Everything Science Grade 12

5.3 CHAPTER 5. THE CHEMICAL INDUSTRY

+ –

anode cathode

NaCl

Cl 2 H 2

NaCl

NaOH

H 2 O

membrane

Figure 5.6: Membrane Cell

At the negatively charged cathode, hydrogen ions in the water are reduced to

hydrogen gas.

2H+(aq)+ 2e−→ H2(g)

The Na+ions flow through the membrane to the cathodecompartment and react

with the remaining hydroxide (OH−) ions from the water toform sodium hy-

droxide (NaOH). The chloride ions cannot pass through, so the chlorinedoes not

come into contact withthe sodium hydroxide inthe cathode compartment. The

sodium hydroxide is removed from the cell. Theoverall equation is as follows:

2NaCl + 2H 2 O→ Cl 2 + H 2 + 2NaOH

The advantage of using this method is that the sodium hydroxide that is produced

is very pure because itis kept separate from the sodium chloride solution. The

caustic soda therefore has very little salt contamination. The process also uses

less electricity and is cheaper to operate.

Exercise 5 - 4

1. Refer to the flow diagram below which showsthe reactions that take place in the
   membrane cell, and then answer the questions that follow.