Download Topic 14 - Fertilisers

Forrester High School. Chemistry Revision Notes
Topic 14 - Fertilisers
Population and Food Needs
The ever increasing world population means more food is needed and fertilisers are used
to grow plants efficiently.


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Organic fertilisers such as animal manure can be used but additional chemical fertilisers are
also needed.
Growing plants need nutrients - soluble compounds containing nitrogen, phosphorus and
potassium which are absorbed by roots.
Fertilisers containing Nitrogen (N), Phosphorus (P) and Potassium (K) are known as NPK
fertilisers.
Fertilisers also need to be soluble to allow them to be absorbed by the plants roots.
This is why the following compounds are important fertilisers as they contain the essential elements
(NPK) that plants need and they are soluble in water.
ammonium salts
(such as NH4Cl),
nitrates
(such as KNO3)
potassium salts
(such as K2SO4)
phosphates
(such as K3PO4)
When crops are harvested, these nutrients are removed from the soil in the plant and need to be
replaced.
Percentage composition
Fertilisers need to contain different proportions of N : P : K for different plants.
When comparing fertilisers it is useful to know the percentage of an element in that fertiliser.
To calculate the percentage of nitrogen in ammonium nitrate we first calculate the GFM
NH4NO3
3x0
1xN
4xH
1XN
= 3 x 16 = 48
= 1 x 14 = 14
=4x1 = 4
= 1 X 14 = 14
GFM
= 80 g
From the total number of nitrogen atoms in ammonium nitrate calculate the total mass of nitrogen.
There are 2 nitrogen atoms in the formula therefore total mass nitrogen = 14 + 14 = 28
Percentage nitrogen = mass of nitrogen x 100
= 28 x 100
GFM
80
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=35%
Forrester High School. Chemistry Revision Notes
The Nitrogen Cycle
Free Nitrogen in the air
Chemical
processes
Denitrifying
bacteria
Lightning /
petrol engines
Fertilisers
Plant
protein
Nitrifying
bacteria
Acid rain
Animal
Protein
Fixed Nitrogen in soluble compounds in Soil
Leguminose plants (peas, beans and clover) can change atmospheric nitrogen into soluble nitrogen
compounds by 'fixing' nitrogen.

This happens because nodules on the roots contain nitrifying bacteria.

It is cheaper than chemical fertilisers and without pollution problems.
Problems caused by the use of fertilisers
Fertilisers need to be soluble to get into plants.
Rain can wash fertilisers into rivers and lochs and cause harmful algae and bacteria to
grow rapidly.
These remove oxygen from the water and kill fish and plants.
Topic 14
Forrester High School. Chemistry Revision Notes
Making fertilisers
Nitrogen
However, nitrogen is un-reactive but not inert. This means it is difficult getting it to react but it can
be done (using electricity i.e. lightening or a spark plug).
The nitrogen will form oxides which dissolve in water forming acids.
Nitrogen dioxide, a brown gas, can be made when air (21% oxygen and 78% nitrogen) is sparked for
about 20 minutes.
N2 + O2
2NO
(colourless)
2NO + O2
2NO2
(brown)
Nitric acid is made if the brown NO2 gas formed shaken with water and oxygen.
2H2O + 4NO2 + O2
4HNO3
The spark plug causes the same reaction in a petrol engine, as does lightning.
Time and cost make this method unsuitable to make large quantities of nitric acid
To make fertiliser we start with nitrogen.
Nitrogen is reacted with hydrogen to make ammonia (NH3).
The ammonia can then be used to make nitric acid.
The nitric acid can then be used to make
Nitrogen
ammonia
nitric acid
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fertiliser
Forrester High School. Chemistry Revision Notes
Making ammonia in the laboratory
In the classroom heating any ammonium compound with alkali produces ammonia.
NH4Cl + NaOH
NH3 + NaCl + H2O
Ammonium compound
+ alkali
moist
pH paper
Heat
This reaction can be used to prove that a compound is an ammonium compound

Ammonia is a colourless gas which turns moist pH paper blue/purple i.e. it is an alkali.
NH3(g) + H2O(l)
NH4OH(aq)
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Ammonia has a characteristic pungent smell.
Ammonia is very soluble in water as shown by the fountain experiment.
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Ammonia reacts with acids to make ammonium salts which can be used as fertilisers.
NH3 + HNO3
NH4NO3
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Forrester High School. Chemistry Revision Notes
The Haber Process (Converting un-reactive nitrogen into ammonia)
Nitrogen (from the fractional distillation of liquid air) and Hydrogen (from natural gas) are mixed
together at a moderately high temperature (400oC) and high pressure with an iron catalyst and
ammonia, (NH3) is made.
N2(g) + H2(g)
NH3(g)
The previous reaction is reversible i.e. it can go in both directions.
To help stop this the ammonia is cooled down to turn it into a liquid.
Any unreacted nitrogen and hydrogen is recycled.
Unreacted N2 and H2 recycled
Nitrogen
Hydrogen
Reaction
Chamber
Fe catalyst
400 0C
200 atm
N2/H2/NH3
Cooler
liquid NH3
The Ostwald Process ( turning ammonia into oxides of nitrogen)
The reaction is an example of oxidation as ammonia reacts with oxygen on the platinum catalyst.
When the reaction begins, a colourless gas is formed and collects in the flask. This gas is called
nitrogen monoxide, NO.
4NH3(g) + 5O2(g)
4NO(g) + 6H2O(g)
Nitrogen monoxide reacts with air (oxygen) to form brown nitrogen dioxide.
2NO(g) + O2(g)
2NO2(g)
This brown acidic gas dissolves in water with oxygen to make nitric acid.
2H2O + 4NO2 + O2
4HNO3
In the Ostwald process the catalyst continues to glow once the reaction starts, because the oxidation
of ammonia is an exothermic reaction.
As a result of this the reaction does not need to be heated once it has started
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Forrester High School. Chemistry Revision Notes
Making Fertilisers by Neutralisation
For fertilisers nitrate salts are formed using nitric acid:
HNO3
+
NaOH
NaNO3
+
H2O
For fertilisers ammonium salts are formed using ammonia:
NH3
+
HNO3
NH4NO3
For fertilisers potassium salts are formed using potassium compounds and an acid:
K2CO3
+
HNO3
KNO3
+ H2O
For fertilisers phosphorus salts are formed using phosphoric acid:
H3PO4
+
(NH4)3PO4
3NH3
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+
CO2