Why reactions happen: reading the IB Chemistry Reactivity 1 guide for SL and HL

Reactivity 1 is the opening sub-topic of the reactivity core in the IB Chemistry guide, and it is the single unit that frames every reaction a student meets later in the course. The central question is deceptively simple: given a mixture of reactants, will a reaction proceed, and if so, how far and how fast? The syllabus answers that question with two distinct sets of ideas — thermodynamics (enthalpy and entropy, brought together in the Gibbs free energy equation) and kinetics (collision theory, the Maxwell–Boltzmann distribution, and the role of a catalyst). For IB Diploma candidates aiming at a 6 or 7, the Reactivity 1 guide is not background reading; it is the analytic vocabulary that examiners test in Paper 1 multiple choice, Paper 2 short-answer and extended response, and in the Internal Assessment. The aim of this article is to read the syllabus text the way a senior teacher would at the whiteboard: line by line, with the question types that actually appear in examination papers attached to each idea.

How the IB Chemistry guide frames Reactivity 1: thermodynamics and kinetics as two lenses

The IB Chemistry syllabus splits the reactivity core into Reactivity 1, 2, and 3, and it is worth being precise about what each one carries. Reactivity 1 is the conceptual foundation: it is where the guide tells you why reactions happen at all. Reactivity 2 then applies those ideas to acid–base and redox chemistry, and Reactivity 3 carries them into organic reaction pathways. A candidate who treats Reactivity 1 as a memorisation unit will survive Paper 1 but will lose the mark bands in Paper 2 that reward explanation of mechanism, equilibrium position, or feasibility. The syllabus statements in Reactivity 1 deliberately cover two distinct families of ideas under one heading, and the assessment papers test them in different question types. Understanding that split is the first preparation step.

Thermodynamics is the part of Reactivity 1 that answers the will it? question. Enthalpy change (ΔH) tells you whether the reaction releases or absorbs heat. Entropy change (ΔS) tells you whether the disorder of the system increases or decreases. The Gibbs free energy equation ΔG = ΔH − TΔS combines the two and gives a single threshold: if ΔG is negative, the reaction is thermodynamically feasible; if positive, it is not, under the stated conditions. Kinetics is the part of Reactivity 1 that answers the how fast? question. Collision theory says that particles must collide with sufficient energy (≥ Eₐ) and the correct orientation. The Maxwell–Boltzmann distribution describes the spread of kinetic energies in a sample and shows why only a fraction of collisions are productive at any temperature. A catalyst provides an alternative pathway with a lower activation energy; it does not appear in the stoichiometry and it does not change ΔH, ΔS, or ΔG.

For preparation strategy, this means the Reactivity 1 study plan must keep these two strands visibly separate before you learn to connect them. A good heuristic is to write two columns at the top of each set of notes: thermodynamic? and kinetic?. When you meet a reaction — the decomposition of hydrogen peroxide, the combustion of methane, the dissolution of ammonium nitrate — you place the facts in the correct column. By Paper 2, examiners will mix the two, and a candidate who can immediately say that is a kinetic argument, not a thermodynamic one scores higher than a candidate who writes a long paragraph that uses the words "feasible" and "fast" interchangeably. The IB marking scheme penalises that conflation explicitly in extended-response questions on rate and equilibrium.

The thermodynamic core: enthalpy, entropy, and the Gibbs free energy threshold

Within the thermodynamic strand of Reactivity 1, the syllabus builds a chain of definitions that examiners reuse across the unit. Enthalpy change (ΔH) is the heat transferred at constant pressure; standard enthalpy of formation, combustion, and neutralisation are tabulated values that students must know how to apply. Hess's law allows the calculation of an enthalpy change that cannot be measured directly by combining known steps. The Born–Haber cycle is the application of Hess's law to ionic lattice formation, and the IB Chemistry HL guide extends this with ionisation energy, electron affinity, and lattice enthalpy. Entropy (S) is a measure of disorder, and entropy change (ΔS) is the change in disorder between reactants and products. The guide expects candidates to predict the sign of ΔS from a reaction: a reaction that produces more moles of gas than it consumes almost always has a positive ΔS, and a reaction that consumes gas almost always has a negative ΔS. Solubility and state changes also drive entropy changes, and these are tested routinely in Paper 1.

