OXIDATION – REDUCTION REACTIONS

By Al D. Deacon – Physicist | Educator | Researcher

📧 al.deaconjr@gmail.com | 🌐 www.goldengeniuzonline.com


1. Investigate the Action of Common Oxidising and Reducing Substances in Everyday Activities

Common Oxidising Agents in Daily Life

  • Bleach (Sodium Hypochlorite, NaClO): Used in cleaning, releases oxygen to break down stains.
  • Hydrogen Peroxide (H₂O₂): Disinfectant, used to clean wounds and bleach hair.
  • Oxygen (O₂): Supports combustion and respiration.
  • Potassium Permanganate (KMnO₄): Used in water purification and antiseptics.

Common Reducing Agents in Daily Life

  • Carbon (C): Used in metallurgy to reduce metal ores (e.g., Fe₂O₃ + 3C → 2Fe + 3CO).
  • Vitamin C (Ascorbic Acid): Prevents oxidation in food and the body.
  • Metals (e.g., Zn, Fe, Al): Reduce other substances by losing electrons.

2. Define Oxidation and Reduction

  • Oxidation: The loss of electrons or increase in oxidation number.
    • Example: Mg → Mg²⁺ + 2e⁻
  • Reduction: The gain of electrons or decrease in oxidation number.
    • Example: Cu²⁺ + 2e⁻ → Cu
  • Redox Reaction: A reaction where oxidation and reduction occur simultaneously.
    • Example: Zn + CuSO₄ → ZnSO₄ + Cu
      • Zn is oxidized (Zn → Zn²⁺ + 2e⁻).
      • Cu²⁺ is reduced (Cu²⁺ + 2e⁻ → Cu).

3. Deduce Oxidation Number from Formulae

  • Oxidation Number Rules:
    1. Free elements (e.g., O₂, N₂, Cl₂) have oxidation number 0.
    2. Simple ions: Equal to their charge (e.g., Na⁺ = +1, Cl⁻ = -1).
    3. Hydrogen: Usually +1, except in metal hydrides (e.g., NaH = -1).
    4. Oxygen: Usually -2, except in peroxides (e.g., H₂O₂ = -1).
    5. Sum of oxidation numbers in a neutral compound = 0.
    6. Sum of oxidation numbers in a polyatomic ion = charge of the ion.

Example Calculations

  1. Determine oxidation number of Cr in Cr₂O₇²⁻:

    • Oxygen = -2 (7 × -2 = -14)
    • Let Cr be x:
      • 2x - 14 = -2
      • 2x = +12
      • x = +6 (Oxidation number of Cr)

  2. Determine oxidation number of Mn in KMnO₄:

    • K = +1, O = -2 (4 × -2 = -8)
    • Let Mn be x:
      • +1 + x - 8 = 0
      • x = +7 (Oxidation number of Mn)

4. Identify Oxidation and Reduction Reactions Including Reactions at Electrodes

Electrolysis Example (Molten NaCl)

  • At the Anode (Oxidation):
    • 2Cl⁻ → Cl₂ + 2e⁻ (Chlorine gas is released)
  • At the Cathode (Reduction):
    • Na⁺ + e⁻ → Na (Sodium metal is formed)

Electrochemical Cell Example (Zn/Cu Daniell Cell)

  • Anode (Oxidation): Zn → Zn²⁺ + 2e⁻
  • Cathode (Reduction): Cu²⁺ + 2e⁻ → Cu

5. Distinguish Between Oxidising and Reducing Agents

  • Oxidising Agent: A substance that causes oxidation by accepting electrons.

    • Examples: O₂, Cl₂, KMnO₄, K₂Cr₂O₇, H₂O₂
    • Reaction: H₂O₂ + 2I⁻ → I₂ + 2H₂O (H₂O₂ is an oxidising agent)

  • Reducing Agent: A substance that causes reduction by donating electrons.

    • Examples: C, H₂, Zn, Fe, Na, SO₂
    • Reaction: Fe²⁺ + MnO₄⁻ → Fe³⁺ + Mn²⁺ (Fe²⁺ is a reducing agent)

6. Perform Tests for Oxidising and Reducing Agents

Tests for Oxidising Agents

  1. Using Potassium Iodide (KI):

    • Oxidising agent oxidizes I⁻ to I₂ (brown solution/blue-black with starch)
    • Example: Cl₂ + 2I⁻ → 2Cl⁻ + I₂ (brown color)

  2. Using Acidified Potassium Manganate (VII) (KMnO₄):

    • If solution turns from purple to colorless, reducing agent is present.
    • Example: SO₂ + MnO₄⁻ + H⁺ → Mn²⁺ (colorless) + H₂O + SO₄²⁻

Tests for Reducing Agents

  1. Using Acidified Potassium Dichromate (VI) (K₂Cr₂O₇):

    • If solution turns from orange to green, reducing agent is present.
    • Example: Fe²⁺ + Cr₂O₇²⁻ + H⁺ → Fe³⁺ + Cr³⁺ (green color)

  2. Using Bromine Water:

    • If solution decolorizes, reducing agent is present.
    • Example: SO₂ + Br₂ + H₂O → H₂SO₄ + 2HBr


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