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The Lovely Hues of Aqueous Copper(II) Complexes

posted Oct 31, 2016, 11:17 PM by Grace Ong   [ updated Nov 22, 2016, 8:43 PM ]
The following post was first posted on Blogger on Saturday, 15 February 2014.



The following solutions were the handiwork of my students during one of their qualitative analysis lab sessions. The colours represent different stages of concentrated hydrochloric acid addition.

solutions of copper(II) complexes

The blue solution on the left contains both Pb2+ and Cu2+ ions before conc HCl is added. In water, Cu2+ ions exist as blue hexaaquacopper(II) complex ions, [Cu(H2O)6]2+. The hydrated Pb2+ ions that are also present in the solution are colourless.

The green solution in the centre is the result of adding some conc HCl to the blue solution on the left. After the acid is added, ligand exchange occurs and water molecules around copper(II) ions are replaced by chloride ions. The resulting complex, the hexachlorocuprate(II) ion (CuCl42−), is yellow. The reason why the solution appears green is that there is now a mixture of both blue [Cu(H2O)6]2+ and yellow CuCl42− complexes. A white precipitate is also observed as insoluble PbCl2 is formed.

The green solution in the centre then changes to the yellow solution on the right after more conc HCl is added. At this point, the solution contains mainly yellow CuCl42− complexes. Notice that there is significantly less white solid in the yellow solution. This is because in the presence of high chloride concentration, some of the solid PbCl2 formed earlier dissolves to form a colourless complex, PbCl42−.
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