COMPREHENSIVE E - NOTE ON ELECTROLYSIS
Edwin Ogie Library
Electrolysis — Comprehensive e-Book (JAMB / WAEC focus)
E-note on Electrolysis
What you’ll learn — objectives & overview
- Introduction to electrolysis — definitions and concepts
- Electrolytes vs non-electrolytes; ionic conduction
- Faraday’s laws & quantitative electrolysis
- Electrode reactions for key electrolytes (CuSO₄, H₂SO₄, NaCl, molten salts)
- Factors affecting discharge and electrode potentials
- Electrochemical cells, electrode potentials and the electrochemical series
- Applications: electroplating, extraction, purification, chlor-alkali industry
- Corrosion and protection (cathodic protection, sacrificial anodes)
- Worked examples + 30-question CBT quiz (with timer)
Introduction to Electrolysis
Electrolysis is the process of using electrical energy to cause chemical change — driving non-spontaneous redox reactions by passing current through an electrolyte. It is central to metal extraction, refining, electroplating, and large-scale chemical production.
Key terms: electrolyte, electrode, anode (oxidation), cathode (reduction), ionic conduction.
Electrolytes & Non-electrolytes — Conduction
Electrolytes dissociate into ions in solution or when molten and thus conduct electricity (e.g., NaCl, H₂SO₄, CuSO₄). Non-electrolytes (sugar, ethanol) do not dissociate and therefore don't conduct.
Ionic conduction occurs as cations migrate to the cathode and anions migrate to the anode under an applied electric field.
Faraday’s Laws (Quantitative Electrolysis)
- Mass of substance deposited ∝ total charge passed.
- Masses of different substances produced by the same charge are proportional to their equivalent weights.
Useful constant: Faraday's constant F ≈ 96485 C per mole of electrons (C·mol⁻¹).
Worked example — copper deposition
Given: 1.00×10⁵ C through Cu²⁺; equivalent weight (Cu) ≈ 31.75 g per equiv.
Mass = (Q / F) × equivalent weight = (1.00×10⁵ / 96485) × 31.75 ≈ 32.9 g.
Electrode Reactions — Common Systems
Which species is discharged depends on reduction/oxidation potentials, concentration, and electrode material.
- Dilute H₂SO₄: cathode → H₂(g); anode → O₂(g).
- Aqueous CuSO₄ (inert anode): cathode → Cu(s) deposited; anode → O₂ produced from water.
- CuCl₂ (aq): cathode → Cu(s); anode → Cl₂(g).
- Molten NaCl: cathode → Na(s); anode → Cl₂(g).
Worked example — aqueous CuSO₄
At cathode: Cu²⁺ + 2e⁻ → Cu(s). At inert anode: 2H₂O → O₂ + 4H⁺ + 4e⁻.
Factors Affecting Ion Discharge
Primary factors: discharge potential (standard electrode potentials), ion concentration, electrode material, applied voltage and overpotential, temperature.
Example: In aqueous NaCl, H₂ is often evolved at cathode (water reduction) rather than Na⁺ because H₂O has a lower (less negative) reduction potential under aqueous conditions.
Electrochemical Cells & Electrode Potentials
The electrochemical series ranks species by standard reduction potential. Cell potential E°cell = E°cathode − E°anode.
Example — zinc / copper cell
If E°(Cu²⁺/Cu)=+0.34 V and E°(Zn²⁺/Zn)=−0.76 V, E°cell = 0.34 − (−0.76) = 1.10 V.
Applications: Purification, Electroplating, Industry
- Electrolytic refining of copper — impure copper anode, pure Cu deposited at cathode.
- Electroplating — coating a workpiece (cathode) with a metal from solution.
- Chlor-alkali industry — manufacture of Cl₂, H₂ and NaOH by electrolysis of brine.
Worked example — purification of copper
Impure copper anode dissolves; Cu²⁺ migrates and is reduced at the cathode to give pure copper deposits.
Corrosion & Protection
Corrosion is an electrochemical (electrolytic) process. Protection methods include cathodic protection (sacrificial anode), coatings, electroplating, and impressed current systems.
Cathodic protection (short)
A more reactive metal (e.g. zinc) is attached as sacrificial anode; it oxidizes instead of the protected metal.
Worked Calculations & Tips
Always balance electrons in half-reactions when doing Faraday calculations. Use F = 96485 C·mol⁻¹ and relate Q (Coulombs) to moles e⁻ then to moles of substance via stoichiometry.
Quick tip
Mass deposited = (Q / F) × (Moles e⁻ per mole product)⁻¹ × molar mass (or use equivalent weight directly).
Summary & Exam Tips
- Understand which ions will be discharged under different conditions (aqueous vs molten; inert vs reactive electrode).
- Practice Faraday calculations and balancing redox half equations.
- Know industrial examples and the role of electrolysis in metal purification and chemical production.
CBT Quiz — How it works
When you click Start Quiz you'll enter a 30-question CBT with a 15-minute timer. Each question is shown one at a time. After submission you'll see a detailed breakdown with explanations.
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