The principle of moderation (Le Chatelier’s principle)

How a system at equilibrium responds to a disturbance

Le Chatelier’s principle describes how a chemical system at equilibrium reacts when disturbed: the system changes in such a way as to partially counteract the disturbance imposed on it.

Example: adding a reactant

Let’s consider a gas-phase equilibrium:

N2 + 3 H2 <=> 2 NH3

If the concentration of N₂ is increased (by adding more), the system is no longer at equilibrium: Qr becomes less than K. The reaction then proceeds in the forward direction, consuming some of the N₂ and H₂ to reform NH₃, until a new equilibrium is established. The system thus ‘moderates’ the increase in concentration that has been imposed on it, without completely cancelling it out.

Example: pressure

For this same reaction, an increase in pressure (at constant temperature) favours the direction that reduces the total number of moles of gas: in this case, the forward direction (from 4 moles of reactants to 2 moles of product), which opposes the imposed increase in pressure.

What the principle of moderation does NOT allow

A shift in equilibrium changes the quantities of substance in the final state, but never changes the value of the constant K, which depends solely on temperature. Similarly, a shift in equilibrium never completely ‘cancels out’ the disturbance: the system finds a new equilibrium; it does not return to its initial state.

Common misconception

The most common mistake is to believe that a catalyst can shift an equilibrium: this is incorrect. A catalyst accelerates the establishment of equilibrium (it is reached more quickly), but the final position of the equilibrium, governed by K, remains strictly unchanged.