7.1 Human influences and industry  (Page 2/3)

The industrial fixation of nitrogen

A number of industrial processes are able to fix nitrogen into different compounds and then convert these compounds into fertilisers. In the descriptions below, you will see how atmospheric nitrogen is fixed to produce ammonia, how ammonia is then reacted with oxygen to form nitric acid and how nitric acid and ammonia are then used to produce the fertiliser, ammonium nitrate.

• Preparation of ammonia (NH ₃ ) The industrial preparation of ammonia is known as the Haber-Bosch process . At a high pressure and a temperature of approximately 500 ${}^0$ C, and in the presence of a suitable catalyst (usually iron), nitrogen and hydrogen react according to the following equation: N ₂ + 3H ₂ $\to$ 2NH ₃ Ammonia is used in the preparation of artficial fertilisers such as (NH ₄ ) ₂ SO ₄ and is also used in cleaning agents and cooling installations.

  Interesting fact

  Fritz Haber and Carl Bosch were the two men responsible for developing the Haber-Bosch process. In 1918, Haber was awarded the Nobel Prize in Chemistry for his work. The Haber-Bosch process was a milestone in industrial chemistry because it meant that nitrogenous fertilisers were cheaper and much more easily available. At the time, this was very important in providing food for the growing human population. Haber also played a major role in the development of chemical warfare in World War I. Part of this work included the development of gas masks with absorbent filters. He also led the teams that developed chlorine gas and other deadly gases for use in trench warfare. His wife, Clara Immerwahr, also a chemist, opposed his work on poison gas and committed suicide with his service weapon in their garden. During the 1920s, scientists working at his institute also developed the cyanide gas formulation Zyklon B, which was used as an insecticide and also later, after he left the programme, in the Nazi extermination camps.Haber was Jewish by birth, but converted from Judaism in order to be more accepted in Germany. Despite this, he was forced to leave the country in 1933 because he was Jewish 'by definition' (his mother was Jewish). He died in 1934 at the age of 65. Many members of his extended family died in the Nazi concentration camps, possibly gassed by Zyklon B.
• Preparation of nitric acid (HNO ₃ ) Nitric acid is used to prepare fertilisers and explosives. The industrial preparation of nitric acid is known as the Ostwald process . The Ostwald process involves the conversion of ammonia into nitric acid in various stages: Firstly, ammonia is heated with oxygen in the presence of a platinum catalyst to form nitric oxide and water.
  4NH ₃ (g) + 5O ₂ (g) $\to$ 4NO(g) + 6H ₂ O(g)
  Secondly, nitric oxide reacts with oxygen to form nitrogen dioxide. This gas is then readily absorbed by the water to produce nitric acid. A portion of nitrogen dioxide is reduced back to nitric oxide.
  2NO(g) + O ₂ (g) $\to$ 2NO ₂ (g) 3NO ₂ (g) + H ₂ O(l) $\to$ 2HNO ₃ (aq) + NO(g)
  The NO is recycled and the acid is concentrated to the required strength by a process called distillation .
• Preparation of ammonium nitrate Ammonium nitrate is used as a fertiliser, as an explosive and also in the preparation of 'laughing gas' which is used as an anaesthetic. Ammonium nitrate is prepared by reacting ammonia with nitric acid:
  NH ₃ + HNO ₃ $\to$ NH ₄ NO ₃