Why does KI dissolve in water spontaneously, even though the process is endothermic?
A reaction will occur spontaneously if its free energy change is negative. The free energy is affected by enthalpy and entropy. Lowering of enthalpy favours reactions, as does increase in entropy. The fact that the process is endothermic indicates that the enthalpy change (positive) does not favour the forward reaction. However, when KI dissolves in water, it goes from the highly-ordered low-entropy ionic crystal to freely dispersed ions in aqueous solution – that is, the entropy increases significantly, which favours the forward reaction. The entropy change is so great that it outweighs the enthalpy change, leaving KI *very* soluble in water.
