Question
Copper dissolves in dilute nitric acid but not in dilute HCl. Explain.
✓
Answer
Since, $\mathrm{E}^0$ of $\mathrm{Cu}^{2+} / \mathrm{Cu}$ electrode $(+0.34 \mathrm{~V})$ is higher than that of $\mathrm{H}^{+} / \mathrm{H}^2$ electrode $(0.0 \mathrm{~V})$, therefore, $\mathrm{H}^{+}$ions cannot oxidise Cu to $\mathrm{Cu}^{2+}$ ions and hence, Cu does not dissolve in dil. HCl .
In contrast, the electrode potential of $\mathrm{NO}_3^{-}$ion, i.e. $\frac{\mathrm{NO}_3^{-}}{\mathrm{NO}}$ electrode $(+0.97 \mathrm{~V}$ ) is higher than that of copper electrode and hence, it can oxidise Cu to $\mathrm{Cu}^{2+}$ ions and hence Cu dissolves in dil. HNO 3 due to oxidation of Cu by $\frac{\mathrm{NO}_3^{-}}{\mathrm{NO}}$ ions and not by $\mathrm{H}^{+}$ions.
In contrast, the electrode potential of $\mathrm{NO}_3^{-}$ion, i.e. $\frac{\mathrm{NO}_3^{-}}{\mathrm{NO}}$ electrode $(+0.97 \mathrm{~V}$ ) is higher than that of copper electrode and hence, it can oxidise Cu to $\mathrm{Cu}^{2+}$ ions and hence Cu dissolves in dil. HNO 3 due to oxidation of Cu by $\frac{\mathrm{NO}_3^{-}}{\mathrm{NO}}$ ions and not by $\mathrm{H}^{+}$ions.
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