Le Chatelier's principle - higher tier
The equilibrium position can be changed by altering the reaction conditions, such as by:
- changing the pressure Force exerted over an area. The greater the pressure, the greater the force exerted over the same area.
- changing 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.
- changing the temperature How warm or cold something is.
Changing the pressure
In a reaction involving gases, if the pressure is increased, the equilibrium position moves in the direction of the fewer molecules of gas.
There are fewer molecules on the right-hand side of the equation for the Haber process:
N2(g) + 3H2(g) 鈬 2NH3(g)
If the pressure is increased, the equilibrium position moves to the right. This increases the yield of ammonia in this equilibrium.
Changing the concentration
In a reaction involving solutions, if the concentration of a solute is increased, the equilibrium position moves in the direction away from this solute. For example, bismuth chloride reacts with water in a reversible reaction:
BiCl3(aq) + H2O(l) 鈬 BiOCl(s) + 2HCl(aq)
The concentration of hydrochloric acid can be increased by adding more hydrochloric acid. When this happens, the equilibrium position moves to the left, away from HCl(aq) in the equation.
Changing the temperature
In a reversible reaction, if the reaction is exothermicA chemical reaction that releases energy. in one direction, it is endothermic in the other direction. If the temperature is increased, the equilibrium position moves in the direction of the endothermicReaction in which energy is taken in. process. For example, sulfur dioxide reacts with oxygen in a reversible reaction:
2SO2(g) + O2(g) 鈬 2SO3(g) (forward reaction is exothermic)
If the forward reaction is exothermic, the backward reaction must be endothermic. Therefore, if the temperature is increased, the equilibrium position moves to the left.
Compromise temperature
A compromise between a reasonable rate of reaction and a decent yield of product is required in industrial processes. The Haber Process is an exothermic reaction in the forward direction so using a low temperature would increase the yield of ammonia. However this would mean a low rate of reaction so a compromise temperature (450 掳C) is used.
| Change | Equilibrium |
| Pressure increased | Position moves towards the fewer molecules of gas |
| Concentration of a reactant increased | Position moves away from that reactant |
| Temperature increased | Position moves in the direction of the endothermic reaction |
| Change | Pressure increased |
|---|---|
| Equilibrium | Position moves towards the fewer molecules of gas |
| Change | Concentration of a reactant increased |
|---|---|
| Equilibrium | Position moves away from that reactant |
| Change | Temperature increased |
|---|---|
| Equilibrium | Position moves in the direction of the endothermic reaction |