Nitrogen and its Compound

1
Chapter 43
Nitrogen and its Compound
43.1 Introduction 43.2 Unreactive Nature of
Nitrogen 43.3 Direct Combination of Nitrogen and
Oxygen leading to Formation of Nitrogen
Oxides 43.4 Ammonia 43.5 Nitric(V)
Acid 43.6 Nitrates(V)
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43.1 Introduction (SB p.110)

• Nitrogen (first member of Group VA)
• Electronic configuration 1s22s22p3
• Complete octet by forming diatomic molecules N
  ? N
• Non-metal, colourless and odourless gas
• Very low melting and boiling points
• Slightly soluble in water and does not support
  combustion

Some information about nitrogen
Covalent radius (nm) 0.074
Melting point (C) 210
Boiling point (C) 196
Bond enthalpy (kJ mol1) 944
First ionization enthalpy (kJ mol1) 1 400
Electron affinity (kJ mol1) 3
Electronegativity 3.0
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43.1 Introduction (SB p.110)

• Nitrogen
• Mainly as free N2 molecules in the atmosphere
  (78 by volume)
• Combine with other elements in the form of
  proteins in all living things
• Liquid N2 is used as coolant
• Raw material for Haber process (manufacture of
  ammonia)
• Ammonia is the major component of nitrogenous
  fertilizers

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43.2 Unreactive Nature of Nitrogen (SB p.111)

• Nitrogen in gaseous state
• As diatomic molecules (N2) which are held by
  weak van der Waals forces
• 2 atoms are joined by extremely strong triple
  covalent bonds
• Bond enthalpy of the triple bond 944 kJ mol1
• Due to extremely strong covalent bonds and
  absence of bond polarity
• ? Nitrogen molecule is very unreactive

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43.2 Unreactive Nature of Nitrogen (SB p.111)
Bond enthalpies of some common covalent bonds
Bond Bond enthalpy (kJ mol1) Bond Bond enthalpy (kJ mol1)
N ? N O O H H C C 944 496 436 348 S S Cl Cl P P F F 264 242 172 158
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43.2 Unreactive Nature of Nitrogen (SB p.111)
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43.3 Direct Combination of Nitrogen and Oxygen
leading to Formation of Nitrogen Oxides (SB
p.112)
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43.3 Direct Combination of Nitrogen and Oxygen
leading to Formation of Nitrogen Oxides (SB
p.112)
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43.3 Direct Combination of Nitrogen and Oxygen
leading to Formation of Nitrogen Oxides (SB
p.112)

• The above reactions are very important in nature
• The NO2 formed dissolves in rainwater to produce
  nitric(V) acid and nitric(III) acid
• 2NO2(g) H2O(l) ?? HNO3(aq) HNO2(aq)

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43.3 Direct Combination of Nitrogen and Oxygen
leading to Formation of Nitrogen Oxides (SB
p.112)

• At high temperatures in car engines, N2 O2
  react to form NO(g) which emitted into air with
  exhausted gas
• The NO formed will be oxidized to NO2

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43.3 Direct Combination of Nitrogen and Oxygen
leading to Formation of Nitrogen Oxides (SB
p.113)

• In laboratory, we use the apparatus shown on the
  right to convert N2 into NO2

• When current is switched on, electric discharges
  occur in the gap between the electrodes
• NO is formed and followed by NO2

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43.3 Direct Combination of Nitrogen and Oxygen
leading to Formation of Nitrogen Oxides (SB
p.113)
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43.3 Direct Combination of Nitrogen and Oxygen
leading to Formation of Nitrogen Oxides (SB
p.113)

• The formation of N2O4 is exothermic
• ? N2O4 predominantes at low temperatures
• ? NO2 predominantes at high temperatures
• ? the colour of mixture fades on cooling,
  darkens on warming

