Molten Salt
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Molten Salt

Electrolysis of Molten Salt

Now let's consider the electrolysis of salts that are not in solution.

Sodium Chloride

The example we will use is the most common of the salts, sodium chloride. An important thing to note is that solid sodium chloride does not conduct electricity. Solid NaCl (not conducting electricity)

However, molten sodium chloride does. In this case we are dealing with molten sodium chloride. That simply means that we've taken sodium chloride and heated it up enough to where it is melted. When it melts, the sodium ions and the chloride ions can separate from one another somewhat, and they are free to move throughout the liquid.

This diagram (example 5 in your workbook) shows that the voltage source is forcing electrons through the wire from right to left. Electrons are forced onto the electrode on the left. The electrons are picked up by the sodium ions. The sodium ions react with electrons at the electrode over on the left to form sodium metal. Since this is reduction, that electrode is called the cathode. Diagram of electrolysis of molten NaCl. Anode:
electrons lost by Cl-
2 Cl- rtarrow.gif (850 bytes) Cl2 + e-
Cathode:
electrons gained by Na+
Na+ + e- rtarrow.gif (850 bytes) Na
Over on the right side, electrons are being pulled off of the chloride ions to form chlorine gas, which would bubble away unless it was somehow captured. Since this is an oxidation process, the electrode on the right side of this particular diagram would be called the anode because that is where the oxidation occurs.

The net result of this reaction is the production of sodium and chlorine from sodium chloride by forcing a current through the cell. That current is forced by the external power supply or voltage source.

Sodium chloride is not the only salt that can be used in this process. We could use any ionic compound that can be melted to free up the ions so that they can move.

Aluminum Oxide

An example is that aluminum metal can be generated from aluminum ores by this process. I would like you to consider the case of passing an electric current through molten aluminum oxide.

The diagram in exercise 4 in your workbook is a blank version of the diagram shown above for sodium chloride. Use it to describe what happens when an electric current is passed through molten aluminum oxide. Presume that the electrodes are inert and won't get involved in the reactions. Refer back to the sodium chloride example (ex. 5 in your workbook) as needed. When you have finished, check your answers below to make sure that you have included all the necessary components in your diagram. Blank diagram for electrolysis of molten salt.

 

Some practical considerations to consider when actually making aluminum metal by this process include these. Are the electrodes really inert? How hot does aluminum oxide have to be to stay molten? Can people work around those temperatures? What voltage and current are needed to make this happen? And, of course, there are the economic and environmental issues of the cost and availability of large amounts of electricity and the price at which aluminum can be sold. If you know or meet someone who has worked in an aluminum plant, you might ask them how some of these practical considerations have been addressed.

Answers

Voltage source: These answers presume the electrons to flow to the left. If you set the electron flow to the right the position of electrodes and reactions will be opposite. Diagram for electrolysis of aluminum oxide.
In the molten salt area of the diagram you should have Al3+ and O2- because these are the ions that are found in Al2O3.
cathode anode
reduction oxidation
Al3+ + 3 e- rtarrow.gif (850 bytes) Al 2 O2- rtarrow.gif (850 bytes) O2 + 4 e-

 

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E-mail instructor: Eden Francis

Clackamas Community College
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