How to score a 7 on IB Chemistry Reactivity 3.1 weak-acid calculations without memorising pKa
IB Chemistry Reactivity 3.1 proton transfer reactions: Brønsted–Lowry, Lewis, Ka, pH curves, and the conjugate-pair writing that decides a 6 from a 7.
Reactivity 3.1 sits at the front of the IB Chemistry acid–base block and frames every later topic in Reactivity 3. The sub-topic tests whether you can identify proton donors and acceptors, write the conjugate pair, choose between Brønsted–Lowry and Lewis descriptions, and link qualitative acid strength to the position of the equilibrium arrow. Across Paper 1, Paper 2, and the Internal Assessment, this is also the sub-topic where most candidates drop one or two marks on what looks like easy content. A 7 in IB Chemistry Reactivity 3.1 depends on diagram discipline, conjugate-pair writing, and the habit of always quoting an acid–base model before naming the species.
What IB Chemistry Reactivity 3.1 actually tests on the exam
The first thing to fix in your head is the scope of the sub-topic. Reactivity 3.1 covers the language of acid–base chemistry: the Brønsted–Lowry model, the Lewis model, conjugate pairs, amphiprotic species, the position of the equilibrium in terms of relative acid/base strength, and the connection between structure (polarity of the H–X bond, electronegativity, charge density) and observed strength. Calculation work in the form of pH, Ka, pKa, and buffer composition belongs to Reactivity 3.2, but the conceptual vocabulary of 3.1 is what makes 3.2 answerable. If you cannot recognise a conjugate base on sight, you cannot label the components of a buffer, and you cannot justify a pH curve shape in Paper 2.
Reactivity 3.1 also acts as a gatekeeper for higher-level understanding of organic chemistry in Structure 3 and for electrochemistry in Reactivity 3.4. The IB frequently tests acid–base ideas on functional groups: the acidic hydrogen on a carboxylic acid, the basic nitrogen on an amine, the zwitterion on an amino acid. A question that begins with “explain why phenol is acidic” looks like a Structure 3 question, but the mark scheme routes the answer through a conjugate-base stability argument that lives inside 3.1. For most candidates reading this, the single highest-value move is to learn the definitions cold, then rehearse them in writing until the answer shape is automatic.
On Paper 1, Reactivity 3.1 appears as one- or two-mark conceptual items: identifying the Brønsted–Lowry acid in an equation, picking the strongest base from a list of conjugate bases, choosing the correct conjugate acid of a polyatomic anion, recognising whether a species is amphiprotic. On Paper 2, the sub-topic feeds the long structured question that links acid strength to structure, often with a small calculation attached. The IA rarely features Reactivity 3.1 in isolation, but a titration-based Internal Assessment will lose marks if the introduction confuses conjugate acid and conjugate base.
The Brønsted–Lowry model: proton donor, proton acceptor, and the conjugate pair you must draw
The IB assesses Brønsted–Lowry as a paired definition. An acid is a proton donor; a base is a proton acceptor. The acid donates H⁺ to form its conjugate base, and the base accepts H⁺ to form its conjugate acid. The two pairs are linked by the proton transfer, which is the only thing the model tracks. If a candidate writes “NH₃ is a base” and stops, the rubric will not award the second mark, because the second mark is reserved for explicitly tying the definition to a proton transfer or a conjugate pair.
Worked example: in the reaction of ammonia with water, NH₃ accepts a proton from H₂O to form NH₄⁺ (the conjugate acid of NH₃), and H₂O donates a proton to form OH⁻ (the conjugate base of H₂O). A top-band answer identifies ammonia as the Brønsted–Lowry base, water as the Brønsted–Lowry acid, NH₄⁺ as the conjugate acid of NH₃, and OH⁻ as the conjugate base of H₂O. The mark scheme also accepts a reverse argument in which water is the base and ammonia the acid, but only if the role is consistent throughout. Examiners penalise “mixed” answers in which the role of one species is correctly identified and the other is not, because the rubric treats the definition as a logical chain.
