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The Chlor-Alkali Process

The Chlor-Alkali Process

Although they are very different chemically, there is a link between chlorine and sodium hydroxide because there is an important electrochemical process that produces the two chemicals simultaneously. The process known as the chlor-alkali process, utilizes sodium chloride, which occurs in large deposits in many parts of the world. This is an electrochemical process to oxidize chloride ion to chlorine and generate sodium hydroxide.

Passing a direct current of electricity through a solution of NaCl causes the chloride ions to migrate to the positive electrode where oxidation to gaseous chlorine occurs when the ion gives up an electron to the electrode:

\(2{\text{Cl}}^{\text{−}}(aq)\;⟶\;{\text{Cl}}_{2}(g)+2{\text{e}}^{-}\phantom{\rule{5em}{0ex}}\text{(at the positive electrode)}\)

The electrons produced travel through the outside electrical circuit to the negative electrode. Although the positive sodium ions migrate toward this negative electrode, metallic sodium does not form because sodium ions are too difficult to reduce under the conditions used. (Recall that metallic sodium is active enough to react with water and hence, even if produced, would immediately react with water to produce sodium ions again.) Instead, water molecules pick up electrons from the electrode and undergo reduction to form hydrogen gas and hydroxide ions:

\(2{\text{H}}_{2}\text{O}(l)+2{\text{e}}^{-}\;\text{(from the negative electrode)}\;⟶\;{\text{H}}_{2}(g)+2{\text{OH}}^{\text{−}}(aq)\)

The overall result is the conversion of the aqueous solution of NaCl to an aqueous solution of NaOH, gaseous Cl2, and gaseous H2:

\(2{\text{Na}}^{\text{+}}(aq)+2{\text{Cl}}^{\text{−}}(aq)+2{\text{H}}_{2}\text{O}(l)\;\stackrel{\phantom{\rule{0.5em}{0ex}}\text{electrolysis}\phantom{\rule{0.5em}{0ex}}}{\to }\;2{\text{Na}}^{\text{+}}(aq)+2{\text{OH}}^{\text{−}}(aq)+{\text{Cl}}_{2}(g)+{\text{H}}_{2}(g)\)

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