3.1.6 Chemical equilibria

• In a reversible reaction at equilibrium (dynamic equilibrium):

  • forward and backward reactions proceed at equal rates
  • the concentrations of reactants and products remain constant
• Dynamic equilibrium only occurs in closed systems.
• Le Chatelier's principle states that if a reaction at equilibrium is subjected to a change in pressure, temperature or concentration, the position of equilibrium will move to counteract the change.
• If we increase the concentration of a reactant or product, the equilibrium will shift towards the reaction that produces less of the reactant or product that was increased.
• If we increase the pressure, the equilibrium will shift towards the reaction that produces the fewest moles of gas to reduce the pressure.
• If we increase the temperature, the equilibrium will shift towards the endothermic reaction to reduce the temperature.
• Catalysts have no effect on the position of equilibrium. A catalyst will speed up the rate of the forward and backward reaction equally.
• A catalyst will speed up the rate at which the equilibrium is reached, but have no effect on yield.
• The value of equilibrium constant (Kc) is not affected by either changes in concentration, pressure or catalysts, but it is affected by temperature.
• If temperature change causes equilibrium to shift to the right, then Kc will increase.
• If temperature change causes equilibrium to shift to the left, Kc will decrease.