2020 H2 Chem P1 Q11 — Thermodynamics Solution

The answer is C (2 only).

\(\Delta G = \Delta H - T\Delta S\), and a reaction is spontaneous when \(\Delta G\) is negative. \(T\) (in K) is always positive.
• Statement 1 is incorrect: \(\Delta H\) is positive and \(\Delta S\) is negative, so \(-T\Delta S\) is also positive. \(\Delta G\) is positive at every temperature, so the reaction is not spontaneous at any temperature.
• Statement 2 is correct: \(\Delta H\) is negative and \(-T\Delta S\) is negative, so \(\Delta G\) is negative at every temperature, and the reaction is spontaneous at all temperatures.
• Statement 3 is incorrect: \(\Delta H\) is negative but \(-T\Delta S\) is positive. At low temperatures the \(\Delta H\) term dominates, so \(\Delta G\) is negative and the reaction is spontaneous; only above \(T = \frac{\Delta H}{\Delta S}\) does \(\Delta G\) become positive.

• A is wrong: it contains statements 1 and 3, which are both incorrect, and leaves out statement 2.
• B is wrong: statement 1 is incorrect, and statement 2 is left out.
• D is wrong: statement 3 is incorrect, and statement 2 is left out.