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GROUP 15 (AS)
NITROGEN
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1.Nitrogen undergoes few reactions
and is unreactive due to
Very strong N N bond
Requires a lot of energy*to break
Or Ea is very high
N N bond energy : + 994 kJmol-1
Most reactions tends to be
endothermic and do not occur at rtp
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However nitrogen does react under
the following conditions :
1) presence of catalyst to lower Ea
2) high temperature to overcome
the high Ea
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2.Reactions of nitrogen
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a. Formation of nitrogen oxides
(NOx)
Conditions : occurs under high
temperature and high pressure
in car engines and during lightning
flashes
Eg : N2(g)+O2(g)
⇔ 2NO (g)
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From air
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Other possible oxides formed :
N2O , dinitrogen oxide and NO2
Enthalpies of formation of NOx are
positive due to the high bond
energy of N2
molecule
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Note : However nitrogen and
oxygen do not react at room
temperature.
Reasons :
1) reaction is endothermic
2) nitrogen has a high bond energy
3) a high Ea is required
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b. Haber process
Equation :N2(g) +3H2(g) ⇔ 2NH3(g)
Conditions : High pressure and high
temperature , catalyst
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c. Formation of nitrides ( N3- )
→
Mg3N2
 Eg : 3Mg + N2
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magnesium
nitride
 Conditions : High temperature and
burning Mg
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AMMONIA
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1. Shape : trigonal pyramidal
3 bond pairs and one lone pair of electrons
Bond angle : 107.30
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2.Presence of lone pair or nonbonding pair of electrons on
nitrogen,
therefore are proton acceptors (
acts as a weak base )
a. aqueous ammonia :
+ +
⇔
NH
OH
NH3 + H2O
4
base
acid
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b. neutralises acids → ammonium
salts
Eg : NH3 + HCl → NH4Cl
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Note :Other rxns/changes involving
lone pair of electrons on nitrogen :
(1)AgCl (s) dissolves in aq NH3
Due to complex formation,
[Ag(NH3)2]+
H3 N
Ag+ NH3
lone pair on N forms dative bond
with Ag+
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(2)Hydrogen bonds between NH3
molecules
H3 N
H-NH2
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H bond
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(4)Formation of dative bonds
BF3
Eg : NH3 + BF3 → H3N
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(3)Formation of NH4
H+
H3 N
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3. Manufactured by Haber process
N2 (g) + 3H2 (g) ⇔ 2NH3 (g)
Conditions :
Fe (s) , 450 – 5500 C , 250 atm
N2 from liquified air and H2 from
natural gas ( CH4)
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Diagram
Hot gases
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Nitrogen &
hydrogen
recycled
Heat
exchanger
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Nitrogen &
hydrogen
Nitrogen,hydrogen,
& ammonia
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condenser
Condensed
ammonia
Note : Heat exchanger warm gas
mixture & speed up reaction
converter
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4.Heat
compounds with an
alkali
NH3 gas evolved
+
NH4 + OH- ⇔ NH3 (g) + H2O
Notes:
+
a)NH4 is acidic/ a proton donor
+
b)Test for NH4 ions
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NH4
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5. Uses of ammonia :
a. manufacture of HNO3(Ostwald Process)
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4NH3 + 5O2 →
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4NO + 6H2O , ∆H=neg
6 atm
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2NO + O2 → 2NO2
4NO2 + O2 + 2H2O → 4HNO3
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Uses of HNO3
: manufacture
of
1)explosives eg TNT
2)organic dyes
3)AgNO3 (used in photography)
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CO2
NH3
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oxidation
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b. manufacture of nitrate fertilisers
Eg N2 + H2 Add directly to soil ammonia
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HNO3
H2SO4
urea
Ammonium
sulphate
H3PO4
Ammonium
phosphate
Ammonium
nitrate
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i) Major nutrients required by plants
: N , P(K)
Nutrients added to soil by use of
fertilisers: Eg
1) Nitrogenous fertilisers (nitrates or
ammonium salts) supplies N
2) Phosphate fertilisers supplies P
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ii) Nitrogeneous fertilisers has to be
added from time to time as
presence of denitrifying bacteria in
