2022 H2 Chem P1 Q11 — Thermodynamics Solution

The answer is D (spontaneous at all temperatures).

A solid forms a gas and a liquid, so disorder increases: \(\Delta S^\circ\) is positive.
\(\Delta G^\circ = \Delta H^\circ - T\Delta S^\circ\). With \(\Delta H^\circ = -124\) kJ mol\(^{-1}\) (negative) and \(\Delta S^\circ\) positive, the term \(-T\Delta S^\circ\) is negative at every temperature, so \(\Delta G^\circ\) is always negative and the reaction is spontaneous at all temperatures.

• A is wrong: spontaneous only at low temperature needs \(\Delta H^\circ\) negative and \(\Delta S^\circ\) negative; here \(\Delta S^\circ\) is positive.
• B is wrong: spontaneous only at high temperature needs \(\Delta H^\circ\) positive and \(\Delta S^\circ\) positive; here \(\Delta H^\circ\) is negative.
• C is wrong: never spontaneous needs \(\Delta H^\circ\) positive and \(\Delta S^\circ\) negative; here both signs favour spontaneity.