IB DP Chemistry Reactivity 3: Mechanisms of Chemical Change -- Revision Notes

Reactivity 3 is the largest single component in the whole DP Chemistry syllabus – 24 hours at SL, 45 hours at HL – though not by a wide margin over the next largest, as set out in the full syllabus guide. These notes work through its four sub-topics with the cross-references to Structure 1 and Structure 2 that questions on this component routinely expect, alongside the subject overview and assessment revision notes already on the site.

Reactivity 3.1 – Proton transfer reactions

Acid-base chemistry, built on the Bronsted-Lowry definition: an acid is a proton (H+) donor, a base is a proton acceptor. Revise conjugate acid-base pairs (a pair differing by one proton, e.g. HCl and Cl-, or NH3 and NH4+) and the distinction between strong and weak acids/bases – strong acids dissociate essentially completely in water, weak acids only partially, which is why a weak acid’s pH cannot be calculated the same way as a strong acid’s from concentration alone. pH itself and the ionic product of water are standard level content, examinable at both levels. At HL, this extends to pOH, the acid and base dissociation constants and their logarithmic forms, the conjugate-pair relations that follow from them, titration curves, salt hydrolysis and buffer solutions, all of which build on the same proton-transfer logic applied quantitatively.

Also revise, at standard level: neutralisation, where an acid and a base react to form a salt and water (acid + base -> salt + water), and amphiprotic species – substances such as HCO3- or H2O that can act as either a proton donor or a proton acceptor depending on what they are reacting with. Note the distinction between a concentrated and a dilute solution (how much acid or base is dissolved in a given volume of water) and a strong and a weak acid or base (what proportion of the dissolved molecules actually dissociate) – these are two entirely separate scales, and a solution can be a concentrated weak acid or a dilute strong acid.

Reactivity 3.2 – Electron transfer reactions

Redox chemistry: oxidation is loss of electrons, reduction is gain of electrons (OIL RIG). Revise oxidation states as the exam-relevant tool for tracking electron transfer across a reaction, even in compounds without simple ionic charges. Rules for assigning oxidation states: an uncombined element is 0; a simple ion’s oxidation state equals its charge; hydrogen is +1 (except -1 in metal hydrides) and oxygen is -2 (except -1 in peroxides); the oxidation states in a compound sum to 0, and in a polyatomic ion they sum to the ion’s overall charge.

Half-equations separate the oxidation and reduction processes so they can be balanced individually before being combined into the full redox equation. Worked example: for the reaction between zinc and copper(II) sulfate, the oxidation half-equation is Zn -> Zn2+ + 2e-, and the reduction half-equation is Cu2+ + 2e- -> Cu; adding them (the electrons cancel) gives the overall equation Zn + Cu2+ -> Zn2+ + Cu.

Revise the reactivity series of metals, which ranks metals by how readily they lose electrons (are oxidised) and lets you predict whether a displacement reaction will occur. Electrochemical cells convert chemical energy to electrical energy or vice versa: a voltaic (galvanic) cell uses a spontaneous redox reaction to generate a current, an electrolytic cell uses an external current to force a non-spontaneous redox reaction (electrolysis), and a fuel cell generates a current directly from the continuous supply of a fuel (such as hydrogen) and an oxidant, without storing the reactants internally. Electrochemical data (standard electrode/cell potentials) is used to predict whether a redox reaction is spontaneous: a positive overall cell potential indicates a spontaneous reaction under standard conditions. This sub-topic connects directly to Structure 2’s ionic and metallic bonding models – electron transfer only makes sense once you can already picture electrons as things atoms genuinely gain, lose or share, which is exactly what Structure 2 establishes.

Reactivity 3.3 – Electron sharing reactions

Radical chemistry – this sub-topic covers homolytic fission, in which a covalent bond breaks so that each fragment keeps one electron, producing two radicals, and free-radical substitution built from initiation, propagation and termination stages. Because each step moves a single electron rather than a pair, these mechanisms are shown using half-headed (single-barbed) arrows, in contrast to the full curly arrows used in Reactivity 3.4. Revise by practising each stage of a free-radical substitution mechanism in order and being able to explain what happens at each one, since exam questions often ask for a named stage rather than the whole mechanism at once.

Reactivity 3.4 – Electron-pair sharing reactions

Organic reaction mechanisms built on shared electron pairs – the syllabus’s most mechanism-heavy sub-topic, covering nucleophilic and electrophilic behaviour in organic reactions, together with Lewis acid-base reactions, coordination bonds and the formation of transition-element complexes, which are also electron-pair-sharing processes. A nucleophile donates an electron pair to form a new bond; an electrophile accepts one. Revise by being able to identify, in a given organic reaction, which species is acting as the nucleophile and which as the electrophile, and to draw the curly-arrow mechanism showing electron-pair movement from the nucleophile to the electrophile. This is where Structure 3.2’s functional-group classification comes in directly – you cannot predict a mechanism without first correctly identifying the functional group involved.

Why this component draws on Structure so heavily

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.

Common exam pitfalls in Reactivity 3

• Confusing oxidation and reduction direction – always double check which species loses electrons (oxidised) and which gains them (reduced) before assigning half-equations.
• Drawing curly arrows from the wrong end in Reactivity 3.4 – a full curly arrow starts at an electron pair (a bond or lone pair) and points to where that electron pair ends up, never from an atom or a positive charge. This rule is specific to electron-pair mechanisms; Reactivity 3.3’s radical mechanisms use half-headed arrows that move a single electron instead.
• Treating strong and weak acids as interchangeable in pH calculations – a weak acid’s degree of dissociation must be accounted for; assuming complete dissociation for a weak acid gives an incorrect pH.
• Misidentifying nucleophiles and electrophiles in unfamiliar organic reactions – go back to electron density: a nucleophile is electron-rich (often has a lone pair or negative charge), an electrophile is electron-poor (often has a partial or full positive charge).

Self-test

1. State the Bronsted-Lowry definitions of an acid and a base.
2. What is a conjugate acid-base pair, and give one example.
3. Using OIL RIG, define oxidation and reduction in terms of electrons.
4. What does a full curly arrow represent in a Reactivity 3.4 mechanism, and where does it start and end? How does this differ from the arrows used in Reactivity 3.3?
5. Define nucleophile and electrophile in terms of electron density.
6. Explain, in one or two sentences, why Reactivity 3 mechanisms cannot be revised independently of Structure 1 and Structure 2.

Answers: 1. An acid is a proton (H+) donor; a base is a proton acceptor. 2. Two species differing by exactly one proton, e.g. HCl (acid) and Cl- (its conjugate base), or NH3 (base) and NH4+ (its conjugate acid). 3. Oxidation Is Loss (of electrons); Reduction Is Gain (of electrons). 4. It represents the movement of a pair of electrons; it starts at an electron pair (a bond or lone pair) and points to where that electron pair ends up (a new bond or a lone pair on an atom). Reactivity 3.3’s radical mechanisms instead use half-headed arrows, each moving a single electron. 5. A nucleophile is electron-rich and donates an electron pair to form a new bond; an electrophile is electron-poor and accepts an electron pair. 6. Because explaining electron-transfer and electron-sharing mechanisms mechanistically requires already understanding electron configurations (Structure 1.3) and the bonding models (Structure 2) that describe how and where electrons are arranged in the first place.

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.