GCSE OCR Gateway Chemistry C6.1d-e Higher tier Haber Process

GCSE OCR Gateway Chemistry C6.1d-e Higher tier 

C6.1d To be able to explain the trade-off between rate of production of a desired product and position of equilibrium in some industrially important processes such as the Haber process

C6.1e To be able to interpret graphs of reaction conditions versus rate

The Haber process

Fritz Haber is the person responsible for inventing a process to fix nitrogen (N₂) from the air as ammonia (NH₃). 

When in WWI Britain’s naval blockage of German seaports prevented the import of fertiliser resources from Chile, Germany could have had problems with both food production and the production of armaments since ammonia was used to make them both. 

Fritz Haber was one of a team of people who solved the problem of ammonia shortage because he found a way to convert very unreactive triple bonded nitrogen molecules (abundant in the air) into the weak base and reactive compound ammonia. 

The conditions usually used today for the process are:

N₂   +    3H₂     ⇌     2NH₃

Pressure 200 atm

Temperature 400^oC or 673K

Iron catalyst

We can see why these conditions are used if we look at the effect of change of temperature and pressure on the yield of ammonia.

 

You can see that at 200 atm and 673 K the yield of ammonia is 40% so 60% of the reactants nitrogen and hydrogen will be recycled. 

Pressure conditions

200 atm is a compromise that gives on the one hand a fast enough rate of reaction but also it is not too high so that there is a less of a risk of the industrial plant breaking or exploding i.e. it is safer to operate at 200 atm than 800 atm.

Temperature conditions

400^oC is a compromise temperature since at this temperature the rate is fast enough to quickly establish the ammonia equilibrium and gives a reasonable yield of 40% if the rest of the reactant gases are recycled.

Operating at a higher temperature might give a faster reaction and faster attainment of equilibrium but it would produce a lower yield whereas at a lower temperature the yield would be higher but the catalyst would not be as effective at speeding up the attainment of the equilibrium.  400^oC is a better temperature for the functioning of the iron catalyst.

See my other blog here for further details of the production of ammonia

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You need to be aware that Haber was a controversial figure in WW1 and afterwards. 

Haber was responsible for the use of biological and chemical weapons and started the gassing of enemy soldiers on the battlefield when he used chlorine on French and British troops.  

It is said that his wife was so horrified at what he was responsible for that she took Haber's service revolver and committed suicide with it.