IB DP Chemistry Reactivity 3: Mechanisms of Chemical Change

This guide covers Reactivity 3 — What Are the Mechanisms of Chemical Change?, the largest single component in the whole DP Chemistry syllabus at 24 hours SL / 45 hours HL, as set out in the full syllabus guide, first assessment 2025.

Where this fits in the syllabus

Reactivity asks “why and how does matter change?”, but answering that question mechanistically requires the Structure strand’s models of what matter is built from. You cannot reason about electron-transfer (3.2) or electron-sharing (3.3, 3.4) mechanisms without a secure grip on electron configurations (Structure 1.3) and the ionic, covalent and metallic bonding models (Structure 2). A common revision mistake is treating Reactivity 3 as a stand-alone topic to memorise mechanism diagrams for, rather than as the point where the whole syllabus’s Structure content gets applied to explain real chemical change.

Syllabus coverage

IB DP CHEMISTRY — REACTIVITY 3: MECHANISMS OF CHEMICAL CHANGE

• 3.1 Proton transfer reactions — acid-base chemistry built on the Brønsted-Lowry definition (an acid is a proton donor, a base is a proton acceptor); conjugate acid-base pairs; strong versus weak acids and bases; neutralisation and amphiprotic species; at HL, extends to pOH, dissociation constants, titration curves, salt hydrolysis and buffer solutions
• 3.2 Electron transfer reactions — redox chemistry (oxidation is loss of electrons, reduction is gain — OIL RIG); oxidation states; half-equations; the reactivity series; electrochemical cells (voltaic, electrolytic, fuel cells) and electrochemical data for predicting spontaneity
• 3.3 Electron sharing reactions — radical chemistry: homolytic fission and free-radical substitution built from initiation, propagation and termination stages, shown using half-headed (single-barbed) arrows
• 3.4 Electron-pair sharing reactions — organic reaction mechanisms built on shared electron pairs: nucleophilic and electrophilic behaviour, Lewis acid-base reactions, coordination bonds and transition-element complex formation, shown using full curly arrows

How to approach it

For 3.1, keep two entirely separate scales distinct: concentrated versus dilute (how much acid or base is dissolved in a given volume) and strong versus weak (what proportion of dissolved molecules actually dissociate) — a solution can be a concentrated weak acid or a dilute strong acid, and conflating the two scales is a common error. For 3.2, always double-check which species loses electrons (oxidised) and which gains them (reduced) before assigning half-equations, and practise the standard rules for assigning oxidation states until they are automatic. For 3.3 and 3.4, the arrow convention is the single most exam-relevant distinction: half-headed arrows move a single electron (radical mechanisms, 3.3), while full curly arrows move an electron pair, starting at a bond or lone pair and pointing to where that pair ends up (3.4) — mixing up which arrow type belongs to which sub-topic is one of the most common Reactivity 3 errors. For 3.4 specifically, correctly identifying the functional group involved (drawing on Structure 3.2) always comes before predicting a mechanism, since the mechanism follows directly from what functional group is present.

Worked example: a redox half-equation pair

Zinc reacts with copper(II) sulfate solution.

Oxidation half-equation:  Zn -> Zn2+ + 2e-
                          (zinc loses electrons -- oxidised)

Reduction half-equation:  Cu2+ + 2e- -> Cu
                          (copper(II) ions gain electrons -- reduced)

Combine (electrons cancel):
                          Zn + Cu2+ -> Zn2+ + Cu

Separating the two half-equations before combining them is the standard method for balancing any redox equation, and is what allows the electron count to be checked and cancelled explicitly rather than balanced by inspection alone.

Distinguishing the three types of electrochemical cell

Within 3.2, keep the three named electrochemical cells precisely distinct rather than treating “electrochemical cell” as a single idea. A voltaic (galvanic) cell uses a spontaneous redox reaction to generate an electrical current – the reaction happens because it is energetically favourable, and the cell simply captures that energy as electricity. An electrolytic cell does the reverse: it uses an external electrical current to force a non-spontaneous redox reaction to occur (electrolysis), meaning energy must be supplied rather than released. A fuel cell generates a current directly and continuously from an external supply of fuel (such as hydrogen) and an oxidant, without storing the reactants internally the way a battery does. Electrochemical data (standard electrode or cell potentials) is used to predict spontaneity: a positive overall cell potential indicates a spontaneous reaction under standard conditions, which is the calculation that determines whether a described cell is behaving as a voltaic cell or would require external current to run as an electrolytic one.

Common mistakes

Confusing oxidation and reduction direction, or misassigning which species is oxidised and which is reduced. Drawing a curly arrow in a 3.4 mechanism from the wrong end (an atom or positive charge instead of an electron pair), or using a full curly arrow where a 3.3 radical mechanism requires a half-headed one. Treating strong and weak acids as interchangeable in pH calculations, when a weak acid’s degree of dissociation must be accounted for separately. Misidentifying nucleophiles and electrophiles in unfamiliar organic reactions — return to electron density: a nucleophile is electron-rich, an electrophile is electron-poor.

Quick revision checklist

• Keep the concentrated/dilute and strong/weak acid scales precisely distinct.
• Have the oxidation-state assignment rules completely automatic.
• Know which arrow type (half-headed vs full curly) belongs to which sub-topic (3.3 vs 3.4).
• Practise identifying nucleophiles and electrophiles by electron density before attempting a mechanism.
• Revise Reactivity 3 alongside Structure 1.3 and Structure 2, not in isolation.

Official syllabus

International Baccalaureate Organization, Chemistry guide, Diploma Programme, first assessment 2025 (published February 2023, updated May 2023) — copy consulted via a school-hosted mirror, not ibo.org. The same source is already cited by the full syllabus guide, which lists this component’s four numbered sub-topics directly from it. Verified 2026-09-06.