Redox Reactions – Key Exam Notes

Redox Basics

• Redox = reduction + oxidation; always occur together
• Everyday processes: respiration, combustion, batteries, metal extraction
• Historical link to reactions with O_2 (Lavoisier)

Definitions

• Oxidation: loss of electrons (OIL)
• Reduction: gain of electrons (RIG)
• Oxidising agent: accepts e⁻, is reduced, causes oxidation
• Reducing agent: donates e⁻, is oxidised, causes reduction

Electron Transfer & Half-Equations

• Represent e⁻ movement separately for each reactant
  • Example: 2Mg + O2 \rightarrow 2MgO • Ox: Mg \rightarrow Mg^{2+} + 2e^- • Red: O2 + 4e^- \rightarrow 2O^{2-}
• Electrons appear on product side for oxidation, reactant side for reduction

Writing Simple Half-Equations (acidic/basic solutions not involved)

1. Write reactant → product
2. Balance elements
3. Balance charge with e⁻
4. Assign states

Writing Overall Equations

1. Write balanced half-equations
2. Multiply to equalise e⁻ lost & gained
3. Add, cancel e⁻ and common species
4. Verify atom & charge balance
   • Example (Na & H₂O):
   Ox: 2Na \rightarrow 2Na^{+} + 2e^-
   Red: 2H2O + 2e^- \rightarrow H2 + 2OH^-
   Overall: 2Na + 2H2O \rightarrow 2Na^{+} + H2 + 2OH^-

Conjugate Redox Pairs

• Pair = oxidised form / reduced form appearing in same half-eqn
  • Zn^{2+}/Zn, Cu^{2+}/Cu etc.
• Two conjugate pairs in every redox reaction

Complex Half-Equations in Acidic Solution

Method (for MnO4^-, Cr2O_7^{2-} etc.)

1. Balance all atoms except O & H
2. Balance O with H_2O
3. Balance H with H^+
4. Balance charge with e⁻
5. Add corresponding oxidation half-eqn, multiply to cancel e⁻
   • Example: MnO4^- + 8H^+ + 5e^- \rightarrow Mn^{2+} + 4H2O
   • Combined with 5Fe^{2+} \rightarrow 5Fe^{3+} + 5e^- gives:
   MnO4^- + 8H^+ + 5Fe^{2+} \rightarrow Mn^{2+} + 4H2O + 5Fe^{3+}

Key Mnemonics & Tips

• OIL RIG: Oxidation Is Loss, Reduction Is Gain (of e⁻)
• Balance charge & mass in every half or full equation
• Metals typically act as reducing agents (tend to lose e⁻)
• Strong laboratory oxidisers: KMnO4, K2Cr2O7, K2CrO4