This section is content heavy and an important sub-section in this chapter. It should
be linked to rate of reactions, equilibrium and acid-base reactions done earlier
in Grade 12.
Two lessons are allocated, but if three are available it would be advisable to spend
this extra time on this sub-section (although the depth of information provided
here is not required from the learners). For this section a poster,
an animation
and accompanying worksheet
are available. These can be used as an introduction to the section as well as a
classwork activity. Play the animation once or twice without the learners
writing anything, then hand out the worksheet, and play the animation again
so that learners can complete the worksheet.
The industrial production of fertilisers involves several processes. Figure 14.3
summarises how a number of different industrial processes are used to manufacture a
variety of fertilisers. In the following sections we will discuss the various processes
indicated on this diagram.
Figure 14.3: The industrial manufacturing of fertilisers
Producing hydrogen: Coal gasification and steam reforming at Sasol (ESCS5)
Sasol is an international company that was founded in Sasolburg, South Africa, in
1950. It employs over \(\text{34 000}\) people in at least \(\text{38}\)
countries and has interests in synthetic fuels, mining, oil, gas and chemistry.
Fossil fuels are the main source of industrial hydrogen. Hydrogen can be generated from
natural gas or coal. These processes are used by Sasol
at their Gas-to-Liquid (GTL) and Coal-to-Liquid (CTL) facilities. Hydrogen is usually
produced by the steam reforming of methane gas (natural gas). At high
temperatures (\(\text{700}\) – \(\text{1 100}\)\(\text{°C}\)), steam
(\(\text{H}_{2}\text{O}\)) reacts with methane (\(\text{CH}_{4}\)) in
an endothermic reaction to yield syngas, a mixture of carbon
monoxide (\(\text{CO}\)) and
hydrogen (\(\text{H}_{2}\)).
You are not required to know this information in as much depth as is provided here,
but the extra information should help your understanding of the subject.
During a second stage, which takes place at a lower temperature of about
\(\text{130}\)\(\text{°C}\), the exothermic reaction generates additional hydrogen.
This is called a water gas shift reaction.
Essentially, the oxygen (O) atom is stripped from the additional water
(steam) to oxidise \(\text{CO}\) to \(\text{CO}_{2}\). This oxidation also provides
energy to maintain the reaction.
Coal can also be used to produce syngas in a similar way to natural gas. The reactions are
shown below:
Remember that yield describes the quantity of product in a container
relative to the maximum product possible.
Obtaining nitrogen: Fractional distillation of liquefied air (ESCS6)
Fractional distillation is a separation method. It uses the difference in boiling
temperatures of the components of a mixture to separate those components.
The mixture is heated to convert the components into the vapour (gas) phase.
The vapour mixture is then pumped into a tall separation column (called a
fractional distillation column), usually at the bottom of the column.
As the vapour mixture moves up the column and cools, the different components
(called fractions) condense as the temperature drops below the various
boiling point temperatures.
These fractions are collected using collection trays.
The fractions can be removed from the mixture in this way and thus the
components are separated.
This process is used to separate the components of air or crude oil. Air is a mixture of
gases, mainly nitrogen (\(\text{N}_{2}\)) and oxygen (\(\text{O}_{2}\)). Liquefied air
(compressed and cooled to \(-\text{200}\)\(\text{°C}\)) is pumped into the
fractional distillation column. Nitrogen gas has the lowest boiling point temperature
and is collected at the top of the column (Figure 14.4).
Figure 14.4: A fractional distillation column for the separation of the
components of air.
Producing ammonia: The Haber process (ESCS7)
Ammonia (\(\text{NH}_{3}\)) plays an important role in the manufacturing process of
fertilisers. The industrial process used to produce ammonia is called the Haber
process. In this reaction nitrogen gas and hydrogen gas
react to produce ammonia gas. The equation for the Haber process is:
This reaction takes place in the presence of an iron (Fe) catalyst under high pressure
(\(\text{200}\) atmospheres (atm)) and temperature (\(\text{450}\) –
\(\text{500}\)\(\text{°C}\)) conditions.
By the \(\text{20}\)th century, a number of methods had been developed to fix
atmospheric nitrogen, in other words to make it usable for plants. Sources of nitrogen
for fertilisers included saltpetre (\(\text{NaNO}_{3}\)) from Chile, and guano.
