Question

Copper can be extracted by hydrometallurgy but not zinc. Explain.

Answer 1

The E° of zinc (Zn²⁺ / Zn = -0.76v) or iron (Fe²⁺ / Fe = -0.44v) is lower than that of copper (Cu²⁺ / Cu = + 0.34v) and hence both Zn and Fe, can displace Cu from solutions of Cu²⁺ ions. Although zinc is a stronger reducing agent than iron, yet iron scrap is chiefty used in hydrometallurgy of copper because it is much cheaper than zinc. 

Fe (s) + Cu²⁺ (aq) → Fe²⁺ (aq) + Cu (s) 

In contrast, to displace zinc from solution of Zn²⁺ ions, we need more reactive metal than it, i.e Al (E° Al³⁺/ Al = -1.66v), Mg (e° Mg²⁺ / Mg = -2.37v), Ca (e° Ca²⁺ / Ca = -2.87) K. (e° k⁺ / k = - 2.93 v) etc. 

But all these metals react with water [2H₂ + 2e^- → H₂(g) + 20n- (ag), = -0.83v] forming their corresponding ions with the evolution of H₂ gas. 

Thus, Al, Mg etc cannot be used to displace zinc from solution of Zn²⁺ ions. Thus copper can be extracted by hydrometallurgy but not zinc.