Why a 7 in IB Chemistry Reactivity 2 starts with a stoichiometry ladder, not a formula

Reactivity 2 in the IB Diploma Programme Chemistry syllabus is the bridge between the descriptive bonding work in Structure and the mechanistic reasoning in Reactivity 3. It answers three deceptively simple questions: how much reagent do you actually have, how fast does the reaction proceed, and how far does it go before equilibrium stops the macroscopic change? Every candidate sitting IB Chemistry SL or HL meets these ideas, but the rubric separates a level 5 from a level 7 on small technical habits: a balanced half-equation, a clean rate expression, a properly inverted equilibrium constant. This article walks through the sub-topics the way a senior tutor would at a whiteboard, with worked examples, the most common mark-losing slips, and the question families that actually appear on Paper 1 and Paper 2.

What Reactivity 2.1, 2.2, and 2.3 actually test on the IB Chemistry papers

Reactivity 2.1 is the stoichiometric core of the topic. Candidates meet the mole, empirical and molecular formulae, reacting masses, solution concentrations in mol dm⁻³, limiting reagent problems, and percentage yield calculations. The skills are not arithmetic; they are translation. The examiner hands you a sentence such as "25.0 cm³ of 0.100 mol dm⁻³ sodium hydroxide is added to 0.200 g of a solid acid" and you are expected to convert each phrase into a number with the right unit before any calculation begins. Most candidates reading this who have lost marks on Reactivity 2.1 lost them at the translation step, not the long division.

Reactivity 2.2 moves from quantity to kinetics. Collision theory is introduced qualitatively, then refined into the rate expression and the rate constant, with temperature dependence explained through the Maxwell–Boltzmann distribution. SL candidates stop at the effect of concentration, surface area and temperature on rate. HL candidates add the Arrhenius equation and the idea of an activation energy extracted from a ln k against 1/T plot. The crucial scoring point is that "collision theory" and "the rate expression" are two different assessment objects, even though they sit in the same sub-topic. The first rewards a labelled diagram and a one-sentence justification; the second rewards a mathematically correct relationship with units that survive substitution.

Reactivity 2.3 closes the trio with equilibrium. The dynamic model, Le Chatelier's principle, the equilibrium constant Kc and its pressure-based cousin Kp all sit here. HL candidates additionally handle the relationship between Kc, Gibbs free energy and the temperature at which a reaction becomes spontaneous. The phrasing "how far" is a number: Kc, not a feeling, not a direction. The mark scheme penalises qualitative hand-waving when a numerical answer is asked for, and it penalises numbers when a qualitative argument is asked for. Reading the command term is half the battle.

Across all three sub-topics, the common thread is the rubric's appetite for clear, ordered working. A correct final answer with no supporting work rarely earns full marks on a Paper 2 extended-response question, while a final answer that is wrong by a single power of ten will often still earn two or three marks if every prior step is laid out. Practising with that in mind changes the way a candidate writes under timed conditions, because each line becomes a deliberate mark-earning device rather than scratch work.

Reactivity 2.1: the mole calculation ladder that decides a 5 from a 7

The single most effective habit in Reactivity 2.1 is to write a vertical "ladder" of conversions on the page: mass ÷ molar mass → moles of substance A → mole ratio from the balanced equation → moles of substance B → × molar mass → mass of B. Every line is a potential mark. When a candidate collapses the ladder into a single line of algebra, they may still reach the right number, but they hand the examiner nothing to award partial credit on if the final line is wrong. The ladder also exposes a mole ratio that has been inverted, which is by far the most common error in this sub-topic.

Consider a typical Paper 2 question: "4.00 g of calcium carbonate reacts with 50.0 cm³ of 1.00 mol dm⁻³ hydrochloric acid. Calculate the mass of carbon dioxide produced and identify the limiting reagent." The ladder is:

• n(CaCO₃) = 4.00 ÷ 100.09 = 0.03996 mol
• n(HCl) = 0.0500 × 1.00 = 0.0500 mol
• Required ratio is 2 mol HCl per 1 mol CaCO₃; available ratio is 0.0500/0.03996 = 1.25, well below 2, so HCl is limiting.
• Maximum n(CO₂) = n(HCl)/2 = 0.0250 mol, mass = 0.0250 × 44.01 = 1.10 g.

