Sodium and potassium are displaced from soap by calcium and magnesium ions in hard water but they are more reactive than calcium and magnesium still they get displaced.Why?
Answer: The reaction soap and calcium or magnesium is not a single displacement reaction, where Ca must be more reactive than Na, what is impossible: Ca + 2C17H35COONa Na + Ca(C17H35COO)2 – incorrect! The reaction soap with calcium or magnesium is a double replacement reaction and precipitation reaction: Soap dissolves in water and forms cations of K+ or Na+ and anions of fatty acid, as example C17H35COO-. Calcium and magnesium ions react with these anions and form insoluble in water salt. C17H35COO-(aq) + Ca2+(aq) → Ca(C17H35COO)2 (s).