Haber Process and Ammonia



Ammonia (NH3) is a compound of nitrogen and hydrogen.

It is a colourless gas with a choking smell, and a weak alkali which is very soluble in water.

Ammonia is used to make fertilisers, explosives, dyes, household cleaners and nylon. It is also the most important raw material in the manufacture of nitric acid.

Ammonia is manufactured by combining nitrogen and hydrogen in an important industrial process called the Haber process.

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The Haber process

The raw materials for this process are hydrogen and nitrogen. Hydrogen is obtained by reacting natural gas - methane - with steam, or through the cracking of oil. Nitrogen is obtained by burning hydrogen in air. Air is 80 per cent nitrogen; nearly all the rest is oxygen. When hydrogen is burned in air, the oxygen combines with the hydrogen, leaving nitrogen behind.

Nitrogen and hydrogen will react together under these conditions:

  • a high temperature - about 450ºC
  • a high pressure - about 200 atmospheres (200 times normal pressure)
  • an iron catalyst
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The reaction is reversible.

nitrogen + hydrogen Equilibrium symbol (http://www.bbc.co.uk/staticarchive/b65c7d8679f3cb0815e4693605278d4d44cd76d7.gif) ammonia

N2(g)       + 3H2(g) Equilibrium symbol (http://www.bbc.co.uk/staticarchive/b65c7d8679f3cb0815e4693605278d4d44cd76d7.gif) 2NH3(g)

Forward reaction is exothermic

Reverse reaction is endothermic

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LOWER TEMPERATURES gives more ammonia because the forward reaction is exothermic. This is because increasing the temperature increases the yield from the endothermic reaction and decreases the yield from the exothermic reaction.

Higher temperatures are actually used because it increases the rate of reaction even though the yield is lower

Temperatures are low enough for a reasonable yield but high enough for a fast reaction.

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HIGHER PRESSURE gives more ammonia because the forward reaction produces a smaller number of molecules. This is because an increase in pressure will favour the reaction that produces the least number of molecules.

High pressure means stronger and more expensive pipes and reaction vessels are needed

The pressure used is low enough not to need expensive reinforced apparatus but high enough for a reasonable yield

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The presence of an iron catalyst speeds up the rate of reaction by lowering the activation energy.

Overall, the Haber process gives about 30 percent yield

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