Playbook
Redox Reactions
44 q - 1.08/paper - 2% HARD. The oxidation number, including its structural exceptions.
- Questions in the bank
- 44
- q/paper (2024–25 papers)
- 1.08
- Tagged HARD
- 2%
- Subtopics
- 3
Strand: Recall
When you’ll see it
An oxidation number to find, a species oxidised or reduced, or a redox equation to balance.
How this chapter is tested
44 q at 1.08 a paper, 2% HARD. Oxidation-number calculation is 26 of them: the sum over a species equals its charge, with O as −2 and H as +1 — except in peroxides (O −1) and metal hydrides (H −1).
The rest reuses it: whichever element's oxidation number rises is oxidised, and it is the reducing agent. A reaction in which no oxidation number changes — CrO₄²⁻ ⇌ Cr₂O₇²⁻ — is not redox.
The sub-skills
The distinct skills inside the chapter, in the order to learn them.
The rules
Elements 0; O −2 (peroxide −1, OF₂ +2); H +1 (metal hydride −1); the sum equals the charge.
Structural exceptions
Fractional averages (S in S₄O₆²⁻ averages +2.5, though the S atoms differ).
Oxidised against reducing agent
The species oxidised IS the reducing agent.
Traps to expect
Distractor shapes this chapter reuses. The Traps page covers the patterns that cut across chapters.
Peroxide oxygen
O in H₂O₂ and Na₂O₂ is −1. Using −2 gives an impossible oxidation state for the other element.
Chromate and dichromate
Cr stays +6 in CrO₄²⁻ ⇌ Cr₂O₇²⁻, so it is not a redox reaction.
Learn it before you drill it
This chapter has full teaching notes — foundations, worked examples, self-checks and a per-subtopic mastery checkpoint. Read the notes once, then drill subtopic by subtopic below.
Redox Reactions notesDrill every redox reactions question
44 questions from the bank, scoped to 3 bundled subtopics.
Drill one subtopic at a time
The 3 subtopics this playbook covers, in catalog order.
Related playbooks
Often paired with this one — the technique, the trap or the taxonomy overlaps. Drill these next.