The Position of Equilibrium (Edexcel IGCSE Chemistry)

•  When a change is made to the conditions of a system at equilibrium, the system automatically moves to oppose the change
  • This is known Le Chatelier’s Principle, but you are not required to know the name for the exam

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

Effects of Temperature

 Example: Iodine Monochloride reacts reversibly with Chlorine to form Iodine Trichloride

ICl              +              Cl₂              ⇌             ICl₃

Dark Brown                                                   Yellow

When the equilibrium mixture is heated, it becomes dark brown in colour. How do we know whether the backward reaction is exothermic or endothermic?

• Equilibrium has shifted to the left as the colour dark brown means that more of ICI is produced
• Increasing temperature moves the equilibrium in the endothermic direction
• So the backward reaction is endothermic

 

Effects of Pressure

Example: Nitrogen Dioxide can form Dinitrogen Tetroxide, a colourless gas

2NO₂             ⇌              N₂O₄

Brown Gas                        Colourless Gas

What is the effect of an increase in pressure on the position of equilibrium?

• • Number of molecules of gas on the left =    2
  • Number of molecules of gas on the right =  1

• An increase in pressure will cause equilibrium to shift in the direction that produces the smaller number of molecules of gas
• So equilibrium shifts to the right

 

Effect of 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