Electrolysis of aqueous solutions: Students should be able to predict the products of the electrolysis of aqueous solutions containing a single ionic compound.

The idea

To predict products, first write the formula of the dissolved compound and list its ions, then add H⁺ and OH⁻ from water. At the cathode, the positive ions compete: usually hydrogen is produced unless the metal is less reactive than hydrogen, in which case the metal is deposited. At the anode, the negative ions compete: usually oxygen is produced from OH⁻ unless a halide ion is present, in which case the halogen is formed. For example, in copper(II) chloride solution, Cu²⁺ and H⁺ compete at the cathode and Cu²⁺ is discharged, giving copper metal; Cl⁻ and OH⁻ compete at the anode and Cl⁻ is discharged, giving chlorine gas. In sodium chloride solution, hydrogen and chlorine are formed, leaving sodium hydroxide in solution. The method is the same for any single ionic compound: identify ions, compare ease of discharge, then name the electrode products.

What earns the marks

• Write the formula of the dissolved compound and list the ions it provides, for example CuCl₂ gives Cu²⁺ and Cl⁻.
• Add H⁺ and OH⁻ from water to the list of ions present in the aqueous solution.
• At the cathode, compare the metal ion with H⁺; hydrogen is usually formed unless the metal is less reactive than hydrogen, when the metal is deposited.
• At the anode, compare the non-metal ion with OH⁻; oxygen is usually formed unless a halide ion is present, when the halogen is formed.
• Name the products and, where required, support each with a half-equation such as Cu²⁺ + 2e⁻ → Cu or 2Cl⁻ → Cl₂ + 2e⁻.
• Recognise that ions left in solution can form a new compound, for example sodium hydroxide remains after electrolysis of aqueous sodium chloride.

Common mistakes and how to put them right

• Predicting the metal at the cathode for every aqueous solution.

  Put it right by checking whether the metal is less reactive than hydrogen before deciding.

• Predicting oxygen at the anode when a halide is present.

  Put it right by applying the rule that a halide ion is discharged in preference to OH⁻.

• Forgetting water as a source of ions.

  Put it right by adding H⁺ and OH⁻ to the ion list before comparing discharge.

Exam tips

• Use a two-column ion list for cathode and anode before writing any product.
• State the rule you are applying, such as 'copper is less reactive than hydrogen, so copper is deposited'.
• Include state symbols or gas names where the product is a gas, and balance half-equations if they are requested.