Unit 31
Nitrogenous fertilizers &
sulphuric acid
How do green plants make their
own food?
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By Photosynthesis
Green plants make
use of carbon
dioxide and water, in
the presence of light
and chlorophyll,
make glucose
(starch) and oxygen.
What are needed for healthy
plant?
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Green plants make their food in
photosynthesis. Any more things needed?
Mineral salts (elements) are needed. What
are they?
They are nitrogen, phosphorus, and
potassium.
These are removed from the soil when crops
are harvested.
Thus, fertilizers have to be added for
restoring minerals to the soil.
Why these elements are good
for plants?
Types of fertilizers
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Natural fertilizers - compost (decaying
organic matter) and manure.
Artificial fertilizers - a mixture of
compounds containing the elements,
nitrogen, phosphorus and potassium.
(e.g., ammonium sulphate or amonium
nitrate)
Changing nitrogen gas (in air)
into compounds
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How?
In thunderstorm (lightning)
(with electric spark),
nitrogen and oxygen
combine to form nitrogen
oxide.
N2(g) + O2(g)  2NO(g)
Nitrogen monoxide then
combines with oxygen in
air to form nitrogen dioxide.
2NO(g) + O2(g)  2NO2(g)
Changing nitrogen gas into
compounds
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Nitrogen dioxide dissolves in rainwater to
form dilute nitric acid.
2NO2(g) + H2O(l)  HNO2(aq) + HNO3(aq)
4NO2(g) + O2(g) + 2H2O(l)  4HNO3(aq)
Sources of nitrate ions (natural
fertilizers).
Nitrogen in soil

Nitrogen-fixing
bacteria in the roots
of peas and beans
for changing nitrogen
into nitrates.
Nitrogenous fertilizers
Nitrogenous compound
Physical
state
Molar mass
(g mol-1)
solubility
gas
17
very high
ammonium nitrate, NH4NO3
solid
80
extremely high
ammonium sulphate, (NH4)2SO4
solid
132
high
ammonium hydrogenphosphate,
(NH4)2HPO4
solid
132
low
Urea, (NH2)2CO
solid
60
high
ammonia, NH3
Percentage by mass of nitrogen
in fertilizers
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Calculate the percentage by mass of
nitrogen in ammonia nitrate.
Formula of ammonium nitrate ?
Mass of 1 mole of ammonium nitrate ?
Mass of nitrogen in 1 mole of ammonium
nitrate ?
Mass percentage of nitrogen in
ammonium nitrate ??
Percentage by mass of nitrogen
in ammonium sulphate
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Formula of ammonium sulphate ?
Molar mass of ammonium sulphate ?
Mass percentage of nitrogen in
ammonium sulphate ??
Calculation example
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1 kg of ammonium sulphate is sold at a
price of $2. Calculate the cost of buying
1 kg of nitrogen by buying ammonium
sulphate.
NPK label
Nitrogen Number
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The mass percentage of nitrogen in the
fertilizer.
If the nitrogen number of a fertilizer is 15,
Calculate the mass of nitrogen in 1000 g
fertilizer.
Phosphorus Number
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the mass percentage of phosphorus in
the form of P2O5 present in the fertilizer.
If the NPK label of a fertilizer is 16.8.24.,
calculate the mass percentage of
phosphorus in the fertilizer.
Potassium number
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the mass percentage of potassium in the
form of K2O present in the fertilizer.
If the NPK label of a fertilizer is 16.8.24.,
calculate the mass percentage of
potassium in the fertilizer
How to prepare a fertilizer
(ammonium sulphate) in the
laboratory?
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How??
What chemicals are needed?
What are their formulae?
Write an equation for the reaction
involved.
Experimental Steps (Procedures)
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Pipette 25.0 cm3 ammonia solution into a
conical flask.
Add a few drops of methyl orange (indicator).
Colour in alkali: yellow
Fill the burette with dilute sulphuric acid.
Titrate the ammonia solution with dilute
sulphuric acid until the methyl indicator turns
from ______ to ______ . (I.e. the end point is
reached.)
Experimental Steps (Procedures)
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Mix 25.0 cm3 of ammonia solution with
the required volume of dilute sulphuric
acid without any indicator.
Warm to saturate the salt solution.
Allow it to crystallize slowly to give large
crystals of ammonium sulphate.
Filter off the crystals and dry crystals
between filter paper.
Haber Process
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Industrial Manufacture of ammonia
By the direct combination of nitrogen and
hydrogen
In the presence of fine divided iron
catalyst.
N2(g) + 3H2(g)  2NH3(g)
What is a catalyst?
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Name an enzyme (a biological catalyst).
A catalyst is a substance that will alter /
change (usually speed up) the rate of a
chemical reaction.
Catalysts are usually transition metals or
compounds of transition metals.
Flow diagram of Haber Process
Raw materials of Haber Process
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Nitrogen – from the fractional distillation
of liquid air
Hydrogen – from the reaction of steam
with methane or naphtha.
CH4(g) + H2O(g)  CO(g) + 3H2(g)
Finely divided iron catalyst
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Fine-divided – in powder form
Why ?
To Increase the surface area of the
catalyst.
‘Poisoning’ of the catalyst
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Impurities, such as sulphides and carbon
monoxide, adhere on the surface of the
solid catalyst. – “poisoning of the
catalyst”.
