Reactivity: What Are the Mechanisms of Chemical Change?
Reactivity 3 (R3.1–R3.4)
Four subtopics, one underlying question: what is actually moving when a bond breaks or forms? A proton, a full electron, one unpaired electron, or a pair of electrons together. Every mechanism question in this topic — MCQ or long-answer — is really asking you to identify which of these four is on the move, then name the species correctly. Get that classification right first and the rest (pH, oxidation states, curly arrows) falls into place.
Overview — The Shape of the Chapter
One topic, four ways to change electron distribution
Reactivity 3 groups reaction mechanisms by what particle transfers. Proton transfer (R3.1) covers acid–base chemistry — nothing changes oxidation state, a hydrogen ion just moves house. Electron transfer (R3.2) is redox — electrons move completely from one species to another, changing oxidation numbers. Electron sharing (R3.3) is radical chemistry — single, unpaired electrons move, formed by homolytic (symmetric) bond breaking. Electron-pair sharing (R3.4) is organic mechanism chemistry — a pair of electrons moves together, via heterolytic (uneven) bond breaking, curly arrows and all.
- Same skill tested four ways: identify the moving particle, name the species correctly (acid/base, oxidising/reducing agent, radical, nucleophile/electrophile), then explain the consequence (pH, colour change, product formed).
- Command terms repeat across all four: 'deduce', 'identify', 'state the role of...' — examiners want the specific species named, with the reasoning shown, not a vague description.
- The four subtopics build in complexity: R3.1–R3.2 are quantitative and calculation-heavy (pH, Ka, oxidation numbers); R3.3–R3.4 are mechanism-diagram-heavy (arrows, radicals, intermediates).
The shape of the chapter
Command terms that dominate this topic
| Command term | What it demands | AO | Mark-earning move |
|---|---|---|---|
| Deduce | reach a conclusion using given data or reasoning chain | AO3 | show the logical steps (e.g. electronegativity → bond polarity → attack site) — the answer alone without the chain loses marks |
| State | give a short, precise answer with no justification needed | AO1 | one line is enough; naming the wrong species here (e.g. wrong conjugate base) scores zero even with correct formatting |
| Suggest | propose a plausible idea where more than one reasonable answer exists | AO3 | any chemically sound explanation is credited — e.g. suggesting why UV light is needed for a radical mechanism |
| Identify | select and name a specific species from the reaction given | AO1 | must give the actual formula/name (e.g. CH₃COO⁻), not a role description like 'the base' |
| Compare | state similarities AND differences explicitly | AO2 | one-sided answers (only listing differences) drop marks — examiners look for 'whereas / but' language |
| Determine | obtain a value showing full working | AO2 | a correct final number with no working shown loses the method mark even if right |
Key point
Overview