JEE Main 2025 Jan 24 Shift 1, Chemistry Q8: Gibbs Energy Change and Equilibrium

JEE Main2025Jan 24, Shift 1Chemistry

Let us consider an endothermic reaction which is non-spontaneous at the freezing point of water. However, the reaction is spontaneous at boiling point of water. Choose the correct option.

A
Both $Δ H$ and $Δ S$ are (+ve)
B
$Δ H$ is ( $-$ ve) but $Δ S$ is (+ve)
C
$Δ H$ is (+ve) but $Δ S$ is ( $-$ ve)
D
Both $Δ H$ and $Δ S$ are ( $-$ ve)

Solution

Spontaneity is governed by $Δ G = Δ H - T Δ S$ , with $Δ G < 0$ for a spontaneous process.

The reaction is endothermic, so $Δ H > 0$ .

It is non-spontaneous at low $T$ but becomes spontaneous on raising $T$ . With $Δ H > 0$ fixed, the only way $Δ G$ can switch sign as $T$ increases is if the $- T Δ S$ term grows more negative, which requires $Δ S > 0$ .

$∴$ Both $Δ H$ and $Δ S$ are positive. Option (1).

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JEE Main 2025 Jan 24 Shift 1, Chemistry Q8: Gibbs Energy Change and Equilibrium

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