JEE Main 2025 Jan 23 Shift 2, Chemistry Q1: Spontaneity

JEE Main2025Jan 23, Shift 2Chemistry

The effect of temperature on spontaneity of reactions are represented as:

	$Δ H$	$Δ S$	Temperature	Spontaneity
(A)	+	$-$	any T	Non spontaneous
(B)	+	+	low T	spontaneous
(C)	$-$	$-$	low T	Non spontaneous
(D)	$-$	+	any T	spontaneous

A
(B) and (D) only
B
(A) and (D) only
C
(B) and (C) only
D
(A) and (C) only

Solution

Using $Δ G = Δ H - T Δ S$ , a process is spontaneous when $Δ G < 0$ .

Row (A): $Δ H > 0$ , $Δ S < 0 \Rightarrow Δ G = (+) - T (-) > 0$ at every $T$ . Non-spontaneous, as listed.

Row (B): $Δ H > 0$ , $Δ S > 0$ . At low $T$ the $- T Δ S$ term is small, so $Δ H$ dominates and $Δ G > 0$ . Reaction is non-spontaneous at low $T$ , contrary to the entry.

Row (C): $Δ H < 0$ , $Δ S < 0$ . At low $T$ , $- T Δ S$ stays small and $Δ H$ dominates giving $Δ G < 0$ . The reaction is spontaneous at low $T$ , contrary to the entry.

Row (D): $Δ H < 0$ , $Δ S > 0 \Rightarrow Δ G < 0$ at every $T$ . Spontaneous, as listed.

$∴$ The entries in rows (B) and (C) are the ones that misrepresent the actual spontaneity.

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