Modern inorganic chemistry

GROUPV 233

NO 2 + H 2 S -> NO 4- H 2 O 4- Sj

NO 2 4- 21" 4- H 2 O -» NO + I 2 + 2OH"

Strong oxidising agents such as acidified potassium manganate(VII)

oxidise NO 2 to the nitrate ion :

2MnO4 + 10NO 2 + 2H 2 O -> 2Mn2+ + 4H+ + 10NO 3 "

Nitrogen dioxide dissolves in water to give a mixture of nitrous
and nitric acids :

2NO 2 + H 2 O -> HNO 2 + HNO 3 (9.1)

The nitrous acid decomposes rapidly at room temperature, thus :

3HNO 2 -> HNO 3 -f 2NO -f H 2 O (9.2)

giving an overall reaction :

H 2 O 4- 3NO 2 -> 2HNO 3 + NOT (9.3)

If this reaction takes place in air, the evolved nitrogen monoxide is
oxidised to the dioxide and this dissolves again as in equation (9.1) ;
hence virtually complete conversion of nitrogen dioxide to nitric
acid can occur (see nitric acid, below). With alkalis, a mixture of
nitrite and nitrate is formed :

2OH- + 2NO 2 -> NO 3 ~ + NO 2 - + H 2 O

Dinitrogen tetroxide, N 2 O 4 , as a liquid, has some power as a
solvent, and appears to dissociate slightly to give nitrosyl nitrate,
thus:

N 2 O 4 ^NO+ +NOJ

If metallic zinc is dissolved in this liquid, the following reaction

occurs :

Zn + 2N 2 O 4 -> Zn(NO 3 ) 2 + 2NOT

i.e. Zn -f 2NO+ -> Zn2+ + 2NOT

(cf. Zn + 2H+ -> Zn2+ 4- H 2 T)

Hence dinitrogen tetroxide (sometimes mixed with an organic
solvent) can be used to prepare anhydrous metal nitrates (many
heavy metal nitrates are hydrated when prepared in aqueous
solution, and they cannot be dehydrated without decomposition).