1.9.3Principles of Chemistry
Writing Half-Equations — Edexcel Linear - OL (Chemistry) Revision Notes & Topical Questions
5 min read 5 parts Lesson 24 of 25 Unit quiz
Part 1 of 5Oxidation and Reduction at Electrodes
Half-equations track the movement of electrons during electrolysis.
Reduction
- Reduction is gain of electrons, and it happens at the cathode where positive ions pick up electrons.
Oxidation
- Oxidation is loss of electrons, and it happens at the anode where negative ions give up electrons.
Half-equation
- A half-equation shows only the change at one electrode, with electrons written as e⁻.
Balancing
- Both the atoms and the total charges must balance on each side of the half-equation.
Part 2 of 5Equations to Learn
Equations to Learn
Molten lead(II) bromide
- Cathode
- Pb²⁺ + 2e⁻ → Pb (reduction).
- Anode: 2Br⁻ → Br₂ + 2e⁻ (oxidation).
Aqueous sodium chloride
- Cathode
- 2H⁺ + 2e⁻ → H₂ (reduction).
- Anode: 2Cl⁻ → Cl₂ + 2e⁻ (oxidation).
Aqueous copper(II) sulfate
- Cathode
- Cu²⁺ + 2e⁻ → Cu (reduction).
- Anode: 2H₂O → O₂ + 4H⁺ + 4e⁻ (oxidation).
Dilute sulfuric acid
- Cathode
- 2H⁺ + 2e⁻ → H₂.
- Anode: 2H₂O → O₂ + 4H⁺ + 4e⁻, or 4OH⁻ → O₂ + 2H₂O + 4e⁻.
| Electrode | Process | Electrons |
|---|---|---|
| Cathode (−) | Reduction | Gained |
| Anode (+) | Oxidation | Lost |
Part 3 of 5How to Write a Half-Equation
A short routine avoids most mistakes.
1
Identify the ion that is discharged at the electrode using the electrolysis rules.
2
Write the ion on the left and the neutral product on the right.
3
For a diatomic product such as Cl₂ or Br₂, write 2 ions on the left to balance the atoms.
4
Add electrons so the total charge is equal on both sides; put them on the left for the cathode and on the right for the anode.
5
Check that atoms and charges balance, then label the process as oxidation or reduction.
Part 4 of 5Linking the Equations Together
Linking the Equations Together
Electron balance
- The electrons lost at the anode equal the electrons gained at the cathode, so the numbers must match in the full process.
Diatomic gases
- Chlorine and bromine form as molecules, which is why two halide ions appear in the anode equation.
Acidic solutions
- In acidic or neutral solutions oxygen forms from water molecules, as hydroxide ions are too scarce to be the source.
Alkaline solutions
- In alkaline solutions hydroxide ions are plentiful, so 4OH⁻ → O₂ + 2H₂O + 4e⁻ is used.
Part 5 of 5Exam Checklist & Tips
1.9.3 – Key Points for the Exam
- Oxidation Loss of electrons.
- Reduction Gain of electrons.
- Half-equation An equation showing electron transfer at one electrode only.
- Discharge Conversion of an ion into a neutral atom or molecule at an electrode.
- OIL RIG A memory aid: Oxidation Is Loss, Reduction Is Gain of electrons.
Common Mistakes
- Writing electrons on the wrong side so the charge does not balance.
- Writing Cl or Br instead of Cl₂ or Br₂ for the gaseous product.
- Mixing up oxidation and reduction; oxidation always occurs at the anode.
- Leaving out state symbols or writing the half-equation without balancing the atoms.
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Chemistry 0620 & Edexcel OL
Product Prediction Rules
Aqueous Electrolysis Rules (Inert Electrodes)
- At Cathode (-): Least reactive cation discharged. If metal is more reactive than hydrogen (Na+, K+, Mg2+), H2 gas is produced:
2H+ + 2e- → H2. If Cu2+ or Ag+, the metal coats the cathode. - At Anode (+): If concentrated halide present (Cl-, Br-, I-), halogen is formed (e.g.
2Cl- → Cl2 + 2e-). If sulfate (SO42-) or nitrate (NO3-), O2 gas is discharged:4OH- → O2 + 2H2O + 4e-.