JEE Main 2025 · 23 January · evening · Single correct · +4 / −1
The effect of temperature on spontaneity of reactions are represented as :
The incorrect combinations are :
| ΔH | ΔS | Temperature | Spontaneity | |
|---|---|---|---|---|
| (A) | + | - | any T | Non spontaneous |
| (B) | + | + | low T | spontaneous |
| (C) | - | - | low T | Non spontaneous |
| (D) | - | + | any T | spontaneous |
- (a) (A) and (C) only
- (b) (B) and (D) only
- (c) (A) and (D) only
- (d) (B) and (C) only
Answer and solution
Answer (d) (B) and (C) only
Solution
Chemistry ma’am · 2 steps
Write Δ G = Δ H - TΔ S and ask which term wins when T is small and when T is large.
- The rule. A reaction is spontaneous when Δ G < 0. At low T the TΔ S term is small, so Δ H decides the sign. At high T the TΔ S term takes over.
Δ G = Δ H - TΔ S
Row Δ H, Δ S Sign of Δ G Stated Verdict (A) +, - (+) - T(-), always + non-spontaneous at any T correct (B) +, + + at low T, - only at high T spontaneous at low T incorrect (C) -, - - at low T, + only at high T non-spontaneous at low T incorrect (D) -, + (-) - T(+), always - spontaneous at any T correct - So. (B) and (C) have the temperature condition reversed: (B) is spontaneous only at high T and (C) is spontaneous at low T. The incorrect combinations are (B) and (C), option (d).
The idea to keep. When Δ H and Δ S have the same sign, temperature decides. Low T favours the Δ H sign and high T favours the Δ S sign. Opposite signs mean T cannot change the outcome.
Answer from the NTA final answer key. Solution by PrepWiser.