The positive standard reduction potential of Cu⁺²/ Cu electrode is due to

The unique behaviour of Cu, having a positive E^o , accounts for its inability to liberate H₂ from acids. Only oxidising acids (nitric and hot concentrated sulphuric) react with Cu, the acids being reduced. The high energy to transform Cu(s) to Cu²⁺(aq) is not balanced by its hydration enthalpy. The general trend towards less negative E^o values across the series is related to the general increase in the sum of the first and second ionisation enthalpies.