The pBlock Element The Nitrogen

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Section 10.2

• The p-Block Element - The Nitrogen

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Different oxidation states of nitrogen
Oxidation States
Compounds
-3
NH3 NH4 N3-
NH2NH2
-2
-1
NH2OH
0
N2
1
N2O
2
NO
3
HNO2
4
NO2 N2O4
5
HNO3
3
Bonding in the nitrogen compounds

• N 1s2 2s2 2px1 2py1 2pz1
• It can only form 3 normal covalent bonds by using
  the 3 singly filled orbitals.
• No octet expansion because there is no underlying
  d-orbital no promotion of electron.
• How about HNO3 ?

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HNO3

• N 1s2 2s2 2px1 2py1 2pz1
• N is sp2 hybridized, 2 of them are singly filled,
  forming normal covalent bond by overlapping with
  orbitals of other atoms. One of them is full
  filled, forming dative bond with oxygen atom.
• Singly filled p orbital is left for the pi bond
  formation.
• For bond length and bond angle refer to section
  4.

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NO2 and N2O4

• Structure

?
ve
2 NO2 N2O4 ?H
Increase in temperature equilibrium
position would shift to
left
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Unreactive Nature of Nitrogen

• Nitrogen occurs mainly as free N2 (N?N)
  molecules.
• Nitrogen is very inert due to
• a very high bond enthalpy 946 kJ mol-1
• absence of bond polarity.
•  
• Both nitrogen and phosphorus belongs to group V.
• Nitrogen is very inert but phosphorus will ignite
  in
• the room condition. Why?

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P4 and N2 is energetically more Stable than P2
and N4 resp.
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Nitrogen

• Nitrogen molecules consist of triple bond but
  phosphorus consists single bond only.N forms 1
  ? bond and 2 ? bonds as the size of N is very
  small so that the effective overlapping of p
  orbitals is possible.Phosphorus does not form ?
  bonds (or form very weak ? bonds) with itself
  because the size of P is larger and thus
  effective overlapping between 3p orbitals is not
  possible (less likely).

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Nitrogen

• The inertness of nitrogen is due to the high bond
  enthalpy (946 kJ mol-1) of N?N triple bond but
  the P-P bond enthalpy is low (209 kJ mol-1)This
  low bond enthalpy of phosphorus is due to the
  high bond angle strain in the P4 molecule.P4 is
  tetrahedral shape, the bond angle is 600 but the
  hybridization of P should be sp3. Therefore the
  P tends to be very reactive.

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Chemical properties of nitrogen

• Direct Combination of Nitrogen and Oxygen
• N?N triple bond is a very strong bond, the
  activation energy of the reaction is thus very
  high.
• lightning
  O2(g) in air
• N2(g) O2(g) 2NO(g) 2NO2(g)
• colourless reddish brown

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Chemical properties of nitrogen

• Lightning can break the N?N triple bond to
• overcome the large activation energy.
• Important in nature because it is one of the
• important source of nitrogen for plants.
• When raining, NO2 will dissolve in the rain and
  fall
• into the ground.
• 2NO2(g) H2O ? HNO3(aq) HNO2(aq)

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Chemical properties of nitrogen

• In Laboratory
• In laboratory, we use a electric arc to do this.

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Chemical properties of nitrogen

• Inside the car engine, similar reaction may occur
  due to the high temperature inside.
• Nowadays, many car install catalytic converter,
  which convert NO to harmless N2 by reacting with
  another poisonous gas CO inside the car engine
• CO(g) 2NO(g) ? CO2(g) N2(g)

Pt / Pd
The exhaust gases react with one another (CO and
NO) on the hot surface of the catalyst in a
series of reactions to produce harmless gases
(CO2 and N2).
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Chemical properties of nitrogen

• Reacts with Li, Mg and Ca
• Forming metal nitride
• 6 Li N2 ? 2 Li3N
• 3 Ca N2 ? Ca3N2
• Why Li (others are group IIA elements)?
• Nitride dissolves in water to form ammonia and
  alkaline solution
• Ca3N2 6 H2O ? 3 Ca(OH)2 2 NH3

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Laboratory preparation of ammonia

• Heating ammonium salts with bases
• e.g. ammonium sulphate and calcium hydroxide
• Ca(OH)2(s) (NH4)2SO4(s) ? 2 NH3 2 H2O
  CaSO4

Common drying agents cannot be used
Conc. H2SO4 why ?
Anhydrous CaCl2 why ?
Use anhydrous CaO
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Industrial preparation of ammonia

• Haber Process
• N2(g) 3H2(g) 2NH3(g)

Fe, 450oC 200 atm.
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Raw materials of Haber process

• Production of H2 and N2
• N2 is come from the fractional distillation of
  liquid air.
• H2 is come from steam reforming of natural gas or
• hydrocarbons.
• Ni
• CH4(g) H2O(g) ? CO(g) 3H2(g)
• CO(g) H2O(g) ? CO2(g) H2(g)
• CO2 and any residual CO would be removed.

