Two species differing by one proton, e.g. CH₃COOH / CH₃COO⁻.
Amphiprotic
Can act as an acid or a base, e.g. H₂O, HCO₃⁻.
Strong acid
Fully dissociates in water (HCl, HNO₃, H₂SO₄); a weak acid only partially dissociates (CH₃COOH, H₂CO₃).
pH 7 is neutral at 298 K. A change of one pH unit is a tenfold change in [H⁺].
The full notes, worked example, interactive diagram and check questions for this subtopic are in the app.
Exam tip: To identify conjugate pairs, look for two species that differ by exactly one H⁺: one on each side of the equation.
R3.2Electron transfer reactions
Key terms
Oxidation
Loss of electrons, or an increase in oxidation state.
Reduction
Gain of electrons, or a decrease in oxidation state.
Oxidizing agent
A species that oxidizes another and is itself reduced.
Voltaic (galvanic) cell
Converts chemical energy from a spontaneous redox reaction into electrical energy.
Electrolytic cell
Uses electrical energy to drive a non-spontaneous redox reaction.
OIL RIG: oxidation is loss, reduction is gain (of electrons).
The full notes, worked example, interactive diagram and check questions for this subtopic are in the app.
Exam tip: Use oxidation states to spot redox: if one element's state rises and another's falls, it is a redox reaction.
R3.3Electron sharing reactions
Key terms
Radical
A species with an unpaired electron; very reactive.
Homolytic fission
Breaking of a covalent bond with one electron going to each atom, forming two radicals.
Substitution
A reaction in which one atom or group is replaced by another.
Alkanes are unreactive (strong, non-polar C–C and C–H bonds) but react with halogens in UV light by free-radical substitution.
The full notes, worked example and check questions for this subtopic are in the app.
Exam tip: In propagation steps, one radical goes in and one comes out; in termination, two go in and none come out.
R3.4Electron-pair sharing reactions
Key terms
Nucleophile
An electron-pair donor that is attracted to an electron-deficient centre, e.g. OH⁻, H₂O, NH₃.
Electrophile
An electron-pair acceptor that is attracted to an electron-rich centre, e.g. H⁺, Br⁺, NO₂⁺.
Heterolytic fission
Breaking of a covalent bond with both electrons going to one atom, forming ions.
Lewis acid / base
Electron-pair acceptor / electron-pair donor.
Nucleophilic substitution: halogenoalkanes react with hydroxide ions to form alcohols: CH₃CH₂Br + OH⁻ → CH₃CH₂OH + Br⁻. The δ+ carbon is attacked by the nucleophile.
The full notes, worked example and check questions for this subtopic are in the app.
Exam tip: Curly arrows must start exactly at a lone pair or a bond and end at an atom (or between atoms for a new bond).