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LearnCambridge97012025–2027Paper 4 · A Level Structured QuestionsChemical energetics

Paper 4 · Module

Chemical energetics

2 subtopics≈ 100 min in total

Start the module: Lattice energy and Born–Haber cycles Open graphing calculator

Lattice energy and Born–Haber cycles
Lattice energy ( $\Delta H_\text{latt}$ ) is the enthalpy change when 1 mol of an ionic solid forms from its gaseous ions (exothermic). It can't be measured directly, so we use a Born–Haber cycle (Hess's law): $\Delta_fH=\Delta_\text{at}H(\text{metal})+\text{IE}+\Delta_\text{at}H(\text{non-metal})+\text{EA}+\Delta H_\text{latt}$ . Lattice energy is more exothermic for smaller ions and higher charges (stronger attraction). Differences between experimental and theoretical (perfect ionic) values show polarisation / covalent character. For dissolving: $\Delta_\text{sol}H=-\Delta H_\text{latt}+\sum\Delta_\text{hyd}H$ .
≈ 50 min
Entropy and Gibbs free energy
Entropy ( $S$ ) measures the number of ways particles and their energy can be arranged; units J K⁻¹ mol⁻¹. $S$ increases from solid → liquid → gas, with more moles of gas, with dissolving and with temperature. $\Delta S=\sum S(\text{products})-\sum S(\text{reactants})$ . Gibbs free energy: $\Delta G=\Delta H-T\Delta S$ (convert $\Delta S$ to kJ). A reaction is feasible when $\Delta G\le0$ . If $\Delta H$ and $\Delta S$ have the same sign, feasibility depends on temperature; the switch occurs at $T=\Delta H/\Delta S$ . A feasible reaction may still be slow (high activation energy): kinetic vs thermodynamic stability.
≈ 50 min