In these trends, you can observe the unique behaviour of copper. Unlike many other metals, the standard electrode potential of copper is positive. This positive value explains why copper does not liberate hydrogen gas when it dissolves in ordinary acids. Other transition metals with more negative values of electrode potential behave differently.
Because of this property, copper does not react with dilute acids like HCl or H2SO4 to release hydrogen gas. Only strong oxidizing acids such as nitric acid (HNO3) and concentrated sulphuric acid (H2SO4) are capable of reacting with copper. In these cases, copper is oxidized, but hydrogen gas is not produced; instead, gases like NO, NO2, or SO2 are formed.
The reason behind this unusual behaviour lies in copper’s hydration enthalpy. The hydration enthalpy of Cu2+ ions is not sufficient to balance the high ionization energy required to transform a copper atom into a Cu2+ ion. As a result, copper remains less reactive compared to metals like manganese, zinc, and nickel.
On the other hand, metals like Mn, Zn, and Ni possess more negative standard electrode potentials. This indicates they are stronger reducing agents and can easily displace hydrogen from dilute acids, readily producing hydrogen gas.
