equilibria 1.6

• Two features of dynamic equilibrium

  1. Forward and backward reactions are occurring at equal rates.

  2. The concentrations of reactants and products stay constant
• dynamic equilibrium means both forward and backward reactions are occurring simultaneously in a closed system at constant temperature and pressure
• Le Chatelier’s principle states that if an external condition is changed the equilibrium will shift to oppose the change (and try to reverse it).
• What effect would increasing temperature have on the yield of ammonia?

  N2 + 3H2 <---->2NH3 = -ve exo

  • If temperature is increased the equilibrium will shift to oppose this and move in the endothermic, backwards direction to try to decrease temperature. The position of equilibrium will shift towards the left, giving a lower yield of ammonia.
• Low temperatures may give a higher yield of product but will also result in slow rates of reaction. Often a compromise temperature is used that gives a reasonable yield and rate.
• Increasing pressure will cause the equilibrium to shift towards the side with fewer moles of gas to oppose the change and thereby reduce the pressure
• What effect would increasing pressure have on the yield of methanol?

  CO (g) + 2H2(g) ⇌ CH3OH (g)

  3 moles 1 mole

  • If pressure is increased the equilibrium will shift to oppose this and move towards the side with fewer moles of gas to try to reduce the pressure . The position of equilibrium will shift towards the right because there are 3 moles of gas on the left but only 1 mole of gas on the right, giving a higher yield of methanol.
• Increasing pressure may give a higher yield of product and will produce a faster rate. Industrially high pressures are expensive to produce ( high electrical energy costs) and the equipment is expensive.

  • so compromise pressure between cost and yield
• A catalyst has no effect on the position of equilibrium, but it will speed up the rate at which the equilibrium is achieved; it speeds up the rates of the forward and backward reactions by the same amount.
• Equilibrium constant Kc

  A) c

  B) D

  C) A

  D) B

  
• 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
• Both the position of equilibrium and the value of Kc will change if temperature is altered
• The position of equilibrium will change if pressure is altered but the value of Kc stays constant as Kc only varies with temperature