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2
CHEMISTRY
NITROGEN AND ITS COMPOUNDS
AMMONIA
S.3
50 Students
Introduction
Nitrogen occurs as combined gas in air. It forms 78% by volume of air. It exists as a
diatomic molecule N2. It occurs naturally in compounds such as proteins, nitrates and
ammonium salts.
LABORATORY PREPARATION OF NITROGEN
Apparatus – delivery tubes, furnace, beaker, trough, Bunsen burner, 2 wash bottles
Chemicals – Air, water, sodium hydroxide
Procedure
Either:
Nitrogen is prepared from the air by removing oxygen and carbon dioxide. Water is used
to push air through sodium hydroxide solution (caustic soda solution) which removes
carbon dioxide.
2 NaOHaq + CO2g
Na2 CO3 + H2O (1)
The remaining gas is passed over heated copper turnings to remove Oxygen.
2Cus + 02 (g)
2 CuO(s)
Nitrogen is collected over water as it is insoluble in water.
OR:
Nitrogen can also be prepared by b heating a mixture of ammonium chloride and sodium
nitrite.
NaNO2(s) + NH4 Cl(s)
NH4NO2(s) + NaCls
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Nitrogen is formed by decomposition of ammonium nitrite
NH4NO2(s)
N2(g) + 2H2O(i)
INDUSTRIAL PREPARATION (LARGE SCALE)
Industrially, Nitrogen is prepared by fractional distillation; Oxygen is obtained at the
same time. Air is purified by removing gas from it.
Then carbondioxide is removed and lastly water vapour. The remaining air is removed
and compressed. The liquidified air is allowed to evaporate. Because Nitrogen has a
lower boiling point, it evaporates first leaving liquid oxygen.
PROPERTIES OF NITROGEN
Physical
- it is colourless gas without smell
- it is a reactive gas
- it does not burn / doesn’t support combustion
- it is neither acidic nor basic
Chemical properties
1. Nitrogen is inert unlike Oxygen, it reacts under special conditions for example
- It reacts with some metals at very high temperatures forming nitrides e.g. Calcium
and magnesium.
3Mg(s) + N2(g)
Mg3N2(S)
3Ca + N2(g)
Ca3N2(s
2. It reacts with hydrogen to form ammonia (Haber’s process)
N2(g) + 3H2g
⇋
2NH3 (g)
AMMONIA (MH3)
Ammonia is a covalent compound. It has a molecular mass of 17.
LABORATORY PREPARATION OF AMMONIA
Apparatus – Round bottomed flask, clump, Bunsen burner, delivery tube, trough, curd
board, wire gauze, lime water.
Chemicals – Calcium hydroxide, Ammonium chloride, Calcium oxide.
Procedure
Ammonia is prepared by heating a mixture of calcium hydroxide and ammonium
chloride.
Ca(OH)2(S) + 2NH4Cl(S)
CaCl2(S) + 2H2O(I) + 2NH3(g)
The tube in which ammonia is generated is fixed in a slanting position to prevent the
water formed from running back and crack the whole tube.
- Concentrated sulphuric acid and anhydrous calcium chloride are not used to dry
ammonia because they react with it. Ammonia is collected by upward delivery
as it is lighter than air.
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HABER PROCESS (INDUSTRIAL PREPARATION)
Apparatus – Compressing chamber, catalytic chamber, cooling chamber, delivery tube.
Chemicals – Finely divided iron, Alminium Oxide, Water, Hydrogen, Nitrogen.
Procedure:
It is manufactured by reacting Nitrogen and hydrogen in the presence of finely divided
iron as a catalyst at temperatures 350ºC - 400ºC at a pressure of about 350 atmospheres.
N2(g) + 3H2(g)
⇋
2NH3(g)
Alminium Oxide is added to the catalyst to improve its performance. It makes it more
porous and this provides a high surface area to the reaction.
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The reaction is reversible hence it is not possible to convert all the reactants into
ammonia.
To separate ammonia from the mixture is cooled, only ammonia liquidfies and it is
separated.
The uncombined Nitrogen and hydrogen are recycled.
Another way of separation is to pass the mixture into water.