That four-line ladder is worth the question. A common pitfall is to identify the solid as limiting by reflex because "there is more of the acid". Reflex answers are exactly what the rubric is designed to test against; the ladder forces the comparison. Paper 1 multiple-choice questions on the same material often give a trap answer in which the wrong reagent is flagged as limiting, and the only way to avoid the trap is to actually compute the ratio.

Concentration calculations in mol dm⁻³

Solution stoichiometry is where unit slippage damages most candidates. Concentration has units of mol dm⁻³, so a volume in cm³ must be divided by 1000 before multiplication. Examiners report year after year that candidates write 50.0 × 1.00 = 0.0500 mol and then lose the mark, even though the numerical manipulation is fine, because the unit is wrong. A simple rule: if the volume in the question is in cm³, the line on the page must explicitly say ÷ 1000. Don't let a correct calculation ride on an invisible conversion; the rubric cannot credit an assumption.

Titration calculations extend the same ladder with an extra step at the start. A typical problem gives the mass of a solid acid, asks for its molar mass via titration against a standard base, then uses the molar mass to deduce the molecular formula. The chain is: mass of acid → moles of base from titre → mole ratio → moles of acid → molar mass → empirical formula → molecular formula. Skipping a step collapses the marks. Practising three or four such chains end-to-end under timed conditions is the only way to internalise the order; rereading notes does not produce the same fluency.

Common pitfalls and how to avoid them in Reactivity 2.1

• Inverted mole ratio. Always re-read the balanced equation before writing the ratio. A useful check: the species you are starting from and the species you are asked about should each appear once on the ratio line.
• Wrong molar mass. Use the periodic table provided in the exam, not the one you memorised. Atomic masses in IB data booklets can differ from a candidate's recall by 0.01, and examiners will not round your way.
• Percentage yield treated as theoretical yield. The percentage is always relative to the theoretical maximum. If the question gives a percentage, multiply the theoretical by it; do not treat it as the final answer.
• Avogadro's number slipped into a mass calculation. Nₐ is for particle counts, not for grams. If the answer is in grams, the line should not contain Nₐ.
• Solution volume not converted. cm³ → dm³ requires an explicit division by 1000. Hide nothing from the examiner.

Reactivity 2.2: collision theory and the rate expression scored as two different skills

Candidates often treat 2.2 as a single block, but the rubric separates the qualitative collision-theory account from the quantitative rate expression. The qualitative side appears most often in Paper 1 as a multiple-choice item, and the rubric awards the mark to candidates who can name the three requirements for a successful collision — correct orientation, sufficient energy (≥ Eₐ), and a collision between reactant particles — and link them to a Maxwell–Boltzmann distribution diagram with the activation energy shaded and the curve correctly shifted when temperature rises. A diagram with a wrong axis, or with Eₐ drawn as a vertical line on the y-axis, is a guaranteed mark loss.

The quantitative rate expression is a different assessment object. Candidates write rate = k[A]ᵐ[B]ⁿ and are expected to determine m and n experimentally from concentration-versus-time data. The orders m and n are not the stoichiometric coefficients; they come from the experiment. A common Paper 2 question gives a table of initial rates for varying concentrations and asks for the orders. The method: compare two rows where one concentration is unchanged and the other doubles or triples, then read off the power from the change in rate. The ladder here is two rows, one ratio, one logarithm:

• Row 1: [A] = 0.10, [B] = 0.10, rate = 1.0 × 10⁻³
• Row 2: [A] = 0.20, [B] = 0.10, rate = 2.0 × 10⁻³
• [A] doubles, rate doubles → first order in A.

Repeat with a pair of rows that hold [A] constant and vary [B] to get the order in B. Then substitute a single row into the rate expression to find k, and write the units of k. The units step is the easiest mark to lose; candidates write "mol dm⁻³ s⁻¹" reflexively, regardless of order. A first-order reaction has units of s⁻¹; a second-order reaction has units of dm³ mol⁻¹ s⁻¹. Skipping this step costs a mark that the rest of the calculation has already earned.

HL extension: the Arrhenius equation and the ln k against 1/T plot

HL candidates extend the rate-expression work with the Arrhenius equation in the form ln k = −Eₐ/RT + ln A. The exam gives either a table of k at different temperatures or a graph to interpret. If a graph, candidates read off two clear points, compute the slope, and multiply by −R to find E₀. The slope is negative, so a positive Eₐ is recovered only if the candidate treats the sign carefully. A typical error is to take the magnitude of the slope and call it Eₐ without the −R step, which produces an answer that is wrong by a factor and a sign.

