3.2.3 - equilibrium

• Dynamic equilibrium occurs when the rate of the forward reaction equals the rate of the reverse reaction
• Dynamic equilibrium occurs when the concentration of reactants and products remain constant
• Dynamic equilibrium occurs in a closed system
• changing three factors affect the position of equilibrium

  • Concentration of reactants or products
  • pressure
  • temperature
• Le chateliers principle says that if a system at dynamic equilibrium is subject to change, the position of equilibrium will shift to minimise the change.
• In an equation with positive delta H in forwards reaction

  increasing the temperature would shift the equilibrium to the right because the forward reaction is endothermic
• CH4 + H2O ——> CO + 3H2

  increasing the pressure would shift the equilibrium to the left because the higher pressure shifts to the side with fewer moles and the reactants only have 2 moles
• CH4 + H2O ——> CO + 3H2

  a high pressure is useed in industry as this 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
• Temperature affects the value of Kc
• For the reaction 2A + 3B + C <==> D + 4E

  the expression for Kc is:

  Kc = [D][E]^4 // [A]^2 [B]^3 [C] the units are mol-1 dm-3
• Kc is relevant for homogenous systems in equilibrium
• When Kc is approximately one = equilibrium lies in the middle

  ratio of concentration of reactants to products is 1:1
• When Kc is greater than 1

  equilibrium lies to the right

  favours the forwards reaction
• When Kc is less than 1

  equilibrium lies to the left

  favours the reverse reaction
• Decreasing the temperature of an endothermic reaction means Kc decreases
• Increasing the temperature of an endothermic reaction means Kc increases
• Decreasing the temperature of an exothermic reaction means Kc increases
• Increasing the temperature of an exothermic reaction means Kc decreases
• The general expression for Kc

  Kc = products / reactants
• Catalysts have no effect on the position of equilibrium but speed up the rate at which equilibrium is established.