Raw materials, nitrogen and hydrogen
are purified, before passing over the redhot catalyst.
Reaction Conditions of Haber
Process
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Nitrogen to hydrogen volume ratio =1:3
Compressed to a pressure of 200
atmospheres.
Pass over the red-hot iron catalyst at
500oC.
N2(g) + 3H2(g)  2NH3(g) Exothermic
Percentage yield : 15%
Ways to improve (increase) the
yield of ammonia
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By removing ammonia (product) from the
reaction mixture.
By liquefying ammonia or by dissolving
ammonia in water.
Removing ammonia and the recycle of
unreacted nitrogen and hydrogen. Increase
the yield of ammonia. (More ammonia)
Heat released in the reaction is absorbed by
the heat exchanger for heating the incoming
new reactants.
Preparing ammonia in the
laboratory
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Ammonia – a weak alkali.
Prepared by heating a salt of weak alkali with
a strong alkali.
Heating ammonium salt with a strong alkali.
NH4Cl + NaOH  NH3 + NaCl + H2O
A colourless gas with irritating smell which
turns moist (wet) red litmus paper blue is
given out.
Ammonia gas is poisonous.
Heating ammonium salt with
sodium hydroxide
Test for ammonia gas
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With a characteristic irritating smell (no a
chemical test)
Turns moist red litmus paper blue.
NH3(aq) + H2O(l)  NH4+(aq) + OH-(aq)
Put a glass rod wetted with concentrated
hydrochloric acid near a gas jar (tube) of
ammonia gas. A dense white fume of
ammonium chloride is formed.
NH3(g) + HCl(g)  NH4Cl(s)
Ammonia solution as a weak
alkali
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What is a weak alkali?
Partly ionized in aqueous solution.
NH3(aq) + H2O(l)  NH4(aq) + OH-(aq)
What do you see when limited amount of
aqueous ammonia is added into
copper(II) sulphate solution?
Pale blue precipitate is formed.
Cu2+(aq) + 2OH-(aq)  Cu(OH)2(s)
Uses of ammonia gas
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To fix atmospheric nitrogen into nitrogen
compounds which are used as fertilizers.
A good solvent for
grease. Used as a glass
(window) cleaner.
Uses of ammonia gas
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Used as a freezing agent (refrigerant) in
refrigerators.
An important starting chemical for
preparing nitric acid and fertilizers.
Industrial manufacture of nitric
acid (Ostwald Process)
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Oxidation number of nitrogen in ammonia ??
Oxidation number of nitrogen in nitric acid ???
By catalytic oxidation of ammonia
Ammonia is oxidized catalytically by oxygen
in the presence finely divided platinum
catalyst.
4NH3(g) + 5O2(g)  4NO(g) + 6H2O(g)
Industrial manufacture of nitric
acid (Ostwald Process)
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Nitrogen monoxide reacts with oxygen
(in air) to form nitrogen dioxide.
2NO(g) + O2(g)  2NO2(g)
Nitrogen dioxide is then dissolved in
water in the presence of oxygen to form
nitric acid
4NO2(g) + O2(g) + 2H2O(l)  4HNO3(aq)
Uses of nitric acid
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For making fertilizers.
For making monomers of the polymer –
nylon.
For making explosives.
For making drugs.
Contact Process for the
manufacture of concentrated
sulphuric acid
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Raw materials: ???
Sulphur dioxide, SO2
By burning sulphur in air
S(s) + O2(g)  SO2(g)
By roasting metal sulphide ore
2ZnS(s) + 3O2(g)  2ZnO(s) + 2SO2(g)
Oxygen from air
Contact Process
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To prevent the ‘poisoning’ of the catalyst
by purifying the raw materials.
Catalyst used
Finely divided platinum – expensive,
(more efficient)
Or vanadium(V) oxide, V2O5 (cheaper)
Reaction conditions
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Sulphur dioxide and oxygen mixed in
the ratio of 2 to 1.
Temperature : 450oC
A pressure of one or two atmospheres.
2SO2(g) + O2(g)  2SO3(g) Exothermic
Percentage yield of SO3 : 90%
Flow diagram of contact
process
Increasing the yield of sulphur
trioxide
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By removing sulphur trioxide.
Not by dissolving sulphur trioxide in
water as the reaction is highly
exothermic and vaporize the acid.
The acid fume is hazardous to the
operator and harmful to the machinery.
Removing sulphur trioxide
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Dissolving sulphur trioxide in
concentrated sulphuric acid to form
oleum (fuming sulphuric acid)
SO3(g) + H2SO4(l)  H2S2O7(l)
Oleum is then carefully diluted in right
proportion to give concentrated sulphuric
acid
H2S2O7(l) + H2O(l)  2H2SO4(l)
Uses of sulphuric acid
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For the manufacturing of fertilizers.
For the manufacture of soapless
detergents and dyestuffs..
Paint Additives
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For the manufacture of paint additives
Metal sulphates are soluble in water
except calcium sulphate, barium
sulphate and lead(II) sulphate
Mixing a soluble calcium / barium salt
solution with a soluble sulphate solution.
Ca2+(aq) + SO42-(aq)  CaSO4(s)
Ba2+(aq) + SO42-(aq)  BaSO4(s)
Uses of concentrated sulphuric
acid