Reactivity: What Are the Mechanisms of Chemical Change?
Reactivity 3
Every reaction in this topic is really the same question in disguise: what moved, and where did it go? A proton hops from acid to base, an electron jumps from reductant to oxidant, a pair of electrons gets shared evenly (covalent, radicals) or unevenly (nucleophile/electrophile). Examiners test whether you can name the moving particle, draw its path with the correct arrow, and predict what forms — not whether you can recite definitions.
Overview: The Logic of Reactivity 3
Reactivity 3 groups reaction types by what species is transferred, not by product class. That's the exam lens: before writing anything, ask 'is a proton moving, an electron moving, or is a pair of electrons being shared?' Get that classification right and the mechanism, arrows, and vocabulary follow automatically.
- Proton transfer (3.1): acid-base chemistry, Brønsted-Lowry pairs, pH/Ka/Kb, buffers (HL).
- Electron transfer (3.2): redox, half-equations, electrochemical cells, Faraday's law calculations (HL).
- Electron sharing (3.3): covalent bonds and radical mechanisms — homolytic fission, fishhook arrows.
- Electron-pair sharing (3.4): nucleophile/electrophile mechanisms — heterolytic fission, curly arrows, SN1/SN2 (HL).
The shape of the chapter
Command terms that trip students in this topic
| Command term | What it demands | AO | Mark-earning move |
|---|---|---|---|
| Deduce | Use given data (pH, Ka, E° values) to reach a stated conclusion — show the logic, not just the number. | AO3 | Mark for the reasoning link, not just the final answer |
| State | One fact, no working — e.g. 'name the conjugate base'. | AO1 | Loses the mark if you add unrequired explanation that contradicts itself |
| Suggest | Propose a plausible mechanism/species when the exact answer isn't taught — used for novel acids/cells. | AO2 | Any chemically sound proposal scores; guessing wildly does not |
| Compare | Give similarity AND difference explicitly (e.g. SN1 vs SN2). | AO2 | One-sided answers (only differences) lose marks |
| Outline | Give a brief account of a mechanism's key steps in order. | AO2 | Missing a step (e.g. termination) costs a mark even if arrows are right |
Key point
Overview