Reversible reactions try to counteract changes

• Le Chatelier’s Principle is the idea that if you change the conditions of a reversible reaction at equilibrium, the system will try to counteract that change

• It can be used to predict the effect of any changes you make to a reaction system

Such as changes to the temperature

• All reactions are exothermic in one direction and endothermic in the other

• If you decrease the temperature the equilibrium will move in the exothermic direction to produce ore heat

• This means you’ll get more products for the exothermic reaction and fewer products for the endothermic reaction

• If you raise the temperature, the equilibrium will move in the endothermic direction to try and decrease it

• You’ll now get more products for the endothermic reaction and fewer products for the exothermic reaction

Pressure

• Changing the pressure only affects an equilibrium involving gases

• If you increase the pressure, the equilibrium tries to reduce it, it moves in the direction where there are fewer molecules of gas

• If you decrease the pressure, the equilibrium tries to increase it, it moves in the direction where there are more molecules of gas

• You can use the balanced symbol equation for a reaction to see which side has more molecules of gas

Concentration:

1. If the concentration of the reactants is raised, the forward reaction will increase its rate in order to get the concentrations back to normal.

2. If the concentration of the products is raised, the backwards reaction will increase its rate in order to get the concentrations back to normal.

Compromise temperature

• A compromise between a reasonable rate of reaction and a decent yield of product is required in industrial processes.

• The Haber Process is an exothermic reaction in the forward direction so using a low temperature would increase the yield of ammonia.

• However this would mean a low rate of reaction so a compromise temperature (450 °C) is used.