The conjugate pair itself must be drawn with a single proton difference. A common error is to write the conjugate base of H₂SO₄ as SO₄²⁻ and skip the intermediate HSO₄⁻. The IB expects you to track the proton one step at a time, especially for diprotic and triprotic acids. For phosphoric acid, the three conjugate bases are H₂PO₄⁻, HPO₄²⁻, and PO₄³⁻, and a question may show any of them. The skill to drill is: given any species in the acid–base block, produce the species that is exactly one proton different, with the charge adjusted by +1 for the conjugate acid or −1 for the conjugate base. In my experience this is the calculation that most candidates skip practising, which is why they then struggle to read pH curves in 3.2.
A further trap appears with amphiprotic species such as HCO₃⁻, HSO₄⁻, and H₂O. The IB wants the candidate to recognise that such species can act as either a Brønsted–Lowry acid or base depending on the partner. The mark scheme does not award full credit for calling the species “amphiprotic” and walking away; the rubric requires a paired example. So in answer, state what HCO₃⁻ does with a strong acid (donates a proton to form H₂CO₃) and what it does with a strong base (accepts a proton to form CO₃²⁻). Two examples, one in each direction, is the pattern that converts a level 4 answer into a level 6.
The Lewis model: when to use it instead of Brønsted–Lowry
The Lewis model defines an acid as an electron-pair acceptor and a base as an electron-pair donor. For the IB, the two models are not interchangeable: Brønsted–Lowry is the default for aqueous proton transfer, and Lewis is the model of choice when no proton is moving, or when a species is acidic without donating a proton. Boron trifluoride reacting with ammonia is the classic example: BF₃ accepts a lone pair from NH₃ even though no H⁺ moves. The IB will mark you down for calling BF₃ a Brønsted–Lowry acid because the question stem will say “no proton transfer occurs”.
A second context where Lewis dominates is metal–ligand coordination. The hydrated aluminium ion, [Al(H₂O)₆]³⁺, behaves as an acid in water because the water ligands donate a lone pair to the small, highly charged Al³⁺ centre, polarising the O–H bond and allowing proton release. A Paper 2 question may show a hydrolysis equation of this form and ask for the role of the metal complex. The model answer says it is a Lewis acid; the O in the water is acting as a Lewis base donating into the empty d orbital of aluminium. For most IB candidates, the ability to switch between models on the same species is what separates a level 6 from a level 7 on a long structured question.
Lewis bases are also a useful frame for nitrogen in amines and oxygen in alcohols and ethers. A common exam item gives the structure of an organic molecule and asks which atom is most basic. The top-band answer is: identify the atom with the available lone pair (often N or O), then justify the choice by referring to the lone pair acting as a Lewis base toward a proton. Just stating “it is a base because it has a lone pair” is worth only one mark; the second mark lives in the explicit connection between the lone pair and a Brønsted–Lowry or Lewis acid partner.
Conjugate acid–base strength and the position of the equilibrium
This is the conceptual heart of Reactivity 3.1 and the place where most candidates lose marks. The rule is: the stronger the acid, the weaker its conjugate base, and the equilibrium always favours the side with the weaker acid and weaker base. The IB expects you to apply this rule to a written equation, and to justify the position of the equilibrium arrow in words, not just to draw the arrow in the right direction.
Worked example: HCl is a stronger acid than CH₃COOH, and Cl⁻ is a weaker base than CH₃COO⁻. When HCl is mixed with sodium acetate, the proton transfers from HCl to CH₃COO⁻, giving CH₃COOH and Cl⁻. The equilibrium lies far to the right. A 7-mark answer writes: “HCl is the stronger Brønsted–Lowry acid, so the proton transfer goes from HCl to CH₃COO⁻; the products (CH₃COOH and Cl⁻) contain the weaker acid and weaker base, so the position of equilibrium favours the right-hand side.” A 4-mark answer writes the equation and points the arrow. The three extra marks live in the explicit comparison of acid strength and the use of the word “weaker” twice.
For polyprotic systems, the rule applies step by step. Sulphuric acid is strong in its first dissociation (HSO₄⁻ is a weak acid, comparable to a moderately strong carboxylic acid), and the second dissociation only proceeds to a limited extent. A Paper 2 question may give a list of Ka values and ask you to rank the strength of the conjugate bases. The skill to drill is: given Ka, write the stronger acid first, then write the conjugate base of that stronger acid on the right, because the weaker conjugate base sits on the product side of the favoured equilibrium.
