Choosing Reaction Conditions

Specification Point 5.21C:

• Explain how, in industrial reactions, including the Haber process, conditions used are related to:
  a) the availability and cost of raw materials and energy supplies
  b) the control of temperature, pressure and catalyst used produce an acceptable yield in an acceptable time

Energy & Raw Materials

• Like all industries, companies that manufacture and sell chemical goods do so to make a profit.
• Part of the industrial process is the decision on how and where to design and implement manufacturing sites.
• The availability and cost of raw materials is a major concern which must be studied well before any decisions are taken. 
• 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.
• For example, the cost of electricity may be very cheap in some countries, however the availability of raw materials may not be adequate or they may need to be transported, adding to costs.

Conditions of The Haber Process

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 less 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. 

Yield, Pressure & Temperature

• The graph below illustrates the effects of changing temperature and pressure on the yield.
• By following any of the curved lines on the graph it can be seen that as the pressure increases, so too does the yield at any given temperature.
• By following any vertical line upwards from the x-axis, the graph shows that as the temperature decreases, the yield actually increases.

The yield of ammonia produced changes with changes made to temperature and pressure

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