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E3.3 · Explain Le Châtelier’s principle and equilibrium shifts

Learn to explain le châtelier’s principle and equilibrium shifts through clear examples and targeted practice.

Ontario Grade 12 Chemistry

Chemical Systems and Equilibrium

Le Châtelier’s principle and how a system responds to change

A sealed container of a gas mixture can change colour when its temperature changes. That visible change can be a clue that the balance between two substances has shifted. To explain such a change, first recall that some reactions can proceed in both directions. At equilibrium, the forward and reverse reactions continue, but their rates are equal. The amounts of substances stay constant over time, though they do not have to be equal. Le Châtelier’s principle helps predict how that balance responds when conditions change.

What you will learn

Describe dynamic equilibrium in a reversible reaction.
State Le Châtelier’s principle in your own words.
Predict the direction of an equilibrium shift when concentration, pressure, volume, or temperature changes.
Distinguish a shift in equilibrium from a change in reaction rate or from equal reactant and product amounts.

1. From reversible reactions to dynamic equilibrium

A reversible reaction can proceed from reactants to products and from products back to reactants. A double arrow represents these opposing directions. For example, nitrogen dioxide can combine to form dinitrogen tetroxide:

\mathrm{2NO_2(g) \rightleftharpoons N_2O_4(g)}

. Nitrogen dioxide is a brown gas, while dinitrogen tetroxide is colourless. The state symbol

(g)

means that a substance is a gas.

In a closed container, the forward reaction can form dinitrogen tetroxide while the reverse reaction forms nitrogen dioxide. Dynamic equilibrium is the condition in which both reactions continue at equal rates. As a result, the amounts of each substance remain steady if the conditions stay unchanged. Steady amounts do not mean equal amounts.

A change in the appearance of a mixture can indicate that the relative amounts of its substances have changed. The particle-level explanation is that the reaction responding in one direction has, for a time, a greater rate than the reaction in the opposite direction. The system then moves toward a new equilibrium.

\mathrm{2NO_2(g) \rightleftharpoons N_2O_4(g)}

Equilibrium is dynamic: both reaction directions continue.
At equilibrium, forward and reverse reaction rates are equal.
The amounts of reactants and products are constant, but need not be equal.

2. Le Châtelier’s principle

Le Châtelier’s principle states that when a system at equilibrium is disturbed, it shifts in the direction that reduces the effect of the disturbance. A shift is a change in the relative amounts of reactants and products as the system moves toward a new equilibrium. The principle predicts the direction of that change; it does not mean the disturbance is completely undone.

For a concentration change, adding a substance tends to shift the reaction away from the added substance. Removing a substance tends to shift it toward the side that makes more of it. For example, adding a reactant favours the forward direction, which uses some of that reactant. Adding a product favours the reverse direction, which uses some of that product.

For gaseous systems, pressure and volume changes can affect the equilibrium position when the two sides of the equation have different numbers of gas particles. Compressing a gas mixture decreases its volume and increases its pressure. The shift is toward the side with fewer gas particles, which reduces the pressure increase. Increasing volume has the opposite effect: the shift is toward the side with more gas particles. Count only gaseous particles when making this comparison.

Temperature changes are treated as adding or removing heat. If the forward reaction releases heat, heat can be treated as a product: raising temperature favours the reverse direction, while lowering temperature favours the forward direction. If the forward reaction absorbs heat, heat can be treated as a reactant, so raising temperature favours the forward direction. The reaction’s heat direction must be known before predicting its temperature response.

A catalyst is not a way to shift equilibrium. It helps both reaction directions reach equilibrium faster, but does not favour one side over the other. This is different from changing the equilibrium position.

A system shifts to reduce the effect of a change.
Concentration changes favour the direction that uses up an added substance or replaces a removed one.
For gases, compare the number of gas particles on each side when pressure or volume changes.
For temperature changes, treat heat as a reactant or product according to the forward reaction.

3. A consistent way to predict a shift

First, write or inspect the balanced reversible equation. Check the state symbols, because pressure and volume comparisons apply to gases. Next, identify exactly what changed: concentration, pressure, volume, or temperature. Then ask which direction would reduce that change. Finally, state the direction of shift and describe which side gains relative amount as the system approaches its new equilibrium.

For pressure and volume, compare the total coefficients of gaseous substances on each side. These coefficients represent relative numbers of particles in the balanced equation. If both sides contain the same number of gas particles, changing pressure or volume alone does not favour either side by this particle-count rule.

A shift describes the system’s response after a disturbance. It is not a claim that reactant and product amounts become equal. Nor does it mean that every particle changes at once. The system moves toward a new balance in which the forward and reverse reaction rates are again equal.

Name the disturbance before deciding on a shift.
Use the balanced equation and relevant state symbols.
Describe the direction of shift, not an unsupported numerical amount.

4. Reading an observable change

In the nitrogen dioxide equilibrium, brown nitrogen dioxide particles and colourless dinitrogen tetroxide particles are interconverted. A greater relative amount of nitrogen dioxide makes the mixture appear more brown; a shift toward dinitrogen tetroxide reduces the relative amount of brown gas. This connects a visible observation with a change in particle amounts.

