The stability of Cu2+(aq) is more than Cu+(aq) is due to the much more negative ∆hyd∘ofCu2+(aq) than Cu+(aq), which more than compensates for second ionisation enthalpy of Cu. ∆hyd H∘ of Cu2+(aq)=−2121kJmol−1 ∆iH10 of Cu=+745kJmol−1 ∆iH20 of Cu=+1960kJmol−1