Ammonia The story behind the gas.


This web site aims to provide a full and comprehensive insight into ammonia production and its history. Covered in this site are the following:

-          A brief insight into the chemistry of ammonia.

-          So how did the Haber-Bosch process come about?

-          So what is the Haber-Bosch process?

-          Why is ammonia so important?

-          Other useful information.








A brief insight into the chemistry of ammonia:


Ammonia has a triangular pyramidal geometry, and boiling points of 77.7*C and 33.5*C. In its pure form ammonia was prepared in 1774 by Joseph Priestly, and its composition was determined in 1785 by Claude-Louis Berthollet. Ammonia has a chemical formula of NH3, and is sp3 hybridised. Ammonia is highly polarised, due to the electronegativity of nitrogen, and as a result, has a large dipole moment. Ammonias polarisation allows it to dissociate in water forming hydroxide and ammonium ions:


NH3 (aq) + H2O (l) NH4 (aq) + OH (aq)


Ammonia solutions are basic, due to the hydroxide ions formed in solution.


Ammonia is commercially produced by the Haber-Bosch process, which is also sometimes referred to as the Haber-Ammonia Process or Synthetic Ammonia Process. Fritz Haber, the German physical chemist, created the process in 1909, and it was further developed by Carl Bosch to make it economically viable. Both chemists won the Nobel prize for their work in this field; Haber in 1918 for its development, and Bosch in 1931 for creating high-pressure conditions which obtained a higher yield, economically.


Left: Fritz Haber,

12.9.1868 - 29.1.1934.


Right: Carl Bosch,

27.8.1874 26.4.1940.


Both images taken without consent from .





So how did the Haber-Bosch process come about?


At the end of the 19th century, Chilean nitrates, were the major source of nitrates at the time. It was clear, that this source would not be able to meet future demands. It was also realised that in the event of a war, any nation cut off from the Chilean supply, would not be able to make adequate amounts of munitions. Germany (Habers native country) was in particular dependent on this source of nitrogen compounds, to manufacture explosives. Following the allied block of the South American ports, this supply was well and truly cut off. An alternative method of producing nitrates was needed. Haber promptly got to work on the problem. In World War 1, had Haber not invented the process, Germany would have been forced to surrender years earlier than it did. As a result the Haber-Bosch process indirectly, cost thousands of people their lives.


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So what is the Haber-Bosch process?


The production of ammonia is achieved by the direct combination of hydrogen and nitrogen, over an iron or aluminium catalyst. Hydrogen is obtained from the decomposition of methane by heating. Nitrogen is obtained from the distillation of liquefied air. It was the first chemical process to use high-pressure conditions. The reaction is shown below:


N2(g) + 3H2(g) 2NH3(g)


The process is highly exothermic with a H value of 92.2 kJ per mole. As a result a compromise has to be made with regards to what temperature is used. A high temperature favours a higher rate of reaction and so equilibrium is reached more quickly; a high temperature however favours the backward reaction since the process is exothermic

(LeCheteliers principal states that any changes made to a reaction mixture will be compensated for by the reaction). Therefore, since the process is exothermic, increasing the temperature promotes the backward reaction since this takes in heat from the surroundings, hence lowering the temperature. As a result, a compromise is made between the two. This compromise temperature is 673* 923* Kelvin.



The catalyst provides an alternative pathway for the reaction to occur, which has lower activation energy. This means that a lower temperature can be used without compensating the rate too much. Carl Bosch found that a mixture of Fe2O3 and Fe3O4 catalyses the reaction best over the temperature range 650 950 Kelvin.




Also due to LeCheteliers principle a high pressure is used. The mol ratio of gases is 4:2, therefore if a high pressure is used the forward reaction is promoted, since at a higher pressure the products hold a smaller volume, hence decreasing the pressure. The pressure used is 200 400 atmospheres, which is quite high.


Below is a diagram of a modern ammonia production site:


Taken from without permission.





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What is ammonia used for?


Ammonia is a very useful product indeed. There are many, many uses for ammonia and its derivatives. It is easily liquefied by compression, and when heated back into its gas phase, absorbs much heat. As a result ammonia is used as a refrigerant. Another common use is as a fertilizer. Fertilizers improve the growth and productiveness of plants, by enhancing the fertility of soil, by replenishing the soils nutrient supply. Most nitrogen fertilizers contain ammonia as either a water solution or as ammonium salts. Ammonia is also used in the manufacturing of nylon, the dyeing of cotton, wool and silk. Ammonia is also used in the Solvay process of producing soda ash, and the Ostwald process of converting ammonia into nitric acid. Ammonia is also found in household detergents and cleaners, and is used in pH control.


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