A7LAM CSCA ACADEMYCSCA Academy / Chemistry / Substances and their reactions
CSCA Chemistry · Lesson 10 of 21Oxidation and reduction
Oxydoréduction · الأكسدة والاختزال
▶ Watch the video lesson (A7LAM students)
In a redox reaction, electrons move from one substance to another. Oxidation numbers show who loses them and who gains them.
Key ideas
- Oxidation: loss of electrons; the oxidation number goes up.
- Reduction: gain of electrons; the oxidation number goes down.
- The reducing agent is oxidised; the oxidising agent is reduced.
- Electrons lost = electrons gained: use this to balance redox equations.
Formulas to know
- Zn + Cu²⁺ → Zn²⁺ + Cu
- Usually O = −2, H = +1; elements = 0
- Oxidation numbers in a compound add up to 0
Worked example
In Fe₂O₃ + 3CO → 2Fe + 3CO₂, which element is oxidised and which is reduced?
- Iron: +3 in Fe₂O₃ → 0 in Fe: gains electrons, reduced
- Carbon: +2 in CO → +4 in CO₂: loses electrons, oxidised
- Agents: CO is the reducing agent, Fe₂O₃ the oxidising agent
Answer: C is oxidised, Fe is reduced
Practice questions
1. Oxidation number of Mn in KMnO₄?
+2 · +4 · +7 ✓ · +5
+1 + Mn − 8 = 0.
2. In Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂, the oxidising agent is:
Cl₂ ✓ · Br⁻ · Cl⁻ · Br₂
Cl₂ gains electrons.
3. Which is NOT a redox reaction?
2H₂ + O₂ → 2H₂O · NaOH + HCl → NaCl + H₂O ✓ · Zn + 2HCl → ZnCl₂ + H₂ · 2Na + Cl₂ → 2NaCl
No oxidation number changes.