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1.9Principles of Chemistry

Electrolysis — Edexcel Linear - OL (Chemistry) Revision Notes & Topical Questions

5 min read 7 parts Lesson 17 of 25 Unit quiz

📚 Edexcel Linear - OL (Chemistry) Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Lesson summary

Electrolysis: Conductivity, Experiments and Half-Equations — EDEXCEL LINEAR – OL (CHEMISTRY) notes: Explain conductivity, describe electrolysis experiments of molten and aqueous electrolytes and write half-equations.An electric current is the flow of charged particles.

  • Worked example 1
  • Worked example 2
  • Worked example 3
  • Worked example 4
  • Electroplating
  • Set up the electrodes in the electrolyte and connect them to a d.c. supply

Section 1 of 7Conductivity

An electric current is the flow of charged particles.

Conductivity
In metals and graphite the charge carriers are delocalised electrons.
In ionic compounds the charge carriers are ions,
  • which can move only when the compound is molten or dissolved in water;
    • the solid has fixed ions and does not conduct.
Simple molecular substances do not conduct since they contain no charged particles (although some, like HCl, form ions when dissolved).

Remember that the external circuit carries electrons from the anode to the cathode through the wires and power supply; inside the electrolyte the flow of charge is by ions, with positive ions moving to the cathode and negative ions to the anode. At the cathode electrons are supplied and gained by ions; at the anode electrons are given up and pushed into the circuit.

Section 2 of 7Electrolysis experiments

Electrolysis breaks down an ionic compound using electricity. Apparatus: a cell containing the molten or dissolved electrolyte, two electrodes (usually inert graphite or platinum), a power supply and wires. Positive ions move to the negative electrode, the cathode; negative ions move to the positive electrode, the anode.

Electrolysis experiments
Set up the electrodes in the electrolyte and connect them to a d.c. supply.
Observe the products at each electrode (a gas, a deposit or a colour change).
Test the gases: hydrogen burns with a squeaky pop; oxygen relights a glowing splint; chlorine bleaches damp litmus paper.

Molten compounds

  • the metal forms at the cathode and the non-metal at the anode (PbBr2 gives lead and bromine;
    • NaCl gives sodium and chlorine).

Electrolysis experiments
Aqueous solutions: at the cathode hydrogen forms if the metal is more reactive than hydrogen, otherwise the metal; at the anode a halogen forms from halide ions, otherwise oxygen.
Concentrated NaCl solution gives hydrogen, chlorine and sodium hydroxide solution; dilute sulfuric acid gives hydrogen and oxygen; copper(II) sulfate gives copper and oxygen.

Section 3 of 7Writing half-equations

A half-equation shows the change at one electrode, with electrons included and balanced for atoms and charge.

ElectrodeTypeExamples
CathodePositive ions gain electrons (reduction)Pb2+ + 2e- to Pb; 2H+ + 2e- to H2; Cu2+ + 2e- to Cu
AnodeNegative ions lose electrons (oxidation)2Cl- to Cl2 + 2e-; 2Br- to Br2 + 2e-; 4OH- to 2H2O + O2 + 4e-

To write balanced half-equations, first write the ion and the product (for example Cu2+ to Cu), then add electrons to balance the charge (Cu2+ + 2e- to Cu).

Writing half-equations
Gases that are diatomic need two ions: 2Cl- to Cl2 + 2e-.
Combine half-equations by multiplying so the electrons cancel, if asked for the overall equation, for example 2NaCl(aq) + 2H2O to 2NaOH + H2 + Cl2.

Section 4 of 7Practical: aqueous solutions

To investigate the electrolysis of aqueous solutions, use graphite electrodes in copper(II) sulfate, sodium chloride and dilute sulfuric acid solutions in turn, collecting gases in test tubes over the electrodes and noting colour changes.

Practical: aqueous solutions
With copper electrodes in copper(II) sulfate the anode loses mass as Cu goes into solution and the cathode gains mass as copper is deposited; this is how copper is purified and how objects are electroplated.
Reasons for the observed products are: ions of lower reactivity are discharged, and water supplies H+ and OH- ions that compete.

Section 5 of 7Worked examples

  1. Why can molten but not solid sodium chloride conduct? [2]2 marks

  2. Write the half-equation at the cathode when copper(II) sulfate solution is electrolysed. [1]1 mark

  3. State the products at each electrode when concentrated sodium chloride is electrolysed. [2]2 marks

  4. Describe the test for oxygen. [1]1 mark

Section 6 of 7Key terms

TermMeaning
ElectrolyteLiquid that conducts by moving ions.
ElectrodeRod through which current enters or leaves.
ReductionGain of electrons.
OxidationLoss of electrons.
ElectroplatingCoating an object with metal by electrolysis.

Section 7 of 7Exam technique

1
Saying electrons travel through the solution.
2
Mixing up cathode and anode.
3
Writing half-equations without electrons.
4
Forgetting that water can be electrolysed too.

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