WJEC Chemistry 3 - Reversible reactions, industrial processes and important chemicals

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Ammonia and the haber process

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.

 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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Equation for the haber process and Temperature

The reaction is reversible.

nitrogen + hydrogen Equilibrium symbol (http://www.bbc.co.uk/schools/gcsebitesize/science/images/gcsechem_equilibrium.gif) ammonia

N2(g)       + 3H2(g) Equilibrium symbol (http://www.bbc.co.uk/schools/gcsebitesize/science/images/gcsechem_equilibrium.gif) 2NH3(g)

The forward reaction in the Haber process is exothermic. This means that if the temperature is increased, the position of equilibrium moves in the direction of the reverse reaction, and less ammonia is formed. If the temperature is too low the rate of reaction will be too low. This would make the process uneconomical. So a compromise temperature is chosen: low enough to get a good yield of ammonia but high enough to obtain a reasonable rate of reaction. The presence of a catalyst does not affect the position of the equilibrium, but it does increase the rate of the reaction. This means the ammonia is produced in a shorter time, reducing the cost of the process. Iron is a cheap catalyst.

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Pressure for the harber process

 If the pressure is increased, the position of equilibrium moves in the direction of the fewest molecules.You can see that for any given temperature the yield of ammonia increases as the pressure increases. You can also see that, for any given pressure, the yield goes down as the temperature increases. This is because the forward reaction is exothermic.

Ammonia can be oxidised to give nitric acid 

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Contact processs

The contact process is the industrial manufacture of sulphuric acid.

Stage One: Sulphur is burned in air to form sulphur dioxide.

S + O2                         SO2   

Stage Two: Sulphur dioxide is reacted with more oxygen. This requires a Vanadium (V) Oxide catalyst and a temperature of 450°C to ensure that the reaction occurs quickly.

2SO2 + O2                                     2SO3 (sulphur trioxide)

Stage 3: Sulphur trioxide is absorbed into concentrated sulphuric acid (98%) and then watered down.

SO3 + H2SO4                                      H2S2O7 (oleum)

H2S2O7 + H2O                          H2SO4

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Reverse Contact process

Vanadium (V) oxide is used as a catalyst for the revisible formation of sulphur trioxide

Sulphur dioxide + Oxygen -> Sulphur Dioxide 

2SO2(g) + O2(g) -> 2SO3(g)

Concentrated sulfuric acid can remove the elements of water from substances such as sugar and hydrated copper sulfate

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Making Fertilisers

We can make solid ferilisers by reacting ammonia (an alkali) with an acid. A salt forms when an acid and a alkali react together.

Ammonium Sulphate:

The Sulphuric acid neutralises the ammonia solution

Ammonium Nitrate:

The nitric acid neutralises the ammonia solution

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Benefits and problems of nitrogenous fertilisers

Fertilisers increase crop growth and yield, have managed to lower food prices and are helping us to feed our growing population.  However, overuse of fertilisers can cause environmental problems, such as eutrophication.

Eutrophication:

·         Excess fertiliser from fields is washed into lakes and rivers.Fertiliser causes increased growth of algae.Algae forms a thick layer on the surface of the water body. This process is called eutrophication. Algae layer blocks out light, and plants on the bottom die (can’t photosynthesise) As the plants are dead, they are no longer releasing Oxygen, while creatures (e.g. fish) are still taking it in, the Oxygen level falls.  Creatures in the water body (e.g. fish) die from lack of oxygen. 


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