The Gibbs free energy equation, ΔG = ΔH − TΔS, is the single equation a candidate must be able to manipulate fluently. At SL, students are expected to calculate ΔG from given ΔH and ΔS values, decide on the sign, and comment on feasibility. At HL, the temperature term T (in kelvin) matters because it amplifies the entropy contribution: at high temperature, the TΔS term can dominate, which is why some endothermic reactions become feasible only above a certain temperature. A frequent Paper 2 question asks candidates to explain, using thermodynamic data, why this reaction is feasible above 500 K but not below. The mark-scheme answer is a four-step argument: state the sign of ΔH, state the sign of ΔS, substitute into ΔG = ΔH − TΔS, and show that the sign of ΔG flips as T increases. Candidates who skip the substitution step and write only because entropy increases with temperature lose the calculation mark.

There is a tactical point worth highlighting. The IB marking scheme treats feasible as a thermodynamic word and spontaneous as an acceptable synonym. It does not treat fast or likely to occur as synonyms. A reaction with a negative ΔG can be so slow that the observer never sees it; this is the case for the conversion of diamond to graphite at room temperature. Examiners reward the candidate who writes thermodynamically feasible but kinetically inert because the two ideas are kept distinct. In a typical extended-response question, the mark for discussing the feasibility of a reaction will split into a thermodynamic half and a kinetic half; the two halves are marked separately, and a candidate who only writes the thermodynamic half scores roughly half the available marks.

The kinetic core: collision theory, the Maxwell–Boltzmann distribution, and catalysis

The kinetic strand of Reactivity 1 is where most candidates lose marks through loose language, because the syllabus asks for an explanatory model, not a vocabulary list. Collision theory has three parts: particles must collide, they must collide with energy greater than or equal to the activation energy Eₐ, and they must collide with the correct geometric orientation. Examiners test all three parts, and the orientation requirement is the one that candidates most often forget. A question that asks why is the reaction between NO and O₃ slower than the reaction between NO and O₂ even though both are termolecular? has its answer in orientation, not in bond enthalpies.

The Maxwell–Boltzmann distribution is the second key idea. It is a curve that plots the number of molecules against their kinetic energy at a given temperature. The area under the curve represents the total number of molecules, and the small portion of the curve to the right of Eₐ represents the fraction of molecules with enough energy to react. The guide requires students to sketch the curve, label both axes, shade the area beyond Eₐ, and explain the effect of changing temperature or adding a catalyst. The standard SL question is use the Maxwell–Boltzmann distribution to explain why increasing temperature increases the rate of reaction. The complete answer needs three moves: state that more molecules now have energy ≥ Eₐ, identify the shaded area beyond Eₐ, and note that the area is larger not because the curve moves but because the curve flattens and shifts to the right.

How a catalyst appears in the distribution

When a catalyst is added, the Maxwell–Boltzmann curve is unchanged, but Eₐ on the x-axis moves to a lower value. The shaded area to the right of the new, lower Eₐ is larger. The catalyst is not consumed, the ΔH of the reaction is unchanged, and the position of equilibrium is unchanged. Candidates who write a catalyst lowers the activation energy, which means the reaction gives out more heat are conflating kinetics and thermodynamics in the exact way the rubric penalises. The correct phrasing is the catalyst provides an alternative pathway with a lower activation energy, so a greater fraction of molecules have sufficient energy to react at the same temperature.

