Changing the equilibrium position - Higher
The can be changed by adjusting:
- the concentrations of
- the pressure of reacting gases
- the temperature at which the process takes place
The effect of changing these can be predicted using the following rule:
Concentration
In a reaction involving , if the concentration of a 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) ⇌ 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.
Question
Iron(III) ions react with thiocyanate ions, SCN-, in a reversible reaction:
Fe3+(aq) + SCN-(aq) ⇌ FeSCN2+(aq)
Predict the effect of adding more iron(III) ions.
Pressure
Increasing the pressure moves the equilibrium position to the side with the fewest . This reduces the effects of the change because the pressure decreases as the number of molecules decreases.
Decreasing the pressure moves the equilibrium to the side with the most molecules. This reduces the effects of the change because the pressure increases as the number of molecules increases.
The shows which side has most molecules. For example nitrogen dioxide NO2 exists in equilibrium with N2O4:
2NO2(g) ⇌ N2O4(g)
Two molecules of NO2 combine to make one molecule of N2O4.
Question
How will an increase in pressure affect the equilibrium position of the above reaction?
Temperature
Increasing the temperature shifts the equilibrium in the direction of the reaction. This reduces the effects of the change because, during an endothermic reaction, energy is transferred from the surroundings.
Decreasing the temperature shifts the equilibrium in the direction of the reaction. This reduces the effects of the change because, during an exothermic reaction, energy is transferred to the surroundings.
Question
The forward reaction, in which two NO2 molecules combine to form a molecule of N2O4, is exothermic:
2NO2(g) ⇌ N2O4(g)
The reverse reaction is endothermic. How will an increase in temperature affect the position of equilibrium?
| Change in conditions | Equilibrium position moves: |
| Pressure increased | Towards the fewest molecules of gas |
| Concentration of a reactant increased | Away from that reactant |
| Temperature increased | In the direction of the endothermic reaction |
| Catalyst added | No change |
| Change in conditions | Pressure increased |
|---|---|
| Equilibrium position moves: | Towards the fewest molecules of gas |
| Change in conditions | Concentration of a reactant increased |
|---|---|
| Equilibrium position moves: | Away from that reactant |
| Change in conditions | Temperature increased |
|---|---|
| Equilibrium position moves: | In the direction of the endothermic reaction |
| Change in conditions | Catalyst added |
|---|---|
| Equilibrium position moves: | No change |
