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  • By how much would the oxidising power of MnO4- /Mn+2 couple change if the H+ ion concentration is decreased 100 times at 25°C?

By how much would the oxidising power of the MnO4- /Mn2+ couple change if the H+ ions concentration is decreased 100 times? 

(a) increases by 189 mV 

(b) decreases by 189 mV 

(c) will increase by 19 mV 

(d) will decrease by 19 mV

Answers (1)

\begin{array}{l} \mathrm{MnO}_{4}^{-}+5 \mathrm{e}^{-}+8 \mathrm{H}^{+} \longrightarrow \mathrm{Mn}^{2+}+4 \mathrm{H}_{2} \mathrm{O} \\ \text { According to Nernst equation, } \\ \mathrm{E}_{\mathrm{red}}=\mathrm{E}_{\mathrm{red}}^{\circ}-\frac{0.059}{5} \log \left[\frac{\left[\mathrm{Mn}^{2+}\right]}{\left[\mathrm{MnO}_{4}^{-}\right]\left[\mathrm{H}^{+}\right]^{8}}\right] \ \ \ \ \ \ \ \mid \ \ \ \ \ \ \ \ \ \text { Let }\left[\mathrm{H}^{+}\right]_{\text {initial }}=\mathrm{X} \end{array}

\begin{aligned} &\mathrm{E}_{\text {red }(\text { initial })}=\mathrm{E}_{\text {red }}^{\mathrm{o}} \frac{-0.0591}{5} \log \left[\frac{\left[\mathrm{Mn}^{2+}\right]}{\left[\mathrm{MnO}_{4}^{-}[\mathrm{X}]^{8}\right.}\right] \mid \quad\left[\mathrm{H}^{+}\right] \text {linal }=\frac{\mathrm{X}}{100}=\frac{\mathrm{X}}{10^{2}}\\ &\mathrm{E}_{\mathrm{red}(\text { final })}=\mathrm{E}_{\mathrm{red}}^{\circ} \frac{-0.0591}{5} \log \frac{\left[\mathrm{Mn}^{2+}\right] \times 10^{16}}{\left[\mathrm{MnO}_{4}^{-}\right] \times[\mathrm{X}]^{8}}\\ &\mathrm{E}_{\text {red }(\text { final })}-\mathrm{E}_{\text {red }(\text { initial })}=\frac{-0.0591}{5} \log 10^{16}=-0.1891 \mathrm{V} \end{aligned}

\\\text{This E red decreases by 0.189 V. The tendency of the half cell to get }\\ \text{reduced is its oxidising power. Hence the oxidising power decreases by } 0.189 \mathrm{V}

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