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Electrolysis

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Electrolysis

Ions of metals in of groups 1 and 2, along with aluminum, are very difficult to reduce; therefore, it is necessary to prepare these elements by electrolysis, an important process discussed in the tutorial on electrochemistry. Briefly, electrolysis involves using electrical energy to drive unfavorable chemical reactions to completion; it is useful in the isolation of reactive metals in their pure forms. Sodium, aluminum, and magnesium are typical examples.

The Preparation of Sodium

The most important method for the production of sodium is the electrolysis of molten sodium chloride; the set-up is a Downs cell, shown in the figure below. The reaction involved in this process is:

\(\text{2NaCl}(l)\;\underset{600\;\text{°C}}{\overset{\phantom{\rule{0.5em}{0ex}}\text{electrolysis}\phantom{\rule{0.5em}{0ex}}}{\to }}\;\text{2Na}(l)+{\text{Cl}}_{2}(g)\)

The electrolysis cell contains molten sodium chloride (melting point 801 °C), to which calcium chloride has been added to lower the melting point to 600 °C (a colligative effect). The passage of a direct current through the cell causes the sodium ions to migrate to the negatively charged cathode and pick up electrons, reducing the ions to sodium metal. Chloride ions migrate to the positively charged anode, lose electrons, and undergo oxidation to chlorine gas. The overall cell reaction comes from adding the following reactions:

\(\begin{array}{l}\text{at the cathode:}\;{\text{2Na}}^{+}+{\text{2e}}^{-}\;⟶\phantom{\rule{0.4em}{0ex}}\text{2Na}(l)\\ \text{at the anode:}\;{\text{2Cl}}^{-}\;⟶\;{\text{Cl}}_{2}(g)+{\text{2e}}^{-}\\ \text{overall change:}\;{\text{2Na}}^{+}+{\text{2Cl}}^{-}\;⟶\;\text{2Na}(l)+{\text{Cl}}_{2}(g)\end{array}\)

Separation of the molten sodium and chlorine prevents recombination. The liquid sodium, which is less dense than molten sodium chloride, floats to the surface and flows into a collector. The gaseous chlorine goes to storage tanks. Chlorine is also a valuable product.

A diagram is shown. At the center of the diagram is a T-shaped structure labeled, “Anode ( positive sign ),” that is on a mesh-like background labeled, “Iron screen.” The anode is connected to a box labeled, “Voltage source,” by a line, and the voltage source is connected to one of two L-shaped structures that surround the anode. They are labeled, “Cathode ( negative sign ).” Above the cathode on the right side of the diagram is a tube connected to a collection chamber labeled, “N a metal,” that has a lower outlet labeled, “N a outlet.” Surrounding all of these components is a blue background labeled, “Molten N a C l,” and a series of tubes that form a square outside the diagram. They have an opening in the upper right corner labeled, “Inlet for N a C l.” At the top of the diagram is a large tube with an upward-facing arrow drawn on it. The tube turns right, has a right-facing arrow on it, and is labeled, “C l subscript 2 outlet.”

Pure sodium metal is isolated by electrolysis of molten sodium chloride using a Downs cell. It is not possible to isolate sodium by electrolysis of aqueous solutions of sodium salts because hydrogen ions are more easily reduced than are sodium ions; as a result, hydrogen gas forms at the cathode instead of the desired sodium metal. The high temperature required to melt NaCl means that liquid sodium metal forms.

The Preparation of Aluminum

The preparation of aluminum utilizes a process invented in 1886 by Charles M. Hall, who began to work on the problem while a student at Oberlin University in Ohio. Paul L. T. Héroult discovered the process independently a month or two later in France. In honor to the two inventors, this electrolysis cell is known as the Hall–Héroult cell. The Hall–Héroult cell is an electrolysis cell for the production of aluminum. The figure below illustrates the Hall–Héroult cell.

The production of aluminum begins with the purification of bauxite, the most common source of aluminum. The reaction of bauxite, AlO(OH), with hot sodium hydroxide forms soluble sodium aluminate, while clay and other impurities remain undissolved:

\(\text{AlO}(\text{OH})(s)+\text{NaOH}(aq)+{\text{H}}_{2}\text{O}(l)\;⟶\;\text{Na}[\text{Al}{(\text{OH})}_{4}](aq)\)

After the removal of the impurities by filtration, the addition of acid to the aluminate leads to the reprecipitation of aluminum hydroxide:

\(\text{Na}[\text{Al}{(\text{OH})}_{4}](aq)+{\text{H}}_{3}{\text{O}}^{\text{+}}(aq)\;⟶\;\text{Al}{(\text{OH})}_{3}(s)+{\text{Na}}^{\text{+}}(aq)+{\text{2H}}_{2}\text{O}(l)\)

The next step is to remove the precipitated aluminum hydroxide by filtration. Heating the hydroxide produces aluminum oxide, Al2O3, which dissolves in a molten mixture of cryolite, Na3AlF6, and calcium fluoride, CaF2. Electrolysis of this solution takes place in a cell like that shown in the figure below. Reduction of aluminum ions to the metal occurs at the cathode, while oxygen, carbon monoxide, and carbon dioxide form at the anode.

A diagram is shown. At the center of the diagram are two black squares, each labeled, “Carbon anode ( positive sign ),” and connected by forked tubes to a horizontal tube labeled with a positive sign. The carbon anodes are submerged in a green liquid labeled, “A l subscript 2 O subscript 3 dissolved in molten N a subscript 3 A l F subscript 6.” It is held in place by a three-sided, double layered container which is labeled, “Steel sheet,” on the outer layer and, “Ceramic,” on the inner layer. The carbon anodes are surrounded by bubbles labeled, “Bubbles of O subscript 2, C O, and C O subscript 2.” Below the green liquids lies a silver layer labeled, “Molten aluminum,” and a black layer labeled, “Carbon cathode ( negative sign ).” Above the diagram is an outlet tube labeled with an upward-facing arrow and the words, “H F and particulates exhaust to filter plant.”

An electrolytic cell is used for the production of aluminum. The electrolysis of a solution of cryolite and calcium fluoride results in aluminum metal at the cathode, and oxygen, carbon monoxide, and carbon dioxide at the anode.

The Preparation of Magnesium

Magnesium is the other metal that is isolated in large quantities by electrolysis. Seawater, which contains approximately 0.5% magnesium chloride, serves as the major source of magnesium. Addition of calcium hydroxide to seawater precipitates magnesium hydroxide. The addition of hydrochloric acid to magnesium hydroxide, followed by evaporation of the resultant aqueous solution, leaves pure magnesium chloride. The electrolysis of molten magnesium chloride forms liquid magnesium and chlorine gas:

\({\text{MgCl}}_{2}(aq)+\text{Ca}{(\text{OH})}_{2}(aq)\;⟶\;\text{Mg}{(\text{OH})}_{2}(s)+{\text{CaCl}}_{2}(aq)\)

\(\text{Mg}{(\text{OH})}_{2}(s)+\text{2HCl}(aq)\;⟶\;{\text{MgCl}}_{2}(aq)+{\text{2H}}_{2}\text{O}(l)\)

\({\text{MgCl}}_{2}(l)\;⟶\;\text{Mg}(l)+{\text{Cl}}_{2}(g)\)

Some production facilities have moved away from electrolysis completely. In the next section, we will see how the Pidgeon process leads to the chemical reduction of magnesium.

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