Contents
1. Oxidation and Reduction
Loss of electrons (OIL — Oxidation Is Loss). Also: gain of oxygen or loss of hydrogen.
Gain of electrons (RIG — Reduction Is Gain). Also: loss of oxygen or gain of hydrogen.
Oxidation and reduction always occur together — a redox reaction. The substance that is oxidised is the reducing agent (it gives electrons). The substance that is reduced is the oxidising agent (it takes electrons).
Fe + CuSO₄ → FeSO₄ + Cu
Fe: 0 → Fe²⁺ (+2) — loses 2 electrons → oxidised (Fe is the reducing agent)
Cu²⁺: +2 → Cu (0) — gains 2 electrons → reduced (Cu²⁺ is the oxidising agent)
2. Electrolysis Basics
The decomposition of an ionic compound (electrolyte) by passing a direct electric current through it in its molten or aqueous state.
| Term | Definition |
|---|---|
| Electrolyte | The ionic compound being decomposed |
| Anode | Positive electrode — attracts anions (negative ions); oxidation occurs here |
| Cathode | Negative electrode — attracts cations (positive ions); reduction occurs here |
PANIC: Positive Anode, Negative Is Cathode. At the Anode, Anions are Attracted and oxidised. At the Cathode, Cations are attracted and reduced.
3. Electrolysis of Molten Compounds
When an ionic compound is melted, the ions are free to move and carry charge. Electrolysis splits it into its elements.
Cathode (−): Pb²⁺ + 2e⁻ → Pb (grey metallic lead deposits)
Anode (+): 2Br⁻ → Br₂ + 2e⁻ (brown bromine vapour produced)
Overall: PbBr₂ → Pb + Br₂
The metal always forms at the cathode; the non-metal forms at the anode.
4. Electrolysis of Aqueous Solutions
In aqueous solution, both the dissolved ions AND water molecules (H⁺ and OH⁻) can be discharged. Which product forms depends on the relative positions in the electrochemical series and concentration.
At the cathode (reduction)
- If the metal is below hydrogen in the reactivity series (Cu, Ag, Au) → metal is deposited.
- If the metal is above hydrogen (Na, Mg, Al, Zn, Fe) → hydrogen gas is produced instead.
At the anode (oxidation)
- If the solution contains high concentration of Cl⁻, Br⁻, or I⁻ → that halogen is produced.
- If the solution is dilute halide, or contains SO₄²⁻ or NO₃⁻ → oxygen gas is produced (from OH⁻ ions).
| Electrolyte | Cathode product | Anode product |
|---|---|---|
| Dilute H₂SO₄ | Hydrogen (H₂) | Oxygen (O₂) |
| Concentrated NaCl(aq) | Hydrogen (H₂) | Chlorine (Cl₂) |
| Dilute NaCl(aq) | Hydrogen (H₂) | Oxygen (O₂) |
| CuSO₄(aq) | Copper (Cu) | Oxygen (O₂) |
| CuCl₂(aq) | Copper (Cu) | Chlorine (Cl₂) |
5. Electroplating
Electroplating deposits a thin layer of metal onto an object using electrolysis.
- Cathode: the object to be plated (connected to negative terminal).
- Anode: the plating metal (e.g. copper, silver, chromium).
- Electrolyte: a solution of a salt of the plating metal.
Cathode: spoon (object to be plated). Anode: pure copper block. Electrolyte: copper sulfate solution.
Cathode: Cu²⁺ + 2e⁻ → Cu (copper deposits on spoon)
Anode: Cu → Cu²⁺ + 2e⁻ (copper anode dissolves to replenish solution)
The concentration of CuSO₄ solution remains constant throughout.
- OIL RIG: Oxidation Is Loss (of electrons). Reduction Is Gain (of electrons).
- Cathode (-): cations are REDUCED (gain electrons). Metal deposited or H2 produced.
- Anode (+): anions are OXIDISED (lose electrons). O2 or halogen gas produced.
- Aqueous electrolysis: at cathode, H2 produced if metal is more reactive than H (above H in reactivity series); metal deposited if below H.
- At anode: Cl2 produced from concentrated HCl/NaCl; O2 from dilute solutions or from OH-.
- Electroplating: article = cathode, plating metal = anode, electrolyte = salt solution of plating metal.
6. Common Exam Traps
This never changes. At the anode, oxidation occurs (anions lose electrons). At the cathode, reduction occurs (cations gain electrons). Reversing these is a very common error.
Concentrated HCl or concentrated NaCl → Cl₂ at anode. Dilute NaCl → O₂ at anode (water is preferentially oxidised). Concentration matters — always check.
Na⁺, Mg²⁺, Al³⁺, Zn²⁺, Fe²⁺ in aqueous solution — H₂ is produced at the cathode, not the metal. The metal is only deposited from a molten compound (no water present), or if the metal is below H in the reactivity series.
Key Terms — Flashcard Review
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Original study notes for Singapore students. Not affiliated with MOE, SEAB or Cambridge.