The `E^(0)(M^(2+)|M)` value for copper is positive (+0.34V). What is possibly the reason for this ? [Hint : consider its high `Delta_(a)H^(0)` and low `Delta_("hyd")H^(0)`]

Explanation

`E^(0)(M^(2+)|M)` for any metal is related to the sum of the energy changes :
(i) enthalpy of sublimation `[Cu(s)to Cu(g)]`,
(ii) enthalpy of ionisation `[Cu(g)to Cu^(2+)(g)]` and
(iii) enthalpy of hydration `[Cu^(2+)(g)to Cu^(2+)(aq)]`. The sum of the enthalpies of sublimation and ionisation (which involve absorption of energy) of copper is not balanced by its hydration enthalpy (release of energy). Thus, `E^(0)(Cu^(2+)|Cu)` is positive.