Comment on the thermodynamic stability of NO (g), given $\begin{aligned} &\frac{1}{2} \mathrm{~N}_2(\mathrm{~g})+\frac{1}{2} \mathrm{O}_2(\mathrm{~g}) \longrightarrow \mathrm{NO}(\mathrm{g}) ; \ &\Delta_{\mathrm{r}} \mathrm{H}^{\circ}=90 \mathrm{~kJ} \mathrm{~mol}^{-1} \ &\mathrm{NO}(\mathrm{g})+\frac{1}{2} \mathrm{O}_2(\mathrm{~g}) \longrightarrow \mathrm{NO}_2(\mathrm{~g}) ; \ &\Delta_{\mathrm{r}} \mathrm{H}^{\circ}=-74 \mathrm{~kJ} \mathrm{~mol}^{-1} \end{aligned}$

Question

Comment on the thermodynamic stability of NO (g), given $\begin{aligned} &\frac{1}{2} \mathrm{~N}_2(\mathrm{~g})+\frac{1}{2} \mathrm{O}_2(\mathrm{~g}) \longrightarrow \mathrm{NO}(\mathrm{g}) ; \ &\Delta_{\mathrm{r}} \mathrm{H}^{\circ}=90 \mathrm{~kJ} \mathrm{~mol}^{-1} \ &\mathrm{NO}(\mathrm{g})+\frac{1}{2} \mathrm{O}_2(\mathrm{~g}) \longrightarrow \mathrm{NO}_2(\mathrm{~g}) ; \ &\Delta_{\mathrm{r}} \mathrm{H}^{\circ}=-74 \mathrm{~kJ} \mathrm{~mol}^{-1} \end{aligned}$,

MARKS App · Accepted Answer

As energy is absorbed in the first reaction, $\mathrm{NO}(\mathrm{g})$ is unstable. As energy is released in the second reaction, $\mathrm{NO}_2(\mathrm{~g})$ is stable. Thus, unstable $\mathrm{NO}(\mathrm{g})$ changes into the stable $\mathrm{NO}_2(\mathrm{~g})$.

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