Standard Grade Chemistry
Summary Notes
Topic 14. Fertilisers
General
Learning Outcomes
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The increasing world population has led to a need for more efficient food
production.
Growing plants require nutrients, including compounds of phosphorous and
potassium and well as nitrogen.
Nitrogen fixing bacteria in root nodules of some plants can convert atmospheric
nitrogen into nitrogen compounds.
You must be able to explain the importance of legumes in increasing soil
fertility.
Fertilisers are substances which restore the essential elements for plant
growth to the soil.
to the soil.
You should be able to give examples of the effect of artificial fertilisers on
lochs and rivers.
You should be able to explain the importance of the decomposition of plant and
animal protein in the recycling of nitrogen.
You should be able to explain in terms of solubility and essential elements why
the following are useful, synthetic fertilisers :
 Ammonium salts
 Potassium salts
 Nitrates
 Phosphates
You should be able to explain the nitrogen cycle in terms of recycling of nitrogen
between plants and animals.
You should be able to describe the various pathways by which nitrogen is gained
and lost in the nitrogen cycle.
Ammonia and nitric acid are nitrogen compounds which are used to make
fertilisers.
You should be able to describe the industrial manufacture of ammonia from
nitrogen and hydrogen (Haber process).
Ammonia can be converted to an ammonium compound.
You should be able to describe the following properties of ammonia :
 Appearance
 Smell
 Solubility
 Effect on damp pH paper/ universal indicator
Nitric acid is formed when nitrogen dioxide, in the presence of air, dissolved in
water,
Nitrogen gas is not very reactive.
Nitrogen dioxide is produced in the air during lightning storms.
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You should be able to explain in terms of energy why nitrogen and oxygen can
combine during lightning storms or in a car engine.
You should be able to explain the effect of nitrogen oxides in terms of :
 Replacing nitrogen compounds in the soil
 Increasing the acidity of the soil
The combination of nitrogen and oxygen does not provide and economical
industrial route nitric acid.
You should be able to describe the catalytic oxidation of ammonia to form
nitrogen dioxide – a step in the industrial manufacture of nitric acid (Ostwald
process).
The catalytic oxidation of ammonia can be carried out in the laboratory.
Credit
Learning Outcomes
 Different crops need fertilisers containing different proportions of nitrogen,
phosphorus and potassium.
 You should be able to work out percentage mass of elements in compounds from
formulae.
 Bacterial methods of fixing nitrogen are cheaper than chemical methods.
 You should be able to explain why the reaction is carried out at moderately high
temperature.
 You should be able to explain why all the nitrogen and hydrogen are not
converted to ammonia.
 Ammonia can be prepared in the laboratory by the reaction of ammonium
compound with alkali.
 You should be able to explain why the catalytic oxidation of ammonia to form
nitrogen dioxide is carried out at a moderately high temperature.
 You should be able to explain why it is not necessary to continue to supply heat
once the catalytic oxidation of ammonia has started.
Nutrients
General
All living things are made mainly from the elements oxygen, carbon, hydrogen and
nitrogen. However, other elements such as sulphur, potassium, phosphorus, iodine,
magnesium, iron and others are essential for growth too.
The major elements, making up over 96% of the human body are oxygen (65%),
carbon (18.5%), hydrogen (9.5%) and nitrogen (3.2%). Remember these in order as
OCHN –oh see him now!
Where do these elements come from?
Plants absorb they need in the form of simple, water soluble compounds. These
elements which provide the elements for a plant to grow are called nutrients.
Animals, such as ourselves, can obtain the essential elements for their bodies by
eating plants or other animals which eat plants.
The three main nutrient elements required by plants are nitrogen, phosphorus and
potassium. The following table gives information on the role of these elements in
plant growth and the effects of any shortage.
Element
nitrogen
N
phosphorus
P
potassium
K
Role In Plant Growth
Essential of making
protein and chlorophyll
(the green chemical in
leaves)
Essential for early root
and stem growth. Involved
in transferring chemical
energy. Required for the
efficient use of nitrogen.
Needed to help make
protein and sugars.
Effect Of Shortage
Plants are stunted. Leaves
are pale green or yellow.
Roots are long and thin.
Slow growth, small seeds
and fruit.
