Reaching Equilibrium (Edexcel GCSE Chemistry)

Predicting the Ideal Conditions

• We have previously seen that ammonia is manufactured using the Haber Process
• The hydrogen and nitrogen react to form ammonia in the following reversible reaction:

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

• The formation of ammonia is exothermic, so using Le Chatelier's Principle we would predict that
  • The reaction will produce a higher yield at low temperatures
  • Using a high pressure would increase the yield as there are fewer moles of gas on the right than the left of the equation

Reaching Equilibrium

• Equilibrium occurs when during the course of a reversible reaction, the rate of the forward reaction equals the rate of the reverse reaction
• This means that products are being formed in the forward reaction as fast as reactants are being formed in the reverse reaction
• It is reached at a faster rate when:
  • A higher pressure is used as there are more successful collisions
  • A higher temperature is used as the particles have greater kinetic energy
  • A higher concentration is used as there are more particles per given volume, hence there are more collisions
  • A catalyst is used as it speeds up the rate of reaction, allowing it to reach equilibrium faster

Economic Considerations

• Like all industries, companies that manufacture and sell chemical goods do so to make a profit
• Part of the industrial process is the economic decision on how and where to design and implement a manufacturing site
• The availability and cost of raw materials is a major consideration which must be studied well before any decisions are taken
• In the Haber Process the raw materials are readily available and inexpensive to purify:
  • Nitrogen - from the air
  • Hydrogen- from natural gas

• If the cost of extraction of raw materials is too high or they are unavailable then the process is no longer economically viable
• Many industrial processes require huge amounts of heat and pressure which is very expensive to maintain
• Production energy costs are also a factor to be considered carefully and alongside the raw materials issue

Temperature: 450ºC

• A higher temperature would favour the reverse reaction as it is endothermic (takes in heat) so a higher yield of reactants would be made
• If a lower temperature is used it favours the forward reaction as it is exothermic (releases heat) so a higher yield of products will be made
• However at a lower temperature the rate of reaction is very slow
• So 450ºC is a compromise temperature between having a lower yield of products but being made more quickly

Pressure: 200 atm

• A lower pressure would favour the reverse reaction as the system will try to increase the pressure by creating more molecules (4 molecules of gaseous reactants) so a higher yield of reactants will be made
• A higher pressure would favour the forward reaction as it will try to decrease the pressure by creating fewer molecules (2 molecules of gaseous products) so a higher yield of products will be made
• However, high pressures can be dangerous and very expensive equipment is needed
• So 200 atm is a compromise pressure between a lower yield of products being made safely and economically

Choosing the conditions for the Haber Process

Catalyst

• The presence of a catalyst does not affect the position of equilibrium but it does increase the rate at which equilibrium is reached
• This is because the catalyst increases the rate of both the forward and backward reactions by the same amount (by providing an alternative pathway requiring lower activation energy)
• As a result, the concentration of reactants and products is nevertheless the same at equilibrium as it would be without the catalyst.

  So a catalyst is used as it helps the reaction reach equilibrium quicker

• It allows for an acceptable yield to be achieved at a lower temperature by lowering the activation energy required
• Without it the process would have to be carried out at an even higher temperature, increasing costs and decreasing yield as the higher temperature decomposes more of the NH₃ molecules

Diagram showing the effect of catalyst on equilibrium position