Question:
A sample of solid chromium(III) hydroxide displays amphoteric character when treated separately with dilute HCl and with dilute aqueous NaOH. Write and ionic equation for each of these reactions. include the formula of each complex ion formed. Describe the changes that would observe in each reaction (5)
Mark scheme:
1.Cr(OH)3 + 3H2O + 3H+ [cr(h2o)6]3+
2.Green / grey−green solid
3.Forms green / purple / ruby / violet solution
4.Cr(OH)3 + 2H2O + OH− [cr(h2o)2(oh)4]
5.Forms green solution

(edited 1 month ago)
are you a level or gcse????
Original post by AnVeryBigIdiot
are you a level or gcse????

aqa a level chemistry (year 13)
Original post by vvwestwood
aqa a level chemistry (year 13)

oh sorry im only a gcse student (i was thinking that looked a bit too difficult lol)
Original post by AnVeryBigIdiot
oh sorry im only a gcse student (i was thinking that looked a bit too difficult lol)

np. thanks anyways
If you dissolve Cr(OH)3 in water then you should get [Cr(H2O)3(OH)3]? Then that is protonated to make [Cr(H2O)6]3+.
And vice-versa for NaOH. That's how I understand it =)
I understand the first one now thank you.
But for the reaction with NaOH, i dont understand why it reacts with 2H20 and just one OH-. Is it something to do with the symmetry of the complex? Like for example whats stopping Cr(OH)3 reacting with 3OH- to make [Cr(OH)6]3-
Original post by vvwestwood
I understand the first one now thank you.
But for the reaction with NaOH, i dont understand why it reacts with 2H20 and just one OH-. Is it something to do with the symmetry of the complex? Like for example whats stopping Cr(OH)3 reacting with 3OH- to make [Cr(OH)6]3-

I would have also guessed that that would be what it forms... is there no mention of it in the MS?

But I'm not 100% sure. Also, I don't think reactions of chromium complexes are in the aqa spec?
Chromium is no longer on the spec, so you do not need to know it!

However, I believe the mark scheme states that you can have Cr(OH)3 react with 3OH- as the coordination number will still be 6. You could even have it react with 1 H2O and 2 OH-.
Original post by Methene
I would have also guessed that that would be what it forms... is there no mention of it in the MS?
But I'm not 100% sure. Also, I don't think reactions of chromium complexes are in the aqa spec?

yeah actually in the MS it does say allow [Cr(OH)6]3-
there is no mention of Cr complexes in the teaching guide:
https://filestore.aqa.org.uk/resources/chemistry/AQA-7405-REACTIONS-OF-METAL-IONS.PDF
However my teacher did say that Cr complexes are still a must know and the exam question is from the same 7405 spec so im not totally sure.
Chromium is no longer on the spec, so you do not need to know it!
However, I believe the mark scheme states that you can have Cr(OH)3 react with 3OH- as the coordination number will still be 6. You could even have it react with 1 H2O and 2 OH-.

well the problem is that these exam questions are from the same 7405 spec and there is still mention of chromium complexes.
and you're correct about the MS. says allow [Cr(OH)6]3-
Original post by vvwestwood
yeah actually in the MS it does say allow [Cr(OH)6]3-
there is no mention of Cr complexes in the teaching guide:
https://filestore.aqa.org.uk/resources/chemistry/AQA-7405-REACTIONS-OF-METAL-IONS.PDF
However my teacher did say that Cr complexes are still a must know and the exam question is from the same 7405 spec so im not totally sure.

It's worth checking again with your teacher...
I just checked both the current spec and the old spec.
The current spec states:

In aqueous solution, the following metal-aqua ions are formed:
[M(H20)6]2+, limited to M = Fe and Cu
[M(H20)6]3+, limited to M = Al and Fe
The acidity of [M(H20)6]3+ is greater than that of
[M(H20)6]2+
Some metal hydroxides show amphoteric character by dissolving in both acids and bases (eg hydroxides of A|3+).

The old spec states:

know that metal-aqua ions are formed in aqueous solution:
[M(H20)6]2+, limited to M = Fe, Co and Cu
[M(H20)6]3+, limited to M = Al, Cr and Fe

One of the main things in the new spec was the removal of Cobalt and Chromium when learning about the reactions of ions in aqueous solution.

Yes, there may be questions about chromium and cobalt in exam questions following the new spec, but it will be on things that you have learnt and could apply to chromium/cobalt.
Original post by vvwestwood
Question:
A sample of solid chromium(III) hydroxide displays amphoteric character when treated separately with dilute HCl and with dilute aqueous NaOH. Write and ionic equation for each of these reactions. include the formula of each complex ion formed. Describe the changes that would observe in each reaction (5)
Mark scheme:
1.Cr(OH)3 + 3H2O + 3H+ [cr(h2o)6]3+
2.Green / grey−green solid
3.Forms green / purple / ruby / violet solution
4.Cr(OH)3 + 2H2O + OH− [cr(h2o)2(oh)4]
5.Forms green solution
“Chromium(III) hydroxide is amphoteric in character” implies draw on your knowledge of aluminium (and its oxide/hydroxide) as that is an element you should have studied as per the AQA syllabus.

Strictly speaking, chromium(III) hydroxide is Cr(OH)3(H2O)3 and it’s the extra protons on the water ligands that are removed when a strong base is added in excess. Since you are expected to draw on your knowledge of aluminium, they want you to deduce that the central ion gets surrounded by 4 hydroxide ligands (and 2 waters). In reality, chromium(III) hydroxide is deprotonated as far as the hexahydroxochromate(III) complex [Cr(OH)6]^3-, but this isn’t something you need to know.

In acidic conditions, it’s just protonation of the hydroxides to water, since hydroxides are basic and so pick up H^+ readily.
I just checked both the current spec and the old spec.
The current spec states:
In aqueous solution, the following metal-aqua ions are formed:
[M(H20)6]2+, limited to M = Fe and Cu
[M(H20)6]3+, limited to M = Al and Fe
The acidity of [M(H20)6]3+ is greater than that of
[M(H20)6]2+
Some metal hydroxides show amphoteric character by dissolving in both acids and bases (eg hydroxides of A|3+).
The old spec states:
know that metal-aqua ions are formed in aqueous solution:
[M(H20)6]2+, limited to M = Fe, Co and Cu
[M(H20)6]3+, limited to M = Al, Cr and Fe
One of the main things in the new spec was the removal of Cobalt and Chromium when learning about the reactions of ions in aqueous solution.
Yes, there may be questions about chromium and cobalt in exam questions following the new spec, but it will be on things that you have learnt and could apply to chromium/cobalt.

Makes sense. Any knowledge on Fe3+ and Al3+ can be applied to Cr3+ and the same with cobalt. Thank you. Saved me a lot of time memorising the cobalt and chromium reactions