Topic 14 – Fertilisers – Learning Outcomes
General Level
o State that the increasing world population has led to a need for better ways to produce food
o State that growing plants require nutrients, including compounds of phosphorus and
potassium as well as nitrogen
o State that nitrogen-fixing bacteria in root nodules of some plants can convert nitrogen in
the air into nitrogen compounds
o Explain the importance of such plants in making the soil more fertile
o State that fertilisers are substances which restore the essential elements for plant growth
to the soil
o Give examples of the effect of artificial fertilisers on lochs and rivers
o Explain the importance of the decomposition of plant and animal protein in the recycling of
nitrogen
o State that bacterial methods of fixing nitrogen are cheaper than chemical methods
o Explain in terms of solubility and essential elements why the following are useful, synthetic
fertilisers: ammonium salts, potassium salts, nitrates, phosphates
o Describe the nitrogen cycle in terms of recycling of nitrogen between plants and animals
o Describe the various pathways by which nitrogen is gained and lost in the nitrogen cycle
o State that ammonia and nitric acid are nitrogen compounds which are used to make
fertilisers
o Describe the industrial manufacture of ammonia from nitrogen and hydrogen (Haber
Process)
o State that ammonia can be changed into an ammonium compound
o Describe the following properties of ammonia: appearance, smell, solubility, effect on damp
pH paper/universal indicator
o State that nitric acid is formed when nitrogen dioxide, in the presence of air, dissolves in
water
o State that nitrogen gas is not very reactive
o State that nitrogen dioxide is produced in the air during lightning storms
o Explain in energy terms why nitrogen and oxygen can combine during lightning storms or in a
car engine
o Explain the effect of nitrogen oxides in rain in terms of replacing nitrogen compounds in the
soil and in increasing the acidity of the soil
o State that the combination of nitrogen and oxygen does not provide an economic industrial
route to nitric acid
o Describe the catalytic oxidation of ammonia to form nitrogen dioxide – a step in the
industrial manufacture of nitric acid (Ostwald Process)
o State that the catalytic oxidation of ammonia can be carried out in the laboratory
Credit Level
o State that different crops need fertilisers containing different proportions of
nitrogen, phosphorus and potassium
o Work out percentage mass of elements in compounds from formulae
o State that bacterial methods of fixing nitrogen are cheaper than chemical methods
o Explain why the catalytic oxidation of ammonia is carried out at a moderately high
temperature
o Explain why it is not necessary to continue to supply heat once the catalytic
oxidation of ammonia has started
Notes
Fertilisers
Fertilisers are important as they help us to produce more food in a given space. This is
important as the world’s population is increasing and we are struggling to feed everyone.
Plants need nitrogen, phosphorus and potassium to grow strong and healthy so good
fertilisers will supply soluble compounds of these. Some plants such as peas, beans and
clover can get their own nitrogen from root nodules. These are little lumps on the roots
that contain nitrogen fixing bacteria that can take nitrogen from the air to give to the
plant. These plants help to make soil more fertile for other plants as well.
Ammonium salts (nitrogen) potassium salts, nitrates and phosphates tend to be used as
fertilisers as they contain essential elements for plants and are soluble in water.
Fertilisers can get washed into rivers and lochs by rain and this causes algae to grow,
this slowly kills life in the rivers and lochs as the algae use all the oxygen in the water.
Nitrogen Cycle
There is a balance on Earth between the nitrogen in the air and nitrogen in the soil and
plants. When living things excrete waste and die, bacteria decompose the material
putting nitrogen into the soil and the air. Fertilisers and bacteria in nodules can put
nitrogen back into the soil for plants and living things to eat. Nitrogen is lost from this
cycle by human waste treatment where the fesses is usually cleaned and washed out to
sea.
Haber Process
The Haber process makes ammonia (NH3) which is an important compound in making
ammonium compounds for fertilisers (also bombs). The Haber process takes nitrogen
and hydrogen and combines them in an iron catalyst to help speed up the reaction. This
reaction is also done at moderate temperature and pressure to give the biggest yield
(make more). Not all the nitrogen and hydrogen will combine to make ammonia so this
gas is recycled back to the start of the reaction so save money and not waste
reactants.
Ammonia is a pungent, colourless gas that can is very soluble in water. When ammonia
dissolves, it forms and alkaline solution (pH greater than 7).
Ostwald Process
This is the catalytic oxidation of ammonia to form nitric acid (HNO3). This is another
important compound in making fertilisers (and bombs). This is formed in the Ostwald
process where ammonia (from the Haber process) is reacted with oxygen from the air in
the presence of a platinum catalyst at 800oC. This reaction does not require to be
heated as it is an exothermic reaction (gives out heat) so it heats itself. This is
important as it saves money as we do not have to keep heating the mixture. This
reaction forms nitrogen monoxide which is reacted with oxygen to from nitrogen
dioxide which is again reacted with oxygen and water to form nitric acid.
This is the best known route to nitric acid as combining nitrogen and oxygen is very
difficult. This only happens with an electrical spark, during lightning storms or the
spark in a car plug. Both these produce nitric acid gas in the air which can be washed
into the soil by rain making the soil acidic.
Past Paper Questions
07 General:
08 General:
09 General:
10 General:
2c, 4a
1c, 7b, 21b
2a, 16a
16
07 Credit:
08 Credit:
09 Credit:
10 Credit:
1b
18
14
7