Question types that recur in Paper 1 and Paper 2 on Reactivity 1

Across examination papers, Reactivity 1 questions cluster into four families. The first is the definitional family: a one-sentence stem in Paper 1 asking for the meaning of activation energy or entropy. These are low-weight but high-frequency, and they reward precise recall. The second is the calculation family: in Paper 2, a short-answer calculation on ΔH from Hess's law, or on ΔG from given ΔH and ΔS. The third is the graphical family: a Maxwell–Boltzmann sketch that the candidate must complete, or a reaction-profile diagram with Eₐ labelled for both catalysed and uncatalysed routes. The fourth is the extended response family: a 4–6 mark question asking the candidate to discuss the feasibility of a reaction using both thermodynamic and kinetic reasoning.

The extended-response family is the one that decides the boundary between a 6 and a 7, and it follows a predictable pattern. The stem usually names a specific reaction, gives the student the thermodynamic data, and asks for a discussion. A high-scoring answer runs through four moves: a thermodynamic feasibility judgement based on ΔG, a kinetic comment based on Eₐ, an explicit mention that the two are independent, and a concluding sentence that brings them together. The mark scheme typically allocates 2 marks for the thermodynamic half, 2 marks for the kinetic half, and 1 mark for the quality of the conclusion. Candidates who write a five-line answer that covers all four moves in plain language typically outscore candidates who write two paragraphs of dense prose but omit one of the moves.

Common pitfalls and how to avoid them

The Reactivity 1 guide is short, but it carries an unusually high density of vocabulary that is easy to confuse. The pitfalls below are the ones I see repeatedly in student work, and each has a concrete fix.

• Conflating feasibility with rate. Writing the reaction is feasible, so it will occur quickly is a mark-losing sentence. Feasibility is a thermodynamic statement (sign of ΔG), and rate is a kinetic statement (Eₐ and the distribution). The fix: keep the words thermodynamically feasible and kinetically viable visibly separate in your writing, even in rough notes.
• Forgetting to convert to kelvin. The temperature term in ΔG = ΔH − TΔS is in kelvin. A Paper 2 calculation that uses 298 in place of 298 K, or that leaves the unit off entirely, loses a method mark. The fix: in every calculation, write T = 298 K explicitly as a separate line before substituting.
• Drawing Maxwell–Boltzmann curves that touch the x-axis. The curve should approach the x-axis asymptotically on both sides, never touch it. The fix: in your sketch, leave a small visible gap between the curve and the x-axis on the high-energy tail.
• Attributing equilibrium shifts to a catalyst. A catalyst does not change the position of equilibrium, only the time taken to reach it. The fix: when discussing a catalyst, use the phrase shortens the time to reach equilibrium but does not affect the position verbatim; this is the wording the mark scheme tends to reward.
• Reading ΔS signs from ΔH signs. A reaction with a negative ΔH can have a positive ΔS (combustion of a hydrocarbon) and a reaction with a positive ΔH can have a positive ΔS (dissolution of ammonium nitrate in water). The fix: derive ΔS from the change in moles of gas, the change in state, and the change in the number of particles, not from ΔH.
• Ignoring the sign convention in ΔG. A negative ΔG means feasible; a positive ΔG means not feasible. Students sometimes invert the sign under time pressure. The fix: write the inequality first — for feasibility, ΔG < 0 — and only then plug in numbers.

One last tactical note. In Paper 2 calculations on ΔG, the units of ΔH are usually kJ mol⁻¹ and the units of ΔS are usually J K⁻¹ mol⁻¹. A candidate who substitutes without converting ΔS to kJ K⁻¹ mol⁻¹ will get a numerical answer three orders of magnitude out, and the method mark may not be awarded. The fix: always write the units beside every numerical substitution, and check them before you move to the next line.

