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.

For AQA GCSE Chemistry, the specific details of how ammonia is made using the Haber process need to be known, as well as an ability to apply the principles on this page to an unfamiliar new equilibrium.

For AQA GCSE Combined Science, the specifics of the Haber process don't need to be known. However, an understanding of these general principles and an ability to apply them to a reaction similar to the Haber process is still needed.

Background to the Haber process

Nitrogen gas is reacted with hydrogen gas to make ammonia gas. The forward reaction is exothermic.

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

Le Chatelier's principle

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

When a change is made to a system at equilibrium, the position of equilibrium moves to counteract the change that was made. For example, if the temperature is increased, the position of equilibrium moves in the endothermic direction to reduce 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, to reduce the pressure.

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

\begin{array}{rcl} N_{2}(g) + 3H_{2}(g) & \rightleftharpoons & 2NH_{3}(g) \\ 1 + 3 = 4~molecules && 2~molecules \end{array}

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

Question

Sulfur dioxide reacts with oxygen to make sulfur trioxide in a reversible reaction: 2SO2(s) + O2(g) ⇌2SO3(g)

Predict the effect of increasing the pressure.

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

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, in the Haber process:

N2(g) + 3H2(g) ⇌ 2NH3(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 in the endothermic direction (to the left) to reduce the temperature. This means that less ammonia (NH3) will be produced.

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.

Changing the concentration

If the concentration of a reactant (on the left) is increased, the equilibrium position moves in the direction away from this reactant, and so more of the products are produced (on the right). If one of the products is removed from a reaction (on the right), then the position of equilibrium moves to the right to make more of that product.

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.

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