The Position of Equilibrium (Edexcel GCSE Chemistry: Combined Science)

• 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 Chaterlier'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 or concentration
• Knowing the energy changes, states and concentrations involved allows us to use the principle to manipulate the outcome of reversible reactions

Changes in 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
• The following table summarises how a temperature change alters the position of equilibrium:

Effect of Temperature Changes on an Equilibrium Table

Changes in Pressure

• Changes in pressure only affects gases so firstly you have to identify all gaseous reactants and products
• The following table summarises how a pressure change alters the position of equilibrium:

Effect of Pressure Changes on an Equilibrium Table

Changes in Concentration

• The following table summarises how a concentration change alters the position of equilibrium:

Effect of Concentration Changes on an Equilibrium Table