Changing the equilibrium position - Higher
This video looks at reversible reactions and dynamic equilibrium
The equilibrium positionA measure of the relative concentrations of substances in an equilibrium, showing if there are more reactants or products at equilibrium. can be changed by adjusting:
- the concentrations of reactantA substance that reacts together with another substance to form products during a chemical reaction.
- 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 solutionMixture formed by a solute and a solvent., if the concentration of a soluteThe dissolved substance in a solution. 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.
Question
Iron(III) ions react with thiocyanate ions, SCN-, in a reversible reaction:
Fe3+(aq) + SCN-(aq) 鈬 FeSCN2+(aq)
Predict the effect of adding more iron(III) ions.
The equilibrium position will move to the right, in the direction away from Fe3+ in the equation.
Pressure
Increasing the pressure moves the equilibrium position to the side with the fewest moleculeA collection of two or more atoms held together by chemical bonds.. 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 balanced chemical equationA chemical equation written using the symbols and formulae of the reactants and products, so that the number of units of each element present is the same on both sides of the arrow. shows which side has most molecules. For example nitrogen dioxide NO2 exists in equilibrium with N2O4:
2NO2(g) 鈬 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?
The equilibrium position will shift to the right, in the direction of the fewest molecules of gas. This reduces the effects of the change because the pressure decreases as the number of molecules decreases.
Temperature
Increasing the temperature shifts the equilibrium in the direction of the endothermicReaction in which energy is taken in. 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 exothermicReaction in which energy is given out to the surroundings. The surroundings then have more energy than they started with so the temperature increases. 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) 鈬 N2O4(g)
The reverse reaction is endothermic. How will an increase in temperature affect the position of equilibrium?
The equilibrium position will shift to the left, in the direction of the endothermic reaction. This will reduce the effects of the temperature increase.
| 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 |