Q1(e). Van't Hoff plots

Answer in brief

A plot of ln K against 1/T has slope -ΔᵣH°/R: negative for endothermic and positive for exothermic reactions.

Key points

• With approximately temperature-independent reaction enthalpy and entropy, ln K=-ΔᵣH°/(RT)+ΔᵣS°/R.
• An endothermic reaction has ΔᵣH°>0; its line slopes down as 1/T increases, meaning K increases on heating.
• An exothermic reaction has ΔᵣH°<0; its line slopes up as 1/T increases, meaning K decreases on heating. The intercept is ΔᵣS°/R.
• For a base-10 plot the slope is -ΔᵣH°/(2.303R). Heat-capacity changes cause curvature over a wide temperature range.