Guano is the droppings of seabirds, bats and also seals. In the early \(\text{20}\)th
century, before the start of the First World War, the Haber process was developed by two
German chemists, Fritz Haber and Karl Bosch. They determined what the best conditions
were in order to get a high yield of ammonia, and found these to be high
temperature and high pressure. During World War I, the
ammonia produced by the Haber process was used to make explosives.
The forward reaction of the Haber process is exothermic, so the forward reaction is
favoured by low temperatures. However, low temperatures slow all chemical
reactions. So, the Haber process requires high temperatures, and the ammonia is
removed as soon as it is formed to prevent it being used in the reverse
reaction.
temp text
Producing nitric acid: The Ostwald process (ESCS8)
The Ostwald
process is used to produce nitric acid from ammonia. Nitric
acid can then be used in reactions that produce fertilisers. Ammonia is converted to
nitric acid in a three-step process.
Firstly ammonia is oxidised by heating it with oxygen, in the presence of a
platinum (\(\text{Pt}\)) catalyst, to form nitrogen monoxide (\(\text{NO}\)) and water.
This step is strongly exothermic, which makes it a useful heat source.
The reaction that takes place is:
Nitrogen monoxide, also known as nitrogen oxide or nitric oxide, is a by-product of this
reaction. The nitrogen monoxide is recycled and the acid is concentrated to the required
strength (e.g., for use in further chemical processes).
temp text
Producing ammonium nitrate (ESCS9)
Nitric acid and ammonia can react together in an acid-base process to form the salt, ammonium
nitrate (\(\text{NH}_{4}\text{NO}_{3}\)). Ammonium nitrate is soluble in water and is
often used in fertilisers. The reaction is as follows:
Figure 14.5: Ammonium nitrate presented as a) the structural
formula, b) a three-dimensional molecular
representation and c) solid crystals.
Producing urea (ESCSB)
Urea (\((\text{NH}_{2})_{2}\text{CO}\)) is a nitrogen-containing compound that is produced on
a large scale worldwide. It has the highest nitrogen content (46,4%) of all solid
nitrogen-containing fertilisers that are commonly used, and is produced by the reaction
of ammonia with carbon dioxide. Two reactions are involved in
producing urea and ammonium carbamate (\(\text{H}_{2}\text{NCOONH}_{4}\)) is an
intermediate product.
Figure 14.6: Urea presented as a) the structural formula,
b) a space-filling three-dimensional model and
c) solid crystals.
In 1828, the German chemist Friedrich Wöhler obtained urea by treating silver
cyanate (\(\text{AgOCN}\)) with ammonium chloride (\(\text{NH}_{4}\text{Cl}\)):
This was the first time an organic compound was artificially synthesised from
inorganic starting materials, without the use of living organisms.
Producing sulfuric acid: The Contact process (ESCSC)
Sulfuric acid (\(\text{H}_{2}\text{SO}_{4}\)) is produced from sulfur, oxygen and water
through the Contact
process. In the first step, sulfur is burned to produce
sulfur dioxide (\(\text{SO}_{2}\)):
Ammonium sulfate (\((\text{NH}_{4})_{2}\text{SO}_{4}\)) can be produced industrially through
a number of processes, one of which is the reaction of ammonia with sulfuric acid to
produce a solution of ammonium sulfate according to the acid-base reaction below:
The ammonium sulfate solution is concentrated by evaporating the water until ammonium sulfate
crystals are formed.
Producing phosphoric acid and super phosphates (ESCSF)
The production of phosphate fertilisers also includes a number of processes. The first is the
production of sulfuric acid through the Contact process. Sulfuric acid
is then used in a reaction with phosphate rock (e.g. fluorapatite
(\(\text{Ca}_{5}(\text{PO}_{4})_{3}\text{F}\))) to produce phosphoric acid
(\(\text{H}_{3}\text{PO}_{4}\)). For more information refer to the Chemical
Industries Resource Pack.
Producing compound fertilisers: The nitrophosphate process (ESCSG)
The nitrophosphate process is one of the processes used to make complex fertilisers. It
involves acidifying calcium phosphate (\(\text{Ca}_{3}(\text{PO}_{4})_{2}\)) with nitric
acid to produce a mixture of phosphoric acid and calcium nitrate
(\(\text{Ca}(\text{NO}_{3})_{2}\)):
If potassium chloride or potassium sulfate is added, the result will be a NPK fertiliser.
