What is Electrolysis? Core Principles Explained

Electrolysis is defined as the decomposition (breaking down) of an ionic compound into its elements when molten or in aqueous solution by the passage of an electric current[citation:1][citation:9]. It's a fundamental concept tested across all major exam boards.

Key Condition: The compound must be ionic and have mobile ions (not in solid state)[citation:1]. This is why solid sodium chloride doesn't conduct electricity, but molten or dissolved NaCl does[citation:9].

Essential Definitions (Write These in Your Notes!)

Electrolyte

The ionic compound in a molten or aqueous state that conducts electricity and is decomposed during electrolysis[citation:1][citation:9]. Examples: molten lead(II) bromide, aqueous sodium chloride.

Electrode

A rod (usually metal or graphite) that conducts electricity into or out of the electrolyte[citation:2].

Anode

The positive electrode. Anions (negative ions) move here to be oxidised (lose electrons)[citation:1][citation:8].

Cathode

The negative electrode. Cations (positive ions) move here to be reduced (gain electrons)[citation:1][citation:8].

Inert Electrode

An unreactive electrode (e.g., graphite/platinum) that does not participate in the reaction[citation:8].

Electroplating

The process of depositing a layer of metal onto an object at the cathode using electrolysis[citation:8][citation:9].

Predicting Products: The Golden Rules

The number one challenge in electrolysis questions is predicting what forms at each electrode. Use this flowchart in your mind for every question.

1. At the CATHODE (Negative Electrode)

• Metals or hydrogen are formed here[citation:1][citation:9].
• If the metal is MORE REACTIVE than hydrogen (e.g., Na, K, Ca, Mg, Al, Zn), then HYDROGEN GAS (H₂) is produced from the reduction of water or H⁺ ions[citation:2].
• If the metal is LESS REACTIVE than hydrogen (e.g., Cu, Ag, Au), then the SOLID METAL is deposited[citation:2].
• Half-equation example (for Cu²⁺): Cu²⁺(aq) + 2e⁻ → Cu(s)[citation:9]

2. At the ANODE (Positive Electrode)

• Non-metals (except hydrogen) are formed here[citation:1][citation:9].
• If the solution contains HALIDE IONS (Cl⁻, Br⁻, I⁻) in high concentration, the HALOGEN is produced (e.g., Cl₂ gas)[citation:5][citation:9].
• If NO halide ions are present, or they are very dilute, then OXYGEN GAS (O₂) is produced from the oxidation of hydroxide ions (OH⁻)[citation:5][citation:8].
• Half-equation example (for OH⁻): 4OH⁻(aq) → 2H₂O(l) + O₂(g) + 4e⁻[citation:9]

Electrolysis of Aqueous Solutions: The Complete Picture

This is where most students get confused. In aqueous solutions, you must consider the ions from BOTH the compound AND water (H⁺ and OH⁻)[citation:3][citation:5]. The products depend on a competition based on reactivity, concentration, and electrode material[citation:5].

Electrolyte (Aqueous)	Cathode Product (Negative)	Anode Product (Positive) - Inert Electrodes	Key Reason / Observation
Concentrated Sodium Chloride (NaCl)	Hydrogen Gas (H₂)	Chlorine Gas (Cl₂)	High [Cl⁻] favours Cl₂ over O₂[citation:5]. Used industrially to make NaOH, H₂, Cl₂.
Dilute Sodium Chloride	Hydrogen Gas (H₂)	Oxygen Gas (O₂)	Low [Cl⁻] means OH⁻ is oxidised instead[citation:9].
Copper(II) Sulfate (CuSO₄) with inert electrodes	Copper Metal (Cu)	Oxygen Gas (O₂)	Cu²⁺ is less reactive than H⁺, so it deposits. SO₄²⁻ isn't discharged, so OH⁻ is[citation:3][citation:5]. Solution becomes acidic.
Copper(II) Sulfate with COPPER electrodes	Copper Metal (Cu)	Copper ions (Cu²⁺)	Active electrodes: The anode dissolves! Cu(s) → Cu²⁺(aq) + 2e⁻. Used for electroplating and purification[citation:3][citation:5].
Dilute Sulfuric Acid (H₂SO₄)	Hydrogen Gas (H₂)	Oxygen Gas (O₂)	Effectively the electrolysis of water. 2H₂O(l) → 2H₂(g) + O₂(g)[citation:5].

Frequently Asked Exam Questions (FAQs)