JEE MAIN - Chemistry (2025 - 28th January Morning Shift - No. 21)
The formation enthalpies, $\Delta \mathrm{H}_{\mathrm{f}}^{\ominus}$ for $\mathrm{H}_{(\mathrm{g})}$ and $\mathrm{O}_{(\mathrm{g})}$ are 220.0 and $250.0 \mathrm{~kJ} \mathrm{~mol}^{-1}$, respectively, at 298.15 K , and $\Delta \mathrm{H}_{\mathrm{f}}^{\ominus}$ for $\mathrm{H}_2 \mathrm{O}_{(\mathrm{g})}$ is $-242.0 \mathrm{~kJ} \mathrm{~mol}^{-1}$ at the same temperature. The average bond enthalpy of the $\mathrm{O}-\mathrm{H}$ bond in water at 298.15 K is _______ $\mathrm{kJ} \mathrm{~mol}^{-1}$ (nearest integer).
Answer
466
Explanation
$$\begin{array}{ll} \frac{1}{2} \mathrm{H}_{2(\mathrm{~g})} \rightarrow \mathrm{H}(\mathrm{~g}) \quad ;\Delta_{\mathrm{f}} \mathrm{H}\left(\mathrm{H}_{(\mathrm{g})}\right)=220 \mathrm{KJ} / \mathrm{mol} \\ \frac{1}{2} \mathrm{O}_{2(\mathrm{~g})} \rightarrow \mathrm{O}(\mathrm{~g}) \quad ; \Delta_{\mathrm{f}} \mathrm{H}\left(\mathrm{O}_{(\mathrm{g})}\right)=250 \mathrm{KJ} / \mathrm{mol} \end{array}$$
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$$\begin{aligned} & \Delta \mathrm{H}_{\mathrm{f}}\left(\mathrm{H}_2 \mathrm{O}_{(\mathrm{l})}\right)=-242=440+250-2(\mathrm{~B} . \mathrm{E} .(\mathrm{O}-\mathrm{H})) \\ & \mathrm{BE}(\mathrm{O}-\mathrm{H})=466 \mathrm{KJ} / \mathrm{mol} \end{aligned}$$
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