23.4.4—Temperature effect on feasibility

Because ΔG° = ΔH° − TΔS°, raising T makes a positive ΔS° more favourable and a negative ΔS° less favourable. The crossover occurs when ΔG° changes sign.

For ΔH°>0 and ΔS°>0, high temperature can make a process feasible; for ΔH°<0 and ΔS°<0, low temperature is favoured. The other sign combinations are usually temperature-independent in the simple model.

If ΔH°=+50 kJ mol⁻¹ and ΔS°=+150 J K⁻¹ mol⁻¹, feasibility begins above T≈333 K because ΔH°/ΔS° sets the threshold.

Do not say “higher temperature always makes reactions spontaneous”; inspect both signs.