Plants stunted. Lower
resistance to frost,
drought and disease.
The Nitrogen Cycle
Animals eat plants in order to obtain some of the essential elements they need for
living and growing. Plants must absorb soluble nitrogen compounds, e.g. nitrates,
from the soil for proper growth. Nitrogen in compounds such as nitrates is called
fixed nitrogen. This fixed nitrogen must be replaced in the soil so that future
crops can be grown successfully.
In Nature, most elements are readily available from the soil. Plants suffer even if
only one necessary nutrient element is missing. Natural plants do not seem to suffer
shortages of nutrients even though they grow on the same soil for years. This is
because , when they die, their elements are returned to the soil, i.e. the nutrient
elements are recycled.
Man continually breaks the cycle by harvesting the crops he has grown and
transporting them to other areas to feed the population. Therefore the nutrients
from the crops are not returned to the soil where they were grown.
Nutrients can be returned to the soil by spreading animal manure or plant compost
on farmland. Decaying dead animals also return nutrients to the soil.
Some plants e.g. peas, beans and clover contain nitrogen fixing bacteria in root
nodules which can successfully convert atmospheric nitrogen gas into nitrates.
The enormous release of electrical energy during lightning can cause unreactive
nitrogen of the air to react with oxygen to for nitrogen dioxide which dissolves in
rain water. This replaces some fixed nitrogen in the soil, but also increases the
acidity of the soil.
The nitrogen cycle shows how one of the main nutrient elements, nitrogen is
recycled between plants and animals and describes the various pathways by which
nitrogen is gained and lost.
Fertilisers
General/Credit
Crops are harvested and transported away to feed town and city dwellers and the
increasing world population in general. The rapidly increasing world population has
led to a need for more efficient food production methods.
Fertilisers have two main roles in improving the efficiency of food production.
They replace the essential elements removed from the soil by harvesting and
they increase the rate of growth of crops.
Nitrogen (N), phosphorus (P) and potassium (K) were the main elements in the
nutrients needed by plants. The fertilisers which supply these three elements are
therefore known as NPK fertilisers. The amount of each element present in a
particular fertiliser is usually given as a percentage. Different fertilisers supply
different NPK proportions because different plants have different NPK needs
and soils vary in their ability to supply nutrients.
A typical bag or packet of fertiliser normally contains a variety of compounds in the
correct proportions to give the exact NPK ratio stated on the packaging. All the
compounds in the fertiliser will be water soluble to allow plants to take them up
through their roots. This high solubility of fertiliser compounds can also be a
problem as they can be washed out of the soil before plants can take them up. This
is called leaching and can be a serious problem for lochs and rivers.
Calculation Of Percentage Mass Of Elements In Compounds
Credit
The percentage mass of each element in a compound, such as a fertiliser, can be
calculated from its formula.
Example
Step 1
Step 2
calculate the percentage mass of each element in potassium nitrate
Write the chemical formula of the compound
potassium nitrate KNO3
Calculate the formula mass of the compound
Step 3
Calculate the percentage mass of each element
( 1 X RAM of K) +
(1 X RAM of N) +
(3 X RAM of O)
= (1 x39) + (1x14) + (3x16)
= 39 + 14 + 48
= 101
%K = mass due to K x 100
formula mass
= 39 x 100
101
= 38.6%
%N = Mass due to N x 100
formula mass
= 14 x 100
101
= 13.9%
% O = Mass due to O x 100
formula mass
= 48 x 100
101
= 47.5%
The Haber Process
General
Although 78% of the air is nitrogen, most plants cannot use it directly. In 14.2 we
learned how nitrogen fixing bacteria in the roots of some plants and lightning play a
role in the return of some fixed nitrogen to the soil.
The sparking of air could therefore be used to make nitrate fertiliser. However,
the large amount of electrical energy needed to make nitrogen and oxygen react,
and its cost, makes this method uneconomical to use industrially on a large scale.
In the latter part of the 19th century, the idea that plants need fertilisers was
becoming more generally understood. In addition, the rapid growth of the world
population and the consequent demand for more and more food supplies made the
need for a cheap, easily produced man-made fertiliser more urgent.
