Nitrogen and Its Compounds

Chapter 4Jordan, Joshua, Kosheila, Poh Ping, Sophia

N N

1s2 2s2 2p3

Achieves stable octet electron formation –covalent triple bonding ◦ N2 molecule

N is for Nitrogen

N Nπ

πσ

Very stable and relatively inert◦ Very strong triple bond◦ Very short bond length◦ High bond length energy

N2 (g) → 2N (g) △H = +945kJmol-1

Bond must be broken with high energy before N can react with other substance ∴ inert

Inert nature of Nitrogen

Linear Non polar Distribution of electrons - symmetrical

∴ absence of polarity

Inert nature of Nitrogen

N●●●

●● N

XXx

Xx

δ + δ - X Xδ + δ - X X

Reactivity of N2

N N N N N NIncreasing reactivity

Very strong triple bond

Single polar bond

Weaker Single bond

δ + δ -

At high temperature & pressure :◦ N2 (g) + H2 (g) → 2NH3 (g)

◦ N2 (g) + O2 (g) → 2NO (g)

◦ N2 (g) + M (s) → MN (s)e.g.

N2 (g) + 2Al (s) → 2AlN (s)

N2(g) + 3Mg (s) → Mg3N2(s)

Reactivity of N2

fractional distillation of liquid air

Extraction of Nitrogen from Air

Ammonia: Its Formation, Properties

and UsesFormation and Properties of Ammonia

By heating a mixture of ammonium salt and a base

NH4+ + OH- NH3 + H20

Ammonia gas is dried by passing it through anhydrous calcium oxide.

Conc.H2SO4 & anhydrous CaCl2 can’t be used because they react with ammonia

Formation of Ammonia from Ammonium Salts

Industrial Preparation of

Ammonia The Haber Process

direct combination of N2 and H2 .

N2(g) + 3H2(g) Fe 2NH3(g)

-N2 from fractional distillation of liquefied air

-H2 from synthesis gas.

Mixture of N2 and H2 passed over ◦ finely divided Fe , or◦ Fe2O3 catalyst with K2O and Al2O3 as promoter ◦ (temp: 450-500o C, P: 200-500 atm).

NH3 condensed in the condenser and collected as liquid ammonia.

Haber Process

Physical Properties of Ammonia

Physical Properties of NH3

colourless pungent-smelling gas common alkaline gas. A covalent compound with a trigonal

pyramidal shape and with a lone pair of electrons.

Properties of Ammonia

a Lewis base (electron pair donor).

Form dative bond by donating electrons to any Lewis acid.

a Bronsted-Lowry base (accepts a proton)ammonium ion.

NH3 (g) + H2O (l) NH4+ (aq) + OH-

(aq) base acid conjugate acid conjugate base

Properties of Ammonia

b.p. - relatively high (presence of hydrogen bond)

soluble in water weak base solution with a low base dissociation constant.

NH3 (g) + H2O (l) H3N.H2O

liquid ammonia undergoes autoionisation NH4

+ and NH2- ions.

Liquid ammonia is a weak conductor of electricity.

2NH3 (l) NH4+ (l) + NH2

- (l)

Properties of Ammonia

Chemical Properties of Ammonia

A lone pair on N atom in the molecule act as proton acceptor

NH3 + H+ NH4+

Dissolved in water, aqueous ammonia undergoes partial dissociation NH4

+ and OH- ions:

H3N.H2O(aq) NH4+ (aq) + OH- (aq)

Aqueous ammonia reacts with acids to form salts NH3(aq) + HCl(aq) NH4Cl(aq)

2NH3(aq) + H2SO4(aq) (NH4)2SO4(aq)

As a weak base

Precipitates insoluble metallic hydroxides from their salts (except Na+, K+ , and Ba2+ )

Cu2+(aq) + 2NH3(aq) + 2H2O(l) Cu(OH)2(s) + 2NH4

+(aq)

Some hydroxides dissolve in excess of NH3

formation of water soluble complexes. Cu(OH)2(s) + 4NH3(aq) [Cu(NH3)4]2+(aq) + 2OH-

(aq)

As a weak base

Form a dative bond by donating the lone pair to the empty orbitals of metallic cations (esp. transition metal ions).

acts as Lewis base / ligand

Examples of complex formation:i. Tetraamminecopper(II), [Cu(NH3)4]2+

ii. Hexaamminenickel (II), [Ni(NH3)6]2+

As Ligands in complex formation

React with some substances◦addition compounds through the formation of

dative bonds.

Examples:i. NH3 + BF3 BF3 . NH3

ii. NH3 + AlCl3 AlCl3 . NH3

Formation of addition compounds

Acts a reducing agent oxidised to nitrogen. 2NH3 N2 + 6H+ + 6e-

Cl2 gas reacts with ammonia at room temp. N2 gas

8NH3(g) + 3Cl2(g) N2(g) + 6NH4Cl(s)

Excess of chlorine dangerously EXPLOSIVE OIL, NCl3 produced:

NH3(g) + 3Cl2(g) NCl3(l) + 3HCl(g)

Reduces aqueous sodium chlorate(I) at room temp. to the Cl- ion:

NH3 + 3ClO-(aq) N2(g) + 3Cl-(aq) + 3H2O(l)

As a reducing agent

- passing the dry gas over certain red hot metallic oxides.

