Chemical equilibrium

• Dynamic equilibrium


  Occurs when the rate of forward reaction equals the rate of reverse reaction and the concentration of reactants and products remain constant in a closed system
• Factors that can alter the position of equilibrium

  • Concentration of reactants or products
  • Pressure
  • Temperature
• Le Chatelier’s principle


  If a system at equilibrium is disturbed, the equilibrium moves in the direction that tends to reduce the disturbance
• Increasing the temperature


  Equilibrium position shifts to the right
• The forward reaction is endothermic
• Yield of hydrogen increases when the equilibrium position shifts to the right
• Increasing the pressure


  Equilibrium position shifts to the left
• The forward reaction produces more moles of gas than the reverse reaction (4 moles of product, 2 moles of reactant)
• The yield of hydrogen decreases when the equilibrium position shifts to the left
• High pressure increases the collision frequency thus increasing the rate of reaction
• This is a compromise pressure between an economically viable rate of reaction and a slightly lower yield of hydrogen
• Catalyst effect on equilibrium


  No effect (because catalyst affects rate of forward and reverse reactions equally)
• Conditions that affect the value of Kc

  • Concentration
  • Catalyst
  • Pressure
  • Temperature
• For the reaction 2[A] + 3[B] + [C] ⇌ [D]+ 4[E], an expression for Kc can be deduced
• Units for the value of Kc are mol-1 dm-3
• Kc relevance


  Homogeneous systems in equilibrium
• Kc greater than 1


  Over to the right
• Kc less than 1


  Over to the left
• Decreasing the temperature of an endothermic reaction

  Kc decreases
• Increasing the temperature of an endothermic reaction

  Kc increases
• Decreasing the temperature of an exothermic reaction

  Kc increases
• Increasing the temperature of an exothermic reaction

  Kc decreases