Everything Science Grade 12

5.3 CHAPTER 5. THE CHEMICAL INDUSTRY

of household bleach. To overcome this problem the chlorine and sodium hydroxide
must be separated fromeach other so that theydon’t react. There are three industrial
processes that have been designed to overcomethis problem, and to produce chlorine
and sodium hydroxide.All three methods involve electrolytic cells (chapter 4).

1. The Mercury Cell
   In the mercury-cell (figure 5.4), brine passes through a chamber which has a
   carbon electrode (the anode) suspended from the top. Mercury flows along the
   floor of this chamber andacts as the cathode. When an electric current isapplied
   to the circuit, chloride ions in the electrolyte areoxidised to form chlorine gas.

2Cl−(aq)→ Cl2(g)+ 2e−

At the cathode, sodiumions are reduced to sodium.

2Na+(aq)+ 2e−→ 2Na(Hg)

The sodium dissolves inthe mercury, forming anamalgam of sodium andmer-

cury. The amalgam is then poured into a separate vessel, where it decomposes

into sodium and mercury. The sodium reacts with water in the vessel and pro-

duces sodium hydroxide (caustic soda) and hydrogen gas, while themercury

returns to the electrolytic cell to be used again.

2Na(Hg)+ 2H 2 O(l)→ 2NaOH(aq)+ H2(g)

Main vessel

Secondary vessel

mercury cathode (-)

Carbon anode (+)

NaCl

NaCl

Cl 2

sodium mer-

cury amalgam

H 2

NaOH

H 2 O

mercury returned to theelec-

trolytic cell

Figure 5.4: The MercuryCell

This method, however,only produces a fractionof the chlorine and sodium hy-

droxide that is used byindustry as it has certain disadvantages: mercury is ex-

pensive and toxic, and although it is returned tothe electrolytic cell, some always