Oxidation & Reduction
Section: 6. Chemical Reactions | Syllabus: Cambridge IGCSE Chemistry (0620)
What is a Redox Reaction?
A redox reaction is a reaction involving oxidation and reduction happening simultaneously - the term combines "reduction" and "oxidation". You cannot have one without the other.
Oxidation and Reduction in Terms of Oxygen
- Oxidation is the gain of oxygen
- Reduction is the loss of oxygen
Worked Example: Magnesium Burning
- 2Mg + O₂ → 2MgO
- Magnesium is oxidised (gains oxygen)
Worked Example: Reducing Copper(II) Oxide
- CuO + H₂ → Cu + H₂O
- Copper(II) oxide is reduced (loses oxygen); hydrogen is oxidised (gains oxygen)
In every redox reaction, one substance gains oxygen while another loses it
Oxidation Numbers and Roman Numerals
An oxidation number (oxidation state) shows the degree of oxidation of an element in a compound, and is written as a Roman numeral in the compound's name.
| Compound | Formula | Oxidation state shown |
|---|---|---|
| Iron(II) chloride | FeCl₂ | iron = +2 |
| Iron(III) chloride | FeCl₃ | iron = +3 |
| Copper(I) oxide | Cu₂O | copper = +1 |
| Copper(II) oxide | CuO | copper = +2 |
Worked Example: Interpreting a Roman Numeral
- Question: nitrogen and oxygen form the compound nitrogen(II) oxide. Which statements about the Roman numeral are correct? 1 - it shows nitrogen's oxidation number is +2. 2 - it shows oxygen's oxidation number is +2. 3 - it shows the formula contains two nitrogen atoms.
- Answer: statement 1 only. The Roman numeral gives the oxidation number of the named element (nitrogen), not oxygen (statement 2 false - oxygen is almost always −2). It is not a count of atoms (statement 3 false) - nitrogen(II) oxide is NO, not N₂O.
Common Mistakes
- ❌ Thinking a Roman numeral counts atoms in the formula
- ✓ the Roman numeral is always an oxidation number/state, never a subscript count - iron(III) oxide is Fe₂O₃, not "3 iron atoms"
- ❌ Applying the Roman numeral to the wrong element
- ✓ the Roman numeral always refers to the FIRST-named element in the compound's name
Identifying a Redox Reaction
A reaction is a redox reaction if one substance gains oxygen while another substance loses oxygen in the same reaction - both oxidation and reduction happen together.
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