VCE Chemistry Question Thread

[keltingmeith]

Thankyou...that method worked for all of them

Yeah, my only issue with 4 is that amino acids do weird things based on pH (something you explore in 3/4 chem), so the question itself is a bit weird to do without knowing the pH. Plus they don't usually exist in the form that the textbook gave. Ah well, whatcha gonna do.

[Maz]

Yeah, my only issue with 4 is that amino acids do weird things based on pH (something you explore in 3/4 chem), so the question itself is a bit weird to do without knowing the pH. Plus they don't usually exist in the form that the textbook gave. Ah well, whatcha gonna do.

well i guess ill see what happens further on in the book... one more thing please...how do you know if and acid is diprotic, tripotic...polyprotic etc?

[keltingmeith]

well i guess ill see what happens further on in the book... one more thing please...how do you know if and acid is diprotic, tripotic...polyprotic etc?

Look at how many protons it has to give out.

For example, H2S has two protons, and so is diprotic. H3PO4 had three protons, and so it's triprotic, etc.

Compare to ammonium, NH4+. It has 4 hydrogens, but can only donate 1 proton, and so is monoprotic. This is a bit of a case of memory work, but the more acid-base stuff that you expose yourself to, you'll start to see some trends as to what makes a proton donateable or not.

[Maz]

Look at how many protons it has to give out.

For example, H2S has two protons, and so is diprotic. H3PO4 had three protons, and so it's triprotic, etc.

Compare to ammonium, NH4+. It has 4 hydrogens, but can only donate 1 proton, and so is monoprotic. This is a bit of a case of memory work, but the more acid-base stuff that you expose yourself to, you'll start to see some trends as to what makes a proton donateable or not.

Thats what i always thought...but then it says in my book that malice acid is diprotic and phenol is monoprotic...i checked on he internet and google is saying the same thing but then that doesn't fit with the principle...?

[zsteve]

well phenol is a bit of a special case, the OH group can actually deprotonate. So the principle is often satisfied but not always.

[Maz]

well phenol is a bit of a special case, the OH group can actually deprotonate. So the principle is often satisfied but not always.

oh okay...is that the same for malic acid? are u just supposed to memorise the exceptions or is there some other way to tell which are rule-abideing and which aren't... thank you btw

[zsteve]

I think usually you'll be told whether an acid is monoprotic, diprotic, etc. for VCE uses. Add this to the usual ones - sulfuric, ethanoic, any alkanoic acids, etc. which you memorise and you'll be fine.

[Maz]

I think usually you'll be told whether an acid is monoprotic, diprotic, etc. for VCE uses. Add this to the usual ones - sulfuric, ethanoic, any alkanoic acids, etc. which you memorise and you'll be fine.

Thankyou
one more thing please...what are the two acid dissociation equilibrium expressions for the ionisation of malic acid (C4H6O5) in water? i don't know how to do this question...im supposed to find which is larger...i know I'm asking a lot of questions but i appreciate the help a lot!
Thankyou in advance

[zsteve]

you don't need the values. It's certain that the first ionisation will occur to a greater extent than the second ionisation, because the acidic nature reduces as it deprotonates.

[Maz]

you don't need the values. It's certain that the first ionisation will occur to a greater extent than the second ionisation, because the acidic nature reduces as it deprotonates.

Thankyou so much

[lzxnl]

If you're interested, look up why these compounds are acidic. The acidity of a molecule is related to the stability of its conjugate base. For carboxylic acids and oxyacids (acids with lits of oxygens like phosphoric, nitric and sulfuric acids), something called resonance helps stabilise the conjugate base.

Same with phenol.
Alcohols are nuch weaker acids because they have little stabilisation. That negative oxygen has nowhere to go

[HasibA]

The following equations are not balanced. Combine the half equations to write a balanced equation.
MnO2(s) + H+(aq) + S(s) → Mn2+(aq) + H2O(l) + SO2(g)
my half equations were: S (s) + 2H2O(l) ® SO2(g) + 4H+(aq) + 4e– which was correct
and
MnO2(s) + 4H+(aq) + 2e- ---> Mn2+(aq) +2H2O(l) which is apparently incorrect? i don't get why though?
apparently it's supposed to be: ii 2MnO2(s) + 4H+(aq) + 2e–  -> Mn2+ + 2H2O(l) but then the Oxygens would not be balanced?
thank you

[keltingmeith]

The following equations are not balanced. Combine the half equations to write a balanced equation.
MnO2(s) + H+(aq) + S(s) → Mn2+(aq) + H2O(l) + SO2(g)
my half equations were: S (s) + 2H2O(l) ® SO2(g) + 4H+(aq) + 4e– which was correct
and
MnO2(s) + 4H+(aq) + 2e- ---> Mn2+(aq) +2H2O(l) which is apparently incorrect? i don't get why though?
apparently it's supposed to be: ii 2MnO2(s) + 4H+(aq) + 2e–  -> Mn2+ + 2H2O(l) but then the Oxygens would not be balanced?
thank you

Yeah, maybe the writers were smoking some sulfur dioxide when they wrote the question. (sidenote: don't smoke sulfur dioxide unless you want to fuck yourself up. Also it smells like egg. ;__;)

I'd go with your answer - it clearly balances, and theirs clearly doesn't, so you're all good.

[lzxnl]

If it's something like this where you can apply a foolproof check to your answer (aka are charges, atoms balanced) and yours meets the check but not some 'authority's ' answer, then have the confidence in yourself to call out the 'authority' on their answer. Doesn't matter if that authority is a textbook, one of us here on AN or your teacher. Trust me, if you get the call right, it's really good for your confidence in the subject.

[Maz]

hey guys...quick question please...
why are indicators most effective over a small pH range?
it says it on the last couple of line on this website http://chemistry.bd.psu.edu/jircitano/abindic.html
if it helps...
Thankyou in advance