Electrolysis of Molten Ionic Compounds

Cathode Reactions (Reduction)

• Process: Cations migrate to the cathode where they gain electrons (reduction) to form neutral atoms. For a metal cation $M^{n+}$, the general half-equation is:

$M^{n+} + ne^- \rightarrow M$

• Observation: A layer of metal typically deposits on the surface of the cathode or sinks to the bottom if it is denser than the melt.

Anode Reactions (Oxidation)

• Process: Anions migrate to the anode where they lose electrons (oxidation) to form neutral atoms or molecules. For a non-metal anion $X^{n-}$, the general half-equation is:

$2X^- \rightarrow X_2 + 2e^-$

• Observation: Non-metals are often released as bubbles of gas (e.g., Chlorine, Oxygen) or as a liquid/solid layer at the anode.

• Molten vs. Aqueous Electrolysis: In molten electrolysis, the only ions present are those from the compound itself, meaning the products are predictable and pure. In aqueous solutions, water molecules compete with the salt ions for discharge at the electrodes, often leading to the production of Hydrogen or Oxygen instead of the salt's constituent elements.

• Predicting Products: Always identify the ions present first. In a molten salt $XY$, the metal $X$ will always form at the cathode and the non-metal $Y$ will always form at the anode.

• State Symbols: When writing half-equations for molten electrolysis, remember that the reactants are usually in the liquid state $(l)$ and the products may be solid $(s)$, liquid $(l)$, or gas $(g)$ depending on the temperature of the cell.

• Diatomic Molecules: Be careful with non-metals like Chlorine ($Cl_2$), Bromine ($Br_2$), and Oxygen ($O_2$). Ensure the half-equation is balanced for both atoms and charge (e.g., $2Cl^- \rightarrow Cl_2 + 2e^-$).

• Common Error: Do not confuse the charge of the electrodes in electrolysis with those in a galvanic (voltaic) cell. In electrolysis, the Anode is Positive (PANIC: Positive Anode, Negative Is Cathode).