The position of equilibrium

The relative amounts of all the reactants and products at equilibrium depend on the conditions of the reaction
This balance is framed in an important concept known as Le Chatelier's Principle,
- Named after Henri Le Chatelier, who was a French military engineer in the 19th century
This principle states that when a change is made to the conditions of a system at equilibrium, the system automatically moves to oppose the change
The principle is used to predict changes to the position of equilibrium when there are changes in:
- Temperature
- Pressure
Knowing the energy changes, states and concentrations involved allows us to use the principle to manipulate the outcome of reversible reactions
For example, if pressure is increased, the position of equilibrium moves in the direction which has the smallest amount of gaseous molecules
The position of equilibrium is said to shift to the right when the forward reaction is favoured
- This means that there is an increase in the amount of products formed
The position of equilibrium is said to shift to the left when the reverse reaction is favoured
- So, there is an increase in the amount of reactants formed

Temperature

Le Chatelier's Principle can be used to predict the effect of changes in temperature on systems in equilibrium
To make this prediction it is necessary to know whether the reaction is exothermic or endothermic
If the temperature of the reaction increases:
- The equilibrium will shift in the direction of the endothermic reaction
If the temperature of a reaction decreases:
- The equilibrium will shift in the direction of the exothermic reaction

Worked example

Iodine monochloride reacts reversibly with chlorine to form iodine trichloride and the forward reaction is exothermic:

ICl (l) + Cl₂ (g) ⇌ ICl₃(s)

dark brown yellow

What colour will the mixture turn when heated? Explain your answer.

Answer:

The system will oppose the increase in temperature
- Increasing the temperature of an equilibrium reaction favours the endothermic reaction
If the forward reaction is exothermic, then the backward reaction must be endothermic
Therefore, the equilibrium will move to the left and produce more reactants
This means that the colour of the mixture will become increasingly brown as the temperature increases

Pressure

We can predict the effect of changes in pressure on systems in equilibrium
Changes in pressure only affects gases
In gaseous reactions:
- An increase in pressure will favour the reaction that produces the least number of molecules
- A decrease in pressure will favour the reaction that produces the greatest number of molecules
If there are the same number of moles of gases on either side of the equation, then there is NO effect on the position of equilibrium when the pressure is changed
- Increasing the pressure will increase the rate of the forward reaction and backward reaction equally which is why the position of equilibrium is unchanged

Worked example

Nitrogen dioxide molecules can dimerise and form dinitrogen tetroxide in the following equilibrium reaction:

2NO₂ (g) ⇌ N₂O₄(g)

dark brown colourless

What will the colour change be if the pressure is increased? Explain your answer.

Answer:

The number of gas molecules produced by the forward reaction = 1
The number of gas molecules produced by the reverse reaction = 2
An increase in the pressure will favour the reaction that produces the least number of molecules
- This is the forward reaction
So, the equilibrium shifts to the right
- This means that the mixture will appear increasingly colourless as the concentration of N₂O₄ increases

Examiner Tip

When the conditions at equilibrium are changed, the system always responds by doing the opposite.

The effect of catalysts on equilibrium

The effect of a catalyst on equilibrium position

The presence of a catalyst does not affect the position of equilibrium but it does increase the rate at which equilibrium is reached
This is because the catalyst increases the rate of both the forward and backward reactions by the same amount (by providing an alternative pathway requiring lower activation energy)
As a result, the concentration of reactants and products is nevertheless the same at equilibrium as it would be without the catalyst

Diagram showing the effect of catalyst on equilibrium position

Graph to show a catalyst increasing the rate of the forward and reverse reaction A catalyst increases the rate of the forwards and reverse reaction but does not alter the position of equilibrium

The Position of Equilibrium (Edexcel IGCSE Chemistry)

Revision Note