Reaching equilibrium - Higher
When a reversible reactionA chemical reaction which can go both ways. happens in a closed container, it reaches a dynamic equilibriumIn chemical reactions, a situation where the forward and backward reactions happen at the same rate, and the concentrations of the substances stay the same.. At equilibrium:
- the forward and backward reactions are still happening
- the forward and backward reactions have the same rate of reaction
- the concentrationThe concentration of a solution tells us how much of a substance is dissolved in water. The higher the concentration, the more particles of the substance are present. of all the reacting substances remain constant (they do not change)
The equilibrium positionA measure of the relative concentrations of substances in an equilibrium, showing if there are more reactants or products at equilibrium. of a reversible reaction is a measure of the concentrations of the reacting substances at equilibrium. For example:
2SO2(g) + O2(g) ⇌ 2SO3(g)
The equilibrium position is:
- to the left if the concentrations of SO2 and O2 are greater than the concentration of SO3
- to the right if the concentration of SO3 is greater than the concentrations of SO2 and O2
The equilibrium position can be changed by changing the reaction conditions.
Rate of attainment of equilibrium
How quickly an equilibrium is reached depends upon:
- the pressureForce exerted over an area. The greater the pressure, the greater the force exerted over the same area. of a reacting gas
- the concentration of a reacting solution
- the temperatureHow warm or cold something is. of the reaction mixture
- the presence of a catalystA substance that changes the rate of a chemical reaction without being changed by the reaction itself.
The table summarises the effects of these factors on the equilibrium position and the time taken to reach equilibrium.
| Change | Movement | Time taken |
| Pressure increased | Equilibrium position moves towards the fewest molecules of gas | Decreased |
| Concentration increased | Equilibrium position moves away from that reactant | Decreased |
| Temperature increased | Equilibrium position moves in the direction of the endothermic reaction | Decreased |
| Catalyst added | No change in equilibrium position | Decreased |
| Change | Pressure increased |
|---|---|
| Movement | Equilibrium position moves towards the fewest molecules of gas |
| Time taken | Decreased |
| Change | Concentration increased |
|---|---|
| Movement | Equilibrium position moves away from that reactant |
| Time taken | Decreased |
| Change | Temperature increased |
|---|---|
| Movement | Equilibrium position moves in the direction of the endothermic reaction |
| Time taken | Decreased |
| Change | Catalyst added |
|---|---|
| Movement | No change in equilibrium position |
| Time taken | Decreased |
Question
Describe the change in reaction conditions which increases the rate of reaching equilibrium, but does not change the position of equilibrium position.
Adding a suitable catalyst.