Structural factors that determine acid strength
Reactivity 3.1 also asks you to justify acid strength in terms of structure. The IB markscheme requires a causal argument, not a list. The argument is built from three structural ideas: the polarity of the H–X bond, the strength of the H–X bond, and the stability of the conjugate base. For most candidates reading this, the third idea is the one that wins marks on long questions, because examiners can ask “why is HCl a stronger acid than HF?” and expect an answer that says H–Cl is weaker than H–F and Cl⁻ is larger and more able to delocalise the negative charge than F⁻.
The argument scales: across a period, increasing electronegativity of X (going from CH₄ to HF, for example) makes the H–X bond more polar and so more easily ionised, which makes the acid stronger. Down a group, bond strength dominates, and the larger atoms form weaker H–X bonds that break more easily, making HBr a stronger acid than HCl, and HI stronger still. For oxoacids, the rule is: more electronegative central atoms and more terminal O atoms withdraw electron density from the O–H bond, making the acid stronger. H₂SO₄ is stronger than H₂SO₃ in part because the extra oxygen stabilises the conjugate base through delocalisation.
For organic acids, the IB uses three ideas: inductive effects, resonance, and hybridisation. The carboxylate ion is stabilised by resonance across two equivalent oxygens, which is why carboxylic acids are much stronger than simple alcohols. Phenol is weakly acidic because the phenoxide ion is stabilised by resonance into the ring, although the negative charge is held on carbon, which is less electronegative than oxygen; this is the reason the rubric gives only one or two marks for a brief answer and three or four for a full resonance sketch. If you're making this mistake right now, swap the verbal answer for a diagram showing the three or four resonance forms of phenoxide; that single move is worth a full band on the long Paper 2 question.
Common pitfalls and how to avoid them in Reactivity 3.1
Across the past two examination cycles, four pitfalls repeat on the Reactivity 3.1 question. The first is mismatched charge bookkeeping on a conjugate pair. Candidates write HSO₄⁻ on the left and SO₄²⁻ on the right as the conjugate base, forgetting that the proton has left; the rubric requires the charge to drop by one. The second is forgetting that water is amphiprotic, so a question asking for the conjugate base of water has two correct answers (OH⁻ when water donates, H₃O⁺ when water accepts) and examiners will not accept a “water is amphiprotic, so no single answer” line as an answer.
The third pitfall is applying Brønsted–Lowry in a non-proton context. A species can accept a lone pair (Lewis) without any proton moving, and the rubric requires the model to match the chemistry. The fourth is ranking strength using pKa in the wrong direction; lower pKa means stronger acid, and the IB frequently writes the numbers without direction hints, expecting the candidate to translate. A short list that converts a 5 into a 7 in this sub-topic looks like this:
- Always state the acid–base model you are using in the first line of the answer, and keep it consistent.
- Always show the conjugate pair on the same side of the equation, with the proton difference visible.
- Always justify the position of the equilibrium with the phrase “weaker acid and weaker base on the right”.
- Always draw at least one resonance form when the conjugate base is delocalised.
- Always write the unit of pKa or Ka when you quote it; the rubric marks the unit.
Comparing the acid–base models the IB tests
The three models you must keep separate are summarised in the table below. Examiners switch between them deliberately to test whether you recognise which model a question wants. A common Paper 2 stem will say “without invoking a proton, explain the acidity of the species”, and the only way to score the marks is to invoke the Lewis definition.
| Model | Acid definition | Base definition | When the IB uses it |
|---|---|---|---|
| Brønsted–Lowry | Proton (H⁺) donor | Proton (H⁺) acceptor | Default for aqueous proton transfer, conjugate pairs, amphiprotic species |
| Lewis | Electron-pair acceptor | Electron-pair donor | Reactions with no proton transfer; metal–ligand coordination; BF₃ + NH₃ |
How Reactivity 3.1 sets up the rest of the IB Chemistry block
Reactivity 3.1 is the vocabulary the rest of Reactivity 3 borrows. In 3.2 you write the Ka expression and the pH calculation, but the conjugate acid and conjugate base come from 3.1. In 3.3 you explain buffer composition in terms of a weak acid and its conjugate base, both identified by 3.1 logic. In 3.4 you connect acid strength to standard electrode potential, and the link only works if you can recognise the protonated and deprotonated forms. Outside the Reactivity block, Structure 3.2 organic chemistry leans on 3.1 for the acidic and basic functional groups, and the IB often pairs the two in a single long question worth 8 to 10 marks.
