IB DP Chemistry Reactivity 3: Mechanisms of Chemical Change -- Practice Questions

These are original questions written for Marlbridge, in the style and at the standard of the examination. They are not reproduced past-paper questions – the IB holds copyright in its own papers. Use these alongside the official past papers available through your school or the IB store.

Related: Reactivity 3 revision notes and the IB DP Chemistry syllabus guide.

Section A

1. State the Bronsted-Lowry definition of an acid. [1]

2. Using OIL RIG, identify which species is oxidised and which is reduced in the reaction Zn + Cu2+ -> Zn2+ + Cu. [2]

3. State whether a nucleophile is electron-rich or electron-poor, and explain why. [2]

Section B

4. Ammonia reacts with hydrochloric acid: NH3 + HCl -> NH4+ + Cl-.

(a) Identify the conjugate acid-base pair on each side of the equation. [2] (b) Explain why this reaction can be classified as a Bronsted-Lowry acid-base reaction. [2]

5. A curly arrow mechanism shows a bromide ion attacking a carbon atom bonded to a leaving group.

(a) State what a curly arrow represents. [1] (b) Explain why the bromide ion in this reaction is acting as a nucleophile. [2] (c) State one property of a good leaving group. [1]

6. A student is asked to distinguish ethanoic acid (a weak acid) from hydrochloric acid (a strong acid) of the same concentration.

(a) Explain, in terms of dissociation, why the two acids have different pH values despite equal concentration. [2] (b) Suggest one experimental method that could distinguish them. [1]

Section C

7. The reaction between propene and hydrogen bromide is an example of an electrophilic addition mechanism.

(a) Identify which species in this reaction acts as the electrophile. [1] (b) Explain, with reference to electron density, why this species behaves as an electrophile. [2] (c) Describe how Structure 2’s covalent bonding model is needed to understand this mechanism. [2] (d) Explain why treating Reactivity 3 mechanisms as a stand-alone topic, without reference to Structure content, is a common revision mistake. [2]

Worked answers

1. An acid is a proton (H+) donor. [1]

2. Zn is oxidised (loses electrons, Zn -> Zn2+ + 2e-); Cu2+ is reduced (gains electrons, Cu2+ + 2e- -> Cu). [1] for each correct identification.

3. A nucleophile is electron-rich. [1] It donates an electron pair (often from a lone pair or negative charge) to form a new bond with an electron-poor species. [1]

4. (a) NH3/NH4+ is one conjugate pair (base/its conjugate acid); HCl/Cl- is the other (acid/its conjugate base). [2] (b) HCl donates a proton to NH3, which accepts it – exactly matching the Bronsted-Lowry proton-donor/proton-acceptor definitions of acid and base. [2]

5. (a) It represents the movement of a pair of electrons, starting at an electron pair (a bond or lone pair) and pointing to where that pair ends up. [1] (b) The bromide ion donates an electron pair (from its negative charge / lone pair) to form a new bond with the carbon atom, which is the defining behaviour of a nucleophile. [2] (c) A good leaving group is able to stabilise the negative charge (or accept the electron pair) once it departs, e.g. it is a weak base / the conjugate base of a strong acid. [1]

6. (a) Hydrochloric acid dissociates essentially completely in water, releasing the maximum possible concentration of H+ ions; ethanoic acid only partially dissociates, so at equal starting concentration it releases far fewer H+ ions, giving a higher pH (less acidic) than the strong acid. [2] (b) Any valid method, e.g. measuring pH with a probe (the strong acid will show a lower pH), or reacting equal volumes with a reactive metal and comparing the rate of gas production (faster for the strong acid). [1]

7. (a) The electrophile is the hydrogen atom of the H-Br molecule (specifically, the H+ end, since Br is more electronegative and polarises the H-Br bond). [1] (b) The H atom is electron-poor (partially positively charged, delta+) because the more electronegative bromine draws the bonding electron density towards itself, leaving the hydrogen able to accept an electron pair from the alkene’s pi bond. [2] (c) Structure 2’s covalent bonding model explains that the C=C double bond in propene consists of a sigma bond and a pi bond, and it is specifically the exposed, higher-energy pi-bond electrons that are available to attack the electrophile – without this structural picture, the mechanism’s starting point cannot be justified. [2] (d) Because the mechanism itself is only a description of how electrons move; predicting which species will attack and why requires already knowing, from Structure 2, which electrons in a molecule are exposed and reactive (such as pi-bond or lone-pair electrons) versus tightly held sigma-bond electrons – so mechanism diagrams memorised without that structural foundation break down on unfamiliar molecules. [2]

How this set is weighted

The four questions worth the most marks (5, 6, 7) are deliberately spread across all four Reactivity 3 sub-topics – electron-pair sharing (5), proton transfer (6), and electron-pair sharing again in a more demanding electrophilic-addition context (7) – because that sub-topic (3.4) is, as the revision notes explain, the syllabus’s most mechanism-heavy component and the one examiners return to most often at Paper 2. Question 7 in particular is written at the level of difficulty a genuine Paper 2 long-answer question would use, requiring the same cross-reference to Structure 2 that the revision notes flag as the single most common source of lost marks in this component.

Official syllabus

International Baccalaureate Organization, Diploma Programme Subject Brief – Sciences: Chemistry, first assessment 2025, published January 2022 – the same source cited by the Reactivity 3 revision notes.