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AS Level OCR A Chemistry Question

Hi guys, I have been stuck on this question for a long time and was wondering if anyone could answer it. Thank you!

Malic acid can be oxidised by heating with acidified potassium dichromate (VI). Write a balanced equation for the reaction showing the structure of the organic product. Use [O] to represent the oxidising agent.
START:-

K2Cr2O7 is empirical formula of potassium dichromate [difficult to tell if its correct cos K+ has a valency of "1" so the ion Cr2O7 must be one minus [Cr is a transition metal so has variable valencies so???? (calling chemistry nerds please - "hello hello testing 3-2-1" :smile:)

Malate = CH2(COOH)CH(OH)COOH [empirically: C4H6O5

Let us try and sort this: what could be products [useful if we already knew): ok google says:- Pot dichromate oxidizes alcohols to aldehydes [just like liver oxidizes whisky [:colondollar:] to acetaldehyde] - formula is CH3CHO so looks like 2 atoms of H have been shed, agreed [from CH3CH2OH]. Oc loss of H is oxidation.

So we might get: FROM: CH2(COOH)CH(OH)COOH
............................TO: CH2(COOH)COCOOH [what is this? [2 acid groups one aldehyde group [??] [C=O is actually a ketone group [ONCE AGAIN: Helloooooooooooooooo chem nerds pls!!] removal o hydrogen is oxidation.

Anyway one product should be C4H4O5 [from bold above]

NOW REACTION: K2Cr2O7 + C4H6O5 -----> C4H4O5 + KCrO4 [???I am making analogous to potassium permanganate: -KMnO4]

If we can balance this, WE KNOW IT IS CORRECT!

I AM SHATTERED - leave you to try to balance!

NOW TO COOK MY BACON EGG + SAUSAGES :yum:

M
(edited 1 year ago)
Reply 2
Original post by macpatgh-Sheldon
START:-

K2Cr2O7 is empirical formula of potassium dichromate [difficult to tell if its correct cos K+ has a valency of "1" so the ion Cr2O7 must be one minus [Cr is a transition metal so has variable valencies so???? (calling chemistry nerds please - "hello hello testing 3-2-1" :smile:)

Malate = CH2(COOH)CH(OH)COOH [empirically: C4H6O5

Let us try and sort this: what could be products [useful if we already knew): ok google says:- Pot dichromate oxidizes alcohols to aldehydes [just like liver oxidizes whisky [:colondollar:] to acetaldehyde] - formula is CH3CHO so looks like 2 atoms of H have been shed, agreed [from CH3CH2OH]. Oc loss of H is oxidation.

So we might get: FROM: CH2(COOH)CH(OH)COOH
............................TO: CH2(COOH)COCOOH [what is this? [2 acid groups one aldehyde group [??] [C=O is actually a ketone group [ONCE AGAIN: Helloooooooooooooooo chem nerds pls!!] removal o hydrogen is oxidation.

Anyway one product should be C4H4O5 [from bold above]

NOW REACTION: K2Cr2O7 + C4H6O5 -----> C4H4O5 + KCrO4 [???I am making analogous to potassium permanganate: -KMnO4]

If we can balance this, WE KNOW IT IS CORRECT!

I AM SHATTERED - leave you to try to balance!

NOW TO COOK MY BACON EGG + SAUSAGES :yum:

M


Thank you so much you've made my life so much easier :h:

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