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Encyclopedia of Physical Science and Technology EN009F-398 July 6, 2001 20:34

Main Group Elements 19

TABLE V Nitrogen Oxides
Formula Name Structure Description
N 2 O Nitrous oxide, dinitrogen N N O “Laughing gas”; colorless; used as an anesthetic and a propellant
monoxide or aerating agent in canned whipped cream
NO Nitric oxide, nitrogen Paramagnetic, odd-electron species; colorless gas; serious air pollutant
N O
monoxide
N 2 O 3 Dinitrogen trioxide O Exists as a blue solid at low temperatures, dissociates into NO 2 and NO
O
N N gases at higher temperatures; formal acid anhydride of nitrous acid:
O
not (older symmetrical O N 2 O 3 + H 2 O → 2HNO 2
O O
representation) N N
NO 2 Nitrogen dioxide Paramagnetic, odd-electron species; brown gas; serious air pollutant
N
O O
N 2 O 4 Dinitrogen tetroxide O O Dimer of NO 2 :
N N 2NO 2 (g) → N 2 O 4 (g) + heat
O O
−
+
N 2 O 5 Dinitrogen pentoxide O O O White ionic solid at room temperature [NO 2 ] [NO 3 ] ; at the formal
N N anhydride of HNO 3 :
O O N 2 O 5 +H 2 O → 2HNO 3
NO 3 Nitrogen trioxide O Transient, odd-electron, paramagnetic species; if recent environmental
studies had not implicated this unstable compound as a serious
N pollutant, it would scarcely merit mention among the oxides
O O

both are also important as the NO x species of air pollu- developed at the University of Wisconsin, and it shows
tion. Recent research is beginning to provide a modified considerable promise as an econimically viable commer-
view of the relative seriousness of SO 2 and NO x as pollu- cial process.
tants. In the past, SO 2 and SO 3 were regarded as the worst In electrogenerative reduction, NO x pollutants are elec-
culprits of air pollution because they produced most of trocatalytically reduced in a special cell to yield both elec-
the acid rain; now, however, studies have shown that NO x trical energy and any of three useful chemical by-products:
and ozone cause serious damage to terrestrial foliage even ammonia, hydroxylamine (NH 2 OH), and nitrous oxide.
though they may not be responsible for “killing” lakes. The percentage of NO x converted is generally high, and
Measures to reduce SO 2 emissions should be continued the relative proportions of the three products can be eas-
and intensified, but corresponding emphasis on reduc- ily adjusted by modifying the reaction conditions. Further
ing NO x , especially from automobiles, should accompany refinements of the electrogenerative reduction technique
these efforts. are still necessary, but any process with three points in its
In fact, a considerable amount of research is under way favor, that is, elimination of NO x pollutants, generation
to find economically sound technologies that will make of electrical energy, and production of valuable chemi-
it possible for industries to comply with the more strin- cals, would seem to have a good chance of commercial
gent regulations expected for NO x emissions. One area of success.
research is concerned with the development of better fur-
naces and burners capable of substantially lowering NO x D. Oxoacids and Oxoanions
formation; another is concerned with the treatment of flue
gases containing high levels of NO x . In the latter approach, Although nitrogen forms several oxoacids, only three are
NO x is reduced to nitrogen by ammonia in the presence discussed here: (meta)nitric acid (HNO 3 ), orthonitric acid
of a highly selective catalyst. (H 3 NO 4 ), and nitrous acid (HNO 2 ).
Unfortunately, the catalytic reduction of NO x in flue The alchemists referred to nitric acid as aqua fortis
gases is not cost effective in the United States, but it has (“strong water”) out of respect for its capacity to dissolve
been employed in Japan. However, a different reduction silver from alloys of gold. Its industrial synthesis from am-
technique, electrogenerative reduction, is currently being monia has been described earlier; however, small amounts