JEE Main · 2025 · 07 Apr 2025 · Shift 2 · Chemical Thermodynamics

The correct statement amongst the following is :

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

Answer: (D)

Solution

A standard state is defined at a pressure of one bar but at a temperature that is stated separately and need not be

0^{\circ} C

273

K, so the statements that fix the temperature are wrong. The standard enthalpy of formation is zero only for an element in its most stable form. Molecular oxygen is the stable form of the element, so its standard formation enthalpy is zero at any temperature, including

500

K, whereas atomic oxygen is not the stable form and its formation enthalpy is not zero. The correct statement is that the standard enthalpy of formation of

O_{2} (g)

500

K is zero.

JEE Main · 2025 · 07 Apr 2025 · Shift 2 · Single correct option

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JEE Main · 2025 · 07 Apr 2025 · Shift 2 · Chemical Thermodynamics

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