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The process

• Dryer and purifier
• Before N2 and H2 are put into the catalytic
  chamber, they must be dried and purified first.
  Why ?
• Avoid poisoning of the catalyst

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The process

• Compressor and Heater
• N2(g) 3H2(g) 2NH3(g) ?H -92.kJ mol-1
• From the above equation, increased / decreased
  pressure will increase the percentage yield of
  product.
• Therefore, the N2 and H2 mixture is compressed
  to 250 - 500 atm. compromise between the yield
  and the cost.

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The process
From the above equation, an increase / decrease
in temperature would increase the yield of the
reaction.
Then why should we increase the temperature ?
Increase the rate of reaction.
N2 and H2 mixture is heated to 450 - 500oC
compromise between the rate and the yield.
What should be the mole ratio of N2 and H2?
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The process

• Catalytic Chamber
• Fe
• N2(g) 3H2(g) 2NH3(g)
• Catalyst finely divided iron catalyst
• The yield of this reaction is low. There is
  only about 10-20 of NH3 produced.
• Heat exchanger
• The gas mixture is cooled by water or by the
  incoming gases.

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Heat exchanger (inside the catalyst chamber)

• Warm up the incoming gases but cool down the
  gaseous product.

Cool N2 and H2
Cool N2 and H2 being warmed up
Tube packed with catalyst
N2 and H2 and NH3
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The process

• Cooler
• Ammonia is liquefied and separated from the
• unloaded mixture of nitrogen and hydrogen gases.
• Lastly, the unreacted gases (about 80 - 90 H2
  and N2 gases) circulate back to the catalytic
  chamber to react once more.

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Laboratory demonstration of Haber process

• Nitrogen and hydrogen gas are passed to and fro
  between the two syringes over heated iron wool.
• The gaseous product is then expelled from the
  syringe onto a moist red litmus paper.
• The litmus paper turns blue, showing the presence
  of ammonia.

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Physical properties of ammonia

• Colourless and alkaline gas
• Pungent smell
• Very soluble in water (Fountain experiment)
• Higher m.p. and b.p. due to formation of H-bond
  (compare with phosphine PH3)

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Chemical properties of ammonia

• Basic character

The lone pair on the nitrogen atom of ammonia has
also caused the molecule to have weakly basic
character because it can accept protons from
water and liberate hydroxide ions NH3 H2O
NH4 OH-
Owing to the release of OH- ions in aqueous
solution, ammonia will form precipitate
(insoluble hydroxide) with metal ion. Some
metal hydroxide will redissolve in excess NH3(aq).
Cu2(aq) 2 OH-(aq) ? Cu(OH)2(s)
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Chemical properties of ammonia

• Formation of complex ion
• It can form complex ions with metal ions by
  serving as ligands, utilizing the lone pair
  electrons in N
• Examples Cu(NH3)2, Ag(NH3)2, Zn(NH3)42
• If the complex ion is stable enough, i.e. with
  large enough Kst, the insoluble hydroxide formed
  would be redissolved in excess ammonia
• Cu(OH)2(s) 4 NH3(aq) ? Cu(NH3)42(aq) 2
  OH-(aq)
• Zn(OH)2(s) 4 NH3(aq) ? Zn(NH3)42(aq) 2
  OH-(aq)

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Chemical properties of ammonia

• With oxygen
• Burn in pure oxygen
• 4 NH3 3 O2 ? 2 N2 6 H2O

Catalytic oxidation 4 NH3 5 O2 ? 4 NO 6
H2O The catalyst surface remain red hot during
the reaction. If copper wire is used as the
catalyst, it will melt and fall down into the
solution, forming Cu(NH3)42.
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Chemical properties of ammonia

• Ammonia as a Reducing Agent
• With chlorine
• A jet of ammonia ignites spontaneously in a
• jar of chlorine.
• 2 NH3 3 Cl2 ? N2 6 HCl
• 6 NH3(g) 6 HCl(g) ? 6 NH4Cl(s)
• 8 NH3 3 Cl2 ? N2 6 NH4Cl
• What would you see ?

