Chemical Equilibrium

• Dynamic equilibrium


  Forward and backward reactions are occurring at equal rates, and the concentrations of reactants and products stay constant
• Position of equilibrium


  The composition of the equilibrium mixture
• Le Chatelier's principle


  If an external condition is changed, the equilibrium will shift to oppose the change and try to reverse it
• If temperature is increased

  The equilibrium will shift in the endothermic direction to try to reduce the temperature
• If temperature is decreased


  The equilibrium will shift in the exothermic direction to try to increase the temperature
• Increasing pressure


  The equilibrium will shift towards the side with fewer moles of gas to oppose the change and reduce the pressure
• Decreasing pressure


  The equilibrium will shift towards the side with more moles of gas to oppose the change and increase the pressure
• If the number of moles of gas is the same on both sides


  Changing pressure will have no effect on the position of equilibrium
• Increasing the concentration of a reactant

  The equilibrium will shift to oppose this and move in the forward direction to remove the excess
• A catalyst has no effect on the position of equilibrium, but it will speed up the rate at which the equilibrium is achieved
• Importance of equilibrium to industrial processes

  • Allows a compromise between yield and rate to be found
  • High pressure leads to high energy costs for pumping and equipment
• Equilibrium constant Kc


  The ratio of the equilibrium concentrations of products to reactants, raised to their stoichiometric coefficients
• The unit of Kc changes and depends on the balanced equation
• Calculating Kc


  1. Work out the equilibrium moles and concentrations

  2. Put the concentrations into the Kc expression
• Equilibrium reactions


  • N2 + 3H2 ⇌ 2NH3
  • H2 + Cl2 ⇌ 2HCl
  • CH2=CH2 + H2O ⇌ CH3CH2OH
  • CO + 2H2 ⇌ CH3OH
• Recycling unreacted reactants can improve the overall yields of industrial processes
• HCl


  Hydrochloric acid
• Initial moles


  The moles of a substance at the start of the reaction
• Equilibrium moles


  The moles of a substance at equilibrium
• Equilibrium concentration (M) = Equilibrium moles / Volume (in dm3)
• Kc


  The equilibrium constant
• If the Kc has no unit then there are equal numbers of reactants and products. In this case you do not have to divide by volume to work out concentration. Equilibrium moles can be put straight into the Kc expression.
• Kc expression


  [HCl (g)]2 / [H2 (g)] [Cl2 (g)]
• The larger the Kc the greater the amount of products. If Kc is small we say the equilibrium favours the reactants.
• Kc only changes with temperature. It does not change if pressure or concentration are altered. A catalyst also has no effect on Kc.
• Effect of temperature on position of equilibrium and Kc

  • If temperature is increased, the reaction will shift to oppose the change and move in the backwards endothermic direction. The position of equilibrium shifts left. The value of Kc gets smaller as there are fewer products.
• Effect of pressure on position of equilibrium and Kc

  • The position of equilibrium will change if pressure is altered but the value of Kc stays constant as Kc only varies with temperature.
• Catalysts have no effect on the value of Kc or the position of equilibrium as they speed up both forward and backward rates by the same amount.
• Calculating the amounts of the equilibrium mixture from Kc

  Use algebra to work out the amounts of each substance at equilibrium using the value of Kc
• This example has been carefully set up to avoid the formation of a quadratic equation. It is not on the A-level syllabus to solve quadratic equations.
• Working out equilibrium constant Kc

  1. Part 1: Prepare the equilibrium mixture

  2. Part 2: Titrate the equilibrium mixture
• The pink colour of the phenolphthalein in the titration can fade after the end-point of the titration has been reached because the addition of sodium hydroxide may make the equilibrium shift towards the reactants.
• Working out initial amount of moles of reactants

  Calculate from densities and volumes of liquids added
• Working out equilibrium amount of moles of acid present from the titre results

  Use the titre results and the initial moles of acid catalyst
• Calculating the equilibrium constant

  Put the equilibrium concentrations into the Kc expression
• In order to confirm that one week was sufficient time for equilibrium to be established, several mixtures could be made and left for different amounts of time. If the resulting Kc is the same value then it can be concluded the time is sufficient.
• Working out equilibrium amount of moles of other substances


  Calculate the equilibrium amounts using the amount of reactant that reacted
• The amount of water at equilibrium would not really be 0 as there would be water present in the acid catalyst.