in Section B of the 2013 exam)
- and, of course as always, spectroscopy and organic reaction pathways.

[IndefatigableLover]

It's okay. I have 'fond' memories of getting that question wrong too!

As for what I predict will feature more heavily on this year's exam:
- acidity constants (this was barely covered last year)
- electrochemistry, particularly in terms of applying your knowledge of electrochemistry to unfamiliar scenarios (the three electrochemistry questions on last year's exam were done horribly)
- calorimetry (not specifically covered last year)
- chromatography, in Section B (only featured in Section A last year, and there was only 2 marks of chromatography in Section B of the 2013 exam)
- and, of course as always, spectroscopy and organic reaction pathways.

I personally think the experimental design question this year will be on calorimetry actually but that's just me LOL but I'll go through this list in the coming days
Thanks jyce (what a champ)!

[jyce]

I personally think the experimental design question this year will be on calorimetry actually but that's just me LOL but I'll go through this list in the coming days
Thanks jyce (what a champ)!

Yes, my bet for the last question is either calorimetry or electrolysis. But don't hold me to these predictions! And you're welcome