Electrolysis

ICSE Class 10 Chemistry • Chapter 06

1. Key Definitions

Term Definition
Electrolysis Chemical decomposition of a compound by passing electric current through it in molten or aqueous state
Electrolyte A substance that conducts electricity when molten or dissolved in water due to presence of free ions
Non-electrolyte A substance that does not conduct electricity (no free ions)
Electrode Conductor through which electricity enters or leaves electrolyte
Anode Positive electrode (connected to +ve terminal)
Cathode Negative electrode (connected to -ve terminal)

2. Strong vs Weak Electrolytes

Strong Electrolytes Weak Electrolytes
Completely ionised in solution Partially ionised in solution
Good conductors Poor conductors
NaCl, HCl, H₂SO₄, NaOH CH₃COOH, NH₄OH, H₂CO₃

Non-electrolytes: Sugar, glucose, urea, alcohol (covalent compounds that don't ionise)

3. Electrolysis Process

At Cathode (Reduction): Cations move here, gain electrons

$M^{n+} + ne^- \rightarrow M$ (Metal deposited)

At Anode (Oxidation): Anions move here, lose electrons

$X^{n-} \rightarrow X + ne^-$ (Non-metal released)

Memory Trick: "AN OX" and "RED CAT"

AN(ode) = OX(idation), RED(uction) = CAT(hode)

4. Electrolysis of Molten Lead Bromide (PbBr₂)

Electrolyte: Molten PbBr₂

Electrodes: Platinum (inert)

Ionisation: PbBr₂ → Pb²⁺ + 2Br⁻

At Cathode: Pb²⁺ + 2e⁻ → Pb (grey metal deposited)

At Anode: 2Br⁻ → Br₂ + 2e⁻ (brown bromine vapours)

5. Electrolysis of Acidified Water

Electrolyte: Water + dilute H₂SO₄ (to increase conductivity)

Electrodes: Platinum

Ionisation: H₂O ⇌ H⁺ + OH⁻

At Cathode: 2H⁺ + 2e⁻ → H₂↑ (colourless gas, pops with burning splint)

At Anode: 4OH⁻ → 2H₂O + O₂↑ + 4e⁻ (colourless gas, relights glowing splint)

Volume Ratio: H₂ : O₂ = 2 : 1

6. Electrolysis of Copper Sulphate Solution

A. With Platinum Electrodes (Inert)

Ionisation: CuSO₄ → Cu²⁺ + SO₄²⁻ and H₂O ⇌ H⁺ + OH⁻

At Cathode: Cu²⁺ + 2e⁻ → Cu (copper deposited)

At Anode: 4OH⁻ → 2H₂O + O₂ + 4e⁻ (oxygen released)

Observation: Blue colour fades as Cu²⁺ ions are deposited

B. With Copper Electrodes (Active)

At Cathode: Cu²⁺ + 2e⁻ → Cu (copper deposited)

At Anode: Cu → Cu²⁺ + 2e⁻ (anode dissolves)

Observation: Blue colour remains constant (Cu removed = Cu added)

Anode dissolves, Cathode gains mass

7. Selective Discharge of Ions

When multiple ions are present, selective discharge occurs based on:

  1. Position in Activity Series (for cations)
  2. Concentration of ions
  3. Nature of electrode

At Cathode: Less active metal ion discharged first

At Anode: Ion that loses electrons easily discharged first

8. Applications of Electrolysis

Application Process
Electroplating Coating metal with thin layer of another metal
Electrorefining Purification of metals (e.g., copper)
Extraction of metals Aluminium from Al₂O₃ (Hall-Héroult process)
Manufacture of chemicals NaOH, Cl₂ from brine (chlor-alkali process)

Electroplating

Object to be plated: Made the cathode

Metal for plating: Made the anode

Electrolyte: Salt solution of the plating metal

Example: Silver plating - Object as cathode, Pure Ag as anode, AgNO₃ solution

Electro-refining of Copper

Anode: Impure copper

Cathode: Pure copper strip

Electrolyte: Acidified CuSO₄ solution

Result: Pure copper deposits at cathode, impurities settle as anode mud

Quick Reference

Electrolyte At Cathode At Anode
Molten PbBr₂ Pb metal Br₂ gas
Acidified water H₂ gas O₂ gas
CuSO₄ (Pt electrodes) Cu metal O₂ gas
CuSO₄ (Cu electrodes) Cu deposited Cu dissolves

Exam Practice (PYQ Trends)

PYQ: 2023

BOARD Give the electrode reactions for electrolysis of acidified water using platinum electrodes.

PYQ: 2022

BOARD In the electrolysis of CuSO₄ using copper electrodes, why does the blue colour remain unchanged?