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4.2.2Chemical Changes

Metal and Acid Reactions as Redox Reactions — AQA GCSE Combined Science Trilogy 8464 - Chemistry Revision Notes & Topical Questions

5 min read 5 parts Lesson 7 of 18

📚 AQA GCSE Combined Science Trilogy 8464 - Chemistry Hub / Topical Revision
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Part 1 of 4Why it is redox (HT only)

Electron transfer
  • Electron transfer The metal atoms give electrons to hydrogen ions in the acid.
  • A transfer of electrons from one species to another is the signature of a redox reaction.
Simultaneous changes
  • Simultaneous changes Loss of electrons (oxidation) and gain of electrons (reduction) happen together in the same mixture.
Typical roles
  • Typical roles Metals tend to lose electrons, so the metal is usually the substance oxidised.
  • The hydrogen ions are reduced to hydrogen gas.

Part 2 of 4Zinc and hydrochloric acid (HT only)

1
Balanced equation: Zn + 2HCl → ZnCl2 + H2.
2
Show the ions: Zn + 2H+ + 2Cl− → Zn2+ + 2Cl− + H2.
3
Remove the chloride ions, which are spectator ions that do not change.
4
Ionic equation: Zn + 2H+ → Zn2+ + H2.
5
Oxidation half equation: Zn → Zn2+ + 2e−.
6
Reduction half equation: 2H+ + 2e− → H2.

Part 3 of 4Summary of the half equations (HT only)

SpeciesChangeElectronsType
Zinc atomsZn → Zn2+Lost (2e−)Oxidised
Hydrogen ions2H+ → H2Gained (2e−)Reduced
Chloride ionsCl− stays Cl−NoneSpectator

Part 4 of 4Magnesium and sulfuric acid (HT only)

1Start
  • Start Mg(s) + H2SO4(aq) → MgSO4(aq) + H2(g).
2Remove spectators
  • Remove spectators Sulfate ions are unchanged on both sides, so they are removed.
3Ionic equation
  • Ionic equation Mg + 2H+ → Mg2+ + H2.
4Assign roles
  • Assign roles Mg → Mg2+ + 2e− is oxidation; 2H+ + 2e− → H2 is reduction.

Key terms

TermMeaning
Redox reactionA reaction in which one species is oxidised and another is reduced.
OxidationLoss of electrons.
ReductionGain of electrons.
Spectator ionAn ion that is unchanged during a reaction and does not appear in the ionic equation.
Half equationAn equation showing the loss or gain of electrons by one species only.

Avoid these mistakes

1
Forgetting that the two half equations must show the same number of electrons so that they cancel.
2
Leaving the chloride or sulfate ions in the ionic equation.
3
Writing the hydrogen product as H instead of H2.
4
Saying the acid is oxidised because it is the 'oxidising' chemical; here H+ is the species that gains electrons and is reduced.

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