Thermodynamics
Thermodynamics tests state functions, the first law, enthalpy, Hess’s law, entropy and Gibbs energy. Expect sign-convention questions and a delta-G calculation linking spontaneity to temperature.
Core rules
- First law: delta U = q + w, with work done on the system positive.
- Enthalpy: H = U + PV; at constant pressure, delta H = qp.
- Hess’s law: enthalpy change is independent of route.
- Entropy: delta S = qrev/T; delta S(total) > 0 for a spontaneous process.
- Gibbs energy: delta G = delta H - T delta S; spontaneous when delta G < 0, equilibrium when delta G = 0.
Worked example
For a reaction, delta H = -92 kJ mol-1 and delta S = -198 J K-1 mol-1. Calculate delta G at 298 K and comment. Reasoning: convert delta H to J: -92,000 J mol-1. delta G = -92,000 - 298(-198) = -92,000 + 59,004 = -32,996 J mol-1 = -33 kJ mol-1. Delta G is negative, so the reaction is spontaneous at 298 K. Because delta S is negative, it becomes non-spontaneous at high temperature; the crossover is T = delta H/delta S = 465 K.
Traps
- Keep kJ and J consistent; the commonest error is mixing them.
- For delta n(gas) not equal to 0, delta H is not equal to delta U.
- Delta G° < 0 means spontaneous under standard conditions, not necessarily at all concentrations.
- At equilibrium delta G = 0, but delta G° is not zero.