Only ammonia dissolves.
FACTORS AFFECTING THE REACTION (PROCESS)
1. Pressure
High pressure causes a better yield of ammonia because it favours the formation of the
smaller products. It also increases the speed of reaction because the reacting molecules
collide more often.
2. Temperature
At low temperatures the yield at equilibrium of Ammonia is higher but the reaction is
slow. At high temperatures the yield of ammonia is low but the reaction is fast, a
temperature of about 500ºC is used. The yield is good but the reaction is still too slow. A
catalyst is therefore necessary to speed up the reaction.
3. Catalyst
A catalyst speeds the reaction but does not affect the equilibrium. The catalyst should be
finely divided because reaction occurs only at the surface.
Properties of Ammonia
1. Ammonia is a colorless gas with a characteristic pungent smell.
2. It turns dump / wet red litmus paper blue. It is the only common alkaline gas.
3. It forms dense white fumes with hydrogen chloride gas
NH3(g) + HCl(g)
NH4Cl(s)
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Ammonia diffuses faster and white dense fumes will be formed near hydrogen
chloride gas – the white dense fume is ammonia chloride.
4. Ammonia is very soluble in water. The great solubility of ammonia can be
demonstrated using the fountain experiment.
Procedure:
A largely dry, round bottomed flask with ammonia is used. The mouth of the flask is
placed under water and a red litmus paper in a trough of gas jar. It is clamped firmly
in position. The spring clip at the end of the long glass tube is opened. Water slowly
rises up the tube until one drop is at the jet at the top.
The drop dissolves so much ammonia that there is a partial vaccum in the flask.
Water is sucked rapidly up the tube and enters the flask as a fountain. The litmus
paper turns blue.
5. Ammonia burns in a lot of air (oxygen). The flame is yellow green.
4NH3(g) + 3O2(g)
6H2O(i) + 2N2(g)
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Nb: Glass wool is to spread out the oxygen in order to bring it into greater contact with
ammonia.
In presence of a catalyst ammonia reacts with oxygen to form nitrogen monoxide.
The monoxide is easily oxidized to dioxide hence if a hot platinum or copper wire is
suspended in a beaker of concentrated ammonia and oxygen is bubbled through the
solution, redish brown fumes are seen. The fumes later turn white. The brown fumes
are due to nitrogen dioxide which turn white as ammonium nitrate is formed.
4NH3 (g) + 5O2(g)
4NO(g) + 6H2O(g)
2NO(g) + O2(g)
2NO2(g)
NO2(g) + O2(g) + 2H2O(l)
4HNO3(g)
HNO3 (g) + NH3(g)
NH4NO3(s)
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6. Ammonia reduces heated copper II oxide to copper i.e. copper turns from black to
brown.
3 CuO(s) + 2NH3(g)
3 Cu(s) + 3H2O(l) +N2(g)
(black)
(brown)
7. Ammonia burns in chlorine forming mist of hydrogen chloride gas. In excess
ammonia, dens white fumes of ammonia chloride are formed.
2 NH3(g) + 3 Cl2(g)
N2(g) + 6HCl(g)
HCl(g) + NH3(g)
NH4 Cl(s)
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8. Ammonia solution (Ammonium hydroxide) contains hydroxyl ions with metal
ions precipitates of the hydroxides are formed. Hence a blue precipitate forms
when aqueous ammonia is added to copper II sulphate solution. The precipitate
dissolves in excess ammonia forming a deep blue solution.
Cu(aq)²+ + 2OH-(aq)
Cu(OH)2(s)
Cu(OH)2(s) + 4NH3(aq)
Cu[NH3]2+4 + 2OH-(aq)
Tetra Ammine Copper II ions (deep blue solution)
Iron II (fe2+) forms a dirty green precipitate with ammonia insoluble in excess Iron III
(fe3+) forms a brown precipitate insoluble in excess.
Uses of ammonia
1. It is used in the manufacture of fertilizers e.g. Ammonium sulphate.
2. It is used in softening water.
3. It is used in making nitric acid.
4. It is used in making plastics.
5. Ammonium chloride is used in dry cells.
6. It is used in making explosives.
Test for Ammonia
It is the only common alkaline gas known. It changes the dump / wet litmus
paper blue.
