Le Chatelier's Principle and Changing Conditions

(Higher Tier only) The whole of this lesson (spec 4.6.2.4 to 4.6.2.7) is assessed at Higher Tier only. Foundation candidates do not need Le Chatelier's Principle.

The relative amounts of reactants and products at equilibrium depend on the conditions. Change a condition and the balance point moves. Le Chatelier's Principle predicts which way.

Le Chatelier's Principle: if a change is made to a system at equilibrium, the system responds to counteract the change.

The position of equilibrium describes whether there are more products or more reactants at equilibrium. Saying it "shifts to the right" means more products form; "shifts to the left" means more reactants form.

Three conditions can be changed: concentration, temperature and pressure. For each one, the principle is the same: the system opposes whatever you do to it. The rest of this lesson applies that idea to each condition in turn.

If you change the concentration of a substance in an equilibrium, the system shifts to oppose that change. Take a general reversible reaction:

$A+B\rightleftharpoons C+D$

• Increase a reactant (add more A): the system removes some of the extra A by shifting right, making more products (C and D).
• Decrease a product (remove C): the system makes more C by shifting right, making more products.
• Increase a product (add more C): the system removes some C by shifting left, making more reactants.

Change made	Equilibrium shifts	Result
Increase concentration of a reactant	Right (towards products)	More product formed
Decrease concentration of a product	Right (towards products)	More product formed
Increase concentration of a product	Left (towards reactants)	More reactant formed
Decrease concentration of a reactant	Left (towards reactants)	More reactant formed

Add more of something and the equilibrium moves away from it; remove something and the equilibrium moves towards it.

Industry uses this by continually removing the product, which keeps pulling the equilibrium to the right and increases the yield.

Consider the equilibrium:

$N_2+3H_2\rightleftharpoons 2N{H}_3$

Question: If more hydrogen is added to this equilibrium, what happens to the amount of ammonia?

Reasoning step by step:

1. Hydrogen (H₂) is a reactant, on the left.
2. Adding more H₂ increases the concentration of a reactant.
3. By Le Chatelier's Principle, the system counteracts the increase by using up some of the added hydrogen.
4. It does this by shifting the equilibrium to the right, towards the products.
5. Shifting right makes more ammonia (NH₃).

Answer: The amount of ammonia increases. Adding a reactant always pushes a Higher Tier equilibrium towards the products, provided the equation is a genuine equilibrium in a closed system.

Temperature is different from concentration because it changes the energy of the system. The direction of the shift depends on whether the forward reaction is exothermic or endothermic.

Increasing temperature shifts the equilibrium in the endothermic direction. Decreasing temperature shifts it in the exothermic direction.

The system opposes the temperature change: if you add heat, it favours the direction that absorbs heat (endothermic); if you remove heat, it favours the direction that releases heat (exothermic).

Change	Endothermic direction	Exothermic direction
Increase temperature	Products increase	Products decrease
Decrease temperature	Products decrease	Products increase

Read the table by asking which direction (forward or reverse) is endothermic. For example, if the forward reaction is exothermic, then increasing temperature favours the reverse (endothermic) direction, so the amount of product falls.

To use this rule you must know whether the forward reaction is exothermic or endothermic. The question will normally tell you, or give the sign of the energy change.

Consider an equilibrium whose forward reaction is exothermic:

$2S{O}_2+O_2\rightleftharpoons 2S{O}_3(\text{forward reaction exothermic})$

Question: What happens to the amount of SO₃ if the temperature is increased?

Reasoning step by step:

1. The forward reaction is exothermic, so the reverse reaction is endothermic.
2. Increasing temperature shifts the equilibrium in the endothermic direction.
3. The endothermic direction here is the reverse (right to left).
4. So the equilibrium shifts left, towards the reactants.
5. Shifting left decreases the amount of SO₃.

Answer: The amount of SO₃ decreases when the temperature is increased, because the reverse reaction is endothermic and is favoured by heating.

Pressure only affects an equilibrium involving gases. The rule depends on counting the number of gas molecules on each side of the balanced equation.

For gaseous equilibria, increasing pressure shifts the position towards the side with fewer gas molecules. Decreasing pressure shifts it towards the side with more gas molecules.

The system counteracts a pressure increase by moving to the side with fewer molecules, which reduces the pressure. Take the ammonia equilibrium:

$N_2+3H_2\rightleftharpoons 2N{H}_3$

Count the gas molecules: the left has 4 molecules (1 + 3); the right has 2 molecules. So:

Change	Shifts towards	Effect on ammonia
Increase pressure	Right (fewer molecules: 2 < 4)	More ammonia formed
Decrease pressure	Left (more molecules: 4 > 2)	Less ammonia formed

If both sides have the same number of gas molecules, changing the pressure has no effect on the position of equilibrium. Only solids and liquids present are ignored; count only the gases.

1. Forgetting this topic is Higher Tier only

Le Chatelier's Principle and the effects of changing conditions on equilibrium are assessed only at Higher Tier. Foundation candidates are not expected to apply them.

2. Getting the temperature direction backwards

Increasing temperature favours the endothermic direction, not the exothermic one. First identify which direction is endothermic, then shift towards it when heating.

3. Applying pressure changes to solutions or solids

Pressure only shifts equilibria that involve gases. Count only the gas molecules in the equation, and ignore any solids or liquids present.

4. Miscounting the gas molecules

Use the big numbers (coefficients) in the balanced equation. In N₂ + 3H₂ ⇌ 2NH₃, the left is 1 + 3 = 4 molecules and the right is 2. Increasing pressure moves it to the side with fewer molecules.

5. Saying the shift changes the rate rather than the yield

For equilibrium questions, describe how the position of equilibrium shifts and whether the amount of product increases or decreases. That is what these Higher Tier questions reward.