Competing Ions in ElectrolysisOxford AQA IGCSE Chemistry: Revision notes

Many electrolytes contain more than one type of positive ion and more than one type of negative ion — for example, sodium chloride solution contains Na⁺, H⁺ (from water), Cl⁻ and OH⁻ (from water). When several ions of the same charge are present, they compete to be discharged (converted to atoms/molecules) at the electrode. Only one product usually forms at each electrode, and which one depends on specific rules.

At the cathode, the product depends on the reactivity of the elements involved:

• If a metal is more reactive than hydrogen, hydrogen gas is produced instead of the metal (the H⁺ ions from water are discharged preferentially)
• If a metal is less reactive than hydrogen, the metal is produced instead of hydrogen

This is because more reactive metals hold onto their electrons more strongly — their ions are harder to reduce back to atoms than hydrogen ions are.

At the anode, the product depends on the relative concentrations of the negative ions present:

• If a halide ion (Cl⁻, Br⁻, I⁻) is present in reasonably high concentration, it is discharged in preference to hydroxide (OH⁻) ions, producing the halogen gas
• If the halide concentration is very low (dilute solution) or there is no halide present, oxygen is produced from the OH⁻ ions instead

So, unlike the cathode (which depends on reactivity), the anode outcome depends on how much of each ion is available.

1. List all the ions present in the electrolyte (from the dissolved compound plus H⁺ and OH⁻ from water)
2. For the cathode, compare the metal's reactivity to hydrogen — the less reactive species is discharged
3. For the anode, compare the concentration of the halide ion to hydroxide — the more concentrated one is usually discharged
4. Write the half-equation for each electrode using the ion actually discharged