Chemistry · Thermodynamics & Thermochemistry

JEE Main 2025 — 7 April, Evening Shift — Question 10

The correct statement amongst the following is

Option A:
$\Delta_{\mathrm{f}} \mathrm{H}_{298}^{0}$ is zero for $\mathrm{O}(\mathrm{g})$
Option B:
$\Delta_{f} H_{500}^{0}$ is zero for $\mathrm{O}_{2}(\mathrm{~g})$
Correct
Option C:
The term 'standard state' implies that the temperature is $0^{\circ} \mathrm{C}$
Option D:
The standard state of a pure gas is the pure gas at a pressure of 1 bar and temperature 273 K

Answer: B

Step-by-step solution

Standard enthalpy of formation of an element in its most stable form at a given temperature is zero. At 500 K, O2(g) is the most stable form of oxygen, so ΔfH°(O2, g, 500 K) = 0. O(g) is not the standard state of oxygen, so its ΔfH° is not zero. Standard state does not imply a specific temperature (0°C or 273 K); it is usually 298 K unless specified, and for gases it is 1 bar pressure at the specified temperature. Thus option B is correct.

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Exam: JEE Main 2025
Paper: 7 April, Evening Shift
Subject: Chemistry
Chapter: Thermodynamics & Thermochemistry
Topic: Thermochemistry and Enthalpy Changes

The hydration energies of K^+ and Cl^- are -x and - y kJ / mol respectively. If lattice energy of KCl is -z kJ / mol , then the heat of…

Answer: B

Step-by-step solution

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