Chapter 15 · Chemical Equilibrium
15.10The Effect of a Temperature Change on Equilibrium
10 min · two checks
Predict
Heat is added to an exothermic reaction at equilibrium. Which way does it shift?
The idea
- Treat heat as a reactant or a product.
- State how K changes with temperature.
Temperature is the disturbance that changes K. Write heat on the product side of an exothermic reaction (ΔH < 0) and on the reactant side of an endothermic reaction (ΔH > 0). Then use Le Châtelier. Heating an exothermic equilibrium shifts it toward reactants and K gets smaller. Heating an endothermic equilibrium shifts it toward products and K gets larger. Cooling does the opposite.
This is not the same statement as “heat speeds the reaction.” Both directions speed up when you heat; the equilibrium constant decides which way the balance tips. Industrial ammonia synthesis is exothermic, so low temperature would favor products, but low temperature makes the reaction too slow. The real process picks a compromise temperature and uses a catalyst. Equilibrium and rate argue, and engineers negotiate.
Keep these
- Exothermic: heat is a product. Endothermic: heat is a reactant.
- Heating an exothermic reaction decreases K.
- Temperature changes K. Concentration and volume changes do not.