A2 Chemistry coursework - Vanadium in 2 different oxidation states

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byakugan_neji
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Hi, I have a final piece of Chemistry coursework to do. Fortunately, I already got 7s for my Planning and Analysis so I don't have to do any difficult and stressful write-ups.

However, I still have to do a bit of research so I know what to expect when I carry out the practical. I'm provided with 0.02 mol dm^-3 ammonium vanadate (V) and 0.01 mol dm^-3 potassium manganate (VII). I have to use 2 methods to reduce the Vanadate (V) to 2 other oxidation states. I then have to titrate it against the potassium manganate (VII), thus oxidising it back to Vanadate (V). By comparing the titres, I should work out which method produces which oxidation state.

I have to record all observations and colour changes and my titration results.

At the moment, what I really need help on is the equations because I'm sure there are some intermediate reactions which show the transfer of electrons etc...and I have to multiply them by a number and insert it back into the general equation. This is important since I need to know the amount of moles of potassium manganate and vanadate ions are used.

So if anyone could provide me with some equations, preferably not typed normally since it's hard to see which numbers are the moles number and which are the oxidation numbers etc.....
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Sahir
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Ok so which equations do you need help with?

I've only done Vanadium and the different oxidation states very recently but i might be able to help if i knew what the equations you needed were..

If you're reducing Vanadium (V) to either (II), (III) or (IV) you should be using reagents based on their E cell values.. i.e.: in order to reduce V (V) to V (IV) you need a specific reagent which has an E cell value between the E cell values of V (V) and V (IV)..

But what the reagent is depends on what you're given.. could be sodium, lead, copper or something else?..
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byakugan_neji
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Well, I have never heard of E-cells before and don't think it's to do with my coursework. It says on one of the methods that I have to add 5g of Zinc to the reduction flask which also contains the aqeuous vanadate (V), water and sulphuric acid.

I don't know what oxidation states I have to reduce the vanadate (V) to since it doesn't say which states on the sheet. It just says I have to reduce it to 2 different oxidation states using the 2 methods given and by comparing the titres and with the use of equations, work out what the oxidation states are. I think my teacher said today that the reduced ions we will get are (IV) and (II).
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elpaw
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zinc reduces V (v) all the way down to V (ii)
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byakugan_neji
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Does it mean I will have to get equations for vanadate (V) reducing to (IV), (III) and (II)? Or is there a more general equation which just shows (V) reducing to (II)?
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elpaw
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(Original post by byakugan_neji)
Does it mean I will have to get equations for vanadate (V) reducing to (IV), (III) and (II)? Or is there a more general equation which just shows (V) reducing to (II)?
they are the same thing because all the V(iv) will reduce to V(iii), and all the v(iii) will reduce to v(ii), so its like doing equations for intermediate steps.
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username9816
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(Original post by elpaw)
they are the same thing because all the V(iv) will reduce to V(iii), and all the v(iii) will reduce to v(ii), so its like doing equations for intermediate steps.
Good night Elpaw.

And thanks for ignoring me as well. :mad:
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byakugan_neji
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(Original post by elpaw)
they are the same thing because all the V(iv) will reduce to V(iii), and all the v(iii) will reduce to v(ii), so its like doing equations for intermediate steps.
You mean like this? http://www.rod.beavon.clara.net/incat9.htm
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Sahir
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hah, yeah Rod Beavon is EdExcel's Chemistry Chief Examiner..

and he rocks!... and he's just saved me LOADS of time explaning this. Yea follow that and you'll have no trouble.
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elpaw
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(Original post by bono)
Good night Elpaw.

And thanks for ignoring me as well. :mad:
when did i ignore you?
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byakugan_neji
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Could someone explain to me how I would use the titration results (the amount of Potassium Manganate needed to neutralise the reaction) to determine which method produces which ion? I'm pretty sure that the first method given to me is (II) while the second method is (IV). I need to know the moles ratio of the reaction in conjunction with the titration results but I don't know how what the general equation is.
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byakugan_neji
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Hi, I have just had the worst day ever. I went to collect my mail from the postal delivery office and since I kinda got lost, I was 30 minutes late for my pratical afterwards! Anyways, I managed to finish it but I really messed up big time, especially on the titrations. Due to not having done enough research and knowing what to expect, I was told that I went way past the end-point.

Anyways, I have the results of the 2 titrations now. I have converted them into moles but what do I do now? Do I compare the mole ratio to that of the equations so that I can identify which ions were produced?
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byakugan_neji
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I have worked out that Method 1 has Vanadium 2+ ions and Method 2 has Vanadium 4+ ions, like I predicted.

I now have to do the Evaluation and so far, I have only 3 points to write about. Can anyone suggest some general potential sources of error which can occur with redox titrations? Thanks.
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