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1.9.4States of Matter

Practical- Investigate the Electrolysis of Aqueous Solutions — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 5 parts Lesson 23 of 30 Unit quiz

📚 Cambridge IGCSE Chemistry 0620 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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Part 1 of 5Apparatus and Set-Up

The same simple cell can be used for several electrolytes, so only the solution changes between tests.

Electrolysis cell
  • Electrolysis cell A beaker of electrolyte with two graphite electrodes connected to a low-voltage DC power pack or battery and fully immersed in the solution.
Inverted test tubes
  • Inverted test tubes Filled with the electrolyte and placed over each electrode so that any gas produced is collected.
Variables
  • Variables The electrolyte is the independent variable, while the observations at each electrode are the dependent variable.
Safety
  • Safety Wear eye protection, keep the power pack set to a low voltage, and keep water and electricity separated on the bench.

Part 2 of 5Testing the Products

Testing the Products
Hydrogen
  • Hydrogen Collected at the cathode; it burns with a squeaky pop when a lighted splint is held to the gas.
Oxygen
  • Oxygen Collected at the anode; it relights a glowing splint pushed into the gas sample.
Chlorine
  • Chlorine Collected at the anode from a chloride solution; it bleaches damp litmus paper.
Metal
  • Metal Deposited as a solid on the cathode, such as pink-brown copper coating the electrode in copper(II) sulfate solution.

Part 3 of 5Results and Conclusions

1
Each aqueous solution contains H⁺ and OH⁻ ions from water, as well as the ions of the dissolved compound.
2
At the cathode, the less reactive ion is discharged, so hydrogen forms when the metal is more reactive than hydrogen.
3
At the anode, halide ions are discharged to form a halogen, and without halide ions hydroxide ions are discharged to form oxygen.
4
Sulfate ions are not discharged in these experiments, so they remain in solution while oxygen forms from water at the anode.
5
Copper forms at the cathode in copper(II) sulfate solution because copper is less reactive than hydrogen.
ElectrolyteCathode productAnode product
Concentrated sodium chlorideHydrogenChlorine
Dilute sulfuric acidHydrogenOxygen
Copper(II) sulfate solutionCopperOxygen

Part 4 of 5Worked Example

Question

  • Dilute sulfuric acid is electrolysed and 24 cm³ of hydrogen is collected at the cathode, both gases measured at room temperature and pressure.
  • Predict the volume of oxygen at the anode.

  1. The overall equation is 2H₂O → 2H₂ + O₂, so hydrogen and oxygen form in a mole ratio of 2 : 1.
  2. Equal volumes of gases at the same temperature and pressure contain equal numbers of molecules, so the volume ratio is also 2 : 1.
  3. Volume of oxygen = 24 ÷ 2 = 12 cm³.

Answer 12 cm³ of oxygen.

Part 5 of 5Exam Checklist & Tips

1.9.4 – Key Points for the Exam

  • Discharge The reaction of an ion at an electrode to form a neutral element or molecule.
  • Inverted test tube A test tube filled with liquid and inverted over an electrode to collect the gas produced.
  • Hydrogen pop test A test in which hydrogen burns with a squeaky pop when a lighted splint is held to it.
  • Halide ion A negative ion of a Group 7 element, such as Cl⁻, Br⁻ or I⁻.
  • Inert electrode An electrode, such as graphite, that does not react with the products of electrolysis.

Common Mistakes

  • Predicting oxygen rather than chlorine at the anode for sodium chloride;
    • halide ions are discharged in preference to hydroxide ions.
  • Testing the gas at the wrong electrode, or expecting the splint test to work for chlorine.
  • Saying the metal is produced at the anode; metals are always deposited at the cathode.
  • Forgetting that water supplies H⁺ and OH⁻, so the cathode product of an aqueous solution is not always the metal.

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