The `E^(ɵ)` of zinc `((Zn^(2+))/(Zn)=-0.76V)` or iron `((Fe^(2+))/(Fe)=-0.44V)` is less than that of copper `((Cu^(2+))/(Cu)=+0.34V)` and hence both zinc and iron can displace copper from solution of `Cu^(2+)` ions. Although zinc is a stronger reducing agent than iron, yet iron scrap is chiefly used in hydrometallurgy of copper because it is much cheaper than
Zinc.
`Fe+Cu^(2+)toFe^(2+)+Cu_(s)`
In contrast, to replace zinc from solution of `Zn^(2+)` ions, a more reactive metal than zinc is required, i.e., Al or Ca. But all these metals react with water
`(2H_2O+2e^(ɵ)toH_(2(g))+2overset(ɵ)(O)H_(aq):E^@-0.83V)`
Forming their corresponding ions with the evolutions of hydrogen gas. Although Al, Mg etc. Cannot be used to displace zinc from solution of `ZN^(2+)` ions. Thus, copper, can be extracted by hydrometallurgy but not zinc.
