Supplement — Topic 6.4.7
Tracking Redox Changes via Oxidation Numbers
Oxidation numbers are the "charge score" of each atom. Watch the score go UP (oxidation) or DOWN (reduction).
⚡ The Core Rules
Free elements
Always = 0
Mg, H₂, O₂
Monatomic ions
= Ion charge
Na⁺ → +1, Cl⁻ → −1
Hydrogen in compounds
+1 (usually)
HCl: H = +1
Oxygen in compounds
−2 (usually)
H₂O, MgO: O = −2
Sum of all ox. numbers
= Overall charge
H₂O: 2(+1) + (−2) = 0 ✓
🌡️ The Thermometer of Charge
Like a thermometer, the oxidation number has a scale. Going UP = Oxidation. Going DOWN = Reduction.
-4-20+2+4+7
🔬 Equation Analyzer
Select an equation, then step through the oxidation number analysis.
⚠️ Examiner's Traps
🚫
Saying neutralisation is redox
Always assign oxidation numbers first. In NaOH + HCl → NaCl + H₂O, ALL numbers stay the same. No change = NOT redox.
📝
Writing "the charge increased" instead of "the oxidation number increased"
Oxidation number ≠ actual charge. Always say oxidation number in your answer.
🔄
Confusing "oxidising agent" with "the thing being oxidised"
The OXIDISING AGENT gets reduced (it accepts electrons). The REDUCING AGENT gets oxidised (it donates electrons). They are OPPOSITES.
📋 Cambridge Keywords — Paper 4
Oxidation
Increase in oxidation number
Reduction
Decrease in oxidation number
Redox reaction
Both oxidation AND reduction occur simultaneously
Oxidising agent
The species that gets reduced (ox. number decreases)
Reducing agent
The species that gets oxidised (ox. number increases)
Non-redox
No change in any oxidation numbers
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