ZIMSEC A Level · Chemistry · Form 6
Oxidation numbers
Oxidation is an increase in oxidation number (loss of electrons); reduction is a decrease (gain). Rules: elements 0; O usually −2; H usually +1; the sum in a compound is 0, in an ion its charge.
Balancing redox equations
Write half-equations, balance atoms (O with H₂O, H with H⁺), balance charges with electrons, equalise electrons and add.
Worked example: Combine MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O with Fe²⁺ → Fe³⁺ + e⁻.
Multiply the iron equation by 5: MnO₄⁻ + 8H⁺ + 5Fe²⁺ → Mn²⁺ + 4H₂O + 5Fe³⁺
Electrode potentials
The standard hydrogen electrode has E° = 0.00 V. A more positive E° means a stronger oxidising agent.
E°cell = E°(right, reduced) − E°(left, oxidised). A positive E°cell means the reaction is feasible (though it may be slow).
Worked example: Zn²⁺/Zn E° = −0.76 V; Cu²⁺/Cu E° = +0.34 V. Find E°cell.
E°cell = 0.34 − (−0.76) = +1.10 V; zinc is oxidised, Cu²⁺ is reduced.
Electrolysis (quantitative)
Q = It; Faraday constant F = 96 500 C mol⁻¹ (charge on one mole of electrons). Moles of electrons = It ÷ F.
Worked example: A current of 2.00 A flows for 30 minutes through copper(II) sulfate. Find the mass of copper deposited (Ar Cu = 63.5).
Q = 2.00 × 1 800 = 3 600 C; moles e⁻ = 3 600 ÷ 96 500 = 0.0373 mol
Cu²⁺ + 2e⁻ → Cu: moles Cu = 0.0187 mol; mass = 0.0187 × 63.5 = 1.18 g
Applications
Batteries and fuel cells, corrosion prevention, electroplating.
Check yourself
- Find the oxidation number of Cr in Cr₂O₇²⁻.
- Which is the stronger oxidising agent: Cl₂ (E° = +1.36 V) or Br₂ (E° = +1.07 V)?
- How many coulombs are needed to deposit 1 mole of silver (Ag⁺ + e⁻ → Ag)?
Show answers
- +6
- Cl₂ (more positive E°).
- 96 500 C
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