The Production of Chlorine

The production of chlorine

The chlorine-alkali (chloralkali) industry is an important part of the chemical industry for the production of chlorine and sodium hydroxide. The most common method involves the electrolysis of a concentrated aqueous solution of sodium chloride (\(\text{NaCl}\)), which is known as brine.

The chemical reactions that take place in this process are:

\(2\text{Cl}^{-}(\text{aq})\) \(\to\) \(\text{Cl}_{2}(\text{g}) + 2\text{e}^{-}\)

\(2\text{H}_{2}\text{O}(\text{l}) + 2\text{e}^{-}\) \(\to\) \(\text{H}_{2}(\text{g}) + 2\text{OH}^{-}(\text{aq})\)

There are also \(\text{Na}^{+}\) ions from the \(\text{NaCl}\) in the solution. If the products are kept separate (to prevent a reaction between the chlorine and hydroxide) the \(\text{Na}^{+}\) will react with the hydroxide ions making the overall reaction as follows:

\(2\text{NaCl}(\text{aq}) + 2\text{H}_{2}\text{O}(\text{l})\) \(\to\) \(\text{Cl}_{2}(\text{g}) + \text{H}_{2}(\text{g}) + 2\text{NaOH}(\text{aq})\)

The uses of chlorine include:

• the purification of water

• as a disinfectant

• in the production of:

  • hypochlorous acid (used to kill bacteria in drinking water), chloroform, carbon tetrachloride

  • paper, food

  • antiseptics, insecticides, medicines, textiles

  • paints, petroleum products, solvents, plastics (such as polyvinyl chloride)

The uses of sodium hydroxide include:

• making soap and other cleaning agents

• purification of bauxite (the ore of aluminium)

• making paper

• making rayon (artificial silk)

If the chlorine and hydroxide ions are not kept separate (i.e. are allowed to react), and the temperature is kept below \(\text{60}\) \(\text{℃}\), then the following occurs:

\(\text{Cl}_{2}(\text{g}) + 2\text{OH}^{-}(\text{aq})\) \(\to\) \(\text{Cl}^{-}(\text{aq}) + \text{ClO}^{-}(\text{aq}) + \text{H}_{2}\text{O}(\text{l})\)

The balanced chemical equation with the inclusion of the \(\text{Na}^{+}\) ions is:

\(\text{Cl}_{2}(\text{g}) + 2\text{NaOH}(\text{aq})\) \(\to\) \(\text{NaCl}(\text{aq}) + \text{NaClO}(\text{aq}) + \text{H}_{2}\text{O}(\text{l})\)

The uses of \(\text{NaClO}\) (sodium hypochlorite) include:

• use in bleaches, disinfectants, and water treatments

• use during root canal surgery and to neutralise nerve agents

If the temperature is above \(\text{60}\) \(\text{℃}\) and mixing occurs, then:

\(3\text{Cl}_{2}(\text{g}) + 6\text{OH}^{-}(\text{aq})\) \(\to\) \(5\text{Cl}^{-}(\text{aq}) + \text{ClO}_{3}(\text{aq}) + 3\text{H}_{2}\text{O}(\text{l})\)

And the balanced chemical equation including \(\text{Na}^{+}\) ions is:

\(3\text{Cl}_{2}(\text{g}) + 6\text{NaOH}(\text{aq})\) \(\to\) \(5\text{NaCl}(\text{aq}) + \text{NaClO}_{3}(\text{aq}) + 3\text{H}_{2}\text{O}(\text{l})\)

The uses of \(\text{NaClO}_{3}\) (sodium chlorate) include:

• making chlorine dioxide

• use as a herbicide

• generating oxygen in chemical oxygen generators

If calcium chloride was used instead of sodium chloride, then the products would be calcium chloride, calcium hypochlorite and calcium chlorate. Similarly, if potassium chloride was used, the products would be potassium chloride, potassium hypochlorite and potassium chlorate.