JEE Main 2023 — Chemistry Physical Chemistry

2023integermedium

At $25^{\circ}C$ , the enthalpy of the following processes are given:

${H}_{2}(g)+{O}_{2}(g)\rightarrow 2\mathrm{OH}(g)\Delta {H}^{o}=78\mathrm{kJ}{\mathrm{mol}}^{-1}$

${H}_{2}(g)+1/2{O}_{2}(g)\rightarrow {H}_{2}O(g)\Delta {H}^{0}=-242\mathrm{kJ}{\mathrm{mol}}^{-1}$

${H}_{2}(g)\rightarrow 2H(g)\Delta {H}^{o}=436\mathrm{kJ}{\mathrm{mol}}^{-1}$

$1/2{O}_{2}(g)\rightarrow O(g)\Delta {H}^{0}=249\mathrm{kJ}{\mathrm{mol}}^{-1}$

What would be the value of X for the following reaction? (Nearest integer)

${H}_{2}O(g)\rightarrow H(g)+\mathrm{OH}(g)\Delta {H}^{o}=X{\mathrm{kJmol}}^{-1}$

Official previous-year question

Held on 1 Feb 2023 · Verified 6 Jul 2026.

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Solution

$2{H}_{2}O(g)\rightarrow 2{H}_{2}(g)+{O}_{2}(g)+(242\times 2){\mathrm{kJmol}}^{-1}$ ....(1)

${H}_{2}(g)+{O}_{2}(g)\rightarrow 2\mathrm{OH}+78\mathrm{kJ}{\mathrm{mol}}^{-1}$ ....(2)

${H}_{2}(g)\rightarrow 2H(g)+436\mathrm{kJ}{\mathrm{mol}}^{-1}$ ....(3)

Using Hess's law of constant heat summation, adding equations(1), (2) and (3) and dividing the net result by 2 we get:

$\Rightarrow {H}_{2}O(g)\rightarrow H(g)+\mathrm{OH}(g)998\times \frac{1}{2}=+499\mathrm{kJ}{\mathrm{mol}}^{-1}$

Thus $X=499$

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Solution

$2{H}_{2}O(g)\rightarrow 2{H}_{2}(g)+{O}_{2}(g)+(242\times 2){\mathrm{kJmol}}^{-1}$ ....(1)

${H}_{2}(g)+{O}_{2}(g)\rightarrow 2\mathrm{OH}+78\mathrm{kJ}{\mathrm{mol}}^{-1}$ ....(2)

${H}_{2}(g)\rightarrow 2H(g)+436\mathrm{kJ}{\mathrm{mol}}^{-1}$ ....(3)

Using Hess's law of constant heat summation, adding equations(1), (2) and (3) and dividing the net result by 2 we get:

$\Rightarrow {H}_{2}O(g)\rightarrow H(g)+\mathrm{OH}(g)998\times \frac{1}{2}=+499\mathrm{kJ}{\mathrm{mol}}^{-1}$

Thus $X=499$

JEE Main 2023 — Chemistry Physical Chemistry

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JEE Main 2023 — Chemistry Physical Chemistry

Did you get this right?

Solution

Your note

JEE Main Physical Chemistry in other years

More JEE Main Physical Chemistry questions

Other Chemistry topics for JEE Main

Organic Chemistry

Inorganic Chemistry

Keep practicing Physical Chemistry