Le Chatelier's Principle for the MCAT: Predicting Equilibrium Shifts

Le Chatelier's principle says that when a system at equilibrium is disturbed, it shifts in the direction that partly counteracts the disturbance. On the MCAT, you apply it by asking one question: which way does the reaction go to use up what was added, or replace what was removed?

The four disturbances

Treat heat as a reactant or product. In an exothermic reaction, heat is a product, so raising the temperature acts like adding product and shifts the reaction left.

Only temperature changes K

Changing concentration, pressure or volume moves the system to a new equilibrium position, but the equilibrium constant K stays the same. Temperature is the one change that alters K. Raising the temperature increases K for an endothermic reaction and decreases K for an exothermic reaction.

Using Q versus K

The reaction quotient Q has the same form as K but uses the current concentrations.

• Q < K: too many reactants, the reaction proceeds forward.
• Q > K: too many products, the reaction proceeds in reverse.
• Q = K: at equilibrium.

This is the same logic as Le Chatelier, but it gives you a number to check.

What does not shift equilibrium

• A catalyst speeds the forward and reverse reactions equally, so equilibrium is reached faster but the position and K do not change.
• Adding an inert gas at constant volume does not change the partial pressures of the reacting gases, so there is no shift.
• Changing the amount of a pure solid or liquid does not shift equilibrium because it does not appear in K.

Worked example

For N2(g) + 3 H2(g) <-> 2 NH3(g), which is exothermic, there are 4 moles of gas on the left and 2 on the right. Increasing the pressure shifts the reaction right, toward ammonia. Raising the temperature shifts it left and lowers K.

Common traps

• Saying a catalyst increases yield. It only speeds up how fast equilibrium is reached.
• Saying K changes when you add reactant. Only temperature changes K.
• Counting liquids or solids when comparing moles of gas. Count gas moles only.

One-minute recap

• The system shifts to oppose the change.
• Pressure favors fewer gas moles; temperature favors the endothermic direction when raised.
• Only temperature changes K; catalysts change neither K nor position.
• Compare Q with K to confirm the direction.