Dense white fume of NH4Cl
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Chemical properties of ammonia

• With Copper (II) oxide
• 2NH3(g) 3CuO(s) ? 3Cu(s) N2(g)
  3H2O(l)

Nitrogen gas
water
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Chemical properties of ammonia

• Thermal decomposition
• It is the reverse reaction of Haber process.
• 2 NH3(g) ? N2(g) 3 H2(g)

2a cm3 of NH3 would decompose to give 4a cm3 of
N2 and H2 gases. These would reduce to a cm3 N2
after passing through copper (II) oxide. From the
results, formula of ammonia can be determined.
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Chemical properties of ammonia

• Formation of ionic amide
• 2 Na(s) 2 NH3(l) ? 2 NaNH2(s) H2(g)
• Test
• It gives dense white fume of ammonium chloride
  with hydrogen chloride gas.
• It also turns moist red litmus paper blue.

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Uses of ammonia

• Making ammonium compounds which are used as
  fertilizers.
• Manufacture of nitric acid.
• Liquid ammonia is used in large scale
  refrigerating plant.
• Use in laundry work and glass cleaning.

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Industrial preparation of nitric acid

• Catalytic Oxidation of Ammonia
• Pt / Rh
• 4 NH3 5 O2 ? 4 NO 6 H2O
• 2NO O2 ? 2 NO2
• 3NO2 H2O ? 2 HNO3 NO OR
• 4 NO2 O2 2 H2O ? 4 HNO3 (overall)

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Industrial preparation of nitric acid

• Catalytic Chamber Pt
• 4NH3(g) 5O2(g) 4NO(g) 6H2O(g)
• ?H -907 kJ mol-1

Pressure 1 atm ToC 900oC Catalyst Pt /
Rh The product gases (NO, H2O) are used to heat
up the incoming gases (NH3, O2).
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Industrial preparation of nitric acid

• Cooler
• Excess O2 will react with NO(g) to form NO2(g).
• 2NO(g) O2(g) ? 2NO2(g) ?H -117 kJ mol-1

According to the Le Chateliers Principle, the
ToC should be high or low if you want to get
higher yield of product?
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Industrial preparation of nitric acid

• Absorption tower
• Dissolve the NO2 into water to obtain HNO3.
• 3NO2(g) H2O(l) ? 2HNO3(aq) NO(g)
• ?H -130 kJ mol-1

The successive oxidation and hydration of
nitrogen dioxide are carried out with continuous
water cooling in a stainless steel absorption
tower. The product is distilled to give
concentrated HNO3 which is an azeotrope,
containing 68 HNO3. The NO given off is
recycled.
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Industrial preparation of nitric acid

• A catalytic chamber
• B purifier and dryer
• C cooler
• D heat exchanger
• E absorption tower
• P hot water
• Q inert gases
• R nitric (V) acid

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Laboratory preparation of nitric acid

• It is prepared by the action of conc. H2SO4 on
  nitrate. And the nitric acid is collected under
  very cold water.
• H2SO4 NaNO3 ? HNO3 NaHSO4
• Nitric acid distilled out is yellow in colour due
  to the presence of NO2 by decomposition of HNO3.
• 4 HNO3 ? 4 NO2 O2 2 H2O

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Physical properties of nitric acid

• Colourless volatile liquid (b.p. 86oC)
• Conc. nitric acid may boil at 105oC to 120oC
  non ideal solution is formed with the formation
  of an azeotrope at 68 HNO3.
• Yellow in colour with dissolved NO2 formed by
  photochemical decomposition of HNO3.
• Conc. nitric acid is stored in brown bottle. (Why
  ? What else would be stored inside brown bottle,
  too ?)

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Chemical properties of nitric acid

• As an acid
• React with alkalis, carbonates,
  hydrogencarbonates as other acids.
• H HCO3- ? CO2 H2O
• BUT
• DO NOT react with metal as normal mineral acid
  do not give H2 as other acids do.
• Instead
• NO3- is a more powerful oxidizing agent than H
  in acidic medium, it is reduced when react with
  metals.
• Only very dilute HNO3 might give H2 gas with
  magnesium.

