Electrolysis of molten ionic compounds and of aqueous solutions including brine, predicting the products formed at the cathode and anode, and writing electrode half-equations.

What this dot point is asking

CCEA Double Award wants electrolysis of molten ionic compounds and of solutions (including brine), how to predict the product at each electrode, and how to write electrode half-equations. The rules for which ion is discharged are the key to scoring on this topic.

What electrolysis is

The two electrodes are the cathode (negative) and the anode (positive). The compound being broken down is the electrolyte.

Where the ions go

Electrolysis of molten compounds

For a molten ionic compound there are only two ions, so the products are simple: the metal forms at the cathode and the non-metal forms at the anode. For molten lead bromide, lead forms at the cathode and bromine at the anode.

The half-equations show the electron transfer at each electrode. At the cathode: Pb2+ + 2e- gives Pb (gain of electrons). At the anode: 2Br- gives Br2 + 2e- (loss of electrons).

Electrolysis of solutions

In a solution, water also provides H+ and OH- ions, so the rules are:

• At the cathode: hydrogen is given off if the metal is more reactive than hydrogen; otherwise the metal is deposited.
• At the anode: a halogen (chlorine, bromine, iodine) is given off if a halide is present; otherwise oxygen is given off.

Electrolysis of brine

Brine is concentrated sodium chloride solution. Electrolysing it gives three useful products: hydrogen at the cathode (sodium is more reactive than hydrogen, so hydrogen forms instead), chlorine at the anode (a halide is present), and sodium hydroxide left in the solution. These are important industrial chemicals.

Examples in context

Example 1. Purifying copper. Copper is purified by electrolysis using an impure copper anode and a pure copper cathode. Copper dissolves from the anode and is deposited pure at the cathode, leaving impurities behind. This uses the cathode-deposition idea for a real industrial process.

Example 2. Why aluminium is expensive to extract. Aluminium is extracted by electrolysis of molten aluminium oxide, which needs a lot of electrical energy to keep it molten and to drive the reaction. The high energy cost, all from electrolysis, is why recycling aluminium saves so much energy.

Try this

Q1. Which electrode do positive ions move towards? [1 mark]

• Cue. The cathode (negative electrode).

Q2. Name the gas formed at the anode when brine is electrolysed. [1 mark]

• Cue. Chlorine.