Summary – Agrochemicals
National 4
Plants need three essential elements to grow successfully. These elements are
nitrogen (N), phosphorus (P), and potassium (K). Plants naturally take these in
through their roots as soluble compounds. The elements are returned to the soil
when the plant dies and decays. When food crops are harvested these elements
never get the chance to return to the soil so farmers need to replace them
using chemicals called fertilisers. Natural fertilisers like garden compost or
farmyard manure can be used but with the growing world population more and
more food crops are required. Chemists can synthesise artificial fertilisers to
meet this demand. The percent of each element in the particular fertiliser can
be calculated. It is written on the label as an NPK value e.g. 10.6, 4.4, 1.7 for the
plant food “Baby Bio”
Percentage Mass Example
Calculate the percent mass of nitrogen in the fertiliser ammonium phosphate.
Step 1 calculate the formula mass (R.F.M.)
Step 2 Calculate the % mass of N
using relative atomic masses (R.A.M.) from %N = Mass of nitrogen
R.F.M.
the data book
(NH4)3PO4
X 100
N x 3 = 14 x 3 = 42
X 100
=
149
H x 12 = 1 x 12 = 12
P x 1 = 31 x 1 = 31
42
=
28%
O x 4 = 16 x 4 = 64
R.F.M.
= 149
The percent mass of Nitrogen in ammonium phosphate is 28%
Artificial fertilisers like ammonium phosphate can be produced in neutralisation
reactions by neutralising phosphoric acid with the alkali ammonium hydroxide.
Fertilisers produced in this way are the “salts” in the neutralisation reaction.
e.g.
phosphoric acid
+ ammonium hydroxide
ammonium + water
phosphate
acid
+
alkali
salt
+ water
Disadvantages
Pollution by fertilisers can be a serious problem. These soluble chemicals can be
washed out of the soil into our rivers by the rain and may even find their way
into our drinking water.
Wildlife in the River Ribble is
under threat by pollution from
untreated farm slurry, it has
been claimed.
Animal fertilisers washed into
the river by heavy rain uses
oxygen in decomposition,
leading fish to suffocate.
Infants below six months who
drink water containing nitrate in
excess of the maximum
contaminant level (MCL) could
become seriously ill and, if
untreated, may die. Symptoms
include shortness of breath and
blue baby syndrome.
Summary – Agrochemicals
National5
The Haber Process
Fritz Haber, 1918
The industrial manufacture of ammonia is carried out by reacting nitrogen from
the air with hydrogen obtained from natural gas in the Haber Process.
Nitrogen
N2
+
hydrogen
+
3 H2
ammonia
2NH3
Unfortunately the reaction is reversible shown by the
arrows and any
ammonia formed can break up again to form hydrogen and nitrogen.
The Haber process maximises the yield of ammonia by




using a catalyst of iron pellets
having a moderately high temperature between 400oC to 500oC
removing liquid ammonia by cooling it as soon as it forms
recycling any unconverted N2 and H2 through the reaction chamber
Ammonia forms the alkali ammonium hydroxide when dissolved in water.
Ostwald Process
Ammonia is also the starting material for the production of nitric acid in the
Ostwald process.
Ammonia is reacted with oxygen to produce nitrogen monoxide. This reaction is
exothermic and uses a platinum catalyst. The nitrogen monoxide then forms
nitrogen dioxide which when added to water will produce nitric acid.
Ammonia + Oxygen
(Pt Catalyst)
Nitrogen Monoxide
Nitrogen dioxide (water)
Nitric acid
The nitric acid is ultimately used to produce ammonium nitrate fertiliser by
neutralising the nitric acid with ammonia. Other nitrate fertilisers can be
produced by reacting nitric acid with various bases.
HNO3 (aq) + NH3 (aq)
NH4NO3 (aq)
HNO3 (aq) + KOH (aq)
KNO3 (aq) + H2O (l)