CC12 Reversible Reactions And Equilibria

• Reversible reactions occur when the backwards reaction takes place relatively easily under certain conditions. A double arrow               is used to show that the forward and backward reactions occur at the same time.
• Dynamic equilibrium is when:

  rate of forward reaction = rate of backward reaction
• In dynamic equilibrium:

  • Appears to be no observable change
  • Although you can’t see anything happening, as fast as the reactants are becoming products, the products are reacting to become reactants.
  • At equilibrium, the concentrations of the reactants and products are constant, but not necessarily equal. 

  It has to be a closed system so that no products can be lost.
• The Equilibrium position gives you an idea of the relative proportion of the products and reactants at equilibrium.
• If the position of equilibrium lies to the left, there will be a more reactants than products. So, the yield of product will be low.
• If the position of equilibrium lies to the right, there will be a more products than reactants. So, the yield of product will be high – Industry conditions are set to favour the forward reaction so the product yield is high.
• Shifting the equilibrium position:

  • If the temperature is decreased then equilibrium will move in the direction that increases the temperature again.
  • If the concentration of reactants is increased then equilibrium will move in the direction that increases the concentration of the products.

  - If the pressure is increased then equilibrium will move in the direction that decreases the pressure again.
• Adding a catalyst reduces the time taken to reach equilibrium, but does not change the position of the equilibrium.

   
• All reactions are exothermic in one direction and endothermic in the other.
• If the concentration of reactant A or B is increased then equilibrium will move in the direction that decreases the concentration of the reactant A or B. So, the position of equilibrium will move to the right. The forward reaction is favoured because it will use up the extra reactant.
• If the concentration of reactant A or B is decreased then equilibrium will move in the direction that increases the concentration of the reactant A or B. So, the position of equilibrium will move to the left. The backward reaction is favoured because it makes more of the reactant.
• Ammonia is made in a reversible reaction by the Haber process:
• nitrogen + hydrogen = ammonia
• N2 (g) + 3H2 (g) = 2NH3 (g)
• The Haber process - conditions:

  • A high pressure of 200atm is used because it increases the rate of reaction and also increases the yield (amount) of ammonia.
  • A low temperature increases the yield of ammonia but is too slow!  A high temperature increases the rate of reaction but decreases the yield too much, so a compromise is used of 450°C.
  • An iron catalyst is then used to increase the rate of reaction.
• in the haber process:

  • Nitrogen is obtained from the air.

  Hydrogen is obtained from natural gas.