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43.3 Direct Combination of Nitrogen and Oxygen
leading to Formation of Nitrogen Oxides (SB
p.114)
Check Point 43-1 (a) Draw the structures of the
following compounds. (i) Dinitrogen
monoxide (ii) Nitrogen monoxide (iii) Dinitrogen
trioxide (iv) Nitrogen dioxide (v) Dinitrogen
tetraoxide (vi) Dinitrogen pentaoxide
Answer
15
43.3 Direct Combination of Nitrogen and Oxygen
leading to Formation of Nitrogen Oxides (SB
p.114)
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43.3 Direct Combination of Nitrogen and Oxygen
leading to Formation of Nitrogen Oxides (SB
p.114)
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43.3 Direct Combination of Nitrogen and Oxygen
leading to Formation of Nitrogen Oxides (SB
p.114)
Check Point 43-1 (b) Arrange N2, O2 and F2 in an
ascending order of reactivity. Explain the order
briefly.
Answer
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43.4 Ammonia (SB p.114)
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43.4 Ammonia (SB p.114)

• Ammonia
• one of the most fundamental raw materials for
  modern industries
• important source of fertilizers and 85 of
  ammonia is used to make nitrogenous fertilizers
  (e.g. (NH4)2SO4, NH4NO3)
• making fibres and plastics (rayon, nylon)
• making nitric(V) acid (used to make fertilizers,
  dyes)
• making household cleaners
• making detergents

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43.4 Ammonia (SB p.115)
Percentages of ammonia used in different
industries
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43.4 Ammonia (SB p.115)
Manufacture of Ammonia by the Haber Process
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43.4 Ammonia (SB p.116)
Flow diagram for the Haber process
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43.4 Ammonia (SB p.116)
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43.4 Ammonia (SB p.116)

• Mixture of CO H2O is mixed with steam and
  passed over a heated catalyst
• CO(g) H2O(l) ?? CO2(g) H2(g)
• The CO2 formed is dissolved in water under
  pressure
• The gases (N2 H2) are purified before
  proceeding to the next stage
• ? Compounds of oxygen and sulphur will poison
  the catalyst

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43.4 Ammonia (SB p.116)

• Purified N2 H2 are mixed in ratio of 3 1 by
  volume
• ? Compressed to 200 1000 atm and heated in
  the heat exchanger
• ? Hot gaseous mixture is passed over iron in
  the catalytic chamber
• ? Gases contain 10 15 of NH3 and unreacted
  N2 and H2 when leaving the chamber
• ? The gases are cooled after passing through
  the heat exchanger
• ? NH3 is liquefied under pressure and unreacted
  gases are recycled

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43.4 Ammonia (SB p.117)
Physico-chemical principles

• According to Le Chateliers principle,
• (1) high pressure will increase the yield
• (2) low temperature will increase the yield

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43.4 Ammonia (SB p.117)

• Apart from increasing yield, the reaction rate
  should be fast
• ? Low temperatures would lower the rate of
  reaction
• ? optimum temperature is around 500C which is
  high enough for reaction to proceed quickly but
  low enough to give satisfactory yield
• Catalyst is used to increase the reaction rate
• ? poisoned by CO, CO2, H2S
• ? Gases entering the catalytic chamber should
  have high purity!!

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43.4 Ammonia (SB p.118)
Chemical Properties of Ammonia
As a base
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43.4 Ammonia (SB p.118)
Reaction with Acids

• Formation of NH4Cl by reacting NH3 with HCl
• NH3(aq) HCl(aq) ?? NH4Cl(s)

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43.4 Ammonia (SB p.118)
Reaction with Metal Salts
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43.4 Ammonia (SB p.119)
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43.4 Ammonia (SB p.119)
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43.4 Ammonia (SB p.120)
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43.4 Ammonia (SB p.120)
As a Reducing Agent
Reaction with Oxygen

• NH3 does not burn in air or support combustion
• It burns in O2 with a yellow flame, forming N2
  and water vapour

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43.4 Ammonia (SB p.120)

• In the presence of catalyst (red hot spiral coil
  of platinum at 800 900C), NH3 is oxidized to
  NO by O2

