The E0 of zinc ( Zn2+ / Zn =−0.76 V ) is lower than that of copper (Cu2+ / Cu =+0.34V ). Hence, zinc can displace Cu from solutions of Cu2+ ions.
Zn( s)+ Cu2+(aq)→ Zn2+(aq)+ Cu(s)
But to displace zinc from solution of Zn2+ ions, we need a metal more Al reactive than it, i.e, Al (E0 Al3+∕Al=−1.66 V ) , Mg (E0 Mg 2+∕Mg=−2.37 V ) , Ca(E0Ca2+∕Ca=−2.87 V ), K(E0 K +∕K=−2.93 V ) etc.
But all these metals react with water forming their corresponding ionswith the evolution of H2 gas. Hence, Al, Mg, etc. cannot be used todisplace zinc from the solution of Zn2+ ions. Thus, copper can be extractedby hydrometallurgy but notzinc.