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3.3.2Movement and Interactions

Redox Reactions of Acids (HT Only) — AQA GCSE Combined Science Synergy 8465 - Physical Sciences Revision Notes & Topical Questions

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📚 AQA GCSE Combined Science Synergy 8465 - Physical Sciences Hub / Topical Revision
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Section 1 of 9Oxidation and reduction (HT only)

Oxidation
  • Oxidation Oxidation is the loss of electrons. Remember OIL: Oxidation Is Loss.
  • A metal atom becomes a positive ion when it is oxidised.
  • For example, in Zn → Zn2+ + 2e− the zinc atom has lost two electrons.
Reduction
  • Reduction Reduction is the gain of electrons. Remember RIG: Reduction Is Gain.
  • Hydrogen ions are reduced to hydrogen molecules.
Redox
  • Redox When one species loses electrons another gains them, so oxidation and reduction occur together.
  • A substance that causes another to be oxidised is itself reduced, so hydrogen ions act as the oxidising agent.

Section 2 of 9Magnesium and hydrochloric acid

1Reactants
  • Reactants Mg(s) + 2H+(aq) with chloride ions present.
  • The reaction mixture contains magnesium atoms, hydrogen ions from the acid and chloride ions that do not change.
2Magnesium oxidised
  • Magnesium oxidised Mg → Mg2+ + 2e−
3Hydrogen ions reduced
  • Hydrogen ions reduced 2H+ + 2e− → H2
4Products
  • Products Magnesium ions with chloride ions form magnesium chloride, and hydrogen gas bubbles off.

Section 3 of 9Writing ionic equations

1
Write the full equation: Mg + 2HCl → MgCl2 + H2.
2
Rewrite the soluble compounds as separated ions.
3
Cancel the chloride ions that appear unchanged on both sides (spectator ions).
4
The ionic equation left is Mg(s) + 2H+(aq) → Mg2+(aq) + H2(g).
5
Check charge balance: both sides total 2+.

Section 4 of 9Redox terms compared

IdeaOxidationReduction
ElectronsLostGained
Species in the exampleMagnesiumHydrogen ions
Change in charge0 to 2+1+ to 0 (in H2)
RoleReducing agentOxidising agent

Section 5 of 9Worked example: zinc and sulfuric acid

1
Write the full equation: Zn + H2SO4 → ZnSO4 + H2.
2
Split into ions: Zn + 2H+ + SO42− → Zn2+ + SO42− + H2.
3
Cancel the sulfate ions, which are spectator ions: Zn + 2H+ → Zn2+ + H2.
4
Oxidation half-equation: Zn → Zn2+ + 2e−.
5
Reduction half-equation: 2H+ + 2e− → H2. The electrons cancel when the two are added.

Section 6 of 9Applying the ideas

Redox in acid reactions
Which metals react
  • Which metals react Metals above hydrogen in the reactivity series lose electrons more easily than hydrogen, so they displace hydrogen from acids.
Reactivity and rate
  • Reactivity and rate The more reactive the metal, the more readily it loses electrons, so the faster the bubbles of hydrogen are produced.
Checking a half-equation
  • Checking a half-equation Every half-equation must balance in atoms and in charge,
    • and electrons appear on the right when oxidation occurs and on the left for reduction.

Section 7 of 9Exam approach

Describe the change in terms of electrons
  • Describe the change in terms of electrons Name the species that loses electrons and the species that gains them,
    • and use the words oxidised and reduced correctly.
Link to observations
  • Link to observations The metal disappears as its atoms become ions in solution, and bubbles of hydrogen form where the hydrogen ions gain electrons.

Section 8 of 9Key terms

TermMeaning
OxidationLoss of electrons by an atom, ion or molecule.
ReductionGain of electrons by an atom, ion or molecule.
Half-equationAn equation showing only the oxidation or reduction part of a redox reaction, including electrons.
Spectator ionAn ion present but not taking part in the reaction.
Ionic equationAn equation showing only the particles that change in a reaction.
Oxidising agentA substance that takes electrons from another substance and is itself reduced.
Reducing agentA substance that gives electrons to another substance and is itself oxidised.

Section 9 of 9Exam technique

1
Mixing up OIL RIG, such as saying that oxidation is gain of electrons.
2
Forgetting to balance charge as well as atoms in a half-equation.
3
Including spectator ions in the ionic equation.
4
Saying the acid is oxidised; the metal is oxidised and the hydrogen ions are reduced.
5
Leaving out the state symbols or charges on ions, so the half-equation does not show what happens to the electrons.

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