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43.4 Ammonia (SB p.121)
Reaction with Copper(II) Oxide
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43.4 Ammonia (SB p.121)
Check Point 43-2 (a) Write chemical equations to
show how hydrogen is produced from (i) the
reaction of natural gas (mainly methane) with
water (ii) the reaction of coal (mainly
carbon) with water.
Answer
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43.4 Ammonia (SB p.121)
Answer
39
43.4 Ammonia (SB p.121)
(b) (i) The forward reaction is exothermic.
According to Le Chateliers principle,
exothermic reactions are favoured at low
temperatures. Therefore, an increase in
temperature will favour the backward reaction,
and thus decrease the yield of ammonia. (ii)
According to Le Chateliers principle, a high
pressure will increase the yield of ammonia as
the forward reaction is accompanied by a decrease
of volume from four to two volumes of the gas.
Therefore, a decrease in pressure will decrease
the yield of ammonia. (iii) Addition of a
suitable catalyst will increase the rate of both
forward and backward reactions to the same
extent. As it does not change the position of the
equilibrium, the yield of ammonia remains
constant.
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43.4 Ammonia (SB p.121)
Check Point 43-2 (contd) (c) Ammonia reacts
with oxygen in two different ways. Give
equations for both of these reactions and
explain how one of them is used industrially to
produce nitric(V) acid.
Answer
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43.4 Ammonia (SB p.121)
(c) In the absence of catalyst, ammonia burns to
give molecular nitrogen and water vapour. 4NH3(g)
3O2(g) ?? 2N2(g) 6H2O(g) Industrially, in the
presence of red hot platinum-rhodium at about
850C, ammonia is catalytically oxidized to
nitrogen monoxide. 4NH3(g) 5O2(s) 4NO(g)
6H2O(g) The nitrogen monoxide formed then reacts
with oxygen from the air to give nitrogen
dioxide. 2NO(g) O2(g) ?? 2NO2(g) The nitrogen
dioxide reacts with excess air and water to
produce aqueous nitric(V) acid. 4NO2(g) O2(g)
2H2O(l) ?? 4HNO3(aq) H2O(l) 3NO2(g) ??
2HNO3(aq) NO(g) The NO(g) is recycled and
subsequently combines with more oxygen and water
to give more nitric(V) acid. Finally, the product
is distilled to give concentrated nitric(V) acid
(containing 68 HNO3).
Pt Rh 850C
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43.5 Nitric(V) Acid (SB p.121)

• Nitric(V) acid
• a very strong acid
• turns yellow on storage as the formation of
  dissolved NO2 from decomposition of some acid
• 4HNO3(l) ?? 4NO2(aq) 2H2O(l) O2(g)
• keep in brown bottles as light will speed up
  decomposition
• used to make explosives, nylon, fertilizers and
  dyestuff synthesis

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43.5 Nitric(V) Acid (SB p.122)
Manufacture of Nitric(V) Acid from the Catalytic
Oxidation of Ammonia
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43.5 Nitric(V) Acid (SB p.122)
2. The NO formed then reacts with O2 to form
NO2 2NO(g) O2(g) ?? NO2(g) 3. The NO2 reacts
with excess air and water to give aqueous
HNO3 4NO2(g) O2(g) 2H2O(l) ?? 4HNO3(aq)
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43.5 Nitric(V) Acid (SB p.122)
Nitric(V) Acid as an Oxidizing Agent

• HNO3 is a strong oxidizing agent, especially when
  concentrated
• NO3 acts as an electron acceptor when H ions
  are present
• HNO3 can be reduced to different nitrogen
  compounds with different oxidation states,
  depending on
• 1. the conc. of HNO3
• 2. nature of substance being oxidized

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43.5 Nitric(V) Acid (SB p.122)

• If dilute or moderately concentrated HNO3 is
  reduced, NO will be formed
• 4HNO3(aq) 3e ?? 3NO3 (aq) 2H2O(l)
  NO(g)or NO3(aq) 4H(aq) 3e ?? NO(g)
  2H2O(l)
• If concentrated HNO3 is reduced, NO2 will be
  formed
• 2HNO3(aq) e ?? NO3 (aq) NO2(g) H2O(l)or
  NO3(aq) 2H(aq) e ?? NO2(g) H2O(l)
• The electrons are supplied by the reducing agent
  in the reaction

