Electrolysis

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Notes

Electrolysis Principles

**Electrolysis** is the breakdown of a molten or aqueous ionic compound by an electric current.
Covalent compounds and solid ionic compounds **cannot** undergo electrolysis because they have no free mobile ions.
**Electrolyte**: molten or dissolved ionic compound that conducts electricity.
**Anode** (positive electrode) attracts **anions** (negative ions).
**Cathode** (negative electrode) attracts **cations** (positive ions).
Use **PANIC**: **P**ositive is **A**node, **N**egative **I**s **C**athode.
Electrons flow from the negative terminal of the power supply to the cathode; they do **not** pass through the electrolyte.
**Reduction** (gain of electrons) occurs at the **cathode**; **oxidation** (loss of electrons) occurs at the **anode**.

Electrolysis of Molten Compounds

Molten binary ionic compounds decompose into their elements: **metal at cathode**, **non-metal at anode**.
Example: molten PbBr₂ → **Pb** (grey metal) at cathode, **Br₂** (brown gas) at anode.
Ions must be free to move – compound must be **molten** (or in solution).
Electrodes must be **inert** (e.g., graphite or platinum) to avoid side reactions.
Half-equation at cathode: Pb²⁺ + 2e⁻ → Pb.
Half-equation at anode: 2Br⁻ → Br₂ + 2e⁻.

Electrolysis of Aqueous Sodium Chloride (Brine)

Brine contains Na⁺, Cl⁻, H⁺, OH⁻ ions.
At **cathode**: H⁺ is discharged (less reactive than Na⁺) → **hydrogen gas**.
At **anode**: Cl⁻ is discharged → **chlorine gas**.
Na⁺ and OH⁻ remain in solution → **sodium hydroxide** forms.
Industrial uses: Cl₂ → bleach; H₂ → margarine; NaOH → soap and detergents.

Electrolysis of Dilute Sulfuric Acid

Contains H⁺, SO₄²⁻, OH⁻ ions.
At **cathode**: H⁺ gains electrons → **hydrogen gas**.
At **anode**: OH⁻ loses electrons → **oxygen gas** (and water).
Volume of H₂ produced is **twice** the volume of O₂.
Gas tests: H₂ – lit splint gives 'squeaky pop'; O₂ – glowing splint relights.

Electrolysis of Aqueous Solutions (Extended)

Water dissociates: H₂O ⇌ H⁺ + OH⁻ – these ions compete with solute ions.
At **cathode**: if metal is **less reactive than hydrogen**, metal is deposited; if **more reactive**, hydrogen gas is produced.
At **anode**: if **halide ions** (Cl⁻, Br⁻, I⁻) are present, halogen forms; otherwise **oxygen** from OH⁻.
**Concentration** matters: concentrated halide → halogen; dilute halide → oxygen.
Example: CuSO₄(aq) with graphite electrodes → **Cu** at cathode, **O₂** at anode; blue colour fades.
With copper electrodes: **anode dissolves** (Cu → Cu²⁺ + 2e⁻), **cathode gains mass** (Cu²⁺ + 2e⁻ → Cu); solution concentration remains constant.

Ionic Half-Equations (Extended)

**Oxidation Is Loss** (of electrons), **Reduction Is Gain** (OIL RIG).
**REDuction at CAThode** (RED CAT); **ANode for OXidation** (AN OX).
Metal formation: Mⁿ⁺ + n e⁻ → M (reduction).
Hydrogen formation: 2H⁺ + 2e⁻ → H₂.
Halogen formation: 2X⁻ → X₂ + 2e⁻ (oxidation).
Oxygen formation: 4OH⁻ → O₂ + 2H₂O + 4e⁻.
Half-equations must balance atoms and charges.

Summary Table of Common Electrolyses

Molten PbBr₂: anode – Br₂; cathode – Pb.
Concentrated NaCl(aq): anode – Cl₂; cathode – H₂.
Dilute H₂SO₄: anode – O₂; cathode – H₂.
CuSO₄(aq) (inert electrodes): anode – O₂; cathode – Cu.

Atomic structure of sodium showing electron configuration (2,8,1). Relevant to understanding ion formation in electrolysis.

Practice questions

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1.What is the name of the process in which a molten ionic compound is broken down by an electric current?
Easy
- AElectrolysis
- BElectroplating
- CElectrorefining
- DElectrolysis of aqueous solutions
2.Which electrode is the positive electrode in an electrolysis cell?
Easy
- AAnode
- BCathode
- CElectrolyte
- DTerminal
3.What type of ion is attracted to the cathode?
Easy
- ACation
- BAnion
- CElectron
- DNeutral atom
4.During electrolysis, what happens at the anode?
Easy
- AOxidation (loss of electrons)
- BReduction (gain of electrons)
- CNo reaction
- DFormation of metal
5.In the electrolysis of molten lead(II) bromide, what is produced at the cathode?
Medium
- ALead
- BBromine
- COxygen
- DHydrogen
6.During the electrolysis of concentrated aqueous sodium chloride (brine), which gas is produced at the anode?
Medium
- AChlorine
- BOxygen
- CHydrogen
- DNitrogen
7.In the electrolysis of dilute sulfuric acid, what is the volume ratio of hydrogen to oxygen produced?
Medium
- A2:1
- B1:2
- C1:1
- D3:1
8.Which of the following is a suitable material for inert electrodes?
Medium
- AGraphite
- BCopper
- CIron
- DZinc

Slides

What is Electrolysis?

