The ionisation enthalpy for change Cu(s) to `Cu^(2+)`(aq) ion is very high. It is not compensated by the hydration enthalpy which is quite low. Therefore `E^(@)` of Cu is +ve. In case of Zn(s) the divalent cations `Zn^(2+)(aq)` is stable ( `3d^(10)` configuration ). Therefore `E^(@)` is negative.
