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4.1.3Electrolysis

Electrolysis of Aqueous Sodium Chloride and Dilute Sulfuric Acid — Combined Science 0653 - Chemistry Revision Notes & Topical Questions

5 min read 6 parts Lesson 5 of 6 Unit quiz

📚 Combined Science 0653 - Chemistry Hub / Topical Revision
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Section 1 of 6Why aqueous solutions are different

Two sources of ions
  • Two sources of ions An aqueous electrolyte contains the ions of the dissolved compound and also H+ and OH- ions from the water itself.
    • At each electrode there is therefore a choice of which ion is discharged.
At the cathode
  • At the cathode If the metal is more reactive than hydrogen, hydrogen ions are discharged and hydrogen gas forms.
    • A metal less reactive than hydrogen, such as copper, is deposited instead.
At the anode
  • At the anode Halide ions (chloride, bromide, iodide) are discharged in preference to hydroxide when the solution is concentrated.
    • If no halide is present, hydroxide ions are discharged and oxygen forms.

Section 2 of 6Concentrated aqueous sodium chloride

NaCl(aq) with inert electrodes
Ions present
  • Ions present Na+ and Cl- from the salt, and H+ and OH- from water.
Cathode (negative)
  • Cathode (negative) Sodium is more reactive than hydrogen, so H+ ions gain electrons and hydrogen gas is released: 2H+ + 2e- → H2.
Anode (positive)
  • Anode (positive) Chloride ions lose electrons and chlorine gas is released: 2Cl- → Cl2 + 2e-.
Solution left behind
  • Solution left behind Na+ and OH- ions remain, so the solution becomes sodium hydroxide, which is alkaline.

Section 3 of 6Dilute sulfuric acid

H2SO4(aq) with inert electrodes
Ions present
  • Ions present H+ and SO42- from the acid, with H+ and OH- from water.
Cathode (negative)
  • Cathode (negative) Hydrogen ions gain electrons, so hydrogen gas forms: 2H+ + 2e- → H2.
Anode (positive)
  • Anode (positive) Sulfate ions are not discharged.
    • Hydroxide ions lose electrons, giving oxygen gas and water: 4OH- → O2 + 2H2O + 4e-.
Overall change
  • Overall change Water is being decomposed, so about twice as much hydrogen as oxygen forms by volume, and the acid slowly becomes more concentrated.

Section 4 of 6Linking to industry

Useful products
  • Useful products The electrolysis of brine gives three valuable products: chlorine for disinfecting water and making plastics, hydrogen for making ammonia and as a fuel,
    • and sodium hydroxide for soap and paper-making.
Dilute versus concentrated
  • Dilute versus concentrated If the sodium chloride solution is very dilute, more hydroxide ions are present and some oxygen may form at the anode alongside chlorine,
    • so the question should state that the solution is concentrated.

Section 5 of 6Key terms

TermMeaning
AqueousDissolved in water.
Inert electrodeAn electrode, such as graphite or platinum, that conducts electricity but does not react.
DischargedDescribes an ion that gains or loses electrons at an electrode to become an atom or molecule.
BrineConcentrated aqueous sodium chloride solution.
Half equationAn equation showing only the gain or loss of electrons at one electrode.

Section 6 of 6Exam technique

1
Saying sodium is formed at the cathode; sodium is more reactive than hydrogen so hydrogen is produced from solution.
2
Forgetting that water supplies H+ and OH- ions as well as the dissolved compound.
3
Writing that chlorine is made at the cathode; negative ions are always attracted to the anode.
4
Stating the products of dilute sulfuric acid as sulfur compounds; sulfate ions stay in the solution and oxygen forms.

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