Electrolysis is the process where a molten ionic compound or an aqueous solution is broken down by an electric current.
Covalent compounds and solid ionic compounds cannot undergo electrolysis because they have no free mobile ions.
When molten or dissolved, ions are free to move and carry charge.

Which of the following can undergo electrolysis?

Solid sodium chloride
Molten lead(II) bromide
Pure water
Solid copper

Key Terms in Electrolysis

Electrode: a rod (usually metal or graphite) that conducts electricity into/out of the electrolyte.
Electrolyte: the ionic compound in molten or dissolved form that conducts electricity.
Anode: positive electrode; attracts anions (negative ions).
Cathode: negative electrode; attracts cations (positive ions).

The ____ electrode attracts negatively charged ions called ____. The ____ electrode attracts positively charged ions called ____.

Products at Electrodes

At the cathode (negative), metals or hydrogen gas are produced. If the metal is less reactive than hydrogen, the metal forms; if more reactive, hydrogen gas forms.
At the anode (positive), non-metals (except hydrogen) are produced, e.g., chlorine, bromine, oxygen.

During electrolysis of molten sodium chloride, what is produced at the cathode?

Chlorine gas
Sodium metal
Hydrogen gas
Oxygen gas

Electrolysis of Molten Compounds

Binary ionic compounds (two elements) always produce their corresponding elements at the electrodes.
Example: molten lead(II) bromide (PbBr₂) produces grey lead metal at the cathode and brown bromine gas at the anode.
Electrodes must be inert (e.g., graphite or platinum) to avoid side reactions.

In the electrolysis of molten lead(II) bromide, the product at the anode is ____ and at the cathode is ____.

Electrolysis of Aqueous Sodium Chloride (Brine)

Brine is concentrated aqueous NaCl. Ions present: Na⁺, Cl⁻, H⁺, OH⁻.
At cathode: H⁺ is discharged (less reactive than Na⁺) to form hydrogen gas.
At anode: Cl⁻ is discharged to form chlorine gas.
Remaining Na⁺ and OH⁻ form sodium hydroxide solution.

What are the products at the electrodes when concentrated aqueous sodium chloride is electrolysed?

Cathode: sodium; Anode: chlorine
Cathode: hydrogen; Anode: chlorine
Cathode: hydrogen; Anode: oxygen
Cathode: sodium; Anode: oxygen

Electrolysis of Dilute Sulfuric Acid

Dilute H₂SO₄ contains H⁺, SO₄²⁻, and also H⁺ and OH⁻ from water.
At cathode: H⁺ gains electrons to form hydrogen gas.
At anode: OH⁻ loses electrons to form oxygen gas and water.
Volume of hydrogen produced is twice that of oxygen.

In the electrolysis of dilute sulfuric acid, what is produced at the anode?

Hydrogen
Oxygen
Sulfur
Sulfur dioxide

Gas Tests for Electrolysis Products

Hydrogen: a lit splint gives a squeaky pop.
Oxygen: a glowing splint relights.
Chlorine: damp litmus paper turns red then bleaches white.

Which of these gas tests are correct?

Hydrogen: lit splint gives a squeaky pop.
Oxygen: a glowing splint relights.
Chlorine: damp litmus paper turns blue.
Chlorine: damp litmus paper turns red then bleaches white.

Electrolysis of Aqueous Solutions (Extended)

Water dissociates: H₂O ⇌ H⁺ + OH⁻. These ions compete with ions from the compound.
At cathode: if metal is less reactive than H⁺, metal deposits; if more reactive, H₂ forms.
At anode: if halide ions (Cl⁻, Br⁻, I⁻) are present, halogen forms; otherwise oxygen forms.
Concentration matters: concentrated halide gives halogen; dilute gives oxygen.

In the electrolysis of aqueous copper(II) sulfate with inert electrodes, the product at the cathode is ____ and at the anode is ____.

Ionic Half-Equations (Extended)

Oxidation is loss of electrons; reduction is gain of electrons (OIL RIG).
At anode (oxidation): anions lose electrons. Example: 2Cl⁻ → Cl₂ + 2e⁻.
At cathode (reduction): cations gain electrons. Example: Cu²⁺ + 2e⁻ → Cu.
For hydrogen: 2H⁺ + 2e⁻ → H₂. For oxygen from OH⁻: 4OH⁻ → O₂ + 2H₂O + 4e⁻.

Which is the correct half-equation for the production of hydrogen at the cathode?

H⁺ + e⁻ → H
2H⁺ + 2e⁻ → H₂
H₂ → 2H⁺ + 2e⁻
2H₂O + 2e⁻ → H₂ + 2OH⁻

Summary Table of Common Electrolysis Reactions

Molten PbBr₂: Anode: Br₂, Cathode: Pb.
Concentrated NaCl: Anode: Cl₂, Cathode: H₂.
Dilute H₂SO₄: Anode: O₂, Cathode: H₂.
Aqueous CuSO₄ (inert): Anode: O₂, Cathode: Cu.

What are the products when dilute sodium chloride solution is electrolysed?

Cathode: H₂; Anode: Cl₂
Cathode: H₂; Anode: O₂
Cathode: Na; Anode: Cl₂
Cathode: Na; Anode: O₂

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