AQA A Level Chemistry复习笔记1.8.2 Le Chatelier's principle

Position of the equilibrium

• The position of the equilibrium refers to the relative amounts of products and reactants in an equilibrium mixture
• When the position of equilibrium shifts to the left, it means the concentration of reactants increases
• When the position of equilibrium shifts to the right, it means the concentration of products increases

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 concentration

• How the equilbrium shifts with concentration changes:

Answer 1:

• • The position of the equilibrium moves to the left and more ethanoic acid and ethanol are formed.
  • The reaction moves in this direction to oppose the effect of added ethyl ethanoate, so the ethyl ethanoate decreases in concentration.

   

Answer 2:

• • The position of the equilibrium moves to the left and more ethanoic acid and ethanol are formed.
  • The reaction moves in this direction to oppose the removal of ethanol so more ethanol (and ethanoic acid) are formed from ethyl ethanoate and water.

   

Answer 3:

• • There is no effect as the water dilutes all the ions equally so there is no change in the ratio of reactants to products.

   

Effects of pressure

• Changes in pressure only affect reactions where the reactants or products are gases
• How the equilibrium shifts with pressure changes:

Answer 1:

• • The equilibrium shifts to the left as there are fewer gas molecules on the left
  • This causes a decrease in pressure

   

Answer 2:

• • The equilibrium shifts to the left as there are no gas molecules on the left but there is CO2 on the right
  • This causes a decrease in pressure

   

Answer 3:

• • The equilibrium shifts to the right as there is a greater number of gas molecules on the right
  • This causes an increase in pressure

   

Effects of temperature

• How the equilbrium shifts with temperature changes:

Answer 1:

• • The reaction will absorb the excess energy and since the forward reaction is endothermic, the equilibrium will shift to the right.

   

Answer 2:

• • The reaction will absorb the excess energy and since this causes a shift of the equilibrium towards the right (as more CO2(g) is formed) this means that the reaction is endothermic.

   

Effects of catalysts

• A catalyst is a substance that increases the rate of a chemical reaction (they increase the rate of the forward and reverse reaction equally)
• Catalysts only cause a reaction to reach equilibrium faster
• Catalysts therefore have no effect on the position of the equilibrium once this is reached