Another common trap is unit confusion on R. The data booklet gives R in J K⁻¹ mol⁻¹, so Eₐ from a slope in K is in J mol⁻¹. The exam often asks for the answer in kJ mol⁻¹. Converting is a single division by 1000, but the line must appear on the page. Candidates who write a J value when the question asks for kJ lose the final mark, even when the chemistry is correct.

Common pitfalls and how to avoid them in Reactivity 2.2

• Orders from stoichiometry rather than experiment. Always check that the question gives experimental data. If it does, the orders come from the data, not from the equation.
• Missing units of k. Write the unit for the order of the reaction you have just determined. First order: s⁻¹. Second order: dm³ mol⁻¹ s⁻¹. Zero order: mol dm⁻³ s⁻¹.
• Wrong sign on Eₐ. The Arrhenius slope is negative; do not strip the sign when reporting the activation energy.
• Collision theory described without a diagram. A mark scheme often gives a mark for a labelled Maxwell–Boltzmann distribution. A description without the diagram is incomplete.
• Catalyst described as "lowering the rate". Catalysts lower the activation energy, which raises the rate. The direction of the effect is a common mark-loss, especially for SL candidates.

Reactivity 2.3: equilibrium constants as the "how far" number, not a feeling

Reactivity 2.3 is where the language of the syllabus tightens. "How far" is a number, and the number is Kc or Kp. Candidates should write the expression as a fraction with products over reactants, each raised to its stoichiometric coefficient, and only including gaseous or aqueous species. Solids and pure liquids are excluded. A common pitfall is to include a solid catalyst in the expression, which is wrong on two counts: the catalyst is not in the equilibrium, and solids are not in Kc anyway.

Consider a heterogeneous equilibrium such as the decomposition of calcium carbonate: CaCO₃(s) ⇌ CaO(s) + CO₂(g). Kp for this reaction is simply P(CO₂), because the solids drop out. Candidates who try to write a full Kc expression with all three species lose a mark for not understanding the rule. The same logic applies to Kp: only gas partial pressures appear, each raised to its coefficient.

For homogeneous equilibria, the calculation is mechanical. The exam gives initial concentrations or amounts, an equilibrium concentration of one species, and asks for Kc. The ladder is: ICE table (Initial, Change, Equilibrium) → equilibrium concentrations → Kc expression → substitution → evaluation with units stripped. Units are not written for Kc; the convention is to leave Kc dimensionless. Stating units costs no mark, but writing a unit that is inconsistent with the order is a small mark loss when the question asks for a numerical value to a specific number of significant figures.

HL extension: Gibbs free energy and the temperature of spontaneity

HL candidates are expected to use ΔG° = −RT ln K to connect Reactivity 2.3 to Reactivity 1.3. A typical question gives a value of ΔG° at a stated temperature and asks for K, or gives K and asks for the temperature at which the reaction becomes spontaneous. The two manipulations are the inverse of each other, and the sign of ΔG° determines the sign of the deviation of K from 1. ΔG° < 0 implies K > 1, meaning products are favoured at equilibrium. ΔG° > 0 implies K < 1. Candidates who mix the signs lose a mark that they could otherwise have earned by re-reading the question.

The relationship also explains Le Chatelier's prediction in quantitative form. If a temperature change shifts K, the magnitude of the shift is calculable from the van 't Hoff equation. The exam does not usually require the integrated form, but a candidate who can read a ΔH sign and predict whether K increases or decreases with temperature has already absorbed the core idea of the topic.

Common pitfalls and how to avoid them in Reactivity 2.3

• Including solids or liquids in Kc or Kp. Re-state the rule on the page if you are unsure. The examiner can then credit a correct exclusion as deliberate rather than accidental.
• Inverted expression. Products go on top. If the question's Kc is "large", products are favoured, and your expression should give a number greater than 1 when the products are the major species.
• Forgetting to use the equilibrium concentration of the species given in the question. The question usually gives one equilibrium value as a hint; use it to back-calculate the others via the ICE table.
• Mixing up Kc and Kp. Kc uses concentrations; Kp uses partial pressures. The choice depends on the data given in the question, not on personal preference.
• Treating Le Chatelier as a quantitative tool. Le Chatelier predicts the direction of shift; the equilibrium constant predicts the new position. The exam will sometimes ask for the new K after a temperature change, which is unchanged, or the new K after a concentration change, which is also unchanged. Only temperature changes the value of K.

