Aqa chem 4/ chem 5 june 2016 thread

So right I can't wait for people to see the paper again because it asked for delta Hf

wtf is the chelate effect?

Solubility one- AGREED

YES! I got this, but people were telling me it's wrong. I think I've dropped 3 marks in total, I don't want a CE and to make it 6!!

Hilarious how people just cannot accept that the question required you to have an appreciation of the fact that 1 mol is formed when standard 😂😂

No, it definitely didn't. It gave you Hf values in a table but it asked you to work out the standard enthalpy change for for the reaction shown.

Do you divide by 2 or not this is doing my head in now

Substitution of unidentate ligands (water in this question) by bidentate (ethane-1,2-diamine in this question) or multidentate ligands leads to a more stable complex because of the increase in the number of particles so disorder increases, entropy increases, delta H is approximately 0 and delta G <= 0

For the dividing by 2, I was looking at the question in the exam trying to figure out whether it was necessary or not! I think that if answers are given as for example: -189 kj for the enthalpy change for the reaction then that's fine, and if given as -94.5 kj mol-1 that's also fine. Both are the same it's just the units that change and I think if you changed units depending on your answer you should get the marks. Agree?

I think you're just a stubborn nerd who can't accept that it was perfectly fine to divide by 2 and work it out

Lets get this straight
1 mark for equation 2:5 ratio
2nd mark for no of moles of Mno4- 0.02x(26.4/1000)
3rd mark for finding moles of C2o4- using the ratio 0.00132
4th mark for x10 to find in original 250 cm3
5th mark for x by 88 Mr of C2O4 to get 1.16 as this is specifically for the total C2o4 present before titre so up to this point definitely right
6th mark for the Mr of C2o4 which is 88
7th mark for dividing by 2.29 or something then times 100% to get the percentage as 50.7
Here are the official 7 marks for this question as both the 50.7 and 94.4% answers concern C2o4 pressent in the original sample 94.4 gives 8 marks whilst this is a 7 marker so why isnt this paper out of 101 dalmations

how many marks do you lose for not converting degrees to kelvin?

Just to point out that the enthalpy change of -198 kj mol-1 still needs the 'mol-1' section to it, because that's the units for enthalpy change. Using 'mol-1' as an argument to justify dividing by 2 is a fallacy because mol-1 is not referring to 'per mole of SO3', it purely denoting 'per such reaction' which is in this case forms 2 moles of SO3

For the gold jewellery one did anyone say:

E (Au+/Au) > E (O2/H2O) so Au+ ions oxidise water to oxygen and Au+ ions are reduced to Au

I seid something like that and also I worked out the Emf and it was negative so reaction was no feasible.

Solubility question stated that solubility was mentioned as the amount of present MgCl2(s) available to dissolve in aq solution, and as the reaction went backwards, more MgCl2(s) was produced, and hence the solubility was increased, not descreased

Either way increase or decrease, as long as your explanation was correct, then I believe you will get a minimum of 2/3

Bumping this because I also forgot to convert to k I think, if you guys give me the answer for using as degrees and answer for kelvin then I can see if I forgot to convert