The Foundations of Chemistry

SOME OXOACIDS OF NITROGEN AND THEIR SALTS

The main oxoacids of nitrogen are nitrous acid, HNO 2 , and nitric acid, HNO 3.

Nitrous Acid (
3 Oxidation State)

Nitrous acid, HNO 2 , is unstable and cannot be isolated in pure form. It is prepared as a
pale blue solution when H 2 SO 4 reacts with cold aqueous sodium nitrite. Nitrous acid is
a weak acid (Ka4.5 10 ^4 ). It acts as an oxidizing agent toward strong reducing agents
and as a reducing agent toward very strong oxidizing agents.
Lewis formulas for nitrous acid and the nitrite ion follow.

N

OO

N

OO

 

nitrite ion

HN

OO

nitrous acid

24-16

24-16 Some Oxoacids of Nitrogen and Their Salts 963

Nitric Acid (
5 Oxidation State)

Pure nitric acid, HNO 3 , is a colorless liquid that boils at 83°C. Light or heat causes it to
decompose into NO 2 , O 2 , and H 2 O. The presence of the NO 2 in partially decomposed
aqueous HNO 3 causes its yellow or brown tinge. The Lewis structure of nitric acid is

HNO 3 is commercially prepared by the Ostwald process.At high temperatures, NH 3
is catalytically converted to NO, which is cooled and then air-oxidized to NO 2. Nitrogen
dioxide reacts with H 2 O to produce HNO 3 and some NO. The NO produced in the third
step is then recycled into the second step. More than 18 billion pounds of HNO 3 was
produced in the United States in 1997.

Nitric acid is very soluble in water (
16 mol/L). It is a strong acid and a strong oxidizing
agent.

4NH 3 (g)

2NO(g)

3NO 2 (g) H
 2 O(
) 2[H


NO 3 ] NO(g)

O 2 (g) 2NO 2 (g)

5O 2 (g) 4NO(g) 6H 2 O(g)

recycle

Pt

1000 °C

cool

HO

O

O

N

O

HO

O

N

Nitrogen also forms hyponitrous acid,

H 2 N 2 O 2 , in which N is in the 
 1

oxidation state, as well as hyponitrite

salts such as Na 2 N 2 O 2.

Nitric acid, HNO 3 , mp 42°C; bp

83°C; bond lengths NXO (terminal)

1.22 Å, NXO (central) 1.41 Å.