Paper 1 versus Paper 2: how Reactivity 2 is actually distributed across the IB Chemistry exam

Paper 1 is the multiple-choice paper, 30 marks for SL and 40 marks for HL, sat in 45 or 60 minutes respectively. Reactivity 2 typically contributes between 8 and 12 marks on Paper 1, with 2.1, 2.2 and 2.3 each taking a share. The questions are diagnostic: they distinguish candidates who can read a stem and apply one idea from those who can read a stem and apply the right idea. The most common Paper 1 trap in 2.1 is the limiting-reagent problem disguised as a mass calculation. The most common Paper 1 trap in 2.2 is the orders problem disguised as a stoichiometric ratio. The most common Paper 1 trap in 2.3 is the Kc expression with a solid in it.

Paper 2 is the extended-response paper, 50 marks for SL and 90 marks for HL, sat in 75 minutes or 135 minutes. Section A contains a data-based question and several short structured questions, while Section B contains a choice of extended-response questions. Reactivity 2 features heavily in Section A, where candidates are expected to perform multi-step calculations with full working. Section B sometimes offers a kinetics or equilibrium question as one of the choices, and the rubric for those questions is unforgiving: a single missing unit or sign costs a mark, and a missing step in the ladder costs two.

Paper 3 is the practical paper, sat by SL and HL candidates, focused on the experimental programme, the qualitative analysis, and the optional themes. Reactivity 2 surfaces in Paper 3 mainly through the rate-determination and titration experiments from the syllabus, where candidates are expected to record data, draw a graph, and interpret a gradient. The mark scheme rewards the interpretation step most heavily: a beautiful graph with no conclusion drawn from it earns only a fraction of the marks, while a scrappy graph with a clear and correct conclusion drawn from it can earn full marks.

Mapping Reactivity 2 to the IB Chemistry rubric: where the bands fork

The IB Chemistry rubric runs from level 1 to level 7, with descriptors published by the IB. A level 7 candidate demonstrates consistent and thorough understanding, applies knowledge to unfamiliar situations, and communicates with precision. A level 5 candidate demonstrates a sound knowledge base and applies it to familiar contexts with some success. Reactivity 2 separates these bands at three specific points.

The first fork is the ladder habit. Candidates who write their working in a vertical ladder on Reactivity 2.1 questions are reliably in the level 6 to 7 band on that question; candidates who write a single line of algebra are reliably in the level 4 to 5 band. The second fork is the units habit. Candidates who write units of k in 2.2 and who drop solids from Kc in 2.3 score in the upper bands on those questions. The third fork is the command-term habit. Candidates who distinguish "state", "explain", "calculate" and "deduce" answer the question that was actually asked, and pick up the marks that the rubric attaches to each verb.

A useful diagnostic after a mock paper is to mark each Reactivity 2 question on three columns: working shown, units included, command term matched. If two of the three are usually present, the candidate is in the upper bands. If only one is present, the candidate is in the middle bands. If none is present, the candidate is being carried by recall and is vulnerable on any application question. The diagnostic takes five minutes and produces a more honest picture of preparation than a single overall percentage.

Comparing the SL and HL demand in Reactivity 2

The table shows where the rubric forks between SL and HL. A candidate who can write Kc but not Kp is correctly placed at SL; a candidate who can also link K to ΔG° is on track for the HL bands. The same is true of the Arrhenius step: the SL plot is sketched, the HL plot is read with a ruler and converted to an activation energy in kJ mol⁻¹.

A preparation strategy that actually moves the bands

Most candidates preparing for IB Chemistry Reactivity 2 fall into one of two traps. The first is rereading the syllabus guide from cover to cover and believing that familiarity is the same as fluency. Familiarity is necessary but not sufficient. The second is doing hundreds of calculation questions without ever looking at the mark scheme, so the same errors recur at the same scale on every timed paper. A working strategy walks the line between the two.

Step one: build a one-page ladder for each of 2.1, 2.2 and 2.3 that lists the steps in order. The 2.1 ladder is mass → moles → ratio → moles → mass (or volume). The 2.2 ladder is data → ratio → order → k → units. The 2.3 ladder is data → ICE table → equilibrium concentrations → Kc expression → value. Writing the ladder once forces the candidate to commit to an order, and that order is what is reproduced under timed conditions.

