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ASChemistryCambridge (CIE)Revision Notes7. EquilibriaBrønsted–Lowry Theory of Acids & BasesBrønsted–Lowry Theory

Brønsted–Lowry Theory (Cambridge (CIE) AS Chemistry): Revision Note

Exam code: 9701

Richard Boole

Last updated

Brønsted–Lowry Theory

  • The Brønsted-Lowry Theory defines acids and bases in terms of proton transfer between chemical compounds

  • A Brønsted-Lowry acid is a species that gives away a proton (H+)

  • A Brønsted-Lowry base is a species that accepts a proton (H+) using its lone pair of electrons

How an acid acts as a Brønsted-Lowry proton donor

Diagram illustrating Brønsted-Lowry acid-base reaction: Proton (H+) transfer from acid (HA) to base (B), forming conjugate base (A-) and conjugate acid (HB+).
The diagram shows a Brønsted-Lowry acid which donates the proton to the Brønsted-Lowry base that accepts the proton using its lone pair of electrons

  • Species that can act both as acids and bases are called amphoteric

    • Eg. water as a Brønsted-Lowry acid

Water acting as a Brønsted-Lowry acid

Chemical equation of water and ammonia as Brønsted-Lowry acid and base, forming hydroxide ion and ammonium ion.
The diagram shows water acting as a Brønsted-Lowry acid by donating a proton to ammonia which accepts the proton using its lone pair of electrons

Dot and cross diagram showing the Brønsted-Lowry behaviour of water with ammonia

Diagram showing water and ammonia molecules reacting. Water forms OH⁻ with extra electron, ammonia forms NH₄⁺, illustrating electron transfer process.
The diagram shows a dot & cross diagram for the reaction of water with ammonia to show how water acts as a Brønsted-Lowry acid and ammonia as a Brønsted-Lowry base

  • E.g. water as a Brønsted-Lowry base

Water acting as a Brønsted-Lowry base

Chemical equation showing hydrochloric acid as a Brønsted-Lowry acid reacting with water, a Brønsted-Lowry base, forming chloride ions and hydronium ions.
The diagram shows water acting as a Brønsted-Lowry base by accepting a proton from hydrochloric acid proton using its lone pair of electrons

Dot and cross diagram showing the Brønsted-Lowry behaviour of water with hydrochloric acid

Diagram showing hydrochloric acid and water reacting. HCl donates H⁺, forming chloride ion and hydronium ion with water's oxygen's electrons.
The diagram shows a dot & cross diagram for the reaction of water with hydrochloric acid to show how water acts as a Brønsted-Lowry base and ammonia as a Brønsted-Lowry acid

  • The Brønsted-Lowry Theory is not limited to aqueous solutions only and can also be applied to reactions that occur in the gas phase

A Brønsted-Lowry acid and base reaction

Chemical equation showing HCl and NH3 gases forming solid NH4Cl, with HCl as a Brønsted-Lowry acid and NH3 as a Brønsted-Lowry base.
HCl acts as a Brønsted-Lowry acid by donating a proton while ammonia acts as a Brønsted-Lowry base by accepting a proton

Examiner Tips and Tricks

  • An atom of hydrogen contains 1 proton, 1 electron and 0 neutrons.

  • When hydrogen loses an electron to become H+ only a proton remains, which is why a H+ ion is also called a proton.

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Richard Boole

Author: Richard Boole

Expertise: Chemistry Content Creator

Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.