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43.5 Nitric(V) Acid (SB p.123)
Reaction with Copper

• Cu reacts with warm dilute HNO3 to give NO3Cu(s)
  8HNO3(aq) ??
• 3Cu(NO3)2(aq) 4H2O(l) 2NO(g)
• The NO formed reacts with atmospheric O2 to give
  NO22NO(g) O2(g) ?? 2NO2(g)

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43.5 Nitric(V) Acid (SB p.123)

• Conc. HNO3 (14 M) reacts with Cu to give NO2 and
  a blue solution of Cu(NO3)2
• Cu(s) 4HNO3(aq) ?? Cu(NO3)2(aq) 2H2O(l)
  2NO2(g)

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43.5 Nitric(V) Acid (SB p.123)
Reaction with Iron(II) Ion

• Conc. HNO3 oxdizes green Fe2 ions to brown Fe3
  ions while itself reduced to NO
• 3Fe2(aq) NO3(aq) 4H(aq)
  ?? 3Fe3(aq) NO(g) 2H2O(l)
• The NO formed reacts with atmospheric O2 to form
  NO2
• 2NO(g) O2(g) ?? NO2(g)

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43.5 Nitric(V) Acid (SB p.123)
Reaction with Sulphur

• Hot concentrated HNO3 oxidizes sulphur to give
  sulphuric(VI) acid and brown fumes of NO2
• S(s) 6HNO3(aq) ??
• H2SO4(aq) 6NO2(g) 2H2O(l)

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43.5 Nitric(V) Acid (SB p.123)
Check Point 43-3 Account for the following
observation by giving a balanced
equation. (a) Nitrogen monoxide turns brown when
exposed to air.
Answer
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43.5 Nitric(V) Acid (SB p.123)
Check Point 43-3 (contd) Account for the
following observation by giving a balanced
equation. (b) Nitric(V) acid turns yellowish
brown on standing.
Answer
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43.5 Nitric(V) Acid (SB p.123)
Check Point 43-3 (contd) Account for the
following observation by giving a balanced
equation. (c) Silver dissolves in dilute
nitric(V) acid, yielding a colourless gas.
Answer
54
43.5 Nitric(V) Acid (SB p.123)
Check Point 43-3 (contd) Account for the
following observation by giving a balanced
equation. (d) The nitrate of a metal ion
decomposed on heat to give the metal.
Answer
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43.6 Nitrates(V) (SB p.124)
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43.6 Nitrates(V) (SB p.124)
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43.6 Nitrates(V) (SB p.124)
Action of Heat on Nitrates(V)
When metal nitrates(V) in solid form are strongly
heated, they decompose differently according to
their thermal stability
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43.6 Nitrates(V) (SB p.124)
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43.6 Nitrates(V) (SB p.125)
Brown Ring Test for Nitrate(V) Ions

• The brown ring test is used to detect nitrate(V)
  ions in aqueous solutions

Procedure 1. Mix a freshly prepared FeSO4
solution with a solution suspected of containing
nitrate(V) ions in a test tube 2. Conc. H2SO4 is
added carefully along the side tothe bottom of
the test tube with the test tube tilted
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43.6 Nitrates(V) (SB p.125)

• Formation of a brown ring confirms the presence
  of nitrate(V) ions in the solution

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43.6 Nitrates(V) (SB p.125)
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43.6 Nitrates(V) (SB p.125)
Check Point 43-4 Give the name of the ion
responsible for the following observation. (a) An
ion produces a blue precipitate with ammonia
solution. The blue precipitate redissolves in
excess ammonia solution to give a clear deep
blue solution.
Answer
63
43.6 Nitrates(V) (SB p.125)
Check Point 43-4 (contd) Give the name of the
ion responsible for the following
observation. (b) An ion produces a dirty green
precipitate with ammonia solution.
Answer
64
43.6 Nitrates(V) (SB p.125)
Check Point 43-4 (contd) Give the name of the
ion responsible for the following
observation. (c) An ion gives a positive result
in the brown ring test.
Answer
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The END