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

$E^\circ(Cu^{2+}/Cu) = +0.34\ V$ (positive) means $Cu^{2+}$ ions are readily reduced in solution — leached copper ore solution can simply be treated with iron scrap: $Fe + Cu^{2+} \rightarrow Fe^{2+} + Cu$. $E^\circ(Zn^{2+}/Zn) = -0.76\ V$ (negative) means Zn is more reactive than hydrogen, so any metal reactive enough to reduce $Zn^{2+}$ in aqueous solution would react with water/acid itself first, making aqueous-solution (hydrometallurgical) displacement impractical for zinc.

Step 1 – What hydrometallurgy requires

In hydrometallurgy, the metal is first brought into solution as its ion (by leaching with a suitable reagent), and then displaced/precipitated as the free metal by adding a more electropositive metal:

This only works cleanly if a metal exists that is reactive enough to reduce $M^{n+}$ but does not itself react destructively with water.

Step 2 – Copper's case

Copper has a positive standard reduction potential, $E^\circ(Cu^{2+}/Cu) = +0.34\ V$. This means $Cu^{2+}$ is a comparatively weak oxidising challenge — many common, cheap metals (notably iron, with $E^\circ(Fe^{2+}/Fe) = -0.44\ V$) are more electropositive than copper and readily reduce $Cu^{2+}$ in aqueous solution:

This is exactly how low-grade copper ore, after leaching, is worked up industrially.

Step 3 – Zinc's case

Zinc has a negative standard reduction potential, $E^\circ(Zn^{2+}/Zn) = -0.76\ V$, which is more negative than that of hydrogen ($0.00\ V$). This means zinc itself is more reactive than hydrogen — to chemically displace $Zn^{2+}$ from aqueous solution, you would need a metal even more reactive than zinc (e.g. Mg, Na). But such highly reactive metals react vigorously with water/H$^+$ ions themselves, rather than selectively reducing $Zn^{2+}$, making the displacement reaction impractical, uncontrollable, and uneconomical in aqueous solution.

Step 4 – Conclusion

Hence hydrometallurgical (solution-based) extraction is practical for copper (mild reduction potential, cheap displacing agents like Fe work well) but not for zinc (its ion is too hard to reduce in water without side reactions from an even more reactive displacing metal).

Compare the standard reduction potentials of $Cu^{2+}/Cu$ (positive) and $Zn^{2+}/Zn$ (negative, more negative than hydrogen) to see whether a practical, cheap metal exists that can displace the ion from aqueous solution without reacting with water itself.

• Ignoring the electrochemical-series/reduction-potential argument and answering only in vague terms like 'zinc is more reactive' without connecting it to why that blocks hydrometallurgy specifically.
• Forgetting to name iron (scrap) as the actual displacing agent used industrially for copper.