1. CLASS PRACTICE 3
a. What four ions are present in sodium chloride solution?
Na+(aq), Cl-(aq), H+(aq), OH-(aq)
b. In Figure 21.3, write the ionic equation for the reactions at:
(i). the anode
2Cl-(aq)  Cl2(g) + 2e-

2. CLASS PRACTICE (ii). the cathode 2H+(aq) + 2e-  H2(g)
c. Explain why sodium hydroxide forms during the electrolysis.
Chloride ions and hydrogen ions are removed, leaving sodium ions and hydroxide ions, i.e. sodium hydroxide.

3. CLASS PRACTICE
4 To prepare a solution of chlorine bleach, dilute sodium chloride solution is not electrolysed. Give the reason.
Oxygen gas is formed instead of chlorine gas. Chlorine is needed to react with the sodium hydroxide solution.

4. CLASS PRACTICE
5 State some differences between the laboratory preparation and the industrial manufacture of bleach.
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. CLASS PRACTICE
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.

6. CLASS PRACTICE
7 The electrolysis of brine is also carried out in a mercury cathode cell.
a. What electrodes does this cell use?
mercury cathode, graphite (sometimes titanium) anode
b. What is formed at each electrode?
Cathode - sodium
Anode - chlorine

7. CLASS PRACTICE c. How is sodium hydroxide formed?
Sodium (dissolved in the mercury) is removed and reacts with water.

8. CLASS PRACTICE 8 a. In the industrial electrolysis of brine, name:
(i). the raw materials; and
solid salt
(ii). all the products.
hydrogen, chlorine, sodium hydroxide solution

9. CLASS PRACTICE End b. Is there any waste substance in this process?
No.
Hydrogen is used to make hydrochloric acid; chlorine and sodium hydroxide form bleach.
End