2017 H2 Chem P1 Q10 — Thermodynamics Solution

The answer is B (1 only).
\(\Delta G^\circ = \Delta H^\circ - T\Delta S^\circ = -1.9 - 298 \times \dfrac{+3.4}{1000} = -1.9 - 1.01 = -2.9 \text{ kJ mol}^{-1}\)
\(\Delta G^\circ\) is negative, so the change from diamond to graphite is thermodynamically feasible. It does not happen at room temperature because the reaction has a very high activation energy, so the rate constant (\(k = A\text{e}^{-E_\text{a}/RT}\)) is extremely small and the rate is negligible.
• Statement 1 is correct and explains the observation: the extremely small rate constant is why the feasible change does not occur.
• Statement 2 is a correct value but does not explain the observation: a negative \(\Delta G^\circ\) shows that the change is feasible, so it cannot explain why diamond does not turn into graphite.
• Statement 3 is false: for the reverse reaction \(\Delta G^\circ = +2.9 \text{ kJ mol}^{-1}\), which is positive, so the reverse reaction is not spontaneous.
• A is wrong: it includes statement 3, which is false.
• C is wrong: it includes statement 3, which is false, and statement 2, which does not explain the observation; it also leaves out statement 1.
• D is wrong: statement 2 does not explain why the change does not happen, and the explaining statement 1 is left out.