Brønsted–Lowry acid–base theory
Acids donate protons; bases accept protons.
The Brønsted–Lowry theory, also known as the proton theory of acids and bases, was independently proposed in 1923 by physical chemists Johannes Nicolaus Brønsted in Denmark and Thomas Martin Lowry in the United Kingdom. It generalizes the earlier Arrhenius theory, which defined acids as substances that release hydrogen cations (H⁺) in water and bases as substances that release hydroxide ions (OH⁻). In the Brønsted–Lowry framework, acids and bases are defined by their mutual reaction: an acid is a proton donor, and a base is a proton acceptor. When they react, the acid forms its conjugate base, and the base forms its conjugate acid through the exchange of a proton (the hydrogen cation). This is represented by the equilibrium equation: acid + base ⇌ conjugate base + conjugate acid. The equilibrium sign indicates reversibility, and most such reactions are fast, leading to dynamic equilibrium.
A key feature is that the theory does not require an acid to dissociate, unlike the Arrhenius theory. Water is amphoteric, acting as either an acid or a base; for example, one water molecule can donate a proton to another, forming hydronium and hydroxide ions. In non-aqueous solvents, analogous reactions occur—liquid ammonia undergoes autoionization to yield ammonium and amide ions, with ammonium salts acting as acids and metal amides as bases. Solvents like dimethylsulfoxide (DMSO) and acetonitrile can accept protons, making some substances acidic in them that are not in water. Conversely, an acidic solvent like liquid hydrogen fluoride can make a dissolved substance more basic; acetic acid, for instance, behaves as a base in hydrogen fluoride. In the same year, G. N. Lewis proposed an alternative theory based on electron pairs, where a Lewis base donates an electron pair to a Lewis acid, explaining proton transfer as a Lewis acid–base reaction.
- field
- Physical chemistry
- known_for
- Brønsted–Lowry acid–base theory (proton theory of acids and bases)
- developed_in
- 1923
- developers
- Johannes Nicolaus Brønsted (Denmark) and Thomas Martin Lowry (United Kingdom)
Lore & Background
The Brønsted–Lowry theory was independently developed in 1923 by Johannes Nicolaus Brønsted in Denmark and Thomas Martin Lowry in the United Kingdom. It defines acids as proton donors and bases as proton acceptors, with reactions involving the transfer of a hydrogen cation (H⁺). When an acid donates a proton, it becomes its conjugate base; when a base accepts a proton, it becomes its conjugate acid. These reactions are reversible and typically reach dynamic equilibrium. This theory generalises the Arrhenius theory, which required acids to dissociate in water to produce H⁺ and bases to produce OH⁻. A key feature is that it does not require an acid to dissociate. Water is amphoteric, acting as either an acid or a base depending on the reaction. In non-aqueous solvents, analogous acid–base behavior occurs; for example, in liquid ammonia, the ammonium ion acts as an acid and the amide ion as a base. Some solvents, like dimethylsulfoxide, accept protons more strongly than water, making certain acids stronger in them. Conversely, acidic solvents like liquid hydrogen fluoride can make dissolved substances behave as bases. The theory contrasts with the Lewis acid–base theory, also proposed in 1923, which defines acids as electron-pair acceptors and bases as electron-pair donors, explaining proton transfer through electronic structure.
Reader's Guide
The Brønsted–Lowry theory broadened the understanding of acid–base reactions beyond aqueous solutions, as it does not require an acid to dissociate. It defines acids and bases by their role in proton transfer: an acid is a proton donor, a base is a proton acceptor. The theory is illustrated by equilibrium equations such as HA + B ⇌ A− + HB+, where HA is the acid, B is the base, A− is the conjugate base, and HB+ is the conjugate acid. Water is amphoteric, acting as either an acid or a base. The theory also applies to non-aqueous solutions, as seen with substances like aluminium hydroxide, which can act as an acid or base depending on the reaction. Most acid–base reactions are fast, with substances in dynamic equilibrium.
Did You Know?
- The theory was developed independently in 1923 by Johannes Nicolaus Brønsted in Denmark and Thomas Martin Lowry in the United Kingdom.
- In the Brønsted–Lowry theory, an acid forms its conjugate base and a base forms its conjugate acid by exchange of a proton (H+).
- Water is amphoteric, meaning it can act as either an acid or a base.
- The theory does not require an acid to dissociate, unlike the Arrhenius theory.
Frequently Asked Questions
Who is Brønsted–Lowry acid–base theory?
It is the proton theory of acids and bases, independently conceived in 1923 by Danish physical chemist Johannes Nicolaus Brønsted and British physical chemist Thomas Martin Lowry. It sits squarely in the field of physical chemistry and was designed to generalise the older Arrhenius framework.
What are Brønsted–Lowry acid–base theory's powers/role?
Its defining 'power' is re-casting every acid–base event as a proton (H⁺) transfer: the acid donates a proton while the base accepts one. Because the definition hinges on proton exchange rather than on water, it can describe reactions in organic solvents, gases, and other non-aqueous media.
How does Brønsted–Lowry acid–base theory's story end?
Rather than a dramatic finale, the theory became a permanent fixture in chemistry curricula worldwide and still serves as the standard introductory model for acid–base behaviour. Later frameworks such as Lewis acid–base theory extend its scope instead of replacing it, so the Brønsted–Lowry story effectively continues as a foundational layer.
Why is Brønsted–Lowry acid–base theory important?
It broadened the acid–base concept far beyond aqueous solutions by anchoring the definition to proton exchange instead of the simultaneous production of H⁺ and OH⁻ ions. That single shift made it possible to discuss acid–base chemistry in virtually any medium, which is why it remains the go-to model taught in every introductory chemistry course.
Who are the developers behind Brønsted–Lowry acid–base theory?
Johannes Nicolaus Brønsted in Denmark and Thomas Martin Lowry in the United Kingdom each published the proton-transfer definition independently in 1923. Because their work appeared in the same year, the community adopted both surnames, giving the theory its dual-name identity.
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