Complete equation to show formation of complex that contains chromium:CrCl3 + 5H2O ->

Why can’t the answer be [Cr(H2O)4Cl2]Cl +H2O
The mark scheme says the answer is [Cr(h2o)5Cl]Cl2
And accept [Cr(H2O)5Cl]2+

Why do we specifically need 5 H2O?

Reply 1

Nitrotoluene

Original post

by meowy2828374748

Why can’t the answer be [Cr(H2O)4Cl2]Cl +H2O
The mark scheme says the answer is [Cr(h2o)5Cl]Cl2
And accept [Cr(H2O)5Cl]2+
Why do we specifically need 5 H2O?

Why not [Cr(H2O)4Cl2]Cl + H2O?

The chromium ion must surround itself with five water molecules to stay stable. The water has to cancel the +3 charge on the chromium. If there were only four water molecules, the complex wouldn’t hold together very well. The fifth is needed to fill what’s called the coordination sphere and keep all of it stable.

So, why five H2O?

The chromium ion, in short, needs a set number of water molecules to remain stable, and five is plenty to hold this pentaaquachloridochromium(III) chloride complex [Cr(H2O)5Cl2]Cl together. The formula [Cr(H2O)5Cl]^2+ is also correct, as it proposes as well: one chromium ion surrounded by five water molecules and one chloride, totalling +2 of charge.

Bye,
Sandro

(edited 11 months ago)

Reply 2

TypicalNerd

Original post

by meowy2828374748

Why can’t the answer be [Cr(H2O)4Cl2]Cl +H2O
The mark scheme says the answer is [Cr(h2o)5Cl]Cl2
And accept [Cr(H2O)5Cl]2+
Why do we specifically need 5 H2O?

They’ve told you one mole of CrCl3 reacts with 5 moles of H2O, so all 5 moles of H2O must be incorporated into the structure. To say that the products side should be [Cr(H2O)4Cl2]Cl + H2O would suggest you have only used 4 moles of water molecules and not 5 for each mole of CrCl3.

Reply 3

TypicalNerd

Original post

by Nitrotoluene

Why not [Cr(H2O)4Cl2]Cl + H2O?

So, why five H2O?

The chromium ion, in short, needs a set number of water molecules to remain stable, and five is plenty to hold this pentaaquachloridochromium(II) chloride complex together. The formula [Cr(H2O)5Cl]^2+ is also correct, as it proposes as well: one chromium ion surrounded by five water molecules and one chloride, totalling +2 of charge.
Bye,
Sandro

I’m going to be a bit nitpicky here.

The charge on the chromium is not cancelled by the water ligands as there is no redox reaction taking place. Ligands do however allow for the charge on the complex to be spread out by the inductive effect, which is a stabilising effect.

Technically speaking, Cr^3+ could support a coordination number less than 5 or 6, but it preferentially supports an octahedral geometry and therefore a coordination number of 6. The reasons why are a bit above A level (which I presume is the level the OP is studying chemistry at), due to a number of factors - namely the crystal field stabilisation energy.

The name you have assigned the complex is almost correct - as I have said, this isn’t a redox reaction and so the oxidation state of the chromium is +3, so it is pentaaquachloridochromium(III). The charge is +2 because the chloride ion has a charge of -1 and so reduces the total charge on the complex by 1, but does not affect the oxidation state of the chromium.

Reply 4

Anonymous #1

Hi I have a question - I wrote [Cr(H20)5]3+ + 3Cl- as the products, why would this be incorrect?

Reply 5

TypicalNerd

Original post

by Anonymous

Hi I have a question - I wrote [Cr(H20)5]3+ + 3Cl- as the products, why would this be incorrect?

In general, transition metal complexes are 6-coordinate (e.g favour an octahedral geometry) with the odd exception (discussed in more depth if you do a chemistry degree, but at A level one important exception you need to be aware of is when you only have large charged ligands like Cl^-, where it instead favours a tetrahedral geometry because fitting 6 chlorides that want to repel away from each other around a metal centre is pretty difficult).

Whilst you could form a 5-coordinate complex of chromium in theory, it wouldn’t be terribly stable (again this is something you need undergrad level chemistry to properly understand) and so would likely pick up another ligand.

Reply 6

Frizzle16523

hi, the question says chromium is in its 3+ oxidation state so i dont get how thats the answer as its 2+? please can someone explain

Reply 7

TypicalNerd

Original post

by Frizzle16523

hi, the question says chromium is in its 3+ oxidation state so i dont get how thats the answer as its 2+? please can someone explain

It remains in its +3 oxidation state, even if the total charge on the complex is +2. Remember chloride ions have a charge of -1 and the water ligands are all neutral. Given the final complex contains one chloride and one chromoum, for a total charge of +2, the chromium must have a charge of +3.

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