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  4. The reaction quotient (Q) for the reduction of O2 to H2O in acid solution, O2(g)+4H+(aq)+4e- arrow 2H2O(l) is [Where , αH+ =activity of H+, pO2 = pressure of O2 when it is present in the reaction, p° = pressure of O2 at standard state ]

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Q. The reaction quotient $ (Q) $ for the reduction of $ O_{2} $ to $ H_{2}O $ in acid solution,
$O_{2}\left(g\right)+4H^{+}\left(aq\right)+4e^{-} \rightarrow 2H_{2}O\left(l\right) $ is
[Where , $ \alpha_{H^{+}} $ =activity of $ H^{+}, p_{O_2} $ = pressure of $ O_{2} $ when it is present in the reaction, $ p^{\circ} $ = pressure of $ O_{2} $ at standard state ]

AMUAMU 2018Equilibrium

Solution:

The half reaction for the reduction is given as
$O_{2}(g)+4H^{+}(aq)+4e^{-} \to 2H_{2}O(l)$
We have, activity for $H_{2}O$ (or any pure liquid or solid) $\propto H_{2}O=1$
Activity for hydrogen ion $\left(H^{+}\right)=\alpha_{H^{+}}$
Activity for oxygen gas $\left(O_{2}\right)=\alpha_{O_2}=\frac{p_{O_2}}{p_{o}}$
Thus, reaction quotient $(Q)$ is given as
$Q=\frac{\alpha_{H_2 O}^{2}}{\alpha_{H^{+}}^{4}\cdot\alpha_{O_2}}=\frac{1^{2}}{\alpha_{H^{+}}^{4}\cdot p_{O_2} p^{o}}$
$=\frac{p^{o}}{\alpha_{H^{+}} \cdot p_{O_2} }$