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CIE A Level Chemistry
12
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Nitrogen & its Compounds
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Reactivity of Nitrogen
Nitrogen is a diatomic molecule and the main unreactive gas in air
78% of air is nitrogen gas
The lack of reactivity of nitrogen gas can be explained by looking at its intramolecular bonds
Intramolecular bonds are the bonds within a molecule
Bonding in nitrogen
The electron configuration of a nitrogen atom is 1s2 2s2 2p3
To achieve a full outer shell of electrons, it needs to gain three electrons
Nitrogen atoms therefore form a triple covalent bond between two nitrogen atoms in which they share
three electrons with each other
Bonding in nitrogen
The diagram shows a triple covalent bond between two nitrogen atoms to achieve a full outer shell of
electrons
The bond enthalpy of the nitrogen triple bond is 1000 kJ mol-1
This means that 1000 kJ of energy is needed to break one mole of N2 triple bond
As it is so difficult to break the nitrogen triple bond, nitrogen and oxygen gas in air will not react with
each other
Only under extreme conditions will nitrogen gas react (e
...
during a thunderstorm)
Polarity of nitrogen
The electrons in a nitrogen molecule are shared equally between the two nitrogen atoms
Therefore, nitrogen molecules are nonpolar molecules
Polarity in a nitrogen bond
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The nitrogen atom in ammonia uses its lone pair of electrons to form a dative bond with a proton to
form the ammonium ion
The ammonium ion has a tetrahedral shape in which all bonds have the same length
Bonding in the ammonium ion
The tetrahedral shape of the ammonium ion
Preparation of ammonia gas from an ammonium salt
Ammonia gas can be prepared from an ammonium salt and a base in an acid-base reaction:
2NH4Cl (s) + Ca(OH)2 (s)
heat
⎯⎯⎯⎯ CaCl2 (s) + 2H2O (l) + 2NH3 (g)
Ammonium chloride (NH4Cl) and calcium hydroxide (Ca(OH)2) are mixed together and then heated
NH4+ acts as an acid (proton donor) and OH- acts as a base (proton acceptor)
This acid-base reaction can be used to test if an unknown solution contains ammonium ions
If the unknown solution does contain ammonium ions, it will react with calcium hydroxide to form
ammonia gas
This ammonia gas will turn damp red litmus paper blue
Producing and testing for ammonia
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Nitrogen Oxides
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Oxides of Nitrogen
Natural occurrence of nitrogen oxides
Due to its lack of reactivity, only under extreme conditions will nitrogen react with oxygen to form
gaseous nitrogen oxides
An example of these extreme conditions is lightning which can trigger the formation of nitrogen(II) and
nitrogen(IV) oxides (NO and NO2 respectively)
The chemical equations for these reactions are:
N2 (g) + O2 (g) → 2NO (g)
N2 (g) + 2O2 (g) → 2NO2 (g)
Man-made occurrence of nitrogen oxides
In the engine of a car, a mixture of air and fuel is compressed and ignited by a spark
Air consists of 78% of nitrogen and 21% of oxygen
Under the high pressure and temperature inside a car engine, nitrogen can react with oxygen to form
nitrogen oxides
These nitrogen oxides are released into the atmosphere through the car’s exhaust fumes
Catalytic removal of nitrogen oxides
The nitrogen oxides released through cars’ exhaust fumes pollute the atmosphere
Many car exhaust systems are therefore fitted with catalytic converters to reduce the pollutants from
motor vehicles
The nitrogen oxides are reduced on the surface of the hot catalyst (eg
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Catalytic converters help reduce the pollutant emissions from motor vehicles
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The diagram shows the formation of PAN from the photochemical reaction between VOCs and nitrogen
oxide
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The diagram shows the formation of acid rain by the oxidation of nitrogen(IV) oxide
Nitrogen oxide as a catalyst
Acid rain also contains dilute sulfuric acid (H2SO4)
Sulfur(IV) oxide (SO2) is another pollutant found in the atmosphere
When SO2 is oxidised, it forms SO3 which reacts with rainwater to form dilute sulfuric acid as follows:
SO3 (g) + H2O (l) → H2SO4 (aq)
Nitrogen oxides can directly cause acid rain but can also act as catalysts in the formation of acid rain
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