Chromium complex goes green to purple? THIS stumped my professor in lecture

Question

Cr 3 (H2O)x > PbSO4 (s)

this happens with a source of sulfate (H2SO4 or NaSO4) and lead (II) ions.

The solution goes from green to purple? What is responsible for this? Is the complex still present?

Answer 1

Don't know what the lead is doing (lead sulphate is insoluble so should form a white ppt). Chromium(iii) sulphate is violet.

Answer 2

the purple is accepted to be [Cr(H2O)6] +3 complex ion

when in solution, the presence of ions like SO4-2  , or Cl-,  & ....

an exchange happens of the neg ion & a negative ion foming a green complex such as the green:

the tetraaquadichlorochromium(III) ion - [Cr(H2O)4Cl2]+

or the pentaaquasulfatochromium(III) ion - [Cr(H2O)5 SO4]+

(perhaps this can also happen with nitrate)

also , in water note that the [Cr(H2O)6] +3 complex ion usually dissociate one of the ligands , forming [Cr(H2O)5 (OH-)] +2   & H3O+

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read about this @:

http://www.chemguide.co.uk/inorganic/tra...

"Ligand exchange reactions involving chloride or sulphate ions

The hexaaquachromium(III) ion is a "difficult to describe" violet-blue-grey colour. However, when it is produced during a reaction in a test tube, it is often green.

We nearly always describe the green ion as being Cr3+(aq) - implying the hexaaquachromium(III) ion. That's actually an over-simplification.

What happens is that one or more of the ligand water molecules get replaced by a negative ion in the solution - typically sulphate or chloride.

Replacement of the water by sulphate ions

You can do this simply by warming some chromium(III) sulphate solution.

One of the water molecules is replaced by a sulphate ion. Notice the change in the charge on the ion. Two of the positive charges are cancelled by the presence of the two negative charges on the sulphate ion.

Replacement of the water by chloride ions

In the presence of chloride ions (for example with chromium(III) chloride), the most commonly observed colour is green. This happens when two of the water molecules are replaced by chloride ions to give the tetraaquadichlorochromium(III) ion - [Cr(H2O)4Cl2]+.

Once again, notice that replacing water molecules by chloride ions changes the charge on the ion"