Electrolysis of Molten Ionic Compounds

Definition and Purpose

Electrolysis is a chemical process that uses an electric current to break down ionic compounds into their elements. When ionic compounds are molten (melted into a liquid), their ions are free to move, allowing the compound to conduct electricity. This movement of ions enables electrolysis to separate the compound into its constituent elements.

Molten ionic compounds conduct electricity because the ions are no longer fixed in a solid lattice and can move freely to carry charge. This is different from solid ionic compounds, which do not conduct electricity as their ions are locked in place.

Electrolysis of molten ionic compounds is used to extract or produce pure elements from their compounds. For example, metals like sodium and aluminium can be obtained by electrolysis of their molten salts.

Process of Electrolysis

To carry out electrolysis of a molten ionic compound:

• The ionic compound is heated until it melts and becomes a liquid.
• Two electrodes (usually made of inert materials like graphite) are placed into the molten liquid.
• The electrodes are connected to a power supply which provides a direct current (DC).
• The positive ions (cations) move towards the negative electrode (cathode).
• The negative ions (anions) move towards the positive electrode (anode).

At the electrodes, the ions gain or lose electrons and form elements. This process breaks down the ionic compound into its elements.

Electrode Reactions

During electrolysis:

• Cations (positive ions) move to the cathode (negative electrode) where they gain electrons (reduction) to form neutral atoms.
• Anions (negative ions) move to the anode (positive electrode) where they lose electrons (oxidation) to form neutral atoms or molecules.

The elements formed at the electrodes are usually metals at the cathode and non-metals at the anode.

For example, in the electrolysis of molten sodium chloride:

• Sodium ions (Na⁺) move to the cathode and gain electrons to form sodium metal.
• Chloride ions (Cl⁻) move to the anode and lose electrons to form chlorine gas.

This process can be summarised as:

• Cathode: reduction (gain of electrons)
• Anode: oxidation (loss of electrons)

Examples of Electrolysis

A common example is the electrolysis of molten sodium chloride (NaCl):

• The molten NaCl contains Na⁺ and Cl⁻ ions.
• At the cathode, Na⁺ ions gain electrons to form sodium metal: $\mathrm{Na^+ + e^- \rightarrow Na}$
• At the anode, Cl⁻ ions lose electrons to form chlorine gas: $\mathrm{2Cl^- \rightarrow Cl_2 + 2e^-}$

Sodium metal collects at the cathode, and chlorine gas bubbles off at the anode.

For instance, if molten sodium chloride is electrolysed, sodium metal and chlorine gas are produced. This is a practical way to obtain sodium, which is too reactive to extract by heating with carbon.

Another example is the electrolysis of molten lead bromide (PbBr₂):

• Pb²⁺ ions move to the cathode and gain electrons to form lead metal.
• Br⁻ ions move to the anode and lose electrons to form bromine gas.

These examples show how electrolysis can separate ionic compounds into their elements.

Example: Calculate the products formed at each electrode during the electrolysis of molten sodium chloride.

Sodium ions (Na⁺) move to the cathode and gain electrons to form sodium metal (Na). Chloride ions (Cl⁻) move to the anode and lose electrons to form chlorine gas (Cl₂).

For instance, when molten sodium chloride is electrolysed, sodium ions gain electrons at the cathode to form sodium metal, and chloride ions lose electrons at the anode to form chlorine gas. This simple transfer of electrons illustrates the fundamental process of electrolysis.