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8.4.3The Periodic Table

Redox Reactions — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 5 parts Lesson 15 of 23 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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Section 1 of 5Half-equations

A redox reaction can be split into an oxidation half and a reduction half.

1
A redox reaction is split into two half-equations, one showing oxidation and one showing reduction.
2
For zinc and copper(II) ions, Zn(s) → Zn²⁺(aq) + 2e⁻ shows zinc losing electrons.
3
The half-equation Cu²⁺(aq) + 2e⁻ → Cu(s) shows copper(II) ions gaining electrons.
4
Adding the two half-equations, with the electrons cancelled, gives the overall ionic equation.

Section 2 of 5Spectator Ions and Overall Equations

Removing spectator ions reveals the species that actually change.

Spectator ions
  • Spectator ions Ions that appear unchanged on both sides of the full ionic equation are removed.
Overall equation
  • Overall equation Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s) is the overall ionic equation for the zinc and copper(II) reaction.
Oxidised species
  • Oxidised species Zinc is oxidised because it loses electrons to form Zn²⁺ ions.
Reduced species
  • Reduced species Copper(II) ions are reduced because they gain electrons to form copper atoms.

Section 3 of 5Balancing Electrons

Half-equations must be balanced for electrons as well as atoms.

Balancing Electrons
Electron balance
  • Electron balance The electrons lost in the oxidation half-equation must equal the electrons gained in the reduction half-equation.
Redox in general
  • Redox in general Every redox reaction has one species oxidised and another reduced at the same time.
State symbols
  • State symbols Include state symbols such as (s), (l) and (aq) in half-equations and ionic equations.
Checking
  • Checking Check the charges on each side of the balanced half-equation before combining them.

Section 4 of 5Worked Example

Question When iron reacts with bromine to form iron(II) bromide, which substance has been reduced?

  1. Fe(s) → Fe²⁺(aq) + 2e⁻: iron loses two electrons, so iron is oxidised.
  2. Br₂(l) + 2e⁻ → 2Br⁻(aq): each bromine molecule gains two electrons, so bromine is reduced.
  3. The two electrons lost by iron equal the two electrons gained by bromine, so the electrons balance.

Answer The bromine, Br₂, has been reduced.

Section 5 of 5Exam Checklist & Tips

8.4.3 – Key Points for the Exam

  • Half-equation An equation showing only the oxidation or the reduction part of a redox reaction, with electrons included.
  • Spectator ion An ion that does not change during a reaction.
  • Overall ionic equation An ionic equation with spectator ions removed.
  • Iron(II) bromide The ionic compound FeBr₂ formed when iron reacts with bromine.

Common Mistakes

  • Forgetting to balance the electrons in half-equations, so the charges do not match on each side.
  • Including spectator ions in the overall ionic equation.
  • Naming the species that gains electrons as oxidised; gaining electrons is reduction.
  • Omitting state symbols when an ionic equation or half-equation requires them.

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