Dynamic equilibria

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• Edexcel GCSE Chemistry
• 5.3 Dynamic Equilibria
• Contents


  • 5.3.1 Reaching Equilibrium
  • 5.3.2 Fertilisers
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• Reaching Equilibrium


  Equilibrium occurs when during the course of a reversible reaction, the rate of the forward reaction equals the rate of the reverse reaction
• This means that products are being formed in the forward reaction as fast as reactants are being formed in the reverse reaction
• Factors that allow equilibrium to be reached faster

  • Higher pressure
  • Higher temperature
  • Higher concentration
  • Use of a catalyst
• Le Chatelier's Principle tells us that any change to a system at equilibrium results in the equilibrium responding by opposing the effect of that change
• Cooling an exothermic reaction results in the equilibrium shifting in the exothermic direction (to produce more heat and raise the temperature)
• Increasing pressure results in the equilibrium shifting to the side with the fewer gas molecules (to decrease the gas pressure)
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• Economic Considerations


  Companies that manufacture and sell chemical goods do so to make a profit
• The availability and cost of raw materials is a major consideration which must be studied well before any decisions are taken
• In the Haber Process the raw materials are readily available and inexpensive to purify: Nitrogen - from the air, Hydrogen- from natural gas
• If the cost of extraction of raw materials is too high or they are unavailable then the process is no longer economically viable
• Many industrial processes require huge amounts of heat and pressure which is very expensive to maintain
• Production energy costs are also a factor to be considered carefully and alongside the raw materials issue
• Temperature: 450ºC


  A higher temperature would favour the reverse reaction as it is endothermic (takes in heat) so a higher yield of reactants would be made
• Temperature: 450ºC


  If a lower temperature is used it favours the forward reaction as it is exothermic (releases heat) so a higher yield of products will be made
• However at a lower temperature the rate of reaction is very slow
• So 450ºC is a compromise temperature between having a lower yield of products but being made more quickly
• Pressure: 200 atm


  A lower pressure would favour the reverse reaction as the system will try to increase the pressure by creating more molecules (4 molecules of gaseous reactants) so a higher yield of reactants will be made
• Pressure: 200 atm


  A higher pressure would favour the forward reaction as it will try to decrease the pressure by creating fewer molecules (2 molecules of gaseous products) so a higher yield of products will be made
• However, high pressures can be dangerous and very expensive equipment is needed
• So 200 atm is a compromise pressure between a lower yield of products being made safely and economically
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• Catalyst


  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
• So a catalyst is used as it helps the reaction reach equilibrium quicker
• It allows for an acceptable yield to be achieved at a lower temperature by lowering the activation energy required
• Without it the process would have to be carried out at an even higher temperature, increasing costs and decreasing yield as the higher temperature decomposes more of the NH3 molecules
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• The reaction conditions chosen for the Haber process are not ideal in terms of the yield but do provide balance between product yield, reaction rate and production cost
• These are called compromise conditions as they are chosen to give a good compromise between the yield, rate and cost
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• Fertilisers


  Compounds containing nitrogen, potassium and phosphorus are used as fertilisers to increase crop yields