The Internal Assessment, especially any titration-based study, opens with a short paragraph on the theory. A 7-band introduction names the acid and base, gives the equation, and identifies the conjugate pair. A 4-band introduction writes a sentence on “how titrations work” with no model and no equation. The IA rubric marks the introduction as part of the “personal engagement” and “exploration” criteria, and the mark is doubled when the chemistry is precise. Treat the IA as a quiet place to rehearse 3.1 language until it sounds automatic, and the Paper 2 long answer will inherit that fluency.
Concrete preparation strategy for Reactivity 3.1
For most IB candidates, three habits convert this sub-topic from a knowledge dump into scored marks. First, rehearse the six core definitions in writing, not just in your head: Brønsted–Lowry acid, Brønsted–Lowry base, conjugate acid, conjugate base, Lewis acid, Lewis base. Aim for a one-sentence definition of each, with an example, on a single index card. Pulling the card out before a study session for 60 seconds each day is enough to keep the words crisp for the exam.
Second, work through a minimum of ten conjugate-pair exercises per week during the Reactivity 3 teaching block. For each exercise, given a species, write its conjugate acid on one line and its conjugate base on a second line, with charges balanced. A 30-second check that you have one proton difference and a charge change of +1 or −1 catches the most common mark loss. For a level-7 answer, also write the role the species plays in two contrasting reactions: one as a Brønsted–Lowry acid, one as a Brønsted–Lowry base. The two examples lock in the amphiprotic idea.
Third, link every acid–base explanation to a structural feature. When you write “HCl is a stronger acid than HF”, follow with a structural reason: the H–Cl bond is weaker than the H–F bond, and Cl⁻ is larger so the negative charge is more delocalised. When you write “H₂SO₄ is a stronger acid than H₂SO₃”, the reason is the extra terminal oxygen which withdraws electron density and stabilises the conjugate base. The IB mark scheme allocates at least one mark to the structural justification in almost every long question on acid strength. For most candidates, the ability to give the structural reason in one sentence is the difference between a level 5 and a level 7.
What the IB examiner reward on a long Reactivity 3.1 answer
The rubric is built from three marks per claim, on average, and the top band is reached only when all three components appear: model identification, conjugate pair, and structural or equilibrium justification. A typical 7-mark Paper 2 question on a strong–weak acid mixture will mark roughly as follows: one mark for naming the acid–base model, one mark for the conjugate pair, one mark for the equation, one mark for the direction of the equilibrium, one mark for the strength comparison in words, one mark for the structural justification, one mark for the final conclusion. Missing any one of those drops the answer to a 4 or 5 out of 7. For most candidates I work with, the missing mark is the structural justification, because they assume the examiner “knows” the structure and skips writing it down.
Final tactical tip: at the end of every Reactivity 3.1 answer, write one sentence that says “the position of equilibrium favours the side with the weaker acid and weaker base”, and underline “weaker” once. Examiners actively scan for that phrase in long answers, and the rubric lists it as a top-band indicator. The same idea in 3.2 becomes the justification of buffer pH, and in 3.4 it is the explanation of why some redox equilibria are pH-dependent. One phrase, threaded through the whole block, is the cheapest mark you will ever pick up.
Conclusion and next steps
Reactivity 3.1 is a vocabulary sub-topic, but it is also the language the rest of the IB Chemistry block speaks. A 7-band answer in this sub-topic is one that names the model, draws the conjugate pair, justifies the position of the equilibrium in words, and ties the explanation back to structure. Rehearse the six core definitions, drill conjugate-pair writing, and pair every claim of strength with a structural reason. In the next study cycle, move on to Reactivity 3.2 and apply the same vocabulary to pH and Ka calculations, where the marks come from the same writing habits you built in 3.1. IB Courses' one-to-one IB Chemistry programme drills conjugate-pair writing on every RL long question and uses a six-card Reactivity 3.1 deck so candidates can rehearse the six acid–base definitions under timed conditions before Paper 2.