If concentrated HCl is added to a solution of hexaaquacopper (II) ions the solution turns from blue to yellow. The reaction is driven by the high concentration of chloride ions and the increase in the number of particles from 5 to 7. The reaction is endothermic but there is little enthalpy change because bonds are of similar strength.[Cu (H2O)6]2+ + 4Cl- ⇌ [CuCl4]2- + 6H2OWhen monodentate ligands are substituted with bidentate ligands the number of particles increases and the entropy is greater. Reactions that increase the total entropy are more likely to occur.Ligand substitution reactions are reversible and the equilibrium constant is called the stability constant.