soil makes the soil poorer in
nitrogen
+
nitrogen in air
NO3 / NH4
However the bacteria has no effect
on phosphates
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Environmental problems
1. Uncontrolled use of nitrate fertilisers
:
If excess fertiliser is added to soil,
Excess nitrates leach out of soil into
streams or lakes
Excessive growth of algae/aquatic plants
When plants/algae die, oxygen is used
up
Fish/aquatic life unable to live
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Process called eutrophication
 Note : also causes general
poisoning of water supplies
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NO-3poisonous )
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To reduce pollution effects :
a) Use less fertiliser or use natural
fertiliser
b) Use other methods of increasing
yield
Eg : use more productive strains of
wheat / grain etc
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2. Nitrogen oxides (NOx) pollution
from car engines :
Under high temperature :
NOx
N2 + O2 → 2NO
Nitrogen oxides
2NO + O2 → 2NO2
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NOx damages the environment in
the following ways :
H2O
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a. + H2O + O2 to form HNO3( acid rain )
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+½O2
→ 2HNO3
b. + other air pollutants to form
ozone ( irritates eyes )
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c. NO2 can react with SO2 ( from burning
of fossil fuel) in the presence of water
vapour in the atmosphere :
1) NO2 + SO2 + H2O → H2SO4 + NO
2) 2NO + O2→ 2NO2
Overall equation:
2SO2(g)+2H2O(g)+O2(g) → 2H2SO4
NO2 (g) functions as homogenous
catalyst
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Overall equation:
2SO2 (g) + O2 (g) → 2SO3
NO2 (g) functions as homogenous catalyst
and oxidising agent
Note : NO2 can also be a reducing agent
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1) SO2 + NO2 → SO3 + NO
+4
oxd agent +6
2) 2NO + O2 → 2NO2
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To prevent NOx pollution :
Use catalytic converter to reduce
pollutants in carexhaust
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Catalytic converters for car exhaust
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1.Air pollutants from motor vehicles /
car exhaust fumes :
a) from incomplete combustion of
hydrocarbon fuel :
CO
Unburnt hydrocarbons
Carbon soot
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b) from reaction between O2 and N2
in air under the high temperature in
car engines:
Nitrogen oxides, NOx (NO and NO2)
Equations: for formation of NOx
i) N2
+ O2 →
2NO
ii)2NO + O2 → 2NO2
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c) from anti knock additives:
Lead oxides
Note :no lead oxide present if
unleaded petrol is used
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2.Effect of pollutants on humans :
 a.Hydrocarbons – breathing
problems
 b.CO –react with haemoglobin /
reduces absorption of oxygen
 c.NOx – acid rain* / breathing
problems / photochemical smog
* main hazard
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3.Use of catalytic converter :
To convert gases which are
environmentally harmful into
harmless or less harmful gases
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4.The catalytic converter contains a
filter of fine meshed aluminium
alloy coated with platinum*or
platinum and rhodium mixture
Fine meshed : greater surface area
therefore faster reaction
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5. Converter speeds up the
following reactions :
a. to remove CO :
CO + nitrogen oxides → CO2
+ N2
+ 2CO → N2 + 2CO2
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Eg : 2NO
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NO + H2
→ H2O + ½N2
2NO+ 2CO → N2 + 2CO2
c. to remove hydrocarbons :
Eg :Hydrocarbons + nitrogen oxides
→ CO2+ N2 + H2O (qualitative eqn)
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b. to remove NOx:
Eg :Hydrocarbons + nitrogen oxides
→ CO2+ N2 + H2O (qualitative eqn)
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6.Diagram
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7.Drawbacks of usage:
a. high cost (*)
b. car must run on lead-free petrol
or the catalyst will be poisoned by
lead or lead may be trapped in the
converter
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c. catalyst system only effective at
sufficiently high temperature
With stop start usage , converter
would be inactive for much of the
time as it would not reach the
operating temp
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8.However exhaust fumes are still
hazardous :Reasons
a. toxic gases not removed until
converter has warmed up
b. too much CO to be completely
removed
c. converter may be less efficient
over a period of time / gets clogged
up
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d. CO2 passes through ,
causes global warming
e. SO2 passes through , causes
acid
rain
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