Chem A2-transition metals

I'm a little stuck on this question, it'd be great if you can help:

An aqueous solution of Chromium(III)Choride
1) reacts with NaOH to form a green gelatinous ppt
2) reacts with Zn(s) and HCl(aq) to form a sky blue solution

Give 2 features of Cr chemistry, illustrated in above, which are typical of a transition element.
Identify the solution in (2)

Incidentally, what is the definition of deprotonation?
Thanks!

first one is a straightforward precipitation of the Cr(OH)3 probably with the structure[Cr(H2O)3(OH)3] derived from the octahedral hexaaqua complex

The second looks like a reduction (Zn and HCl form nascent hydrogen - a reducing agent) will reduce the CrIII to CrII. I suspect that the sky blue colour is the complex is [Cr(H2O)6]2+

deprotonation = removal of a hydrogen ion (proton) - in the first reaction the OH- ions deprotonate the H2O ligands (three of them) of the hexaaqua chromium III complex

Reply 2

Frogsofhope

2

Would the sky blue not be [Cr(H2O)5Cl]2+ ?

If this were the case then the zinc would be redundant in the question.

Unless the concentration of Cl- ions is very high then there would be little reason to replace water ligands

Reply 4

Mimo

OP

charco

first one is a straightforward precipitation of the Cr(OH)3 probably with the structure[Cr(H2O)3(OH)3] derived from the octahedral hexaaqua complex

The second looks like a reduction (Zn and HCl form nascent hydrogen - a reducing agent) will reduce the CrIII to CrII. I suspect that the sky blue colour is the complex is [Cr(H2O)6]2+

deprotonation = removal of a hydrogen ion (proton) - in the first reaction the OH- ions deprotonate the H2O ligands (three of them) of the hexaaqua chromium III complex

How can the structure be [Cr(H2O)3(OH)3] when you start with chromium(III)chloride?
The complex [Cr(H2O)6]2+ is green, isn't it? So, it can't be that...

Reply 5

BCHL85

12

Mimo

How can the structure be [Cr(H2O)3(OH)3] when you start with chromium(III)chloride?
The complex [Cr(H2O)6]2+ is green, isn't it? So, it can't be that...

In solution, Chromium(III) chloride is dissociated, then Cr(3+) ions are surrounded by H2O (dative bonds are formed by pair of electrons in oxygen atoms). So the solution is [Cr(H2O)6](3+)
Then the ligand exchange reaction take places
[Cr(H2O)6](3+) + 3OH- ---> [Cr(H2O)3(OH)3] (ppt).

Cr(3+) is green, Cr(2+) is blue.

Reply 6

Mimo

OP

BCHL85

In solution, Chromium(III) chloride is dissociated, then Cr(3+) ions are surrounded by H2O (dative bonds are formed by pair of electrons in oxygen atoms). So the solution is [Cr(H2O)6](3+)
Then the ligand exchange reaction take places
[Cr(H2O)6](3+) + 3OH- ---> [Cr(H2O)3(OH)3] (ppt).

Cr(3+) is green, Cr(2+) is blue.

ahh... thanks very much- that explains it all!
But wait...
what are the 2 features of chromium chemistry then?

Chromium is typical of transition metals in that it has:

variable oxidation state
complex ion formation
coloured compounds

and also (but not shown in this question):

magnetism (paramagnetic due to unpaired electrons)
catalysis
cluster compounds

Reply 8

Mimo

OP

charco

Chromium is typical of transition metals in that it has:

variable oxidation state
complex ion formation
coloured compounds

and also (but not shown in this question):

magnetism (paramagnetic due to unpaired electrons)
catalysis
cluster compounds

Thanks!
Ok, last question in relation to the first....

Evapourating cold aqueous chromium(III)chloride produces violet crystals. However when hot aqueous chromium(III)chloride crystallises, green crystals result. Empirical formula of both salts are Cr(H2O)6Cl3.
Addition of excess silver nitrate to aqueous solutions of either salts produces a ppt of sliver chloride. All violet salt is pptd but only 2/3 of chloride from the green salt.
Suggest formula for the 2 salts.

Thanks again.

silver salts in solution will precipitate only those chloride ions that are not complexed.
i.e. it WILL precipitate AgCl from [Cr(H2O)6]Cl3
but NOT from [Cr(H2O)3Cl3]3+

So you can see from the question that if 2 moles of Cl are precipitated as AgCl then those two Cl- ions form part of the balancing anions and NOT part of the complex ion.

Hence the violet salt has all three chlorides as part of the balancing anions and is [Cr(H2O)6]Cl3 whereas the other salt has only two thirds of the chloride precipitated and is therefore [Cr(H2O)5Cl]Cl2

In the second salt one of the chloride ions forms part of the complex ion and is unaffected by the addition of silver ions

Reply 10

Mimo

OP

charco

silver salts in solution will precipitate only those chloride ions that are not complexed.
i.e. it WILL precipitate AgCl from [Cr(H2O)6]Cl3
but NOT from [Cr(H2O)3Cl3]3+

So you can see from the question that if 2 moles of Cl are precipitated as AgCl then those two Cl- ions form part of the balancing anions and NOT part of the complex ion.

Hence the violet salt has all three chlorides as part of the balancing anions and is [Cr(H2O)6]Cl3 whereas the other salt has only two thirds of the chloride precipitated and is therefore [Cr(H2O)5Cl]Cl2

In the second salt one of the chloride ions forms part of the complex ion and is unaffected by the addition of silver ions

thanks again! I'll stop bothering you now...

Chem A2-transition metals

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