The standard reduction potentials at $298 \mathrm{~K}$ for the following half reactions are given against…
The standard reduction potentials at $298 \mathrm{~K}$ for the following half reactions are given against each $\mathrm{Zn}^{2+}(a q)+2 \mathrm{e}^{-} ightleftharpoons \mathrm{Zn}(s) ;-0.762 \mathrm{~V}$
$\mathrm{Cr}^{3+}(a q)+3 \mathrm{e}^{-} ightleftharpoons \mathrm{Cr}(s) ;-0.740 \mathrm{~V}$
$2 \mathrm{H}^{+}(a q)+2 \mathrm{e}^{-} ightleftharpoons \mathrm{H}_{2}(g) ; 0.00 \mathrm{~V}$
$\mathrm{Fe}^{3+}(a q)+\mathrm{e}^{-} ightleftharpoons \mathrm{Fe}^{2+}(a q) ; 0.770 \mathrm{~V}$
Which is the strongest reducing agent?
$\mathrm{Zn}(s)$
$\mathrm{Cr}(s)$
$\mathrm{H}_{2}(g)$
$\mathrm{Fe}^{3+}(a q)$
Solution
\(\mathrm{E}_{\mathrm{OP}}^{\circ}\) for \(\mathrm{Zn}=+0.762 \mathrm{~V}\) (maximum amongst the given values)
More the value of \(\mathrm{E}_{\mathrm{op}}^{\circ}\), more is the tendency for the species to get oxidised, and hence the strongest reducing agent.
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