AQA CHEM5 A2 Chemistry - 19th June 2013

The emf of the reaction "VO2+(aq) + 2H+(aq) + e– → VO2+(aq) + H2O(l)" is less than the one for MnO4- so V2+ makes V3+ which makes VO(2+) which makes VO2(+). The MnO4- must be reduced.

VO2+ +5 + -4 = +1

And the equation should be pretty straight forward once you know the final species because you just balance the oxygen and water etc.

Hope that helped because that first part is always hard to explain!

But it says it was added to a solution containing $V^{2+}$, then wouldn't you have to use the first equation making the species $V^{3+}$?

Same I'm doing chem 2, 4 and 5 but I'm worried I'm leaving chem 5 too late

Yeah because the MnO4- species has a higher emf it is reduced and the other is oxidised so the equation is reversed. Bare in mind that they're all written as reductions in the electrochemical series.

Sorry, I still don't get it .

The way I see it is as you add it into a solution containing $V^{2+}$ ions, the two equations must be

$V^{2+} \rightarrow V^{3+}+e^-$ and

$MnO_4^{-}+8H^++5e^- \rightarrow Mn^{2+}+4H_2O$

which would the produce an overall equation of

$5V^{2+}+MnO_4^{-}+8H^+\rightarrow 5V^{3+}+Mn^{2+}+4H_2O$

This then gives $V^{3+}$ as the final Vanadium species and so I can't see where the $VO_2^{+}$ is coming from.

In the same table V³⁺ can be oxidised to VO²⁺ because the emf is still less than that of MnO₄^- and you carry on until you can no longer make another product.

From the Chem5 jan 12 paper, question 8.e:

Why doesn't the diaminoethane form a co-ordinate bond with the aluminium ion, yet it does with the cobalt ion in the following question?

It can. It told you diaminoethane can act as a ligand or a base. It asked for the equation where it acts as a base.

The question DOES NOT specify whether the diaminoethane acts as a base or a ligand, nor does it in the following question. It just states the fact that it could.

From the Chem5 jan 12 paper, question 8.e:

Why doesn't the diaminoethane form a co-ordinate bond with the aluminium ion, yet it does with the cobalt ion in the following question?

Fair point. I think you were suppose to make the assumption they wanted the reaction where it acted as a base since the reaction was with an acid and also that it asked for the appearance of the product. I suppose 'colourless solution' could have been the appearance for the substitution reaction but, since they provide you with an acid, I'd again assume they're asking for an acid-base reaction.

You know it can act as a ligand, what would be the point of mentioning it can act as a base if that wasn't the point of the quesiton?

The important thing in this question is where it says LIKE AMMONIA so you treat it as you would ammonia acting as a base.

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I see what you mean, I put it down to to a little negligence on AQA's part.

As for your equation in 8e)ii), I also gave the same answer when I did the paper earlier and only awarded myself a single mark. You should gain the mark for having the correct Co(III) species but i'm not sure whether oxygen can be shown to be reduced like that. I remember in a similar question in another past paper where it said you need to show H20 or H+ in the equation when O2 is being reduced. But i'm not entirely sure if you'd lose the mark for definite. But in future just add the H20 or H+ to guarantee the mark.

what exactly is the cisplatin structure? it seems that the aqa nt book mixes it up with trans and im not sure which it is?