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Electrolysis of Molten Ionic Compounds

Instructor: Hemnath (Vikash) Seeboo

Taught Science (mainly Chemistry, Physics and Math) at high school level and has a Master's Degree in Education.

This lesson looks into how molten ionic compounds can be electrolyzed. It also provides an understanding on how metals such as aluminum and sodium can be extracted from molten compounds using electrolysis.

Electrolysis and Ionic Compounds

Aluminum is the most abundant metal on earth. But did you ever wonder how we obtain this versatile metal? Aluminum actually doesn't occur in the natural state, and we have to resort to the process of electrolysis, or, more precisely, electrolysis of a molten ionic compound.

Usually an ionic compound, with general formula MN, is formed by strong electrostatic bonds that exist between positive and negative ions. These oppositely charged ions are locked into a rigid structure where they are not free to move. This structure does not allow the ionic solid to conduct electricity, and hence, electrolysis cannot take place.

One way in which this rigid structure can be made to conduct electricity is to heat it up to its melting point. At the melting temperature both the cations (Mx+) and anions (Ny-) acquire enough energies that allow them to move freely. This resulting liquid is now known as an electrolyte. If the electrolyte is connected to a power supply through inert electrodes, electrolysis occurs.

Electrolytic arrangement for ionic compounds
Electrolytic arrangement for ionic compounds.

The cations (Mx+) will be attracted to the negative electrode or the cathode where they will gain electron(s) to become atoms. Reduction takes place at the cathode according to the following equation:

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The anions (Ny-) will be attracted to the positive electrode or the anode where they will lose electron(s) to become atoms. Oxidation takes place at the anode according to this equation:

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Electrolysis of molten Lead (II) bromide

Let us now consider the electrolysis of molten lead (II) bromide using inert electrodes.

At room temperature, if we connect the white powdered lead (II) bromide through electrodes to an electrical supply, nothing appears to be happening as solid lead (II) bromide is unable to conduct electricity. However, at a temperature around 370 degrees Celsius, lead ions and bromide ions are released from the solid lead (II) bromide structure. The molten lead (II) bromide allows conduction of electricity and thereafter chemical reactions to occur. So what happens at both the anode and cathode?

At the cathode, a silvery deposit of lead is seen.

Lead ions migrate towards the cathode where each cation gains two electrons to form lead atom.

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At the anode, a brown gas is seen. This gas produced is bromine.

Bromide ions move to the anode where each ion loses an electron to form a bromine atom. Then two of the bromine atoms combine to form bromine gas.

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Now let us turn to the applications of electrolysis that involve molten ionic compounds. We will next discuss the extractions of aluminum and sodium from molten alumina and molten sodium chloride respectively.

Extraction of Aluminum from Molten Aluminum Oxide

Aluminium oxide or alumina, Al 2 O3, is obtained from bauxite, an ore, which also contains impurities such as iron (III) oxide, Fe2 O3, and silicon dioxide, SiO2. The purification is carried out by a process known as the Bayer process.

Summary of the Bayer process.

The purified alumina is then electrolyzed in molten cryolite, Na3 AlF6 , by a process commonly known as the Hall-Heroult process. Cryolite not only lowers the melting point of alumina from around 2000 degrees Celsius to about 980 degrees Celsius but also increases its conductivity. The electrolysis is carried out in an electric furnace, using carbon electrodes.

Extraction of Aluminium
Extraction of Aluminium

Molten alumina contains Al3+ and O2- ions.

At the cathode, which includes the bottom and sides of the electrolysis cell, aluminum ions are reduced to aluminum metal.

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At the anode, the oxide ions are oxidized to oxygen gas.

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At that operating temperature, oxygen produced reacts with the carbon anodes to produce both carbon dioxide. You will also find that the anodes burn away and there is need to continuously replace them.

The overall equation for the extraction of Aluminum is summarized by the chemical equation:

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