Connecting Reactivity 1 to the rest of the IB Chemistry syllabus

The reactivity core is structured so that Reactivity 1 supplies the analytical vocabulary for Reactivity 2 and 3. In Reactivity 2, the same thermodynamic and kinetic language is used to discuss acid–base equilibria, redox feasibility, and the operation of electrochemical cells. In Reactivity 3, the same language is applied to organic reaction mechanisms, where the concept of activation energy becomes the explanation for why a primary halogenoalkane reacts faster than a tertiary one in an Sₙ2 pathway, and where the Maxwell–Boltzmann distribution is the explanation for why an increase in temperature accelerates substitution and elimination reactions in parallel. A candidate who has internalised the Reactivity 1 vocabulary will read Reactivity 2 and 3 as applications, not as new material, and this is the single biggest shift in study efficiency between a 5 and a 7.

What the Internal Assessment expects from Reactivity 1

The Internal Assessment is the third assessment component where Reactivity 1 appears, and it is often underestimated. A common IA topic is the effect of temperature on reaction rate, or the effect of concentration on the rate of the iodine clock reaction. The assessment criteria reward a candidate who explains the observed trend using collision theory and the Maxwell–Boltzmann distribution, and who explicitly discusses Eₐ and orientation. A candidate who records beautiful data and writes a procedure in clear English but never mentions Eₐ will land at the lower end of the analytical band, regardless of the quality of the data. The fix: when planning the IA, choose a topic that allows a kinetic or thermodynamic explanation, and draft the explanation paragraph before you run the experiment.

A short comparative read of the SL and HL expectations

The table below summarises the most common differences between SL and HL on Reactivity 1. It is not exhaustive, but it is the comparison that matters for question choice and time allocation in the examination.

Building a six-week Reactivity 1 preparation plan

For a candidate starting from cold, a six-week plan is the realistic window. Weeks 1 and 2 should be the thermodynamic strand: build a single A4 sheet that lists the standard enthalpy definitions on one side and the entropy rules on the other, then do a calculation set of around 30 Hess's law and ΔG problems. The aim is fluency, not novelty. Weeks 3 and 4 should be the kinetic strand: draw at least 20 Maxwell–Boltzmann curves by hand, with three temperature variants and a catalyst variant, and rehearse the verbal explanation aloud. The act of speaking the explanation is the closest substitute for the exam room, where you cannot re-read your own paragraph. Week 5 should be the cross-over: a set of extended-response questions that mix thermodynamic and kinetic arguments, marked against the official rubric. Week 6 should be revision through past-paper questions on Reactivity 1 specifically, with the wrong answers logged in a separate column and re-attempted after a 48-hour gap.

For scoring, the most efficient gain is in the calculation mark for unit conversion in ΔG. A candidate who loses 1 mark on 8 out of 10 calculation questions is dropping a full band on the final grade, and the fix is a single repeated drill. The second most efficient gain is in the Maxwell–Boltzmann sketch mark, which is awarded for axis labels, the Eₐ marker, and the shaded area. Candidates who rehearse the sketch 10 times will produce a drawing that scores full marks in under 90 seconds in the exam room, which is a meaningful time saving in a 1-hour Paper 1. Together, these two drills close the gap between a 5 and a 6 for most candidates, and they free up cognitive load in the extended-response question for the analytical moves that decide the 6-to-7 boundary.

Conclusion and next steps

Reactivity 1 is the unit in which IB Chemistry tells you, in the most compact form, the language you will use for the rest of the course. The thermodynamic strand answers will the reaction happen? through ΔH, ΔS, and ΔG. The kinetic strand answers how fast will it happen? through collision theory, the Maxwell–Boltzmann distribution, and catalysis. The exam rewards candidates who keep these strands separate in their writing, who can do the ΔG calculation with correct units, and who can sketch and explain the Maxwell–Boltzmann curve in under two minutes. A focused six-week plan that drills calculation fluency and graphical rehearsal, then crosses into extended-response practice, is the most reliable path from a 5 to a 7 on this unit. IB Courses' IB Chemistry HL programme builds a per-paper error log that flags unit-conversion slips on ΔG and axis-label omissions on Maxwell–Boltzmann sketches, and turns a 7 target on Reactivity 1 into a concrete week-by-week preparation plan.

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