Advantages and disadvantages of inorganic fertilisers (ESCSH)
Advantages
Organic fertilisers generally do not contain high levels of nutrients and
might not be suitable to sustain high intensity crop production.
Large scale agricultural facilities prefer
inorganic fertilisers as they provide a more accurate
control over their nutrient supply. Inorganic fertilisers are supplied
in a water-soluble form which ensures that they are easily absorbed by
plants. Much less inorganic fertiliser can therefore be applied to have
the same result as organic fertilisers.
Disadvantages
The two major disadvantages of inorganic fertilisers are that:
Inorganic fertilisers must be manufactured industrially. This
involves cost in terms of both chemicals and the energy
involved in the production. Air pollution is also a
result of these industrial processes.
Nutrients which are not taken up by plants,
will either accumulate in the soil therefore
poisoning the soil, or leach
into the ground water where they will
be washed away and accumulate in water sources like dams
or underground rivers. This is discussed in more detail
in Section 14.6.
Reproductions of the worksheets are given in the teachers guide.
The industrial production of fertilisers
Textbook Exercise 14.2
Use the following processes to create your own mind map of the
manufacturing of fertilisers:
Fractional distillation
Steam reforming
Haber process
Contact process
Ostwald process
you can, and should, add to this list
Include what the reactants and the products are for each process and
how these link with the other processes involved.
Research the process of fractional distillation (from old school
science textbooks, from your teacher, or the internet) and write
a paragraph on how this process works.
Fractional distillation is a separation method that uses the
difference in boiling temperatures of the components of a
mixture to separate those components. The mixture is heated to
convert the components into the vapour (gas) phase. The vapour
mixture is then pumped into a tall separation column (called a
fractional distillation column), usually at the bottom of the
column. As the vapour mixture moves up the column and cools, the
different components (called fractions) condense as the
temperature drops below the various boiling point temperatures.
The fractions are collected using collection trays. The
fractions can be removed from the mixture in this way and thus
the components are separated. This process is used to separate
the components of air or crude oil.
The reaction of hydrogen and nitrogen is an exothermic reversible
reaction.
Use your knowledge of chemical equilibrium and explain what
effect the following will have on this equilibrium
reaction:
raising the temperature
When the temperature of any reaction
is increased, the rate of both the forward and
the reverse reactions will be increased.
However, the forward reaction is exothermic.
Raising the temperature would favour the
reaction which would remove the excess heat. The
reverse reaction will therefore be favoured more
than the forward reaction. This will result in
more \(\text{NH}_{3}\) decomposing to form
\(\text{H}_{2}\) and \(\text{N}_{2}\). The
overall result will be that more
reactants will be formed.
raising the pressure
Raising the pressure will shift the
equilibrium to decrease the pressure. This would
favour the reaction which produced fewer
molecules. The forward reaction produces 2 moles
of \(\text{NH}_{3}\) (compared to the 4 moles of
the reverse reaction). The forward reaction will
therefore be favoured. The overall result will
be that more products will be
formed.
Ammonia and nitric acid react to form the compound A. Compound A is
soluble in water and can be used as a fertiliser.
Write down the name and formula for compound A.
ammonium nitrate, \(\text{NH}_{4}\text{NO}_{3}\)
What type of reaction takes place to form compound A.
Acid-base reaction
Write a balanced equation for the reaction to form compound
A.
Carbon monoxide or \(\text{CO}\). \(\text{CO}\) causes
\(\text{H}_{2}\text{O}\) to be reduced.
Which compound is the oxidising agent?
Water or \(\text{H}_{2}\text{O}\). \(\text{H}_{2}\text{O}\)
causes \(\text{CO}\) to be oxidised.
Identify two conditions that will ensure a high yield of
hydrogen.
Remove \(\text{CO}_{2}\) and \(\text{H}_{2}\) (the products)
from the reaction vessel. More product will therefore be
formed from the same amount of reactants.
Decrease the temperature. This will favour the forward
(exothermic) reaction and more product will be formed
with the same amount of reactants, meaning an increased
yield.
(Increasing the concentration of \(\text{CO}\) or
\(\text{H}_{2}\text{O}\) (steam) (the reactants) will
increase the amount of product formed, but not the
yield. More reactants means that more product must be
formed to get the same yield because the maximum yield
possible has increased).
