Chemistry, Third edition

REACTIONS OF THE HYDROXIDE ION 95

Acids produced when gases CO 2 , SO 2

and NO 2 dissolve in water

Carbon dioxide and sulfur dioxide gases are both water soluble. Some of the dis-

solved gas molecules then react with water producing hydrogencarbonate and

hydrogensulfite ions, respectively. The reactions are:

CO 2 (aq)H 2 O(l) HCO 3 (aq)H(aq) hydrogencarbonate ion

SO 2 (aq)H 2 O(l) HSO 3 (aq) + H(aq) hydrogensulfite ion

The\==\sign shows that both reactions do not go to completion, i.e. attempts to react

the gases completely with water will always produce an equilibrium mixture of un-

reacted gas and hydrogencarbonate or hydrogensulfite and hydronium ions. Under

the same conditions, more SO 2 molecules react with water than do CO 2 molecules.

This, and the fact that SO 2 (g) is more soluble in water than CO 2 (g), means that satu-

rated solutions of SO 2 are more acidic (contain a higher Hconcentration) than

solutions of CO 2. This explains why CO 2 (g) only turns damp blue litmus paper a

purple/red colour, whereas SO 2 (g) produces a much more definite red colour.

In the presence of oxygen, a solution of sulfur dioxide is very slowly converted to

dilute sulfuric acid at room temperature:

SO 2 (aq)^1 ⁄ 2 O 2 (aq)H 2 O(l)SO 42 (aq)2H(aq)

In the atmosphere, the conversion of SO 2 (aq) to sulfuric acid within clouds occurs

faster, and is the main source of acid rain (see page 413).

If CO 2 or SO 2 are dissolved in alkalinesolutions, all the hydrogencarbonate and

hydrogensulfite ions are converted to carbonates and sulfites, respectively:

HCO 3 (aq)OH(aq)CO 32 (aq)H 2 O(l)

HSO 3 (aq)OH(aq)SO 32 (aq)H 2 O(l)

Summarizing, when CO 2 and SO 2 gases are bubbled into a beaker of water, a

mixture of undissolved gas and hydrogencarbonate or hydrogensulfite ions are pre-

sent. However, in the presence of OH(aq), the only species present are carbonate or

sulfite ions.

Nitrogen dioxide (NO 2 (g)), is a toxic brown gas which dissolves in water produc-

ing a highly acidic cocktail of hydronium ions, nitrate ions (NO 3 (aq)) and nitric

oxide gas (NO):

3NO 2 (g)H 2 O(l)2H(aq)NO(g)2NO 3 (aq)

Reactions of the hydroxide ion

The hydroxide ion in solution is normally written as OH(aq).

Effect on litmus

Litmus turns blue in basic solution.

6.7

6.6