Changing the equilibrium position - Higher

This video looks at reversible reactions and dynamic equilibrium

The equilibrium position can be changed by adjusting:

  • the concentrations of reactants
  • 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:

curriculum-key-fact
Any change made to a reaction which is in equilibrium will result in the equilibrium position moving to reduce the effect of the change made.

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) ⇌ 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.

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 molecules. 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 equation 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?

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 endothermic 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 exothermic reaction. This reduces the effects of the change because, during an exothermic reaction, energy is transferred to the surroundings.

curriculum-key-fact
If given, the energy change for a reversible reaction is always for the forward reaction.
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?

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 conditionsEquilibrium position moves:
Pressure increasedTowards the fewest molecules of gas
Concentration of a reactant increasedAway from that reactant
Temperature increasedIn the direction of the endothermic reaction
Catalyst addedNo change
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