Edexcel A Level Chemistry:复习笔记1.10.2 Le Chatelier's Principle

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 equilibrium shifts with concentration changes

7.2-Effects-of-concentration-table

Worked Example

Changes in equilibrium position

Using the reaction below:

CH3COOH (I) + C2H5OH (I)  ⇌  CH3COOC2H5 (I) + H2O (I)

Explain what happens to the position of equilibrium when:

   1. More CH3COOC2H5 is added

   2. Some C2H5OH is removed

Using the reaction below:

Ce4+ (aq) + Fe2+ (aq) ⇌ Ce3+ (aq) + Fe3+ (aq)

Explain what happens to the position of equilibrium when

   3. Water is added to the equilibrium mixture

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

7.2-Effects-of-pressure-table

Worked Example

Changes in pressure

Predict the effect of increasing the pressure on the following reactions:

   1. N2O4 (g) ⇌ 2NO2 (g)

   2. CaCO3 (s) ⇌ CaO (s) + CO2 (g)

Predict the effect of decreasing the pressure on the following reaction:

   3. 2NO2 (g) ⇌ 2NO (g) + O2 (g)

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 equilibrium shifts with temperature changes

7.2-Effects-of-temperature-table

Worked Example

Changes in temperature

Using the reaction below:

H2 (g) + CO2 (g) ⇌  H2O (g) + CO (g)    ΔH = +41.2 kJ mol-1

   1. Predict the effect of increasing the temperature on this reaction

Using the reaction below:

Ag2CO3 (s)  ⇌  Ag2O (s) + CO2 (g)

   2. Increasing the temperature increases the amount of CO2(g) at constant pressure. Is this reaction exothermic or    endothermic?

Explain your answer

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

 

 

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