Discharge – IGCSE Chemistry Definition

In the context of electrolysis, discharge means an ion gains or loses electrons at an electrode, converting from an ion into a neutral element. The 0620 syllabus (Supplement) expects you to apply selective discharge rules to predict which ions are discharged at each electrode in aqueous solutions.

The 0620 definition

Discharge (in electrolysis) is the process by which an ion gains or loses electrons at an electrode to form a neutral atom, molecule, or element.

• At the cathode: cations gain electrons (reduction) — they are discharged
• At the anode: anions lose electrons (oxidation) — they are discharged

Selective discharge rules (Supplement)

In aqueous solutions, water provides additional ions (H⁺ and OH⁻), so more than one type of cation or anion is present. The rules for which is preferentially discharged:

At the cathode (choosing between metal ion and H⁺)

Metal reactivity	Which cation is discharged	Product
Metal more reactive than hydrogen (e.g. Na, K, Ca, Mg, Al, Zn)	H⁺ is discharged	Hydrogen gas
Metal less reactive than hydrogen (e.g. Cu, Ag)	Metal ion is discharged	Metal deposited

At the anode (choosing between non-metal ion and OH⁻)

Condition	Which anion is discharged	Product
Concentrated halide (Cl⁻, Br⁻, I⁻) present	Halide ion discharged	Halogen gas
Dilute halide or no halide	OH⁻ discharged	Oxygen gas
Sulfate (SO₄²⁻) or nitrate (NO₃⁻) present	OH⁻ discharged (sulfate/nitrate not discharged)	Oxygen gas

Example: electrolysis of dilute NaCl vs concentrated NaCl

Solution	Cathode product	Anode product
Dilute NaCl	H₂ (Na too reactive)	O₂ (Cl⁻ too dilute)
Concentrated NaCl	H₂ (Na too reactive)	Cl₂ (Cl⁻ concentrated enough)

Worked exam question

Copper(II) sulfate solution is electrolysed with carbon electrodes. (a) Which ion is discharged at the cathode? Explain why. (2) (b) Which ion is discharged at the anode? Explain why. (2)

Mark scheme

(a) Cu²⁺ [1]; copper is less reactive than hydrogen / below hydrogen in the reactivity series, so Cu²⁺ is preferentially discharged [1]

(b) OH⁻ [1]; sulfate ions (SO₄²⁻) are not discharged / OH⁻ is discharged in preference to sulfate [1]

Common exam mistakes

• Applying the wrong rule at the wrong electrode. Reactivity series is used at the cathode (for cations). Concentration of halide is used at the anode (for anions).
• Saying sulfate or nitrate ions are discharged. They are never discharged at IGCSE level — OH⁻ is discharged instead, producing oxygen.
• Confusing “discharged” with “attracted”. Ions are attracted to the opposite electrode, but being discharged means they actually gain or lose electrons there.