What Products Form When You Electrolyze an Aqueous Solution vs a Molten Salt? (HL)

A molten salt contains only its ions, so the cation is reduced to the element at the cathode and the anion is oxidized at the anode. In an aqueous solution, water also participates, so competing electrode reactions determine the products.

How the products are determined

Electrolysis uses electrical energy to drive a non-spontaneous redox reaction. Reduction occurs at the negative cathode, while oxidation occurs at the positive anode.

For molten sodium chloride, only sodium and chloride ions are present:

Cathode: $\text{Na}^+(l) + e^- \rightarrow \text{Na}(l)$

Anode: $2\text{Cl}^-(l) \rightarrow \text{Cl}_2(g) + 2e^-$

The products are sodium metal and chlorine gas.

For concentrated aqueous sodium chloride with inert electrodes, sodium ions compete with water at the cathode. Water is reduced more readily:

$2\text{H}_2\text{O}(l) + 2e^- \rightarrow \text{H}_2(g) + 2\text{OH}^-(aq)$

Chloride ions are oxidized at the anode:

$2\text{Cl}^-(aq) \rightarrow \text{Cl}_2(g) + 2e^-$

Therefore, the products are hydrogen and chlorine, while sodium hydroxide remains in solution. In more dilute chloride solutions, oxygen formation from water may compete more successfully.

By contrast, aqueous copper(II) chloride can deposit copper because copper ions are reduced more readily than water:

$\text{Cu}^{2+}(aq) + 2e^- \rightarrow \text{Cu}(s)$

This comparison shows that water is a competitor, not automatically the species discharged; the preferred process depends on the ions and conditions.

For aqueous electrolysis, identify every species present, including water, then compare the possible half-equations. Standard electrode potentials provide a thermodynamic guide, but concentration, overpotential and electrode material can affect the observed product.

A common misconception is that electrolysis of any salt produces its metal and non-metal. This is only reliably true for a molten salt; in aqueous solutions, water may be discharged instead.

Exam technique

For a “deduce” question, label the cathode and anode, write balanced half-equations with state symbols, and justify any competing reaction. Unless stated otherwise, assume inert electrodes; reactive electrodes can themselves participate.