Thursday, November 27, 2008

Manufacture of ammonia and fertilisers-GCSE /IGCSE CHEMISTRY

Manufacture of ammonia and fertilisers

The chemical reaction used to make ammonia

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

350 atmospheres; high pressure increases yield

about 450ÂșC ; high temperature cuts yield but increases rate

and the use of a catalyst, which is usually iron; increases rate

ammonia is:nitrogen + hydrogen --> ammoniaN2 + 3H2 ---> 2NH3Ammonia 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.

Temperature, pressure and the position of dynamic equilibriumTemperatureThe 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.PressureOn the left hand side of the equation there are 4 moleculesOn the right hand side there are only 2 molecules which take up less space than 4 moleculesPressure 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.

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/mol2H2(g) + O2(g) = 2H2O(g) ; DH = - 280kJ/molCH4(g) + H2O(g) = CO(g) + 3H2(g) ; DH = + 40kJ/mol

Reaction rates and equilibriumUsing 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.

Neutralising ammonia with nitric acidWhen nitric acid reacts with ammonia (an alkali) the acid is neutralised and a salt is formed.acid + alkali ---> salt + waternitric acid + ammonium hydroxide ---> ammonium nitrate + waterHNO3(aq) + NH4OH(aq) ---> NH4NO3(aq) + H2O(l)

Ammonium nitrate contains fixed (chemically combined) nitrogen and so is a good fertiliser.

Write equations for the neutralisation of ammonium hydroxide, potassium hydroxide and ammonia by nitric acid HNO3, sulphuric acid H2SO4, phosphoric acid H3PO4.

Nitrogenous fertilisers and plant growthPlants 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.

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.

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