Changing the position of equilibrium - Higher

This video looks at reversible reactions and dynamic equilibrium

The equilibrium position of a reversible reaction is a measure of the concentrations of the reacting substances at equilibrium. Using the Haber process, which makes ammonia, as an example:

nitrogen + hydrogen ⇌ ammonia

N2(g) + 3H2(g) ⇌ 2NH3(g)

The equilibrium position is:

  • to the left if the concentrations of N2 and H2 are greater than the concentration of NH3
  • to the right if the concentration of NH3 is greater than the concentrations of N2 and H2

The equilibrium position can be changed by changing the reaction conditions by:

  • changing the pressure
  • changing the concentration
  • changing the temperature

Changing the pressure

If the pressure is increased in a reaction involving gases, the equilibrium position moves in the direction of the fewest molecules of gas.

There are fewer molecules on the right hand side of the equation for the Haber process:

N2(g) + 3H2(g) ⇌ 2NH3(g)

1 + 3 = 4 molecules / 2 molecules

If the pressure is increased, the equilibrium position moves to the right.

Question

Calcium carbonate decomposes when it is heated: CaCO3(s) ⇌ CaO(s) + CO2(g)

Predict the effect of increasing the pressure.

The equilibrium position will move to the left, in the direction of the fewest molecules of gas.

Changing the concentration

If the concentration of a solute is increased in a reaction involving solutions, 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.

Changing the temperature

In a reversible reaction, if the reaction is exothermic in one direction, it is endothermic in the other direction.

If the temperature is increased, the equilibrium position moves in the direction of the endothermic process. For example, sulfur dioxide reacts with oxygen in a reversible reaction:

2SO2(g) + O2(g) ⇌ 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.

Question

Hydrogen can be manufactured by reacting carbon with steam:

C(s) + H2O(g) ⇌ H2(g) + CO(g) (forward reaction is endothermic)

Predict the effect of increasing the temperature.

The equilibrium position will move to the right, in the direction of the endothermic reaction.

Summary

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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