OXIDES OF NITROGEN
Nitrogen forms the oxides N2O, NO, N2O3, N2O4, N2O5 and N2O6
Nitrous Oxide N2O the three atoms in one straight
line. Nitrous oxide resembles carbon dioxide (O=C=O)in physical
properties.
Nitric Oxide NO
Dinitrogen Trioxide N2O3 , this compound is the least stable (with the
exception of N2O6) of all oxides of nitrogen.
Dinitrogen Tetraoxide N2O4
Dinitrogen Pentaoxide, N2O5
Dinitrogen Hexaraoxide N2O6 , this compound is stable only below 142˚ and at higher temperature it decomposed in to O2 and NO2.
Preparation of Nitrogen Oxides:
Nitrous Oxide N2O , is prepared by cautiously heating ammonium
nitrate.
NH4NO3=N2O+2H2O
It is collected over hot water. The gas produced by this reaction contains
nitrogen , higher oxides of nitrogen. Very pure nitrous oxide can be
prepared by mixing solutions of hydroxylamine hydrochloride and
sodium nitrate.
NH2OH.HCl +NaNO2=N2O+NaCl+2H2O
Properties:
Nitrous oxide is a colorless gas , B.Pt.-88.5˚ . It is fairy soluble in water
to yield a solution neutral to litmus.1 volume of water dissolves 1 volume
of nitrous oxide at 6˚.It is chemically not very active and the
decomposition of nitrous oxide in to its elements is exothermic:∆H=17,000 calories
Nitrous oxide is stable at ordinary temperatures and decomposed at
600˚.The heat of decomposition of nitrous oxide, as well as the heat of
combustion , is available for raising the temperature of the burning
substance, which therefore burns as vigorously as it would in oxygen.
Nitric Oxide, NO, is obtained when mixture of oxygen and nitrogen is
subjected to a spark discharge . It is prepared in Kipps apparatus by the
action of cold dilute nitric acid on copper turning :
3Cu + 8 HNO3= 3Cu(NO3)2 + 4H2O + 2NO
Avery pure gas is made by adding a saturated solution of ferrous sulphate
to mixture of concentrated sulphuric acid and powdered potassium
nitrate.
Properties:
Nitric oxide is a colorless gas , B.Pt.-152˚. It is slightly soluble in water
,the coefficient of solubility at 15˚ is 0.05.Although its decomposition is
exothermic:∆H=-21,600 calories.
It does decomposed unless heated to about 1000˚, and therefore it is not a
ready supporter of combustion.
Dinitrogen Trioxide N2O3, may be formed at very low temperature in
the form of blue crystals . At -120˚ these crystals melt and at once begin
to dissociate:
2N2O3=2NO + N2O4
At room temperature the gas contains 10 per cent of undissociated
molecules of N2O3, at 100 ˚ dissociation is complete. A mixture of nitric
oxide and nitrogen dioxide in the proportions required to give nitrogen
trioxide is conveniently prepared by distilling arsenious oxide with an
equal weight of 60 per cent of nitric acid:
As2O3 + HNO3 = As2O5 + N2O3 +H2O
Dinitrogen Trioxide dissolves in aqueous alkalis to give nitriles. With
water it gives a blue solution which rapidly changed to nitric acid and
nitric oxide.
Nitrogen Dioxide NO2, is preparing by heating lead nitrate:
2Pb(NO3)2 = 2PbO +4NO2 +O2
Properties:
Nitrogen dioxide is a brown gas which condenses to a yellow liquid , B.P.
21˚.Expermintes on the density of nitrogen dioxide show that below 150
the gas consist of N2O4 and NO2 in equilibrium. And the proportion of
N2O4 increased as the temperature falls.
Dinitrogen Pentaoxide N2O5,is prepared by the dehydration of nitric
acid with phosphorus pentaoxide:
HNO3 + P4O5 = 2N2O5 + 4 HPO3
Phosphorus pentaoxide is added in small quantities at a time , to a little
less than its own weight of purified nitric acid cooled in a freezing
mixture. The pasty mass is transferred to a retort and gently distilled. The
distillate dinitrogen pentaoxide is collected in ice cooled bottles where it
condenses as colorless crystals.
Properties:
Dinitrogen Pentaoxide is hydroscopic, and dissolves in water with hissing
sound , forming nitric acid . Sodium , phosphorus and potassium burn in
liquid dinitrogen pentaoxide if warmed .
Crystalline N2O5 has been shown by X-ray analysis to be [NO2]+[NO3]- .