What topics are included in A-level thermodynamics?

For AQA and OCR A, Year 13 thermodynamics includes ionic energy cycles, entropy and Gibbs free energy. Both boards use Born–Haber cycles and Gibbs calculations; the exact lattice-enthalpy convention and some required interpretations differ by board.

Why must entropy units be converted in Gibbs calculations?

Standard entropy changes are usually given in J K−1 mol−1 while enthalpy changes are usually in kJ mol−1. Convert one so ΔH and TΔS use the same energy unit before subtracting.

Year 13 · AQA & OCR A

Thermodynamics

Connect energy cycles, entropy and Gibbs free energy. Decide which quantity matters, keep the signs and units under control, and explain what the answer means chemically.

Both boards AQA OCR A

Choose what you need to practise

Energy cycle

Born–Haber cycles

Build ionic-energy cycles, keep stoichiometric factors visible and calculate a missing enthalpy term.

Practise Born–Haber cycles

Ionic energetics

Lattice, hydration & solution

Connect lattice enthalpy with hydration and solution enthalpy, then explain trends using ionic charge and radius.

Practise ionic energetics

Feasibility

Entropy & Gibbs free energy

Calculate ΔS and ΔG, diagnose unit traps and decide whether a change is thermodynamically feasible.

Practise entropy & Gibbs

Temperature

Temperature & feasibility

Find crossover temperatures and read ΔH, ΔS and feasibility directly from a ΔG against T relationship.

Practise temperature & feasibility

Try a linked thermodynamics paper

Work through a linked mini-paper where later parts use the same data and earlier results. Choose your board first so board-specific material stays on the right course.

Start an exam set

How the thermodynamics ideas connect

Hess's law gives the bookkeeping rule. Born–Haber and hydration cycles apply it to ionic substances. Entropy adds the dispersal of energy, and Gibbs free energy combines ΔH, ΔS and temperature into a feasibility test.

1
Build the energy routeTrack the direction and sign of every enthalpy term.
2
Calculate ΔSUse products − reactants and keep every coefficient.
3
Put the units togetherΔH and TΔS must use the same energy unit.
4
Interpret ΔGDecide feasibility at the stated temperature, then state what that does and does not tell you.

AQA and OCR A are close, with a few important differences

AQA · AQA 3.1.8

AQA accepts lattice enthalpy defined as lattice formation or lattice dissociation; the sign convention must therefore be stated clearly.
AQA explicitly requires comparison of Born-Haber lattice enthalpies with perfect-ionic-model values as evidence for covalent character.
AQA explicitly requires entropy calculations, Gibbs calculations, crossover temperature and extracting ΔH and ΔS from a ΔG against T graph.

Show AQA scope →

OCR A · OCR 5.2.1 · 5.2.2

OCR defines lattice enthalpy as formation of one mole of ionic lattice from gaseous ions.
OCR explicitly requires qualitative effects of ionic charge and radius on lattice enthalpy and hydration enthalpy.
OCR explicitly requires the kinetic limitation of ΔG predictions and has assessed ΔG against T graph interpretation.

Show OCR A scope →

Refresh Hess's law first if the cycle arithmetic is shaky

Practise reversing and scaling enthalpy changes until the route arithmetic feels routine. Born–Haber calculations then become much easier.

Practise Hess's law