Manufacture of ammonia and fertilisers

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C6.01 Conditions used in the Haber process
The optimum (best) conditions for the Haber process that turns nitrogen and hydrogen into ammonia are:

• 350 atmospheres; high pressure increases yield
• about 450ºC ; high temperature cuts yield but increases rate
• and the use of a catalyst, which is usually iron; increases rate

Story C6.01

C6.02 Reversible reactions and dynamic equilibrium
The chemical reaction used to make ammonia is:
nitrogen + hydrogen --> ammonia
N₂         +      3H₂    ---> 2NH₃
Ammonia can also decompose to form nitrogen and hydrogen:
2NH₃ ---> N₂ +3H₂
This reaction is reversible.  The reaction is never complete but does reach a state when no more change can be seen.  This state is called equilibrium.

Although no change is seen at equilibrium the reaction still carries on with some ammonia molecules being made and some decomposed.  This is called dynamic equilibrium.

C6.03 Temperature, pressure and the position of dynamic equilibrium
Temperature
The reaction below is exothermic, energy is given out when ammonia forms, but energy is taken in if ammonia breaks up.
N₂(g)  +   3H₂(g)    ---> 2NH₃(g)  ;  DH = - 92kJ/mol
Reactions resist changes.

If the temperature goes up the reaction tries to prevent this by taking in energy.
The reaction can take in energy by breaking up ammonia. 
The position of the equilibrium moves to the left.
So if the temperature goes up then ammonia breaks up which is not helpful.

If the temperature goes down the reaction tries to prevent this by giving out energy.
The reaction can give out energy by forming ammonia. 
The position of the equilibrium moves to the right.
So if the temperature goes down then ammonia form which is helpful.

Pressure
On the left hand side of the equation there are 4 molecules
On the right hand side there are only 2 molecules which take up less space than 4 molecules
Pressure is reduced if the position of the equilibrium moves to the right, so an increase in pressure causes a shift to the right so more ammonia is formed.  As high pressure favours a big yield of ammonia so 200 atmospheres pressure is used.
TaskC6.03 State and explain the effect of (a) increasing the temperature and (b) decreasing the pressure on the following reactions:
2NO(g) + O₂(g) = 2NO₂(g) ;  DH = +57kJ/mol
2H₂(g) + O₂(g) = 2H₂O(g) ;  DH = - 280kJ/mol
CH₄(g) + H₂O(g) = CO(g) + 3H₂(g) ;  DH = + 40kJ/mol

C6.04 Reaction rates and equilibrium
Using a high pressure gives a small increase in rate because the gases are more concentrated.  Using a low temperature gives a low rate so it takes a long time to reach equilibrium.  A catalyst like iron in the Haber process is needed to speed up the reaction.

C6.05 Neutralising ammonia with nitric acid
When nitric acid reacts with ammonia (an alkali) the acid is neutralised and a salt is formed.
acid + alkali ---> salt + water
nitric acid + ammonium hydroxide ---> ammonium nitrate + water
HNO₃(aq) + NH₄OH(aq) ---> NH₄NO₃(aq) + H₂O(l)
Ammonium nitrate contains fixed (chemically combined) nitrogen and so is a good fertiliser.
Task C6.05 Write equations for the neutralisation of ammonium hydroxide, potassium hydroxide and ammonia by nitric acid HNO₃, sulphuric acid H₂SO₄, phosphoric acid H₃PO₄.

C6.06 Nitrogenous fertilisers and plant growth
Plants grow well when they can obtain fixed nitrogen from the soil.  Only a few plants like peas and beans can make use of nitrogen in the air.  Most plants need fixed nitrogen in compounds like nitrates.  Plants need nitrogen to make proteins which gives them strong stems and healthy leaves.  Some nitrates find their way into the soil naturally but intensive farming removes a lot when crops are harvested.  Fertilisers containing nitrogen are used to replace nitrogen lost from the soil during farming.

C6.07 The leaching of artificial fertilisers
1. Fertilisers are very soluble.
2. Fertilisers dissolve in rain water.
3. Fertilisers are leached from the soil and washed into rivers.
4. Water plants grow very well in fertilised river water.
5. The over growth of plants like algae at the surface cuts out the light to plants below.
6. Plants without light stop growing and die.
7. Dead plants rot due to bacteria that use a lot of oxygen
8 The amount of oxygen in the water drops.
9 Fish and other animals start to die because of a lack of oxygen.
10 The process is called eutrophication.