Exam

JEE Main

Year

2025

Date

28 January

Shift

Morning Shift

Subject

Chemistry

Chapter

Thermodynamics (C)

JEE Main 2025 · 28 January · Morning Shift · Chemistry · Thermodynamics (C)

The formation enthalpies, Δ H_ f for H_ (g) and O_ (g) are 220.0...

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).

Step-by-step solution

$$\begin{aligned} & \frac{1}{2} \mathrm{H}_{2(\mathrm{~g})} \rightarrow \mathrm{H}(\mathrm{g}) \quad ; \quad \Delta_{\mathrm{f}} \mathrm{H}\left(\mathrm{H}_{(\mathrm{g})}\right)=220 \mathrm{KJ} / \mathrm{mol} \\ & \frac{1}{2} \mathrm{O}_{2(\mathrm{~g})} \rightarrow \mathrm{O}(\mathrm{g}) \quad ; \quad \Delta_{\mathrm{f}} \mathrm{H}\left(\mathrm{O}_{(\mathrm{g})}\right)=250 \mathrm{KJ} / \mathrm{mol}\end{aligned}$$
$$\begin{aligned} & \Delta \mathrm{H}_{\mathrm{f}}\left(\mathrm{H}_2 \mathrm{O}_{(\mathrm{l})}\right)=-242=440+250-2(\mathrm{~B} . \mathrm{E} .(\mathrm{O}-\mathrm{H})) \\ & \mathrm{BE}(\mathrm{O}-\mathrm{H})=466 \mathrm{KJ} / \mathrm{mol}\end{aligned}$$