2019 H2 Chem P1 Q13 — Thermodynamics Solution

The answer is D (2 only).

For \(2\text{H}_2\text{O}_2\text{(l)} \rightarrow 2\text{H}_2\text{O(l)} + \text{O}_2\text{(g)}\): \(\Delta H\) is negative (-196 kJ mol\(^{-1}\)) and \(\Delta S\) is positive, since 0 mol of gas becomes 1 mol of gas.
• Statement 1 is false: with \(\Delta H\) negative and \(\Delta S\) positive, \(\Delta G = \Delta H - T\Delta S\) is negative at every temperature, including 20 °C. The decomposition is energetically feasible.
• Statement 2 is true: a feasible reaction can still be extremely slow if its activation energy is very high, because very few molecules have enough energy to react at room temperature. This explains why \(\text{H}_2\text{O}_2\) keeps for weeks (it is kinetically stable).
• Statement 3 is false: forming a gas from liquids increases disorder, so \(\Delta S\) is positive, not negative.
Only statement 2 is correct.

• A is wrong: it includes statements 1 and 3, which are both false.
• B is wrong: statements 1 and 3 are both false.
• C is wrong: it includes statement 3; \(\Delta S\) is positive.