Electrolysis: a clear study guide

What happens during electrolysis?

An ionic compound must be molten or dissolved so its ions can move. A power supply creates a negative cathode and a positive anode, and oppositely charged ions move towards them.

Positive ions are attracted to the cathode, where they gain electrons in reduction. Negative ions are attracted to the anode, where they lose electrons in oxidation.

• Cathode: negative electrode; reduction happens here.
• Anode: positive electrode; oxidation happens here.
• Remember: PANIC — Positive Anode, Negative Is Cathode.

How to predict the products

First write the ions in the electrolyte. In a molten compound, the ions from that compound are the only ions available. In an aqueous solution, water also contributes hydrogen and hydroxide ions, so competition must be considered.

At the cathode, a less reactive metal may be produced instead of hydrogen. At the anode, halide ions can form halogens; otherwise oxygen is often produced from hydroxide ions.

• Molten lead bromide produces lead at the cathode and bromine at the anode.
• Aqueous copper sulfate with inert electrodes produces copper and oxygen.
• State the ion, the product and the half-equation when the question asks for detail.

Exam technique

Show the charge on each ion before deciding where it travels. Check that your half-equation balances both atoms and charge, then use the observations in the question as evidence for the product.

If a question asks why the electrolyte must be molten or aqueous, explain that the ions must be free to move and carry charge.

Quick check

Common questions