This discussion is now closed.

Check out other Related discussions

Chromium

Hi!

We have to explain the colours of chromium ions during an oxidation reaction such as turning an aldehyde into an acid using CrO3 in acid, and acetone solvent. I'm not sure on these colours mainly because I am not sure what is going on within the reaction, is there anyone who could help me please :smile:

CrO3 is a 6+ oxidation state for chromium - when this was added to the aldehyde, it turned initially green - the +4 and +5 state together?

Then blue, the +4 state?

Then ended up green again, is this the 3+ state?

Or am I getting it totally wrong :smile:

Reply 1

hidden088

OP

2

Ok I put:

This is due to the chromium ions being reduced, They first turn blue from orange because the chromium (VI) ions are being reduced to 3+ ions which are green but as this has only partially happened the solution appears blue with a mix of 3+ ions and 6+ ions. It eventually turns green as all ions are reduced to the 3+ state. This can be shown by the following redox equation:

3RCHO + Cr2O72- + 8H+ → 3RCOOH + 2Cr3+ + 4H2O

Is this correct?

Reply 2

JayEm

14

Everything you need to know, and more here: http://www.chemguide.co.uk/inorganic/transition/chromium.html

Reply 3

hidden088

OP

2

I couldn't really find anything that gives detailed info on the different colours of chromium ions during a oxidation of a aldehyde reaction and why on that website :frown:

Anyone else have any other info on this?

At this level you just need to know that orange Cr(VI) changes to green Cr(III)

Reply 5

hidden088

OP

2

charco

At this level you just need to know that orange Cr(VI) changes to green Cr(III)

Ok thanks. The question says it goes from orange > blue > green. What would the blue be? Is it just a mix of ions before they have all been reduced like I said?

Reply 6

EierVonSatan

21

hidden088

Ok thanks. The question says it goes from orange > blue > green. What would the blue be? Is it just a mix of ions before they have all been reduced like I said?

I think:

CrO3 + H3O+ --> H2CrO4 (orange)
H2CrO4 (orange) + RCH2OH ---> H2CrO3 (blue?) + RCHO
H2CrO3 (blue?) ---> Cr3+(aq) (green)

Reply 7

hidden088

OP

2

EierVonSatan

I think:

CrO3 + H3O+ --> H2CrO4 (orange)
H2CrO4 (orange) + RCH2OH ---> H2CrO3 (blue?) + RCHO
H2CrO3 (blue?) ---> Cr3+(aq) (green)

Edit: Yeah I think that is right, chromous acid has a blue colour to it.

hidden088

Edit: Yeah I think that is right, chromous acid has a blue colour to it.

Chromous is the name given to the (II) oxidation state of chromium - this isn't be formed in this transformation from (VI) to (III).

The mechanisms of oxidation have been extensively studied (Cotton and Wilkinson) suggesting that the chromium passes through (V) and (IV) oxidation states as well as forming carboxylate complexes.

There is no way that you are required to state specific compounds for this level as they remain uncertain. The examiners would expect you to suggest an intermediate oxidation state of chromium between (VI) and (III).

Reply 9

hidden088

OP

2

charco

Chromous is the name given to the (II) oxidation state of chromium - this isn't be formed in this transformation from (VI) to (III).

The mechanisms of oxidation have been extensively studied (Cotton and Wilkinson) suggesting that the chromium passes through (V) and (IV) oxidation states as well as forming carboxylate complexes.

There is no way that you are required to state specific compounds for this level as they remain uncertain. The examiners would expect you to suggest an intermediate oxidation state of chromium between (VI) and (III).

Ah ok many thanks! Is there any level for which this would be required? Or is it to uncertain.

Chromium

Related discussions

Latest

Trending

Trending

Articles for you