The Haber Process - choice of conditions
The Haber process is an important industrial process which needs to be understood for A-level .
These details and conditions need to be remembered.
You must also be able to USE the ideas on other unfamiliar equilibria.
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EFFECT ON THE POSITION OF EQUILIBRIUM
Temperature:
The forward direction is exothermic (-ve enthalpy change value).
Increasing the temperature will favour the endothermic (reverse) direction.
Increasing the temperature will move the equilibrium in favour of the reactants.
The concentration of the reactants will be increased.
Pressure:
The forward direction goes with a decrease in the number of moles of gas.
High pressure favours the direction with a decrease in moles of gas.
Increasing the pressure will favour the products because this side has the fewer number of gaseous molecules.
EFFECT ON RATE OF REACTION
Temperature:
High temperature increases the rate of a reaction.
Pressure:
High pressure increases the rate of a reaction.
COSTS:
Temperature:
Any temperature other than ambient (the surrounding) temperature requires some expense.
Energy costs can be particularly important for very high temperatures.
Pressure:
Any pressure other than ambient (the surrounding) pressure requires some expense.
The equipment is more expensive (thicker pipes, stronger tanks etc) and more energy is needed for high pressure work.
COMPROMISE:
Temperature:
A low temperature favours formation of the products, but will mean that reaction will happen slowly.
A compromise temperature is used (around 450 oC).
Pressure:
A high pressure would favour the formation of the products and would help to give a fast rate of reaction but too high a pressure would have excessively high energy costs and expensive, thicker walled pipes and reactor would be needed so a compromise pressure is used (around 2.0 x 104 kPa).
A finely divided iron catalyst is used to speed up the reaction, and the unconverted reactants are recirculated. Care is taken to ensure that the gas stream is clean (dust can damage the catalyst) and free of carbon monoxide (which can damage the catalyst).
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