IBDP · Chemistry HL
Reactivity: How Much, How Fast, and How Far?
Reactivity 2
42 min readAdvancedCore HL strand — equilibrium (Kc/Qc), stoichiometry and kinetics questions appear on every paper; ICE tables and Arrhenius graphs are near-guaranteed HL Paper 2 content
Reactivity 2 is really three separate questions bolted together under one heading, and IB loves testing whether you can tell which one is actually being asked. How far means equilibrium — does the reaction actually go anywhere, and how much of each side is present when it stops moving (, , Le Chatelier). How much is pure stoichiometry — moles, masses, gases, yields. How fast is kinetics — rate laws, orders, activation energy. None of the maths overlaps: mixing up a unit with a rate constant unit is the single most common HL slip on this topic, so pin down the boundaries between the three before you touch a single equation.
Overview — Three Questions, One Topic
The three questions, one topic
- Extent (, , Le Chatelier) tells you the position of equilibrium at a fixed temperature — it says nothing about how quickly that position is reached.
- Amount (stoichiometry, moles, yield, atom economy) is about counting particles through a balanced equation — no time dependence, no equilibrium, just ratios.
- Rate (rate law, order, Arrhenius) is about the pathway and speed from reactants to products — a fast reaction can have a tiny equilibrium constant, and a thermodynamically favourable one can be kinetically dead slow (diamond → graphite is the classic example).
- HL adds the quantitative machinery to all three strands: numeric / problems with ICE tables, integrated rate expressions and half-life, and the Arrhenius equation in its linear form — SL only goes qualitative on all three.
The shape of the chapter
Command terms that decide the mark scheme here
| Command term | What it demands | AO | Mark-earning move |
|---|---|---|---|
| Calculate | Obtain a numerical answer showing the relevant stages in the working | AO2 | Full method mark for a correct Kc/rate expression even with an arithmetic slip; the final answer needs correct units where Kc/k actually has units, or that mark is lost even with the right number. |
| Determine | Obtain a numerical answer using given data, usually with some working shown | AO2/AO3 | Examiners expect the given data to be used explicitly — quoting a memorised textbook value instead of the one in the question scores zero, even if it's 'more accurate'. |
| Deduce | Reach a conclusion from the information provided | AO3 | Must reference the specific numbers/trend given (e.g. comparing Qc to Kc) — a correct answer with no justification from the data loses the mark. |
| Predict | Give an expected result based on a pattern, without full justification | AO2 | A one-line answer is enough, but it must match the direction implied by Le Chatelier/Arrhenius, not just a vague 'increases'. |
| Sketch | Represent by means of a graph, roughly in proportion with correct general shape | AO2 | Axes must be labelled and intercept/asymptote behaviour correct (e.g. Maxwell–Boltzmann curve through the origin) — a freehand shape starting in the wrong place loses the mark even with the right overall trend. |
| Suggest | Propose a solution, hypothesis or explanation for an unfamiliar context | AO3 | Credit given for chemically sound reasoning even if not the 'official' answer — leaving it blank because the exact scenario wasn't taught is the costliest mistake here. |
Key point
The whole of Reactivity 2 is one idea in three costumes: same reaction, but a question asks either 'how far will it go' (Kc), 'how much forms' (stoichiometry), or 'how quickly' (rate). Identify which one first — confusing Kc units with rate constant units is the single most common HL slip on this topic.
Overview