Chemistry Examination Discussion

[Stick]

I didn't even think of that..

Fingers crossed haha

LOL I guess we did the same thing? XD I much prefer it when I get something wrong with someone else I know (I don't know why ).

[Edward21]

Hey would it matter if that 1 mark diagram for the energy profile was done with bits in pencil??? I didn't want to stuff it up and not be able to erase a terrible drawing!

[achre]

Hey would it matter if that 1 mark diagram for the energy profile was done with bits in pencil??? I didn't want to stuff it up and not be able to erase a terrible drawing!

Should be fine. I stuffed it up the first time and just redrew the little energy axis below it and put a line through my original answer.
They weren't asking for an exact value of enthalpy change, right, just an indication? Molar enthalpies of ethyl propanoate and propanoic acid weren't in the data book.

[Mr Keshy]

LOL I guess we did the same thing? XD I much prefer it when I get something wrong with someone else I know (I don't know why ).

Sharing is caring! hahaha

[brightsky]

Hey would it matter if that 1 mark diagram for the energy profile was done with bits in pencil??? I didn't want to stuff it up and not be able to erase a terrible drawing!

haha I did the whole of section B in 2B pencil. I had a word with my chem teacher before the exam, and she said that pencil is fine, provided that the graphite content is sufficient (lol). the only reason why vcaa wants us to use blue/black pen is because the booklet is scanned...I don't see any issue with using pencil, provided that the pencil is not 2H or something.

Should be fine. I stuffed it up the first time and just redrew the little energy axis below it and put a line through my original answer.
They weren't asking for an exact value of enthalpy change, right, just an indication? Molar enthalpies of ethyl propanoate and propanoic acid weren't in the data book.

nup. I think they were just testing whether or not we knew that combustion reactions are exothermic.

[eddybaha]

i did the whole thing in pencil LOL

[beastly101]

Just realised I drew my energy profile graph as an endothermic reaction, noob move!
But I put the activation energy correctly, FML.

[Edward21]

With the H3PO4, did you use the Ka1, or the multiple of 3...I USED ALL 3 talk about ambiguity. Because you combine the equations into one, and multiply each of the K values; what did you make of this MC question?

[rany]

With the H3PO4, did you use the Ka1, or the multiple of 3...I USED ALL 3 talk about ambiguity. Because you combine the equations into one, and multiply each of the K values; what did you make of this MC question?

I used only k1 because it had the largest ka value. the others had tiny ka values and hence the % ionisation would be so low that its insignificant so it wont affect the ph. The question asked APPROXIMATE ph so thats why k2 and k3 were irrelevant

[Edward21]

I used only k1 because it had the largest ka value. the others had tiny ka values and hence the % ionisation would be so low that its insignificant so it wont affect the ph. The question asked APPROXIMATE ph so thats why k2 and k3 were irrelevant

Gah that's so picky 

[beastly101]

I used only k1 because it had the largest ka value. the others had tiny ka values and hence the % ionisation would be so low that its insignificant so it wont affect the ph. The question asked APPROXIMATE ph so thats why k2 and k3 were irrelevant

I did only K1.

[rany]

Gah that's so picky 

Ye it is, but the first equation would have a high % ionization meaning that the [H+] would be the most in that, the others dont really affect the concentration of H+ ions as the equilbria favours the back reaction.

[achre]

I nearly did this one as well, but ended up just doing methyl propanoate to be safe.
Surely you'll still get the mark for doing this?

Well, if they're just asking for a structure that matches the new C NMR data and the molecular formula, I can't see why not.
But it really did seem like they were leading us towards a C=O and a C-O bond. I think it was just one mark though, so I'm not too worried if I lose it, god knows I've dropped enough.

[Edward21]

Ye it is, but the first equation would have a high % ionization meaning that the [H+] would be the most in that, the others dont really affect the concentration of H+ ions as the equilbria favours the back reaction.

*dies*

[lzxnl]

Haha yeah i noticed that too. Had to appeal to ka for ethanol like four times...

Just saying, ethanol is a weaker acid than water.
You mean ethanoic acid?

With the H3PO4, did you use the Ka1, or the multiple of 3...I USED ALL 3 talk about ambiguity. Because you combine the equations into one, and multiply each of the K values; what did you make of this MC question?

The only issue with that is that the first Ka is small; you form little H2PO4-. Then, the second Ka is even smaller, and as you initially have a tiny amount of H2PO4-, you form even less HPO4-. And so forth. I don't think VCAA intended for us to use all three Ka's.