The Electrolysis of Molten Sodium Chloride

In molten sodium chloride, the ions are free to migrate to the electrodes of an electrolytic cell. A simplified diagram of the cell commercially used to produce sodium metal and chlorine gas is shown in the figure below. Sodium is a strong reducing agent and chlorine is used to purify water, and is used in antiseptics and in paper production. The reactions are

\(\begin{array}{l}\underset{¯}{\begin{array}{l}\text{anode:}\phantom{\rule{4.91em}{0ex}}2{\text{Cl}}^{\text{−}}(l)\phantom{\rule{0.2em}{0ex}}⟶\phantom{\rule{0.2em}{0ex}}{\text{Cl}}_{2}(g)+{\text{2e}}^{\text{−}}\phantom{\rule{4em}{0ex}}{E}_{{\text{Cl}}_{2}\text{/}{\text{Cl}}^{\text{−}}}^{°}=\text{+1.3 V}\\ \text{cathode:}\phantom{\rule{2.13em}{0ex}}{\text{Na}}^{\text{+}}(l)+{\text{e}}^{\text{−}}\phantom{\rule{0.2em}{0ex}}⟶\phantom{\rule{0.2em}{0ex}}\text{Na}(l)\phantom{\rule{4em}{0ex}}{E}_{{\text{Na}}^{\text{+}}\text{/Na}}^{°}=\text{−2.7 V}\end{array}}\\ \hline \text{overall:}\phantom{\rule{0.2em}{0ex}}2{\text{Na}}^{\text{+}}(l)+{\text{2Cl}}^{\text{−}}(l)\phantom{\rule{0.2em}{0ex}}⟶\phantom{\rule{0.2em}{0ex}}\text{2Na}(l)+{\text{Cl}}_{2}(g)\phantom{\rule{4em}{0ex}}{E}_{\text{cell}}^{°}=\text{−4.0 V}\end{array}\)

The power supply (battery) must supply a minimum of 4 V, but, in practice, the applied voltages are typically higher because of inefficiencies in the process itself.

This diagram shows a tank containing a light blue liquid, labeled “Molten N a C l.” A vertical dark grey divider with small, evenly distributed dark dots, labeled “Porous screen” is located at the center of the tank dividing it into two halves. Dark grey bars are positioned at the center of each of the halves of the tank. The bar on the left, which is labeled “Anode” has green bubbles originating from it. The bar on the right which is labeled “Cathode” has light grey bubbles originating from it. An arrow points left from the center of the tank toward the anode, which is labeled “C l superscript negative.” An arrow points right from the center of the tank toward the cathode, which is labeled “N a superscript plus.” A line extends from the tops of the anode and cathode to a rectangle centrally placed above the tank which is labeled “Voltage source.” An arrow extends upward above the anode to the left of the line which is labeled “e superscript negative.” A plus symbol is located to the left of the voltage source and a negative sign it located to its right. An arrow points downward along the line segment leading to the cathode. This arrow is labeled “e superscript negative.” The left side of below the diagram is the label “2 C l superscript negative right pointing arrow C l subscript 2 ( g ) plus 2 e superscript negative.” At the right, below the diagram is the label “2 N a superscript plus plus 2 e superscript negative right pointing arrow 2 N a ( l ).”

Passing an electric current through molten sodium chloride decomposes the material into sodium metal and chlorine gas. Care must be taken to keep the products separated to prevent the spontaneous formation of sodium chloride.

This lesson is part of:

Electrochemistry

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