2NH3(g) + 3CuO(s) 3Cu(s) + N2(g) + 3H2O(l)

2NH3(g) + 3PbO(s) 3Pb(s) + N2(g) + 3H2O(l)

Magnesium burns in dry ammonia magnesium nitride &H2 gas

3Mg(s) + 2NH3(g) Mg3N2(s) + 3H2(g)

Oxidation to nitrogen and steam

Dry ammonia burns in oxygen to produce nitrogen and steam.

4NH3(g) + 3O2(g) 2N2(g) + 6H2O(g)

When dry ammonia and oxygen is passed over heated platinum, nitrogen monoxide is formed.

4NH3(g) + 5O2(g) 4NO(g) + 6H2O(g)

- Involved in the Ostwald process .

Combustion of ammonia

Decompose when heated. Non-oxidising acids decompose to NH3 and the

corresponding acids. NH4Cl (s) NH3(g) + HCl(g)

(NH4)2CO3(s) 2NH3(g) + CO2(g) + H2O(l)

Oxidising acids decompose to either N2 or oxides of nitrogen.

NH4NO2(s) N2(g) + 2H2O(l)

(NH4)2Cr2O7(s) N2(g) + Cr2O3(s) + 4H2O(l)

p/s : All these reactions can be EXPLOSIVE!!!

Action of heat on ammonium compounds

A reagent to identify cations in qualitative analysis.

All cations metal hydroxides (precipitates) except Na+, K+, and NH4

+.

Reactions of aqueous ammonia on certain cations

Cation Observations Solubility in excess NH3

Ag+ White ppt → light brown √

Zn2+ White ppt √

Mn2+ White ppt →brown X

Pb2+ White ppt X

Mg2+ White ppt X

Al3+ White ppt X

Cu2+ Blue ppt √(dark blue)

Cr3+ Great blue ppt X

Fe2+ Dirty green ppt X

Fe3+ Reddish brown ppt X

Ni2+ Green ppt √(light blue)

USES OF AMMONIA Nitrogenous fertilisers.

Manufacture of nitric acid in Ostwald process. catalytic oxidation of ammonia oxidation of nitrogen monoxide to nitrogen dioxide formation of nitric acid from nitrogen dioxide

Cooling agent / refrigerant

Cleaning agent

Formation of Oxides of Nitrogen

Nitrogen Monoxide colourless neutral gas, insoluble in water,

reducing agent

heptet electron configuration ( unpaired electron delocalised), paramagnetic

unstable, easily oxidised 2NO(g) + O2(g) 2NO2 (g)

brown acidic gas, pungent smell

V-shaped, heptet configuration (unpaired electron at nitrogen atom)

double and dative bond delocalised actually resonance hybrid structure

Nitrogen Dioxide

in the environment

Nitrogen, N2(g) Oxygen, O2(g)

2 NO(g)

(disassociated by lightning)

(Nitrogen monoxide)

2 NO(g)

Oxygen, O2(g)

2 NO2(g)

(Nitrogen dioxide)

IN THE INTERNAL COMBUSTION ENGINE

Volvo

2800˚C

N2(g) + O2(g) => 2 NO(g)

2 NO2(g) NxOy

ATMOSPHERIC POLLUTANTS FROM COMBUSTION OF FOSSIL

FUELS

Nitrogen oxides

Sulphur dioxides

Carbon monoxide

Unburnt hydrocarbon

Lead bromide(lead

petrol)

EFFECTS OF OXIDES OF NITROGEN ON AIR POLLUTION

Acid Rain Photochemical Smog

ACID RAIN

2 NO2(g) + H20 -> HNO2 + HNO3

4 NO2(g) + 2 H20 + O2-> 4 HNO3

Oxides of nitrogen dissolve in rain water to produce nitrous acid and nitric acid.

(one of the causes of acid rain)

Sulphur oxides and oxides of nitrogen are the primary pollutants of the atmosphere.

pH less than 5

ACID RAIN

Nitrogen dioxide also catalyses the oxidation of sulphur dioxide to produce sulphur trioxide.

NO2 + SO2 -> NO + SO3

NO + ½ O2 -> NO2

SO2 + ½ O2 -> SO3

(also one of the causes of acid rain)

ACID RAINThe oxides of sulphur dissolve in rain water to produce acid rain.

SO2 + H20 -> H2SO3 SO3 + H20 -> H2SO4

Sulphurous acid

Sulphuric acid

ACID RAIN

PHOTOCHEMICAL SMOG

PHOTOCHEMICAL SMOG

Nitrogen dioxide

Hydrocarbons

Smoke & Fog

Smog

NO2(g)

2 Stages:

NO(g) + O (g)

Stage1

Stage2

O (g) + O2(g)

O3(g)

uv light

Ozone molecule

PHOTOCHEMICAL SMOG

Ozone molecule

Unsaturated hydrocarbon (from incomplete

combustion)

combines

Organic radicles that form smog when combined with nitrogen monoxide

oxidize

Less volatile organic products that condenses on air particles to form smog

View of Beijing 2005

After rain Before rain

Tetranitrogen N4

Hydrogen Cyanide

HCN

Laughing Gas

N2O

Coolant agent Aircraft fuel

Further Uses of Nitrogen