Le Chatelier’s Principle

In a reversible reaction, the forward and reverse reactions can both continue at the same time. At equilibrium, they happen at the same rate, so the amounts of reactants and products stay constant. This does not mean the reaction has stopped. It means the two opposite changes balance each other.

The balance can change if the reaction conditions change. The equilibrium position is the relative amount of reactants and products present at equilibrium, and this position is not fixed under all circumstances.

Le Chatelier’s Principle states that if a system at equilibrium is disturbed by a change in conditions, the equilibrium shifts in the direction that tends to oppose that change and reduce its effect.

This is a qualitative idea. It helps you predict which way the equilibrium moves after a disturbance: toward the reactants or toward the products.

Le Chatelier’s Principle: what a shift means

Consider a general equilibrium:

$$\ce{A + B <=> C + D}$$

If the equilibrium shifts to the right, the forward reaction is favoured for a time, so more products form. If it shifts to the left, the reverse reaction is favoured for a time, so more reactants form.

The system does not cancel the change completely. It only shifts in a way that partly opposes the disturbance until a new equilibrium is established.

Le Chatelier’s Principle: using the idea to predict direction

A useful way to think about the principle is: what change would reduce the disturbance?

• If more reactant is added, the system tends to use up some of that extra reactant, so the equilibrium shifts towards the products.
• If some reactant is removed, the system tends to replace some of it, so the equilibrium shifts towards the reactants.
• If more product is added, the system tends to use up some of that extra product, so the equilibrium shifts towards the reactants.
• If some product is removed, the system tends to replace some of it, so the equilibrium shifts towards the products.

In each case, the equilibrium moves in the direction that reduces the effect of what was done to the system.

Le Chatelier’s Principle: the core mental model

The principle is not about memorising a phrase. It is about seeing equilibrium as a dynamic balance that responds to change.

1. Start with a reaction at equilibrium.
2. Change a condition, so the balance is disturbed.
3. Ask which direction would oppose that change.
4. Predict that the equilibrium shifts in that direction.

The key idea is simple: changing the conditions changes the equilibrium position, and the shift is in the direction that counteracts the imposed change.