A-Level Chemistry: Thermodynamics

Thermodynamics is the most mathematically demanding physical chemistry topic at A-level, and its questions punish both sloppy definitions and sloppy units. This quiz covers the Year 2 apparatus in full.

The Born-Haber questions work at both levels the exam does. Definitionally, the first electron affinity is pinned down precisely — gaseous atoms gaining electrons to form gaseous ions, the step students most often mislabel in a cycle. Computationally, a full magnesium chloride cycle must be assembled and solved for the lattice enthalpy, handling the doubled chlorine atomisation and electron affinity terms that make MgCl₂ the classic stumbling block.

Lattice enthalpy reasoning is tested through its two determinants — ionic charge and ionic radius — and applied comparatively: why magnesium oxide, with its 2+/2− ions, out-bonds every 1+/1− competitor. The solution chemistry questions connect lattice dissociation and hydration enthalpies into the enthalpy of solution, with the always-exothermic character of hydration explained through ion-dipole attraction.

Entropy is covered from prediction (melting and dissolving raise it; a gas-consuming reaction like hydrogen combustion lowers it) through to the Gibbs equation, where the feasibility-temperature calculation — T = ΔH/ΔS with the kilojoule-to-joule conversion done correctly — lands at 333 K. That unit conversion is precisely where this question type is usually lost.

Together with the Energetics quiz in this collection, this set completes the enthalpy story from Year 1 cycles to Year 2 feasibility.

• Identify and define every step in a Born-Haber cycle
• Solve a full Born-Haber cycle for lattice enthalpy, handling doubled anion terms
• Explain lattice enthalpy magnitudes through ionic charge and radius
• Connect lattice, hydration and solution enthalpies
• Predict entropy signs and calculate feasibility temperatures with ΔG = ΔH − TΔS

Aligned to the thermodynamics content of the DfE A level science subject content as delivered in current specifications: Born-Haber cycles, lattice enthalpy, enthalpies of solution and hydration, entropy and Gibbs free energy.