AQA Chemistry Unit 2: 9 Equilibria


9.1 The Idea of Equilibrium

  • Equilibrium mixture: The mixture formed when a reversible reaction proceeds in a closed container until no further change occurs. The backward and forward reactions are proceeding at the same rate

Setting up an Equilibrium

  • Dynamic Equilibrium: Forward and Backward reactions are occurring at the same rate so the mixture composition doesn't change


  • Can only be reached in a closed system
  • The final equilibrium position will be the same
  • It is a dynamic process where the forwards and backwards reactions occur at the same time
  • When the density, concentration, colour and pressure are the same equilibrium has been reached
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The Idea of Equilibrium 2

Chemical Equilibria

  • Equalibrium can be formed of any proportions of reactant and products
  • Proportions can change due to conditions like temperature, pressure and concentration
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9.2 Changing Conditions

The Equilibrium Mixture

  • Changing the proportions of reactants to products is a way of being able to obtain a greater yield of product
  • This is changing the position of equilibrium
  • If the proportion of product is increased the equilibrium position shifts to the right
  • If the proportion of reactants is increaased the equlibrium position shifts to the left

Le Chatelier's Principle

  • If a system at equilibrium is disturbed, the equilibrium position moves to oppose it

Changing Concentrations

  • If you increase the concentration of a reactant then the equilibrium position shifts to reduce this concentration
  • The equilibrium position moves to the right increasing the yield of product
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Changing Conditions 2

Temperature Increase

  • Equilibrium position shifts to decrease the temperature
  • The position will move in the endothermic direction
  • It will shift to the right

Increase in Pressure

  • The position shifts to decrease pressure
  • Equilibrium position shifts from the higher moles to the lower moles side of the reaction
  • For: CO + 2H2 -> CH3OH
  • The right side has less pressure so it shifts to the right


  • No effect on position of equilibrium
  • Work by producing alternative route for reaction with lower activation energy
  • They allow equilibrium to be reached quicker
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9.3 Equilibrium Reactions in Industry: Ammonia

  • Raw Materials:
    • Nitrogen (from air), coal (to condense to liquid then for fractional distillation), water and natural gas
  • Hydrogen: Achieved by H20 + CH4 -> CO + 3H2
  • Conditions:
    •  20,000kPa and 670K (15% conversion to ammonia so gases are recycled back into the process)
    • Iron Catalyst (allows continuous flow of gases but becomes poisoned after 5 years)
  • Reasons:
    • High temperature would give lower yield
    • Low temperature would have slow rate of reaction
    • Higher pressure is more dangerous and uses expensive equipment
    • Lower pressure gives low yield
  • Uses:
    • 80% is used in fertiliser, rest makes nylon, explosives, drugs and dyes
  • Equation:
    • N2(g) + 3H2(g) ->2NH3(g), -92Kjmol-
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Making Ethanol

  • Raw Materials:
    • Ethene from crude oil obtained by fractional distillation and cracking
    • Reactants are all gaseous
  • Conditions:
    • Made by hydration of ethene using catalyst: phosphoric acid absorbed on silica
    • 570K and 6500kPa (give conversion of 5%, unreacted ethene is reused)
  • Reasons:
    • Lower temperature reduces rate of reaction
    • High pressure causes ethene to polymerise to polyethene and increases the costs of the building
    • Too much steam dilutes the catalyst
  • Uses:
    • Cosmetics, drugs, detergents, inks, motor fuel and alcohol
  • Equations:
    • Fermentation: C6H12O6(aq) -> 2C2H5OH(aq) + 2CO2(g)
    • Ethene: H2C=CH2(g) + H2O(g) -> CH3CH2OH(g)
    • -46Kjmol-
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Making Methanol

  • Raw Materials:
    • Excess hydrogen used to make more methanol
    • Carbon monoxide 
  • Conditions:
    • 500K and 10000kPa (produces around 5-10% yield)
  • Reasons:
    • Higher temperature used for faster rate of reaction
    • Lower pressure used to reduce building costs
  • Uses:
    • Chemical feedstock (starting material for making other chemicals)
    • Manufacture of methanal which is used for plastics like Bakelite
    • Forms terylele and perspex or as a motor fuel
  • Equation:
    • CO(g) + 3H2(g) -> CH3OH(g)
    • -91Kjmol-1
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