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Chemistry · Thermodynamics & Thermochemistry

JEE Main 2025 — 28 January, Morning Shift — Question 49

The formation enthalpies, ΔHf⊖\Delta \mathrm{H}_{\mathrm{f}}^{\ominus} for H(g)\mathrm{H}_{(\mathrm{g})} and O(g)\mathrm{O}_{(\mathrm{g})} are 220.0 and 250.0 kJ mol−1250.0 \mathrm{~kJ} \mathrm{~mol}^{-1}, respectively, at 298.15 K , and ΔHf−\Delta \mathrm{H}_{\mathrm{f}}^{-}for H2O(g)\mathrm{H}_{2} \mathrm{O}_{(\mathrm{g})} is −242.0 kJ mol−1-242.0 \mathrm{~kJ} \mathrm{~mol}^{-1} at the same temperature. The average bond enthalpy of the O−H\mathrm{O}-\mathrm{H} bond in water at 298.15 K is \qquad kJmol−1\mathrm{kJ} \mathrm{mol}^{-1} (nearest integer).

Answer: 466

Numerical answer — enter this value.

Step-by-step solution

12H2( g)→H(g);ΔfH(H(g))=220KJ/mol\quad \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}

12O2( g)→O(g);ΔfH(O(g))=250KJ/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}

figure

ΔHf(H2O(l)=−242=440+250−2(\Delta \mathrm{H}_{\mathrm{f}}\left(\mathrm{H}_{2} \mathrm{O}_{(\mathrm{l}}\right)=-242=440+250-2( B.E. (O−H))(\mathrm{O}-\mathrm{H}))

BE(O−H)=466KJ/mol\mathrm{BE}(\mathrm{O}-\mathrm{H})=466 \mathrm{KJ} / \mathrm{mol}

Answer key and solution verified before publishing.

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Exam
JEE Main 2025
Paper
28 January, Morning Shift
Subject
Chemistry
Chapter
Thermodynamics & Thermochemistry
Topic
Thermochemistry and Enthalpy Changes

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