Chemistry, Third edition

90 6 · REACTIONS OF IONS IN SOLUTION

Heating a mixture of ammonium and hydroxide ions causes them to recombine

making ammonia gas and water. This works whatever the source of the ammonium

ions. For example, heating ammonium chloride with sodium hydroxide solution

produces ammonia. This is used as a laboratory test to confirm the presence of the

ammonium ion. The ammonia gas is detected by its very characteristic smell (usually

described as pungent) and by the fact that it turns damp red litmus blue (Fig. 6.4 and

Exercise 6H).

Fig. 6.4Testing a solution to

see if it contains ammonium

(NH 4 ) ions. If ammonia is

detected, the presence of the

ammonium ions is confirmed.

Reactions that produce ammonia gas

Which of the following would be expected to produce ammonia gas when heated with

sodium hydroxide solution?

(i) lead(II) nitrate (ii)ammonium carbonate (iii)calcium carbonate.

Write an ionic equation for the reaction(s) that are involved.

Exercise 6H

Acids and bases

Acids

An acid is a substance that produces H+(aq) ions when dissolved in water.

The commonest acids in the laboratory are sulfuric acid (H 2 SO 4 ), hydrochloric acid

(HCl) and nitric acid (HNO 3 ). These are known as the mineral acids.

Another common acid is ethanoic acid (acetic acid, CH 3 COOH), a smelly liquid

which boils at 118 °C. Ethanoic acid is the chemical that gives vinegar its sour taste.

Its full structure is as follows:

Only one of the hydrogen atoms in this molecule makes a hydrogen ion in solution,

and for this reason the hydrogen atom is called an acidic hydrogen. In ethanoic acid,

the acid hydrogen is the one bonded to the oxygen atom. The ionization of ethanoic

acid may then be represented by the equation

H 2 O

CH 3 COOH(l) CH 3 COO(aq)H(aq)

ethanoate

(or acetate) ion

or,

The sign shows that only some of the ethanoic acid molecules are ionized in

solution.

6.4

Ammonia causes

damp red litmus

to turn blue

Suspected
ammonium salt

• sodium
  hydroxide