IB DP Chemistry: HL复习笔记7.1.5 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 at dynamic equilibrium, the position of the equilibrium moves to minimise 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

Effects of Concentration Table

Answer:

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

Effects of Pressure Table

Answer:

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

Effects of Temperature Table

Answer:

Answer 1:

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

Answer 2:

• • The reaction will absorb the excess heat 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 (because endothermic reactions favour the products).

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 its equilibrium faster
• Catalysts therefore have no effect on the position of the equilibrium once this is reached