Step two: do three or four past-paper questions per sub-topic, but mark them with the rubric in hand. The IB publishes mark schemes that are publicly available; using them in preparation is not optional, it is the only way to align a candidate's working with the examiner's expectations. After marking, write a single sentence on the page that captures the lesson: "I keep dropping solids from Kc", or "I keep forgetting the units of k for second-order reactions". That sentence is the input for step three.

Step three: a week later, repeat one of the questions from step two without notes, then mark it. If the same error recurs, the candidate has not practised the lesson; the candidate has practised the question. Switch to a different question in the same family, because the family is the unit of practice, not the individual item. Two or three cycles of this is the difference between a level 5 and a level 7 on the topic.

Step four: a fortnight before the exam, run a single timed Paper 2 Section A in full, with the rubric beside the paper. Mark it within an hour and read the comments on every lost mark. The patterns that emerge from a single timed paper under realistic conditions are the patterns that the final paper will repeat. If the candidate has six marks down on units in a single section, the units habit is still missing, and step two needs to be revisited.

Tying Reactivity 2 to the rest of the IB Chemistry syllabus

Reactivity 2 is rarely examined in isolation. On Paper 2, the same extended-response question can move from a stoichiometric calculation into a Kc expression and finish with a brief comment on Le Chatelier. The marks then span 2.1, 2.3 and the bridging idea of position of equilibrium. On Paper 3, the rate experiments feed into 2.2 directly, and the acid-base titrations feed into 2.1 with an equilibrium flavour when the indicator choice is discussed. A candidate who treats Reactivity 2 as a self-contained topic will lose these cross-topic marks; a candidate who treats it as the bridge between Structure and Reactivity 3 will collect them.

The link to Reactivity 1 is also worth practising. A question on enthalpy change in a reaction can be paired with a question on Kc, and the relationship between ΔG°, ΔH° and ΔS° then locks the two sub-topics together. The HL-only material makes this link explicit through ΔG° = −RT ln K; the SL material hints at it through qualitative Le Chatelier arguments. Either way, the candidate who can read a thermodynamic prompt and find the equilibrium constant has crossed the bridge, and the rubric can see it.

The link to Reactivity 3 is structural. Mechanisms in Reactivity 3.1 to 3.4 are written in terms of curly arrows, but the rate of each step is governed by the kinetics in 2.2 and the position of equilibrium in 2.3. A SN1 mechanism, for instance, depends on the rate-determining ionisation step; a question that asks for the rate expression of that step in 3.3 is really asking the candidate to apply 2.2 in a mechanism context. The candidate who has practised 2.2 with a mechanism in mind will answer those questions faster and more accurately.

Final preparation checklist for IB Chemistry Reactivity 2

Before the exam, the candidate should be able to do all of the following without notes: balance a half-equation in acid without losing the spectator ions; write a mole ladder and identify the limiting reagent in two minutes; write the rate expression from a table of initial rates and quote the units of k; sketch a Maxwell–Boltzmann distribution with the activation energy shaded; write Kc and Kp expressions for a homogeneous and a heterogeneous equilibrium; use ΔG° = −RT ln K to find K at a given temperature; convert a slope on a ln k against 1/T plot to an activation energy in kJ mol⁻¹ with the correct sign.

If any of those steps is shaky, the candidate should revisit the relevant sub-topic before the exam. The point of a checklist is to convert a vague feeling of preparation into a specific list of practiced and un-practised skills. Most candidates reading this who feel "mostly ready" will find that two or three items on the list are still under-practised; those two or three items are the difference between the band they have and the band they want.

IB Chemistry Reactivity 2 rewards candidates who treat small technical habits as the unit of progress. The mole ladder, the units of k, the solids-out rule for Kc, the sign on the Arrhenius slope — each of these is worth a mark or two on the day, and the marks accumulate across the paper. A preparation strategy that drills those habits in isolation, then recombines them under timed conditions, is the strategy that produces level 7s on Reactivity 2.

For candidates aiming at the top band, IB Courses' one-to-one IB Chemistry HL programme builds a per-student Reactivity 2 ladder from Paper 1 and Paper 2 error logs, then rehearses the limiting-reagent, rate-expression and Kc-expression families until the small habits become automatic. Reach out for a diagnostic mock and a tailored preparation plan.

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