Changes in Temperature & Pressure (AQA A Level Chemistry)

Changes in Temperature & Pressure

• We can apply Le Chatelier's Principle to gaseous equilibria in the same way it is applied to aqueous systems

• Here's a reminder of how the principle works

Le Chatelier’s principle

• Le Chatelier’s principle says that if a change is made to a system in dynamic equilibrium, the position of the equilibrium moves to counteract this change

• The principle is used to predict changes to the position of equilibrium when there are changes in temperature, pressure or concentration

Predicting changes in Temperature & Pressure

Effects of temperature

• How the equilibrium shifts with temperature changes:

Effect on the value of K_p

• For a reaction that is exothermic in the forward direction, increasing the temperature pushes the equilibrium from right to left

• Therefore, the value of K_p will decrease as the ratio of [ products ] to [ reactants ] decreases

• Conversely, if the temperature is raised in an endothermic reaction, the value of K_p will increase

Effects of pressure

• Changes in pressure only affect reactions where the reactants or products are gases

• How the equilibrium shifts with pressure changes:

Effect on the value of K_p

• The value of K_p is not affected by any changes in pressure.

• Changes in pressure cause a shift in the position of equilibrium to a new position which restores the value of K_p 

• This is analogous to what happens to K_c when you change concentration in an aqueous equilibrium; a shift restores equilibrium to a new position maintaining K_c

Presence of a catalyst

• If all other conditions stay the same, the equilibrium constant K_p is not affected by the presence of a catalyst

• A catalyst speeds up both the forward and reverse reactions at the same rate so the ratio of  [ products ] to [ reactants ] remains unchanged

• Catalysts only cause a reaction to reach equilibrium faster

• Catalysts therefore have no effect on the position of the equilibrium once this is reached