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Haber process

The Haber Process (aka Haber-Bosch process) is the reaction of nitrogen and hydrogen to produce ammonia.

The nitrogen and hydrogen are reacted over an iron catalyst under conditions of 200 atmospheres, 450°C:

N2(g) + 3H2(g) <--> 2NH3(g) + ΔH ...(1)

The process was developed by Fritz Haber and Carl Bosch in 1909 and patented in 1910. It was first used on an industrial scale by the Germans during World War I: Germany had previously imported nitrates from Chile, but the demand for munitions and the uncertainty of this supply in the war prompted the adoption of the process. The ammonia produced was oxidised for the production of nitric acid in the Ostwald process, and the nitric acid for the production of various explosive nitro compounds used in munitions.

The nitrogen is obtained from the air, and the hydrogen is obtained from water natural gas in the reaction:

CH4(g) + H2O(g) → CO(g) + 3H2(g) ...(2)

Equilibrium and the Haber Process

The reaction of nitrogen and hydrogen (1) is reversible, meaning the reaction can proceed in either the forward or the reverse direction depending on conditions. The forward reaction is exothermic, meaning it produces heat and is favored at low temperatures. Increasing the temperature tends to drive the reaction in the reverse direction, which is undesirable if the goal is to produce ammonia. However, reducing the temperature reduces the rate of the reaction, which is also undesirable. Therefore, an intermediate temperature high enough to allow the reaction to proceed at a reasonable rate, yet not so high as to drive the reaction in the reverse direction, is required.

The forward reaction favours high pressures because there are fewer molecules on the right side. So the only compromise in pressure is the economical situation trying to increase the pressure as much as possible.

The iron catalyst has no effect on the position of equilibrium, however it does increase the reaction rate. This allows the process to be operated at lower temperatures, which as mentioned before favors the forward reaction. Other catalysts are also active for this reaction, in fact the first Haber-Bosch reaction chambers, used osmium and uranium catalysts.

The ammonia is formed as a gas but on cooling in the condensor liquefies at the high pressures used, and so is removed as a liquid. Unreacted nitrogen and hydrogen is fed back in to the reaction.

Notwithstanding its original adoption as a military necessity, the Haber process now produces about half of all the nitrogen used in agriculture: billions of people are alive and fed from its use.



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