A-Level Chemistry OCR Notes
3.2.3 Chemical equilibrium
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Chemical Equilibrium
- Reversible reactions at equilibrium are denoted by the equilibrium symbol: ⇌
- Many chemical reactions are reversible
- A chemical system is in dynamic equilibrium when all three of the following conditions are met:
- The concentration of reactants and products is constant
- The rate of the forwards reaction is the same as the rate of the backwards reaction
- The reaction is in a closed system
- Le Chatelier’s principle- when a system in dynamic equilibrium is subjected to change, the position of the equilibrium will shift to oppose the change
- If the conditions of a reaction are changed, the position of equilibrium will shift to favour either the forwards or backwards reaction to oppose the change.
- The effect of a change in temperature on the position of equilibrium will depend on the enthalpy change of the reaction. If the forward reaction is exothermic and the backwards reaction is endothermic:
- Increasing the temperature will cause the position of equilibrium to shift in the endothermic direction to absorb the added heat
- Decreasing the temperature will cause the position of equilibrium to shift in the exothermic direction to add more heat
- Changing the concentration of a reactant or product will cause the position of equilibrium to shift to oppose this change
- Increasing the concentration of the reactants will cause the position of equilibrium to shift to make more of the product
- Increasing the concentration of the products will cause the position of equilibrium to shift to remove the extra product, making more reactants
- Changing the pressure of a system will only change the position of the equilibrium if the reaction involves gases.
- Increasing the pressure of the system will cause the position of equilibrium to shift to the side with the fewest moles of gas
- Decreasing the pressure of the system will cause the position of equilibrium to shift to the side with more moles of gas
- Adding a catalyst does not change the position of equilibrium, but increases the rate at which the equilibrium is established
Equilibrium Constant
- The equilibrium constant, Kc , indicates where the equilibrium lies – it is the ratio of the concentration of products and reactants in a reversible reaction
- The concentration, in mol dm ^-3 , of a species X involved in the expression for Kc is represented by [X]
- For the general reaction: aA + bB ⇌ dD + eE
- The units of the equilibrium constant will vary depending on how many species take part in the reaction and the stoichiometry
- The magnitude of Kc indicates the extent of the reaction:
- Kc = 1: equilibrium lies halfway between the reactants and the products
- Kc > 1: equilibrium lies further to the right and the products are favoured
- Kc < 1: equilibrium lies further to the left and the reactants are favoured
Haber Process
- Nitrogen gas reacts with hydrogen gas to form ammonia in the Haber process
- The optimum conditions for the Haber process are:
- High pressure- increasing the pressure will cause the position of equilibrium to shift to the right, favouring the forwards reaction as there are fewer moles on the right (two moles) than on the left (four moles). Therefore, more ammonia is produced, and the pressure will be reduced, opposing the change
- Low temperature- the forward reaction is exothermic, so decreasing the temperature will cause the position of equilibrium to shift to the right, favouring the forwards reaction. Therefore, more ammonia is produced and the temperature increases, opposing the change
- These optimum conditions are not favourable because:
- Maintaining high pressures is expensive and unsafe
- Low temperature would mean a slow rate of reaction
- In industry, conditions must be used that strike a balance between obtaining a good yield and being economically feasible
- Compromise conditions must be used in industrial processes to ensure a good yield whilst considering the following factors:
- The rate of reaction
- Cost and risks of equipment
- Side reactions
- The industrial conditions used today in the Haber process are a compromise and are:
- 400-500°C- allows a reasonable rate of reaction and yield
- 200 atm- allows a high yield without costing too much or posing a safety risk
- Iron catalyst increases the rate of reaction and allows equilibrium to be established more quickly at a lower temperature, saving energy and increasing profits
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