a. What four ions are present in sodium chloride

solution?

3

Na+(aq), Cl-(aq), H+(aq), OH-(aq)

b. In Figure 21.3, write the ionic equation for the

reactions at:

2Cl-(aq) Cl2(g) + 2e-

(i). the anode

2H+(aq) + 2e- H2(g)

(ii). the cathode

Chloride ions and hydrogen ions are removed,

leaving sodium ions and hydroxide ions, i.e.

sodium hydroxide.

c. Explain why sodium hydroxide forms during the

electrolysis.

Oxygen gas is formed instead of chlorine gas.

Chlorine is needed to react with the sodium

hydroxide solution.

4 To prepare a solution of chlorine bleach, dilute

sodium chloride solution is not electrolysed. Give

the reason.

1. Different electrodes.

2. No porous membrane in laboratory cell.

3. Three products separated in industrial cell.

4. Bleach formed inside the laboratory cell,

separately from the industrial

cell.

5. Industrial product is purer (cf. Class

Practice 6).

5 State some differences between the laboratory

preparation and the industrial manufacture of

bleach.

6 In the industrial manufacture, the chlorine at the

anode is not mixed with the sodium hydroxide

solution inside the cell.

Why not?

The bleach formed is mixed with other

substances (e.g. NaOH, NaCl) and so is not pure.

mercury cathode, graphite (sometimes titanium)

anode

7 The electrolysis of brine is also carried out in a

mercury cathode cell.

a. What electrodes does this cell use?

Cathode - sodium

Anode - chlorine

b. What is formed at each electrode?

Sodium (dissolved in the mercury) is removed

and reacts with water.

c. How is sodium hydroxide formed?

a. In the industrial electrolysis of brine, name:8

solid salt

(i). the raw materials; and

(ii). all the products.

hydrogen, chlorine, sodium hydroxide solution

No.

Hydrogen is used to make hydrochloric acid;

chlorine and sodium hydroxide form bleach.

b. Is there any waste substance in this process?