Chemical changes are at the heart of Chemistry. This guide covers everything you need to know about oxidation, reduction, and displacement reactions for your GCSE exam.
1. Oxidation and Reduction
Oxidation
Oxidation is the loss of electrons or gain of oxygen.
Example: Magnesium + Oxygen → Magnesium oxide
[ 2Mg + O_2 → 2MgO ]
Magnesium loses electrons (is oxidised).
Reduction
Reduction is the gain of electrons or loss of oxygen.
Example: Copper(II) oxide + Carbon → Copper + Carbon monoxide
[ CuO + C → Cu + CO ]
Copper oxide loses oxygen (is reduced).
> Memory Trick: OIL RIG - Oxidation Is Loss, Reduction Is Gain (of electrons)
2. Metal Reactions
Metals with Oxygen
[ \text{Metal} + \text{Oxygen} → \text{Metal Oxide} ]
- Potassium, Sodium, Lithium, Calcium: React vigorously (even explosively)
- Magnesium, Zinc, Iron: React steadily
- Copper, Silver, Gold: Little to no reaction
Metals with Water
Very reactive metals (K, Na, Ca) react with cold water:
[ 2Na + 2H_2O → 2NaOH + H_2 ]
Moderately reactive metals (Mg, Zn, Fe) react with steam:
[ Mg + H_2O → MgO + H_2 ]
Metals with Dilute Acids
[ \text{Metal} + \text{Acid} → \text{Salt} + \text{Hydrogen} ]
Example: Zinc + Hydrochloric acid
[ Zn + 2HCl → ZnCl_2 + H_2 ]
3. Displacement Reactions
A more reactive metal displaces a less reactive metal from its compound.
[ \text{More reactive metal} + \text{Less reactive metal's compound} → \text{New compound} + \text{Less reactive metal} ]
Reactivity Series (Higher Tier)
[ \text{K} > \text{Na} > \text{Ca} > \text{Mg} > \text{Al} > \text{Zn} > \text{Fe} > \text{Cu} > \text{Ag} > \text{Au} ]
Example: Iron + Copper(II) sulfate
[ Fe + CuSO_4 → FeSO_4 + Cu ]
Iron is more reactive than copper, so it displaces copper.
4. Electrolysis
Electrolysis uses electrical energy to break down compounds.
At the Cathode (Negative Electrode)
Positive ions gain electrons (are reduced):
- Aluminium ions: Al³⁺ + 3e⁻ → Al
- Copper ions: Cu²⁺ + 2e⁻ → Cu
At the Anode (Positive Electrode)
Negative ions lose electrons (are oxidised):
- Oxide ions: O²⁻ → O₂ + 4e⁻
- Chloride ions: 2Cl⁻ → Cl₂ + 2e⁻
5. Useful Equations to Memorise
| Reaction | Equation |
|---|---|
| Sodium + water | 2Na + 2H₂O → 2NaOH + H₂ |
| Magnesium + oxygen | 2Mg + O₂ → 2MgO |
| Zinc + hydrochloric acid | Zn + 2HCl → ZnCl₂ + H₂ |
| Iron + copper sulfate | Fe + CuSO₄ → FeSO₄ + Cu |
| Aluminium extraction | Al³⁺ + 3e⁻ → Al |
Exam Tips
- Always write balanced equations - the examiner will check!
- Use the reactivity series to predict displacement reactions
- Remember OIL RIG for oxidation/reduction definitions
- State observations - e.g., "magnesium ribbon bubbles vigorously"
- Higher Tier: Be able to explain electron transfer in redox reactions
Practice Questions
Question 1 (Foundation)
Describe what happens when magnesium is placed in dilute hydrochloric acid. Include a balanced equation.
Question 2 (Higher)
Explain why aluminium is extracted by electrolysis rather than by displacement from its ore.
Question 3 (Higher)
In the reaction: Fe₂O₃ + C → Fe + CO₂
Identify which substance is oxidised and which is reduced. Explain your answer in terms of electron transfer.
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