Reversible reactions and equilibria

• Reversible reaction


  Reaction where the product molecules can react with each other or decompose to form the reactant molecules again
• Representing a reversible reaction

  1. Use two opposing arrows to indicate the forward and reverse reactions occurring at the same time

  2. Each arrow has half an arrowhead - the top one points right, the bottom one points left
• Dynamic equilibrium

  When the rate of the forward reaction equals the rate of the reverse reaction, the overall reaction is in a state of equilibrium
• Equilibrium can only be reached in a closed system
• The Haber Process


  1. Stage 1: H2 and N2 obtained from natural gas and air

  2. Stage 2: Gases compressed to 200 atm

  3. Stage 3: Pressurised gases pumped into tank with iron catalyst at 450°C, N2 + 3H2 ⇌ 2NH3

  4. Stage 4: Unreacted H2 and N2, and product NH3 pass into cooling tank, NH3 liquefied and stored

  5. Stage 5: Unreacted H2 and N2 recycled back into the system
• Conditions for the Haber Process

  • Temperature: 450°C (compromise between rate and yield)
  • Pressure: 200 atm (compromise between yield and cost)
  • Catalyst: Iron to speed up reaction
• Le Chatelier's Principle


  When a change is made to the conditions of a system at equilibrium, the system automatically moves to oppose the change
• Effects of changes on equilibrium

  • Temperature increase: Favours endothermic reaction
  • Temperature decrease: Favours exothermic reaction
  • Pressure increase: Favours reaction with fewer gas molecules
  • Pressure decrease: Favours reaction with more gas molecules
  • Concentration increase: Favours reaction that consumes that substance
  • Concentration decrease: Favours reaction that produces that substance