Equilibrium Constants & Changing Conditions (OCR A Level Chemistry)

• 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

Effects of temperature

• How the equilibrium shifts with temperature changes:

Effect on the value of K_c

• 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_c will decrease as the ratio of [ products ] to [ reactants ] decreases
• Conversely, if the temperature is raised in an endothermic reaction, the value of K_c will increase

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_c

• The value of K_c is not affected by any changes in pressure as this does not involve gases.

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

Answer

• • Only a change in temperature will affect the value of K_c and any other changes in conditions would result in the position of the equilibrium moving in such way to oppose this change.
  • Adding a catalyst will increase the rate of reaction meaning the state of equilibrium will be reached faster but will have no effect on the position of the equilibrium and therefore K_c is unchanged.