Limitations of Predictions

Le Chatelier's principle is a useful rule, but its predictions have limitations.1. It is qualitative. It predicts the direction in which the position of equilibrium shifts, but not how far it shifts. To calculate the new equilibrium composition you need the equilibrium constant, K_c.2. It says nothing about rate. It predicts the equilibrium position, not how quickly equilibrium is reached. A low temperature may favour the products of an exothermic reaction, but the reaction may then be so slow that equilibrium is never reached in a useful time.3. It only applies to a closed system at equilibrium. Many industrial reactors are open, continuous systems. Gases pass over the catalyst and leave before equilibrium is reached, and the product is removed. The actual yield per pass is set by rate as well as by the equilibrium position.4. 'Pressure' must be interpreted carefully. Adding an unreactive gas such as argon to a container of fixed volume increases the total pressure, but the concentrations of the reacting gases do not change, so the position of equilibrium does not move.5. It gives no reason. The principle is a summary, not an explanation. The underlying reason is that K_c is constant at a given temperature. When a concentration or pressure changes, the concentrations readjust until the ratio in the K_c expression equals K_c again. Only a temperature change alters K_c itself.So the best industrial conditions cannot be chosen from Le Chatelier's principle alone. Rate, cost, energy and safety must also be considered.