Ammonia forms dense fumes of ammonium chloride when brought into
contact with fumes of hydrogen chloride from concentrated hydrochloric acid.
Subject
:
Chemistry
Topic
:
Nitrogen and its compounds
Sub-topic
:
Ammonia
Class
:
S.3
Size
:
50 students
In groups of 10 students
Time:
4 doubles a week
Course content and outline
A student is required to know ammonia, its state – covalent compound
Its monocular mass (17)
Its formula MH3
Its preparation on small scale (in the laboratory)
The reagents used (calcium hydroxide and ammonium chloride)
Relevant equation for ammonia formation
Precautions used in its preparation e.g. the tube in which ammonia is
generated is fixed in a slanting position to prevent the water from running
back and crack the whole tube.
Concentrated sulphuric acid and an hydrous calcium chloride are not used to
dry ammonia because they react with it.
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-
Its method of collection – by upward delivery – as it is lighter than air.
Diagramic illustration
Its preparation on large (industrial scale) Haber process
The reagents used e.g. N2 & H2, catalyst finely divided iron, temp 350ºc 400ºc, pressure about 350 atmospheres
-
⇋
N2(g) + 3H2(g)
2NH3(g)
To explain the factors affecting the reaction in ammonia formation.
Pressure, temperature, catalyst,
To explain the properties of ammonia, using diagrams where possible, both
physical and chemical properties.
To explain the uses of ammonia.
Organization advice
Student experiments can be done in groups. Each group should be able to
carry out assigned experiments perfectly.
Ensure that the group is small (5-6), manageable and maximum discipline
must be observed. When grouping them, choose a leader for each group,
he/she must be of average IQ (Intelligent quotient).
Practice team teaching whenever possible.
Make prior arrangements to produce the necessary resource materials.
Try out the experiment before giving them to students. This will help you (the
teacher) to get solutions to any short comings.
Improvise where necessary.
Relate the experiments to daily life.
Organize project work on Haber process.
Bear in mind the safety precautions
Get into the minds of students and think about the questions they are likely to
ask
Teach students to learn to make genuine conclusions from the experiments
carried out
Time should be set and followed
References
1.
Holderness and Lambert, (1987), A new certificate Chemistry, 6th edition,
pages 187.
2.
Wilford L. DR, (1984), Chemistry for first Examinations, pages 211-217.
3.
Sinclair M and Afolayan S. A., (1990), “O” level Chemistry for East Africa.
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Sample lesson plan
Topic:
Nitrogen and its compound
Sub topic:
Ammonia
Step
I
II
III
IV
V
Teacher’s activity
Teacher greets students and takes
roll call.
5 min Revision
of Ask questions on previous work
previous work
covered.
50 min Topic and notes Teacher dictates notes, draws
relevant diagrams.
15 min Tasks
Teacher gives students class
work / home work
5 min End
Teacher collects the students’
tasks.
Time
Content
10 min Introduction
Exercise
1.
2.
3.
4.
5.
6.
7.
8.
9.
Student activity
Respond when they
are called.
Answer questions
asked.
Take
notes
attentively.
Do the tasks (class
work / home work).
Students hand in
their tasks written
either in their books
or papers.
What is a catalyst?
What is a covalent compound?
Give any precaution taken during the manufacture of ammonia.
How is ammonia gas collected? Give a reason to your answer.
Give and explain the factors affecting the reaction during the manufacture of
ammonia.
Perform a simple experiment to show that Ammonia
(i)
forms dense white fumes with hydrogen chloride.
(ii)
is very soluble in water
(iii)
burns in a lot of air (oxygen)
(iv)
reduces heated Copper (II) oxide to copper
Give the industrial uses of ammonia.
Perform a simple test how to detect the ammonia gas using the litmus paper.
Ammonia reacts with lend (II) oxide according to this equation
2MH3(g) + 3PbO(s)
N2(g) + 3Pb(s) + 3H2O(i)
Calculate the volume of ammonia at room temperature that would be required
to completely react with 2.5g of lend II oxide.
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