Edexcel IAL Chemistry · Unit 2
Redox Chemistry & Acid–Base Titrations
The Big Idea: Every chemical reaction where electrons move (redox) can be tracked using oxidation numbers like a scorecard — and every acid-base titration is just a carefully measured "electron-free" reaction used to find an unknown concentration.
Quick Summary — Everything in this Chapter
- Oxidation numbers are a "bookkeeping charge" — the charge an atom would have if all bonding were 100% ionic.
- OIL RIG: Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons).
- Six simple rules let you calculate the oxidation number of any atom in any compound or ion.
- Roman numerals (Stock notation) show the oxidation state of a variable element, e.g. iron(III) oxide.
- An oxidising agent gets reduced itself (it "steals" electrons); a reducing agent gets oxidised itself (it "gives away" electrons).
- Disproportionation = the SAME element is oxidised AND reduced in one reaction.
- Half-equations show electron gain/loss separately; combine two half-equations (matching electrons) to build a full ionic equation.
- Acid-base indicators are weak acids/bases that change colour sharply at the equivalence point of a titration.
- Concentration = moles ÷ volume (in dm³). Titration calculations combine this with reaction stoichiometry.
- Uncertainties from multiple pieces of equipment (balance, pipette, burette, flask) are added together to get total experimental uncertainty.
1. Oxidation Numbers — Introduction
Think of oxidation number as a label tag you stick on every atom in a compound, telling you "how it would behave" if the compound were fully ionic. It's not a real charge in covalent molecules — it's a bookkeeping tool that lets chemists track electron movement without drawing every single bond.
Oxidation vs Reduction — Three Definitions
Depending on the type of reaction, oxidation and reduction can be defined in three equivalent ways:
| Oxidation | Reduction |
|---|
Addition of oxygen
2Mg + O₂ → 2MgO | Loss of oxygen
2CuO + C → 2Cu + CO₂ |
Loss of hydrogen
CH₃OH → CH₂O + H₂O | Addition of hydrogen
C₂H₄ + H₂ → C₂H₆ |
Loss of electrons
Al → Al³⁺ + 3e⁻ | Gain of electrons
F₂ + 2e⁻ → 2F⁻ |
Memory HookOIL RIG — Oxidation Is Loss, Reduction Is Gain (of electrons). This is the definition used almost everywhere in A-Level redox questions.
Oxidation Numbers of Simple Ions
For a simple (monatomic) ion, the oxidation number is literally just the charge on the ion. No calculation needed.
| Type | Example | Why |
|---|
| Atoms | Na in Na = 0 | Neutral already — no electrons need adding or removing |
| Cations | Na in Na⁺ = +1 | Need to add 1 electron to make Na⁺ neutral |
| Anions | Cl in Cl⁻ = −1 | Need to take 1 electron away to make Cl⁻ neutral |
Worked Example
Q: What are the oxidation numbers of the elements in: (a) C (b) Fe³⁺ (c) Fe²⁺ (d) O²⁻ (e) He (f) Al³⁺?
A: a) 0 b) +3 c) +2 d) −2 e) 0 f) +3
The Six Oxidation Number Rules
| # | Rule | Example |
|---|
| 1 | An uncombined element's oxidation number is zero | H₂, Zn, O₂ all = 0 |
| 2 |
Many atoms have fixed oxidation numbers in compounds |
| 3 |
A mono-atomic ion's oxidation number = its charge |
| 4 |