The Foundations of Chemistry

OXOACIDS OF SULFUR

Sulfurous Acid, H 2 SO 3

Sulfur dioxide readily dissolves in water to produce solutions of sulfurous acid, H 2 SO 3.

The acid has not been isolated in anhydrous form.

H 2 O
SO 234 H 2 SO 3

The acid ionizes in two steps in water

H 2 SO 334 H

HSO 3  Ka11.2 10 ^2

HSO 3  34 H

SO 32  Ka26.2 10 ^8

When excess SO 2 is bubbled into aqueous NaOH, sodium hydrogen sulfite, NaHSO 3

is produced. This acid salt can be neutralized with additional NaOH or Na 2 CO 3 to

produce sodium sulfite.

NaOH
H 2 SO 3 88nNaHSO 3 
H 2 O

NaOH
NaHSO 3 88nNa 2 SO 3 
H 2 O

The sulfite ion is pyramidal and has tetrahedral electronic geometry as predicted by the

VSEPR theory.

24-12

956 CHAPTER 24: Some Nonmetals and Metalloids

H O O

O

H

S

H O O



O

S O

O

OS

2 

Sulfurous acid, H 2 SO 3 Hydrogen sulfite ion, HSO 3  Sulfite ion, SO 32 

Sulfuric Acid, H 2 SO 4

More than 40 million tons of sulfuric acid are produced annually worldwide. The contact

process is used for the commercial production of most sulfuric acid. The solution sold

commercially as “concentrated sulfuric acid” is 96–98% H 2 SO 4 by mass and is about 18

molar H 2 SO 4.

Pure H 2 SO 4 is a colorless, oily liquid that freezes at 10.4°C and boils at 290 to 317°C

while partially decomposing to SO 3 and water. There is some hydrogen bonding in solid

and liquid H 2 SO 4.

Tremendous amounts of heat are evolved when concentrated sulfuric acid is diluted.

This illustrates the strong affinity of H 2 SO 4 for water. H 2 SO 4 is often used as a dehy-

drating agent. Dilutions should always be performed by adding the acid to water to avoid

spattering the acid.

Sulfuric acid is a strong acid with respect to the first step of its ionization in water.

The second ionization occurs to a lesser extent (Example 18-17).

H 2 SO 434 H

HSO 4  Ka1very large

HSO 4  34 H

SO 42  Ka21.2 10 ^2

Pouring concentrated H 2 SO 4 into
an equal volume of H 2 O liberates a
lot of heat—enough to raise the
temperature of the resulting solution
from room temperature to 121°C.