The standard reduction potentials at $ 298 \mathrm{~K} $ for the following half reactions are given against each $$ \begin{array}{ll} \mathrm{Zn}^{2+}(\mathrm{aq})+2 \mathrm{e} \rightleftharpoons \mathrm{Zn}(\mathrm{s}) & -0.762 \ \mathrm{Cr}^{3+}(\mathrm{aq})+3 \mathrm{e} \rightleftharpoons \mathrm{Cr}(\mathrm{s}) & -0.740 \ 2 \mathrm{H}^{+}(\mathrm{aq})+2 \mathrm{e} \rightleftharpoons \mathrm{H}_{2}(\mathrm{~g}) & 0.000 \ \mathrm{Fe}^{3+}(\mathrm{aq})+\mathrm{e} \rightleftharpoons \mathrm{Fe}^{2+}(\mathrm{aq}) & 0.770 \end{array} $$ Which is the strongest reducing agent?

Question

The standard reduction potentials at $ 298 \mathrm{~K} $ for the following half reactions are given against each $$ \begin{array}{ll} \mathrm{Zn}^{2+}(\mathrm{aq})+2 \mathrm{e} \rightleftharpoons \mathrm{Zn}(\mathrm{s}) & -0.762 \ \mathrm{Cr}^{3+}(\mathrm{aq})+3 \mathrm{e} \rightleftharpoons \mathrm{Cr}(\mathrm{s}) & -0.740 \ 2 \mathrm{H}^{+}(\mathrm{aq})+2 \mathrm{e} \rightleftharpoons \mathrm{H}_{2}(\mathrm{~g}) & 0.000 \ \mathrm{Fe}^{3+}(\mathrm{aq})+\mathrm{e} \rightleftharpoons \mathrm{Fe}^{2+}(\mathrm{aq}) & 0.770 \end{array} $$ Which is the strongest reducing agent?, $ \mathrm{Fe}^{2+} $ (aq), $ \mathrm{H}_{2}(\mathrm{~g}) $, $ \operatorname{Cr}(\mathrm{s}) $, $ \operatorname{Zn}(\mathrm{s}) $

MARKS App · Accepted Answer

For an element or an Ion to act as a reducing agent, it should itself get oxidised. Hence, the species which has the highest negative value of reduction potential will be most easily oxidised. As Zn 2+ has the highest negative value for reduction, it will have the highest positive value for oxidation and thus will be the best reducing agent among the given options. The correct order of reducing character is : Zn 2 + ( E ° = - 0 .762 ) > Cr 3 + ( E ° = − 0 .740 ) > H + ( E ° = 0 ) > Fe 3 + ( E ° = 0 .770 )

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