Reactivity: How Much, How Fast, and How Far?
Reactivity 2
Three separate exam personalities live inside this topic, and examiners love mixing them in the same question: give you a reaction, ask you to work out moles first (How Much), then predict what happens to the mixture over time (How Fast), then hand you an equilibrium concentration and demand (How Far). Learn each skill set in isolation — mole arithmetic, ICE tables, collision-theory reasoning — then practise spotting which one a command term is actually testing before you write a single line.
Overview: three questions, one reaction
Why these three ideas are grouped together
Every chemical change can be interrogated with the same three questions: how much product forms, how fast it forms, and how far the reaction actually goes before it stops changing. These aren't three unrelated chapters — they're three lenses examiners apply to the exact same reaction, often in the same question. A methanol synthesis question might ask you to find moles of limiting reagent in part (a), predict a rate change in part (b), and calculate from an ICE table in part (c).
- How Much? (Reactivity 2.1) — pure stoichiometric bookkeeping: mole ratios, limiting reagent, yield, atom economy, concentration.
- How Fast? (Reactivity 2.2) — kinetics: collision theory, factors affecting rate, activation energy, catalysts.
- How Far? (Reactivity 2.3) — equilibrium: dynamic equilibrium, Le Chatelier's principle, the equilibrium constant .
- Temperature is the one variable that genuinely touches all three: it changes , it reshapes the energy distribution driving rate, and (HL only) it appears explicitly in the Arrhenius equation.
The shape of the chapter
Command terms that dominate this topic
| Command term | What it demands | AO | Mark-earning move |
|---|---|---|---|
| Determine | Obtain a numerical value using given data, with working shown. | AO2 | Method mark for correct ICE table/set-up survives even if the final arithmetic slips — always show every row. |
| Deduce | Reach a conclusion from evidence/information provided, not from memory alone. | AO3 | Must reference the specific data given (e.g. compare Q to Kc) — a bare direction with no justification scores zero. |
| Predict | State a likely outcome without extended justification. | AO2 | One correct statement is enough — don't burn time over-explaining a 1-mark predict. |
| Explain | Give a reasoned account linking cause to effect. | AO3 | Marks awarded per linked idea in the chain — e.g. higher T → more particles with E≥Ea → more successful collisions → faster rate is three separate mark points. |
| Sketch | Draw an approximately correct diagram with key features and labels, not to scale. | AO2 | Wrong curve shape (e.g. an M-B curve touching the x-axis) loses marks even if the general trend is right. |
| Calculate | Obtain a numerical answer showing all working. | AO2 | Correct units and sensible significant figures are separate mark points from the number itself. |
Key point
Overview