Le Chatelier's Principle

Core Concept

Le Chatelier's principle states that if a system at equilibrium is disturbed, the system shifts in the direction that counteracts (partially offsets) the disturbance, until a new equilibrium is established. It is a qualitative way to predict the response of an equilibrium to changes in concentration, pressure, or temperature — and it is fully consistent with the quantitative Q-versus-K comparison.

Key Ideas

• Adding a reactant or removing a product shifts the system toward products (and vice versa).
• Increasing pressure (decreasing volume) shifts toward the side with fewer moles of gas.
• Temperature changes change the value of K; concentration and pressure changes do not.
• A catalyst speeds both directions equally and does not change the equilibrium position or K.

Equations and Variables

The principle is a verbal rule, but it maps onto the reaction quotient Q. For aA + bB ⇌ cC + dD:

Q = [C]^c[D]^d[A]^a[B]^b,   K = constant at fixed T

A disturbance changes Q (concentration, pressure) or K (temperature). The system shifts so that Q returns to K.

The temperature dependence of K is governed by the van 't Hoff relation:

lnK₂K₁ = -ΔH^°R(1T₂ - 1T₁)

How It Works

Concentration. Add more reactant → Q drops below K → the forward reaction is favored (shift right). Remove product → same response. Add product or remove reactant → shift left. K stays fixed because temperature is unchanged.

Pressure/volume. For gases, decreasing volume increases pressure and favors the side with fewer gas molecules (reduces the total number of particles, lowering pressure). For N₂ + 3 H₂ ⇌ 2 NH₃ (4 mol gas → 2 mol gas), increasing pressure shifts right toward NH₃. If Δn = 0, pressure has no effect on the position.

Temperature. This is the one disturbance that changes K. For an exothermic reaction (ΔH° < 0), heat is a product: raising T shifts the system left (reactant-favored) and K decreases. For an endothermic reaction (ΔH° > 0), heat is a reactant: raising T shifts right and K increases. Treat heat as a chemical you add or remove.

Catalyst. A catalyst lowers Ea for both directions equally, so it reaches the same equilibrium faster — no shift, no change in K.

Worked Example

Consider N₂(g) + 3 H₂(g) ⇌ 2 NH₃(g), ΔH° = −92 kJ/mol (exothermic).

(a) Increase [N₂]. Q decreases, so the system shifts right, consuming N₂ and H₂ to form more NH₃. K is unchanged.

(b) Increase pressure (decrease volume). 4 moles of gas → 2 moles of gas, so the system shifts right, toward fewer gas molecules. K is unchanged.

(c) Increase temperature. The reaction is exothermic, so heat is a product. Adding heat shifts the equilibrium left, toward reactants, and K decreases.

(d) Add a catalyst. No shift; the system simply reaches the same equilibrium faster. K is unchanged.

How it works

Concentration. Add more reactant → Q drops below K → the forward reaction is favored (shift right). Remove product → same response. Add product or remove reactant → shift left. K stays fixed because temperature is unchanged.

Pressure/volume. For gases, decreasing volume increases pressure and favors the side with fewer gas molecules (reduces the total number of particles, lowering pressure). For N₂ + 3 H₂ ⇌ 2 NH₃ (4 mol gas → 2 mol gas), increasing pressure shifts right toward NH₃. If Δn = 0, pressure has no effect on the position.

Temperature. This is the one disturbance that changes K. For an exothermic reaction (ΔH° < 0), heat is a product: raising T shifts the system left (reactant-favored) and K decreases. For an endothermic reaction (ΔH° > 0), heat is a reactant: raising T shifts right and K increases. Treat heat as a chemical you add or remove.

Catalyst. A catalyst lowers Ea for both directions equally, so it reaches the same equilibrium faster — no shift, no change in K.

Common confusions

• Thinking all disturbances change K. Only temperature changes K.
• Treating pressure like temperature. Pressure/volume changes shift the position but leave K fixed.
• Forgetting the sign of ΔH°. Exothermic → heat is a product; endothermic → heat is a reactant.
• Expecting a catalyst to increase yield. It only shortens the time to the same equilibrium.

Quick review

1. State Le Chatelier's principle.
2. Which way does an equilibrium shift when a product is removed?
3. For a gas reaction, what effect does increasing pressure have, and on which side?
4. Which single variable changes the value of K?
5. Why doesn't a catalyst affect the equilibrium position?