[T]

There was this question I saw recently:

The dichromate ion Cr₂O₇^2– is orange in aqueous solution whereas Cr³⁺ is green. An acidified solution of potassium dichromate is thus a useful reagent for identifying many chemical species.
The following five aqueous solutions were prepared, and a few drops of acidified potassium dichromate solution were added to each one. Three of them turned green; one turned a murky brown colour, and one remained orange.
Which solution remained orange?

A SnCl₂

B KI

C CH₃CH₂OH

D NaNO₂

E Al₂(SO₄)₃

You are not given the electrochemical series table.

To turn the dichromate ion into Cr³⁺ you will need to reduce it. So therefore the solution that will remain orange is the one that won't reduce the dichromate ion. However, I can't tell which metal can reduce the dichromate ion. Can someone give me some hints?

Thanks

[Burner]

To turn the dichromate ion into Cr³⁺ you will need to reduce it. So therefore the solution that will remain orange is the one that won't reduce the dichromate ion. However, I can't tell which metal can reduce the dichromate ion. Can someone give me some hints?

Good point here, but there's another approach: Which only chemical can oxidize the dichromate ion(i.e. act as an oxidizing agent?). Note that oxidizing agent usualy do not reduce other chemicals, since it reduces itself instead.

[T]

Can't A, B, D, and E all reduce? Since they are all metal ions, they can gain electrons. Thanks.

[AWK]

Dichromate is a powerful oxidizer. In which compund one or more element does not show the highest possible oxidation number? In which one the highest ON?

[Burner]

Can't A, B, D, and E all reduce? Since they are all metal ions, they can gain electrons. Thanks.

So therefore the solution that will remain orange is the one that won't reduce the dichromate ion. However, I can't tell which metal can reduce the dichromate ion.

Note that an oxidizing agent oxidize other species by reducing the oxidizing agent itself.
A reducing agent reduce other species by oxidizing itself.

[Borek]

Which only chemical can oxidize the dichromate ion(i.e. act as an oxidizing agent?)

There is no such reagent between those listed.

Actually I am not sure it is at all possible to further oxidize Cr(VI) in compounds.

[T]

Dichromate is a powerful oxidizer. In which compund one or more element does not show the highest possible oxidation number? In which one the highest ON?

E, Al has a oxidation number of 3+

[AWK]

This is a good answer, but you should check all elements in these 5 compounds (though you may neglect H and O - why?) since one of these compound contain 2 reductors.

[Burner]

Which only chemical can oxidize the dichromate ion(i.e. act as an oxidizing agent?)

There is no such reagent between those listed.

Actually I am not sure it is at all possible to further oxidize Cr(VI) in compounds.

Al³⁺ is a weak oxidizing agent. An oxidizing agent can't reduce Cr₂O₇^2-.

Perhaps my wordings are not so clear. And the question asks us to find the chemical which don't change colour with Cr₂O₇^2-, i.e. the one that won't reduce the dichromate ion.

[Borek]

Al³⁺ is a weak oxidizing agent.

Al³⁺ is quite stable. Sure, in some exotic conditions it can be reduced to Al, and that technically makes it an oxidizer, but for all practical purposes Al³⁺ is perfectly inert. Na⁺ and K⁺ can be reduced as well, would you call them weak oxidizing agents, or would you classify them as inert cations?

Perhaps my wordings are not so clear. And the question asks us to find the chemical which don't change colour with Cr₂O₇^2-, i.e. the one that won't reduce the dichromate ion.

Things that are inert won't reduce the dichromate.

[T]

This is a good answer, but you should check all elements in these 5 compounds (though you may neglect H and O - why?) since one of these compound contain 2 reductors.

A SnCl2 -- Sn (+2) Cl (-1) Sn cannot oxidise anymore so won't reduce the dichromate. The Cl can be oxidised into Cl₂ so that means it can reduce the dichromate. A is not the answer

B KI -- K (+1) I (-1) K cannot oxidise anymore so won't reduce the dichromate. The I can be oxidised into I₂ so that means it can reduce the dichromate. B is not the answer

C CH3CH2OH -- I learnt somewhere that the C-H, C-O, O-H bonds are all strong so I don't think it will react. A possible answer.

D NaNO2 -- Na (+1) NO₂^- (1-) Na cannot oxidise anymore so won't reduce the dichromate. NO₂^- I am not too sure

E Al2(SO4)3 -- Al (+3) SO₄^2- (2-) It can't be oxidised so it is a possible answer.

Thanks

[Borek]

A SnCl2 -- Sn (+2) Cl (-1) Sn cannot oxidise anymore

Check it out. What are common oxidation numbers for Sn?

C CH3CH2OH -- I learnt somewhere that the C-H, C-O, O-H bonds are all strong so I don't think it will react. A possible answer.

Do you know what kind of an organic molecule it is? Its functional group is -OH, quite a characterstic one.

NO₂^- I am not too sure

What acids with N as a central atom do you know?

[T]

Check it out. What are common oxidation numbers for Sn?

Another oxidation number for Sn is 4+

Do you know what kind of an organic molecule it is? Its functional group is -OH, quite a characterstic one.

It is an alcohol, ethanol.

What acids with N as a central atom do you know?

I know nitric acid and nitrous acid and that's about it.

Thanks

[Burner]

It is an alcohol, ethanol.

What redox reactions can alcohol undergo? Oxidation or reduction?

I know nitric acid and nitrous acid and that's about it.

Are nitric acid and nitrous acid oxidizing agent(oxidzer) or reducing agent(reductor)?

[T]

What redox reactions can alcohol undergo? Oxidation or reduction?

Alcohol can oxidise into aldehyde then carboxylic acids.

Are nitric acid and nitrous acid oxidizing agent(oxidzer) or reducing agent(reductor)?

They are reductants.