5.3.2 Le Chatelier's Principle

• 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 Chatelier'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 and forms the basis for much of the work of chemical engineers to increase the percentage yield
• To increase the percentage yield, the position of the equilibrium needs to move towards the right side, which is the product side, and therefore make a higher concentration of product at the point of dynamic equilibrium
  • If the equilibrium moves to the left, the reactant side, then the concentration of reactant will be higher at the equilibrium point and the percentage yield will be lower, as there is less product
  • If a chemical reaction is being carried out then the point is to make product and not be left with unreacted reactant, so the principle is key to keeping the costs of chemical products low and is often discussed as 'chemical economics'

Changing Concentration

• Le Chatelier's Principle can be used to predict the effect of changes in concentration on systems in equilibrium
• The following table summarises how a concentration change alters the position of equilibrium:

Table to show the Effect of Changes in Concentration to the Equilibrium Position

Answer

• • An increase in the concentration of ICl or Cl₂ causes the equilibrium to shift to the right (product), so more of the yellow solid, ICl₃, is formed. The reaction would be increasingly yellow!
    • This is because when the concentration of a reactant increases, the equilibrium moves to oppose the change and create more product from the excess reactant
  • A decrease in the concentration of ICl or Cl₂ causes the equilibrium to shift to the left (reactant), so less of the yellow solid, ICl₃, is formed. The reaction would be increasingly brown!
    • This is because when the concentration of a reactant decreases, the equilibrium moves to oppose the change and create more reactant from the now in excess product

Changing 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:

Table to show the Effect of Changes to Temperature on the Equilibrium Position

Answer

• • When a system is heated, the system opposes the change by doing more of the endothermic reaction
  • Since the forward reaction is exothermic, the backward reaction is endothermic
  • Therefore the equilibrium will move to the left and produce more of the reactants
  • The colour of the mixture will become increasingly brown as more ICl is produced

Changing Pressure

• Le Chatelier's Principle can be used to predict the effect of changes in pressure on systems in equilibrium
• Changes in pressure only affects only gases, so firstly you have to identify all gaseous reactants and products
• If there are the same number of moles of gases on either side of the equation, then there is NO effect on the position of equilibrium when the pressure is changed
  
  • Increasing the pressure will increase the rate of the forward reaction and backward reaction equally which is why the position of equilibrium is unchanged
• The following table summarises how a pressure change alters the position of equilibrium:

Table to show the Effect of Changes to Pressure on the Equilibrium Position

Answer

• • Number of gas molecules on the left side = 2
  • Number of gas molecules on the right side = 1
  • An increase in the pressure will cause the equilibrium to shift in the direction that produces the smaller number of molecules of gas so the equilibrium shifts to the right
  • This means that the mixture will appear increasingly colourless as the concentration of N₂O₄ increases