The equation has two gas particles on the left for every one on the right. Therefore, a pressure increase caused by decreasing volume favours the side with fewer gas particles. This example shows how the particle model and the balanced equation work together: the colour gives a possible observation, and the equation helps predict the direction of shift.

Use this reasoning only when the system is at equilibrium before the change and the stated disturbance applies. If information about the reaction’s heat direction is not given, do not guess how a temperature change affects it.

The observed colour can reflect the relative amount of coloured gas.
The balanced equation supplies the gas-particle comparison.
Temperature predictions require knowing whether the forward reaction releases or absorbs heat.

Disturbance and predicted response

Change to an equilibrium system	Favoured response
Add a reactant	Shift toward products
Add a product	Shift toward reactants
Remove a reactant	Shift toward reactants
Remove a product	Shift toward products
Increase pressure by decreasing volume	Shift toward fewer gas particles, if the sides differ
Decrease pressure by increasing volume	Shift toward more gas particles, if the sides differ
Raise temperature	Shift in the direction that absorbs heat
Lower temperature	Shift in the direction that releases heat

Worked example

Predicting the effect of compression

A mixture is at equilibrium in a sealed container according to

\mathrm{2NO_2(g) \rightleftharpoons N_2O_4(g)}

. The container’s volume is decreased at constant temperature. Predict the direction of the shift and the expected change in the relative amounts of the two gases.

Identify the disturbance
Decreasing the container’s volume compresses the gas mixture and raises its pressure. The temperature is unchanged, so use the gas-particle comparison rather than a temperature rule.
Compare gas particles
The left side has two gas particles for each reaction event, while the right side has one. The right side therefore has fewer gas particles.
$2 > 1$
Apply the principle
The pressure increase favours the side with fewer gas particles. The equilibrium shifts right, toward dinitrogen tetroxide. As the system approaches its new equilibrium, the relative amount of dinitrogen tetroxide increases and the relative amount of nitrogen dioxide decreases.
$\mathrm{2NO_2(g) \rightarrow N_2O_4(g)}$

Answer: The equilibrium shifts right, toward

\mathrm{N_2O_4(g)}

. Its relative amount increases, while the relative amount of

\mathrm{NO_2(g)}

decreases.

Check: The equation is balanced: it has two nitrogen atoms and four oxygen atoms on each side. The shift is toward the side with fewer gas particles.

Common mistakes and how to avoid them

Assuming equilibrium means equal amounts of reactants and products.

Correction: Equilibrium means the forward and reverse reaction rates are equal. The amounts can be different.

Saying a pressure increase always shifts an equilibrium to the products.

Correction: For a gas equilibrium, compare the numbers of gas particles on both sides. The shift is toward fewer gas particles when pressure rises through compression.

Predicting a temperature shift without knowing whether the forward reaction releases or absorbs heat.

Correction: Identify the heat direction first. A temperature increase favours the direction that absorbs heat.

Treating a catalyst as a substance that shifts equilibrium.

Correction: A catalyst does not favour either side. It helps the system reach equilibrium faster.

Lesson summary

At dynamic equilibrium, forward and reverse reaction rates are equal, and amounts remain constant over time.
Le Châtelier’s principle predicts that an equilibrium shifts in the direction that reduces the effect of a disturbance.
Concentration changes favour the direction that uses up added material or replaces removed material.
For gases, pressure and volume shifts depend on the number of gas particles on each side.
Temperature shifts depend on which direction releases or absorbs heat.

Check your understanding

Question 1

At equilibrium, which statement is correct?

Reactant and product amounts must be equal.
The forward and reverse reaction rates are equal.
Both reaction directions have stopped.
Only the forward reaction continues.

Show answer and explanation

The forward and reverse reaction rates are equal.

At dynamic equilibrium, both directions continue at equal rates. The amounts do not have to be equal.

Question 2

For

\mathrm{2NO_2(g) \rightleftharpoons N_2O_4(g)}

, what shift is favoured when pressure is increased by decreasing volume?

Left, toward the side with more gas particles.
Right, toward the side with fewer gas particles.
No shift, because both sides have the same number of gas particles.
The direction cannot be predicted from the equation.

Show answer and explanation

Right, toward the side with fewer gas particles.

There are two gas particles on the left and one on the right. Higher pressure favours the side with fewer gas particles.

Question 3

A catalyst is added to a mixture at equilibrium. What is the expected effect on the equilibrium position?

It shifts toward products.
It shifts toward reactants.
It does not shift in either direction.
It makes the reactant and product amounts equal.

Show answer and explanation

It does not shift in either direction.

A catalyst does not favour either side, so it does not shift the equilibrium position.

Key terms

Reversible reaction: A reaction that can proceed from reactants to products and from products back to reactants.
Dynamic equilibrium: A condition in a reversible reaction where both directions continue at equal rates and the amounts of substances remain constant.
Equilibrium shift: A change in the relative amounts of reactants and products as a system responds to a disturbance.
Le Châtelier’s principle: The principle that an equilibrium shifts in the direction that reduces the effect of a disturbance.

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