Equilibrium (1) Justifying Haber Process conditions

Edexcel A level Chemistry (2017)

Topic 10: Equilibrium I:

Here is the fourth learning objective:

10/I/3. Be able to evaluate data to explain the necessity, for many industrial processes, to reach a compromise between the yield and the rate of reaction

The key term here is compromise.

There are competing features in all chemical processes.

Here are a couple of key operating principles:

• Companies want to minimise energy costs and that means using the lowest possible pressure and temperature.

• But companies also want to maximise yield and that also means minimising the time taken to reach an equilibrium yield i.e. the rate of reaction. 

Looking at the Haber process data graph we can see that minimising energy costs is in conflict.

Reducing the operating pressure reduces the maximum yield, but reducing the operating temperature increases the maximum yield.

Similarly, operating the process at too low a temperature increases the time take to reach equilibrium and so reduces yield.

Adding a catalyst of iron to the reaction vessel can increase the rate of attainment of the equilibrium and for a relatively small additional cost produce a large gain in reaction rate.

Running the process under compromise conditions of temperature and pressure means using 200atm and 350^OC.

• 350^oC is hot enough to maintain a good reaction rate with the addition of a catalyst without reducing the yield too far.

• 200atm is less energy intensive than say 400atm or an even higher pressure and also probably safer with less risk of explosion and less wear and tear on the plant process equipment.