Edexcel IGCSE Chemistry 复习笔记 3.3.3 The Position of Equilibrium

•  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

ExampleIodine 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

ExampleNitrogen 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

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