Equilibrium + Industry

• effect of temperature in eqm

  • forward reaction is exothermic (gives out heat)
  • reverse reaction is endothermic (takes in heat)
• increasing temperature causing the position of eqm to move in the direction that decreases the temperature

  • the system opposes the change by taking in heat + position of eqm shifts
  • the position of equilibrium moves in the endothermic, +ΔH, direction
• decreasing temperature causing the position of eqm to move in the direction that increases the temperature

  • the system opposes the change by releasing heat + position of eqm shifts
  • the position of eqm moves in the exothermic, -ΔH, direction
• effect of a catalyst

  does not alter the position of eqm or composition of an eqm system

  • a catalyst speeds up the forward + reverse equally
  • catalyst increases rate at which eqm is established but does not affect the position of eqm
• EQM vs Yield

  equilibrium systems include :

  • the preparation of ammonia from nitrogen + hydrogen in the Haber Process
  • the conversion of sulphur dioxide into sulphur trioxide in Contact Process
• The Haber Process

  N2 (g) + 3 H2 (g) ⇌ 2 NH3 (g) ΔH = -92 kJ mol-1

  raw materials must be readily available :

  • N is obtained from air by fractional distillation
  • H is prepared by reacting together methane (from natural gas) + water
• Ammonia is produced by forward reaction in this eqm.

  The optimum conditions are high pressure + low temperatures due to :

  • forward reaction produces fewer gas molecules, so favoured by using high pressure
  • forward reaction is exothermic -ΔH, so favoured by using low temperature
• Drawbacks of Haber Process:

  • although low temperatures should produce high eqm yield, it would take place at a very low rate; comparatively few N2 + H2 molecules have enough energy to overcome required activation energy
  • high pressure increases concentration of gases, increasing rate of reaction; so high pressure should produce both a high eqm yield + high rate however large quantities of energy are required to compress gases, adding significantly to running costs

  -> safety implications -any failure in the systems could potentially allow chemicals to leak into environment endangering worker
• the modern ammonia plant

  needs to produce a sufficient yield of ammonia at a reasonable cost + as short a time as possible

  In practice, a compromise is made between yield + rate :

  • temperature - this must be high enough to allow reaction to proceed a realistic rate, whilst still producing an acceptable eqm yield ( temp at 400-500 degrees)
  • pressure - high pressure must be used, but it must not be so high that the workforce is put in danger or environment threatened (pressure of 200 atm)
• the modern ammonia plant 2 :

  • catalyst - an iron catalyst is added to speed up rate of reaction, allowing equilibrium to be established faster + lower temperatures to be used. Less energy is needed to generate heat, reducing costs

  actual compromise is used to convert only 15% of N + H into ammonia

  • ammonia produced is removed + liquefied
  • unreacted N + H gases are then passed b=through reactor again. Eventually, virtually all N + H will have been converted into ammonia