Chemical equilibria and Le Chatelier's principle

• Le Chatelier’s principle can be used to predict the effects of changes in temperature, pressure and concentration on the yield of a reversible reaction. This has important consequences for many industrial processes
• Le Chatelier's principle can be used to predict the effects of changes in temperature, pressure and concentration on the position of equilibrium in homogeneous reactions.
• Many chemical reactions are reversible. Reversible reaction are where reactions can go forward and backwards.
• What is Le Chatelier's principle?

  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.
• In a reversible reaction at equilibrium:

  • Forward and reverse reactions proceed at equal rates.
  • The concentrations of reactants and products remain constant.
• Factors which may affect the position of equilibrium are changes in temperature, pressure or concentration of a particular reactant or product.
• Le Chatelier's principle predictions of the effects of changes on the position of equilibrium in homogeneous reactions: Pressure

  • An increase in pressure at constant temperature shifts the position of equilibrium to the side with the smaller gas volume, the right side. Which would result in an increase in the concentration of the products.
  • A decrease in pressure, at constant temperature, shifts the position of the equilibrium to the side with the larger gas volume, the left side.
• Le Chatelier's principle predictions of the effects of changes on the position of equilibrium in homogeneous reactions: Temperature

  The position of the equilibrium can be altered by changing the temperature but this depends on whether the reaction is exothermic (forward reaction) or endothermic (reverse reaction).
• Le Chatelier's principle predictions of the effects of changes on the position of equilibrium in homogeneous reactions: Temperature

  • Increase in temperature where the forward reaction is exothermic would shift the position of the equilibrium to the left side.
  • Decrease in temperature where the forward reaction is exothermic would shift the position of the equilibrium to the right side.
  • Increase in temperature where the forward reaction is endothermic would shift the position of the equilibrium to the left side.
  • Decrease in temperature where the forward reaction is endothermic would shift the position of the equilibrium to the right side.
• Le Chatelier's principle predictions of the effects of changes on the position of equilibrium in homogeneous reactions: Concentration

  If more of a reactant or product is added to or removed from the equilibrium system at constant temperature and pressure, the equilibrium will adjust to replace any substance that has been changed.
• Le Chatelier's principle predictions of the effects of changes on the position of equilibrium in homogeneous reactions: Concentration

  • If you increase (add more) the concentration of the reactants, the position of the equilibrium will shift to the right side.
  • If you decrease (remove) the concentration of the products, the position of the equilibrium will shift to the right side.

  For example: $N_2(g)+$$3H_2(g)\leftrightarrow2NH_3(g)$

  Increase $N_2$ = Equilbirum shifts right = More $N_2$ molecules react which will increase the yield of ammonia.

  Decrease $2NH_3$= Equilibrium shifts right to form more $2NH_3$. This will increase the yield of $2NH_3$and lower the concentration of $N_2$ and $3H_2$.
• A catalyst has no effect on the position of equilibrium but allows the reaction to get to equilibrium faster. A catalyst increases the rate of the forward and reverse reactions equally.
• Concentrated phosphoric (or sulfuric) acid is used as a catalyst in the industrial production of ethanol as it ensures that equilibrium is attained more quickly so making the product more rapidly.
• A high pressure between 5 MPa and 10 MPa is used in the industrial production of ethanol as it increases the yield of ethanol, this range is a compromise as high pressure is expensive to apply due to increased electrical pumping costs and this requires expensive strong-walled vessels/ expensive valves, equipment to withstand the pressure.
• The temperature used is between $300^{\circ}C$ and $600^{\circ}C$ which is a comprimise tempreture between rate and yield. It increases the rate of reaction at the expense of a loss in some yield at equilibrium.