Although nitrogen was an obvious starting point for making fertiliser compounds,
early chemists had great difficulty getting nitrogen to react. The breakthrough was
brought about by a German chemist called Fritz Haber. Starting in 1904, he tried
to combine nitrogen and hydrogen gases to make ammonia gas. After about 4
years research he discovered a successful method for.
N2 (g) + 3H2(g)  2NH3 (g)
This method is now know as the Haber process in his honour.
The Haber process used hydrogen obtained from methane and nitrogen obtained
from the fractional distillation of liquid air. The reaction is an example of what is
known as a reversible reaction. This is because some of the ammonia molecules
decompose and change back into nitrogen and hydrogen. In order to make ammonia
gas as efficiently as possible, the following steps are taken in the manufacturing
process :An iron catalyst is used to speed up the reaction.
The gases are reacted under high pressure (200atm)
A moderately high temperature (450oC) is used. The higher the temperature, the
faster the reaction, but the lower the percentage of conversion to ammonia –
therefore the compromise of a moderately high temperature is used.
The ammonia gas is removed by cooling, soon after it forms, to stop it
decomposing back into hydrogen and nitrogen.
Ammonia
General
Ammonia is a covalent compound of nitrogen and hydrogen. It has the chemical
formula NH3. Ammonia is a colourless gas. So, however are hydrogen, oxygen,
methane and other gases. It is ammonia’s other properties that make it different
from these other gases.
A convenient way to make ammonia in the laboratory is to heat together an
ammonium compound and an alkali.
e.g. ammonium chloride + calcium hydroxide  calcium chloride + water + ammonia
Experiments prove that ammonia dissolves very well in water to form an alkali.
The alkali formed is called ammonium hydroxide (NH4OH). The solution contains
the ions NH4+ and OH-.
The ammonia molecule (NH3) has taken an H+ from a water molecule to form the
NH4+ ion. An OH- ion also forms from the water molecule.
NH3(g) + H2O  NH4+(aq) + OH-(aq)
As ammonia solution (ammonium hydroxide) is an alkali, it can react with acids
neutralising the acid. As in all other acid and alkali neutralisation reactions the
products of the reaction are salt and water.
e.g. ammonium hydroxide + hydrochloric acid  ammonium chloride + water
Nitric Acid
General/ Credit
About 80% of the ammonia produced industrially in the Haber process is used to
make fertilisers. The remaining ammonia is used to produce nylon, household
cleaners, other chemicals, but most importantly nitric acid.
Nitric acid is a very important industrial chemical. It is also used in the production
of fertilisers and to make explosives e.g. nitroglycerine and TNT.
Nitric acid is produced industrially by a process called the Ostwald process, the
first stage of which involves the catalytic oxidation of ammonia.
The Ostwald Process
General/Credit
The industrial conversion of ammonia (NH3) into nitric acid (HNO3) is called the
Ostwald process. There are three steps in the conversion.
STEP 1
A dry mixture of air and ammonia is passed over a hot platinum catalyst. The
ammonia and oxygen from the air react to form the colourless gas nitrogen
monoxide and water. As the reaction between ammonia and oxygen of the air is
exothermic it is not necessary to heat the catalyst after the reaction has started.
ammonia + oxygen  nitrogen monoxide + water
4NH3
+
Pt catalyst
~800oC
5O2 
4NO
+
6H2O
STEP 2
The colourless gas, nitrogen monoxide (NO) is cooled. It then reacts very easily
with more oxygen for the air to form a brown coloured gas, nitrogen dioxide.
nitrogen dioxide + water + oxygen  nitric acid
NO2 + H2O + O2  HNO3
Salt Properties
General
Salts formed from ammonia and / or nitric acid for use as fertilisers must be
soluble in water. An insoluble compound containing essential elements will be no use
as a fertiliser as plant roots will be unable to absorb the nutrients.
However, if the compound is too soluble, then it may be very quickly washed out of
the soil by rain. This means that the farmers have to pay close attention to
weather forecasts before they decide to spread fertilisers for their crops.
Too dry – the fertiliser will not dissolve. It will not be absorbed by the plant when it
most needs it.
Too wet – much of the fertiliser will be washed away. This again will not help the
crop.
Problems of pollution arise if too much fertiliser gets washed out of farmland soils.
The levels of nitrates and phosphates in rivers and lochs are often carefully
monitored.