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Chemical properties of nitric acid

• HNO3 (Or nitrate ion, in the presence of H) is a
  strong oxidizing agent. Products may be NO2, NO
  and NH4.

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Chemical properties of nitric acid

• React with metals
• Conc. HNO3 - NO2 produced
• Cu(s) 4H(aq) 2NO3-(aq) ?Cu2(aq) 2NO2(g)
  2H2O(l)
• Dilute HNO3 - NO(g) produced
• 3Cu(s) 8H(aq) 2NO3-(aq) ?3Cu2(aq) 2NO(g)
  4H2O(l)
• Very dilute HNO3 - no reaction
• H cannot oxidize metals which are lower than H
  in
• ECS.

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Chemical properties of nitric acid

• Special cases
• Metals lower in ECS Au and Pt, reacts only in a
  mixture of conc. HNO3 and conc. HCl
• With the formation of complex ion AuCl4-,
  PtCl42-.
• Al, Fe and Cr would not react with nitric acid as
  the surface of these metals would be coated with
  a non-porous oxide layer the acid would render
  it passive.

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Chemical properties of nitric acid

• With non-metals
• Hot conc. HNO3 oxidizes sulphur and carbon to a
  high oxidation state giving off brown fumes of
  nitrogen dioxide at the same time.
• S(s) 6HNO3(aq) ? H2SO4(aq) 6NO2(g)
  2H2O(l)
• C(s) 4HNO3(aq) ? CO2 4NO2 2H2O

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Chemical properties of nitric acid

• React with Fe2 salt
• Conc. HNO3 oxidizes green Fe2 to brown Fe3 salt
  on warming.
• 3Fe2(aq) NO3-(aq) 4H(aq) ? 3Fe3(aq)
  NO(g) 2H2O(l)
• 2NO(g) O2(g) ? 2NO2(g)

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Chemical properties of nitric acid

• Action of Heat on Nitrate
• Do you remember what is the product of thermal
• decomposition of nitrates of group I and group
• II metals, e.g. NaNO3, Ca(NO3)2 ? What is the
• Difference between them, if any ?

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Thermal decomposition of nitrates
 
Equation
Products
K
2KNO3(s) ? 2KNO2(s) O2(g)
metal nitrite O2
Na
2NaNO3(s) ? 2NaNO2(s) O2(g)
Ca
2Ca(NO3)2(s) ? 2CaO(s) 4NO2(g) O2(g)
metal oxide NO2 O2
Mg
2Mg(NO3)2(s) ? 2MgO(s) 4NO2(g) O2(g)
Al
4Al(NO3)3(s) ? 2Al2O3(s) 12NO2(g) 3O2(g)
Zn
2Zn(NO3)2(s) ? 2ZnO(s) 4NO2(g) O2(g)
Fe
2Fe(NO3)2(s) ? 2FeO(s) 4NO2(g) O2(g)
Pb
2Pb(NO3)2(s) ? 2PbO(s) 4NO2(g) O2(g)
Cu
2Cu(NO3)2(s) ? 2CuO(s) 4NO2(g) O2(g)
Hg
Hg(NO3)2(s) ? Hg(l) 2NO2(g) O2(g)
metal NO2 O2
Ag
2AgNO3(s) ? 2Ag(s) 2NO2(g) O2(g)
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Thermal decomposition of nitrates

• NH4NO3
• NH4NO3(s) ? N2O(g) 2H2O(g)
• Ammonium nitrate is a slightly unstable
• compound which can decompose readily to give
• large volumes of gaseous products. It is
  therefore
• an explosive.

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Chemical tests for nitrate (V) ions

• Brown Ring Test
• Test for the NO3- ion in aqueous solution.
• Add freshly prepared iron (II) sulphate
• solution
• Tilt the test tube
• Add conc. H2SO4 carefully to the bottom of the
• test tube along the side.
• A brown ring should appear at the junction of the
  two
• layers of solution if nitrate ion is present.

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Chemical tests for nitrate (V) ions

• NO3-(aq) H2SO4(l) ? HNO3(aq) HSO4-(aq)
• HNO3(aq) 3Fe2(aq) 3H ? 2H2O(l) NO(g)
  3Fe3(aq)
• FeSO4(aq) NO(g) ? FeSO4?NO(aq)
• (a simplified formula of
• 
  the brown complex)