Question #66506
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?
Expert's answer
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).
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).
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