Question

When an aqueous solution containing [Cr(H₂O)6]³⁺ ions is warmed in the presence of

Cl⁻ ions, [Cr(H₂O)₅Cl]²⁺ ions are formed and the colour of the solution changes. Name this type of reaction.
Suggest, in terms of electrons, why the colours of the complex ions are different.

Answer 1

The type of reaction that occurs is a substitution reaction. The difference in color of the complex ions is due to the different arrangements of ligands around the central chromium ion.

Step-by-step explanation:

The type of reaction that occurs when an aqueous solution containing [Cr(H₂O)6]³⁺ ions is warmed in the presence of Cl⁻ ions, resulting in the formation of [Cr(H₂O)₅Cl]²⁺ ions and a change in color, is called a substitution reaction. In this reaction, the chloride ions (Cl⁻) replace some of the water molecules (H₂O) in the complex ion [Cr(H₂O)₆]³⁺, resulting in the formation of [Cr(H₂O)₅Cl]²⁺.

The difference in color between the complex ions is due to the different arrangements of the ligands around the central chromium (Cr) ion. The ligands in the [Cr(H₂O)₆]³⁺ complex ion are water molecules, while in the [Cr(H₂O)₅Cl]²⁺ complex ion, one of the water molecules is replaced by a chloride ion. The presence of the chloride ion alters the electronic structure and bonding in the complex ion, resulting in a different absorption spectrum and therefore a different color.