6.2.4 The effect of changing conditions on equilibrium
A system at equilibrium shifts to oppose any change you make
Definition
Dynamic equilibrium
The state reached in a closed system when the forward and reverse reactions occur at equal rates, so the concentrations of reactants and products remain constant.
- If a system at equilibrium has one of its conditions changed, the equilibrium moves to counteract that change.
- This idea is known as Le Chatelier's principle.
- The conditions that can be changed are the concentration, the temperature and the pressure.
- The principle lets you predict which way the equilibrium will move.
Key Idea
- Change a condition and the equilibrium moves in the direction that opposes the change.
- This lets you predict whether more product or more reactant will form.
Reading the shift as more product or more reactant
Definition
Forward reaction
The reaction in which the reactants shown on the left of a reversible equation form the products shown on the right.
Definition
Reverse reaction
The reaction in which the products shown on the right of a reversible equation react to form the substances shown on the left.
- A shift towards the right favours the forward reaction, so more product forms.
- A shift towards the left favours the reverse reaction, so more reactant forms.
- To predict the shift, work out which direction cancels out the change you made.
- The next articles apply this to changes in concentration, temperature and pressure.
Exam technique
- Start by stating that the equilibrium moves to oppose the change.
- Then say which direction that is, and whether it makes more product or more reactant.
- Only change one condition at a time when predicting a shift.
Self review
- What happens to a system at equilibrium when a condition is changed?
- What is the name of this principle?
- Which three conditions can be changed to move an equilibrium?
- What does a shift to the right do to the amount of product?
