Question: 59: At 298 K , the standard electrode potentials of \mathrm{Cu}^{2+} / \mathrm{Cu}, \mathrm{Zn}^{2+} / \mathrm{Zn}, \mathrm{Fe}^{2+} / \mathrm{Fe} and \mathrm{Ag}^{+} / \mathrm{Ag} are 0.34 V , -0.76 \mathrm{~V},-0.44 \mathrm{~V} and 0.80 V , respectively. On the basis of standard electrode potential, predict which of the following reaction cannot occur?
(1) \mathrm{FeSO}_{4}(\mathrm{aq})+\mathrm{Zn}(\mathrm{s}) \rightarrow \mathrm{ZnSO}_{4}(\mathrm{aq})+\mathrm{Fe}(\mathrm{s})
(2) 2 \mathrm{CuSO}_{4}(\mathrm{aq})+2 \mathrm{Ag}(\mathrm{s}) \rightarrow 2 \mathrm{Cu}(\mathrm{s})+\mathrm{Ag}_{2} \mathrm{SO}_{4}(\mathrm{aq})
(3) \mathrm{CuSO}_{4}(\mathrm{aq})+\mathrm{Zn}(\mathrm{s}) \rightarrow \mathrm{ZnSO}_{4}(\mathrm{aq})+\mathrm{Cu}(\mathrm{s})
(4) \mathrm{CuSO}_{4}(\mathrm{aq})+\mathrm{Fe}(\mathrm{s}) \rightarrow \mathrm{FeSO}_{4}(\mathrm{aq})+\mathrm{Cu}(\mathrm{s})
Answer: Option (2)
Explanation:
A redox reaction will occur spontaneously only if the standard cell potential is positive.
The metal with lower standard reduction potential can displace the metal ion with higher standard reduction potential from its salt solution.
The given standard reduction potentials are:
\mathrm{Zn}^{2+} / \mathrm{Zn} = -0.76\,\mathrm{V},\; \mathrm{Fe}^{2+} / \mathrm{Fe} = -0.44\,\mathrm{V},\; \mathrm{Cu}^{2+} / \mathrm{Cu} = 0.34\,\mathrm{V},\; \mathrm{Ag}^{+} / \mathrm{Ag} = 0.80\,\mathrm{V}In reaction (1), zinc has a lower reduction potential than iron, so zinc can displace iron from \mathrm{FeSO}_{4}. The reaction is feasible.
In reaction (2), silver has a higher reduction potential than copper.
Hence, silver cannot displace copper from \mathrm{CuSO}_{4}.
The standard cell potential for this reaction is negative, so the reaction cannot occur.
In reaction (3), zinc has a much lower reduction potential than copper,
so zinc can easily displace copper from \mathrm{CuSO}_{4}.
The reaction is feasible.In reaction (4), iron has a lower reduction potential than copper,
so iron can displace copper from \mathrm{CuSO}_{4}.
The reaction is feasible.
Therefore, the reaction that cannot occur is option (2).