The $E^{0}$ of zinc $(\mathrm{Zn}^{2+}$ / $\mathrm{Zn}$ $=-0.76\ \mathrm{V}\ )$ is lower than that of copper $(\mathrm{Cu}^{2+}$ / $\mathrm{Cu}$ $= +0.34\ \mathrm{V}$ ). Hence, zinc can displace Cu from solutions of $\mathrm{Cu}^{2+}$ ions. $\mathrm{Zn}_{(s)} + \mathrm{Cu}^{2+}_{(aq)} \rightarrow \mathrm{Zn}^{2+}_{(aq)} + \mathrm{Cu}_{(s)}$But to displace zinc from solution of $\mathrm{Zn}^{2+}$ ions, we need a metal more Al reactive than it, $\text{i.e.},$ Al $\left( E^{0}_{\mathrm{Al}^{3+}/\mathrm{Al}} = -1.66\ \mathrm{V} \right)$ , Mg $\left( E^{0}_{\mathrm{Mg}^{2+}/\mathrm{Mg}} = -2.37\ \mathrm{V} \right)$ , $\mathrm{Ca}\ \left( E^{0}_{\mathrm{Ca}^{2+}/\mathrm{Ca}} = -2.87\ \mathrm{V} \right),$ $\mathrm{K}\ \left( E^{0}_{\mathrm{K}^{+}/\mathrm{K}} = -2.93\ \mathrm{V} \right)$ etc.But all these metals react with water forming their corresponding ionswith the evolution of $\mathrm{H}_2$ gas. Hence, Al, Mg, etc. cannot be used todisplace zinc from the solution of $\mathrm{Zn}^{2+}$ ions. Thus, copper can be extractedby hydrometallurgy but notzinc.