Cathode – IGCSE Chemistry Definition

The cathode is the negative electrode in electrolysis. It is where reduction occurs — cations migrate to it and gain electrons. Knowing what is produced at each electrode is a core requirement for the 0620 syllabus, and half-equation writing at the cathode is tested on Papers 2 and 4.

The 0620 definition

The cathode is the negative electrode. During electrolysis, positively charged ions (cations) are attracted to the cathode and are discharged by gaining electrons. This is reduction.

Memory aids

• Cathode = Cations go here
• OILRIG: Reduction Is Gain (of electrons) — happens at the cathode
• RED CAT: REDuction at the CAThode

What happens at the cathode

Cations are attracted to the negative cathode. When they reach it, they gain electrons (are reduced) and are discharged.

Electrolyte	Ion at cathode	Half-equation	Product
Molten lead(II) bromide	Pb²⁺	Pb²⁺ + 2e⁻ → Pb	Lead metal
Copper sulfate solution	Cu²⁺	Cu²⁺ + 2e⁻ → Cu	Copper metal
Dilute sulfuric acid	H⁺	2H⁺ + 2e⁻ → H₂	Hydrogen gas
Sodium chloride solution	H⁺ (from water)	2H⁺ + 2e⁻ → H₂	Hydrogen gas

Selective discharge at the cathode (Supplement)

When more than one cation is present (e.g. in aqueous solutions), the ion that is preferentially discharged depends on the reactivity series:

• If the metal ion is less reactive than hydrogen, the metal is deposited (e.g. Cu²⁺ → Cu)
• If the metal ion is more reactive than hydrogen, hydrogen gas is produced (e.g. Na⁺ stays in solution, H⁺ is discharged)

Observations at the cathode

• Metal deposited: a metallic coating appears on the cathode (shiny solid)
• Hydrogen gas: bubbles of gas at the cathode; squeaky pop with a burning splint

Worked exam question

Dilute copper(II) sulfate solution is electrolysed using carbon electrodes. (a) Name the product at the cathode. (1) (b) Write the half-equation at the cathode. (2) (c) Explain why copper is deposited rather than hydrogen. (1)

Mark scheme

(a) Copper [1]

(b) Cu²⁺ + 2e⁻ → Cu [1 for formula, 1 for balance]

(c) Copper is less reactive than hydrogen / copper is below hydrogen in the reactivity series, so Cu²⁺ ions are preferentially discharged [1]

Common exam mistakes

• Confusing cathode with anode. Cathode is negative, anode is positive. Positive ions (cations) go to the cathode.
• Forgetting to check the reactivity series for aqueous solutions. In molten electrolytes, the metal cation is always discharged. In aqueous solutions, you must compare the metal to hydrogen.
• Writing “oxidation” at the cathode. Reduction (gain of electrons) always occurs at the cathode.