Introduction to Proton Transfer Reactions
Acids and bases are among the most fundamental concepts in chemistry, touching everything from the digestion of food to the chemistry of the oceans. The framework that best captures their behaviour at the chemical level is the Brønsted–Lowry theory, which focuses on the transfer of protons (H⁺ ions) between species.
Before diving in, it helps to know why we need a specific theory at all. The earlier Arrhenius model defined an acid as a substance that produces H⁺ ions in water, and a base as a substance that produces OH⁻ ions in water. While useful, this definition is restricted entirely to aqueous solutions. The Brønsted–Lowry approach is more powerful because it works in any solvent , and even in the gas phase.
Think of a proton (H⁺) as a hot potato being passed between players. The acid is the player who wants to get rid of it; the base is the player ready to catch it. Every acid–base reaction is simply a game of proton toss.
This subtopic builds from defining acids and bases, through conjugate pairs, pH calculations, and the behaviour of strong vs. weak species, up to neutralisation reactions and titration curves.
Brønsted–Lowry Acids and Bases
Brønsted–Lowry acid: A Brønsted–Lowry acid is any species that can donate a proton (H⁺) to another species.
Brønsted–Lowry base: A Brønsted–Lowry base is any species that can accept a proton (H⁺) from another species.
The theory was introduced in 1923 and is not restricted to aqueous (water-based) solutions, making it far more versatile than the earlier Arrhenius definition.
Key examples in aqueous solution:
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Hydrochloric acid donating a proton to water:
HCl is the acid (proton donor); H₂O is the base (proton acceptor). -
Ammonia accepting a proton from water:
NH₃ is the base (proton acceptor); H₂O is the acid (proton donor).
In IB Chemistry, both and are acceptable representations of the proton in aqueous solution. Strictly speaking, a free proton does not exist independently in water , it bonds immediately with a water molecule to form the hydronium ion, :
Bases vs. Alkalis , an important distinction:
Alkali: An alkali is a base that dissolves in water and produces OH⁻ ions in solution.
- A base is any species that accepts a proton (includes insoluble substances such as CuO).
- An alkali is a base that dissolves in water to produce OH⁻ ions.
For example, sodium hydroxide (NaOH) is both a base and an alkali because it dissolves readily and releases OH⁻. Ammonia (NH₃) is also soluble in water (it dissolves readily, ~900 g/L at 0°C) and produces OH⁻ in solution, so it is also considered an alkali , but only a weak one because the reaction is reversible and only partial:
The key distinction between a base and an alkali is not solubility alone , it is whether the species produces OH⁻ ions in aqueous solution. Insoluble metal oxides (e.g. CuO) are bases but not alkalis because they do not dissolve to give OH⁻. The degree to which a dissolved base produces OH⁻ determines whether it is a strong or weak alkali.
