Chapter 5. Microscale Gas Chemistry:

Nitrogen Oxides Information

A. Appearance
 
   Nitric oxide is the common name for nitrogen monoxide, NO(g).  Nitric oxide is a colorless, paramagnetic gas.  Nitric oxide has an acrid odor and has very low solubility in water.  Nitric oxide is the simplest thermally stable odd-electron molecule known in chemistry.  It is extremely reactive and readily oxidizes in air to produce nitrogen dioxide:

2NO(g) + O2(g)  ---> 2 NO2(g)

      Nitrogen dioxide is a red-brown gas that is highly reactive with water.  The dynamic interconversion between NO2 and N2O4 makes it impossible to study either of these species alone:

  2NO2(g) ---> N2O4(g)

In both forms, the nitrogen is in the +4 oxidation state.  Dinitrogen tetroxide is a colorless, diamagnetic gas that dissociates reversibly as shown above.  Its melting point is -11.2 oC and its boiling point is 21.2 oC. Nitrogen dioxide is a brown, paramagnetic, reactive gas with the structure shown at right.

B. Physical Properties of NO and NO2

 
Nitric oxide, NO
Nitrogen dioxide, NO2
Atomic mass: 30.01 g/mol 46.01 g/mol
melting point -163.6 oC  -11.2 oC
boiling point -151.8 oC  +21.2 oC
C. History
     Both gases were discovered by Joseph Priestley in the 1770s.

D. Natural Abundance
     Electrical discharges such as lightening produce NO from nitrogen and oxygen.  Automobiles also form NO.  Nitric oxide rapidly oxidizes to NO2 and these two gases are collectively called NOx.  Tolerable levels of NOx are less than 3 - 5 ppm.  In Los Angeles where photochemical smog is particularly bad, levels have reached 0.9 ppm.  A level of 500 ppm is fatal.
 

E. Industrial Production
     Nitric oxide is formed from the combustion of ammonia.  W. Ostwald studied the production of nitric acid and by 1901 had perfected the following sequence of reactions, the first of which uses a platinum catalyst at 850 oC:

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

Nitrogen dioxide forms upon contact with air:

2 NO + O2  ---> 2 NO2

F. Industrial Uses
     The primary use of these gases is the production of nitric acid for the ultimate production of fertilizers and a wide range of useful chemicals.  Nitric acid is formed by reacting NO2 with water.  The NO produced by this disproportionation reaction reacts with air to form more NO2 which, in tern, reacts with water.

3NO2 + H2O(l) ---> 2 HNO3 (aq) + NO(g)

In terms of quantities, most of the nitric acid produced is reacted with ammonia to form ammonium nitrate for use as a fertilizer.
 

G. Gas Density of NO and NO2
      The density of NO is 1.226 g/L at 25 oC and 1 atm or 3% heavier than air.  Nitrogen dioxide is 59% heavier than air with a density of 1.880 g/L

H. Gas Solubility of NO and NO2
     NO is not very soluble in water.  Only 0.00983 g NO dissolve per L at 0 oC.  This means that 14 mL of water will dissolve 1 mL NO(g).  Nitrogen dioxide is extremely soluble in water Ñ and actually reacts with water to produce nitric acid and NO:

3 NO2(g) + H2O(l)  ---> 2 HNO3(aq) + NO(g)
 

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