6.2.7 The effect of pressure changes on equilibrium
Pressure changes only affect equilibria involving gases
Definition
Equilibrium position
The relative amounts of reactants and products present when a reversible reaction reaches equilibrium in a closed system.
- Changing the pressure moves the equilibrium position only when gases take part in the reaction.
- To predict the shift, count the number of gas molecules on each side of the equation.
Higher pressure favours the side with fewer gas molecules
Definition
Coefficient
A large whole number placed before a chemical formula to show the relative number of particles or moles in a balanced equation.
- Increasing the pressure moves the equilibrium to the side with fewer gas molecules.
- Decreasing the pressure moves the equilibrium to the side with more gas molecules.
- If both sides have the same number of gas molecules, changing the pressure has no effect.
- You find the number of gas molecules from the coefficient in front of each gas in the balanced equation.
Reading the equation for the number of gas molecules
Definition
Equilibrium yield
The amount or percentage of a product present when a reaction mixture has reached equilibrium under stated conditions.
- Add up the gas molecules shown on each side of the balanced equation.
- Take N2(g)+3H2(g)⇌2NH3(g)\text{N}_2\text{(g)} + 3\text{H}_2\text{(g)} \rightleftharpoons 2\text{NH}_3\text{(g)}N2(g)+3H2(g)⇌2NH3(g), with four gas molecules on the left and two on the right.
- Increasing the pressure moves this equilibrium to the right, raising the equilibrium yield of ammonia.
Exam technique
- Check that gases are involved before saying pressure has any effect.
- Count the gas molecules on each side using the balancing numbers.
- Say a higher pressure favours the side with fewer gas molecules, then name the product whose yield changes.
Self review
- When does a change in pressure move an equilibrium?
- Which side does a higher pressure favour?
- What happens if both sides have the same number of gas molecules?
- In N2+3H2⇌2NH3\text{N}_2 + 3\text{H}_2 \rightleftharpoons 2\text{NH}_3N2+3H2⇌2NH3, which way does a higher pressure move the equilibrium?
