How does a base accept a proton?

It uses a lone pair of electrons to form a dative covalent bond to H⁺.

Why is ammonia a base even though it contains no OH⁻?

It accepts a proton (using its nitrogen lone pair) to form NH₄⁺.

What is the conjugate base of an acid?

The species formed when the acid donates a proton.

What is the conjugate acid of a base?

The species formed when the base accepts a proton.

Name the conjugate acid–base pairs in NH₃ + H₂O ⇌ NH₄⁺ + OH⁻.

NH₄⁺/NH₃ and H₂O/OH⁻.

What do acid–base equilibria involve?

Transfer of protons in both the forward and reverse directions.

In HCl + H₂O → H₃O⁺ + Cl⁻, what role does water play?

It is the base, because it accepts a proton from HCl.

Does an acid–base reaction need water?

No. HCl(g) + NH₃(g) → NH₄Cl(s) is a proton transfer with no water present.

Bronsted-Lowry acids and basesAQA A-Level Chemistry: Flashcards

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What is a Brønsted–Lowry acid?

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What is a Brønsted–Lowry acid?: A proton (H⁺) donor.
What is a Brønsted–Lowry base?: A proton (H⁺) acceptor.
What is a proton in acid–base chemistry?: A hydrogen ion, H⁺.
What ion forms when a proton joins a water molecule?: The oxonium ion, H₃O⁺.
How does a base accept a proton?: It uses a lone pair of electrons to form a dative covalent bond to H⁺.
Why is ammonia a base even though it contains no OH⁻?: It accepts a proton (using its nitrogen lone pair) to form NH₄⁺.
What is the conjugate base of an acid?: The species formed when the acid donates a proton.
What is the conjugate acid of a base?: The species formed when the base accepts a proton.
Name the conjugate acid–base pairs in NH₃ + H₂O ⇌ NH₄⁺ + OH⁻.: NH₄⁺/NH₃ and H₂O/OH⁻.
What do acid–base equilibria involve?: Transfer of protons in both the forward and reverse directions.
In HCl + H₂O → H₃O⁺ + Cl⁻, what role does water play?: It is the base, because it accepts a proton from HCl.
Does an acid–base reaction need water?: No. HCl(g) + NH₃(g) → NH₄Cl(s) is a proton transfer with no water present.

Exam questions on Bronsted-Lowry acids and bases

Ammonia gas dissolves in water to form a solution that turns red litmus blue. The equilibrium in the solution can be written as NH₃ + H₂O ⇌ NH₄⁺ + OH⁻.
Explain, in terms of proton transfer, why ammonia acts as a base in water.2 marks
Sodium hydrogencarbonate is added to water. The hydrogencarbonate ion, HCO₃⁻, reacts with acids and with alkalis according to the equations: (1) HCO₃⁻ + H₃O⁺ → H₂CO₃ + H₂O and (2) HCO₃⁻ + OH⁻ → CO₃²⁻ + H₂O.
Use the two equations to explain why the hydrogencarbonate ion can act as both an acid and a base.2 marks
Two acid–base reactions are studied. Reaction 1: hydrogen chloride gas and ammonia gas meet in a dry tube and form a white smoke of solid ammonium chloride, HCl(g) + NH₃(g) → NH₄Cl(s). Reaction 2: in the nitrating mixture used to make nitrobenzene, concentrated nitric acid is mixed with concentrated sulfuric acid, HNO₃ + H₂SO₄ ⇌ H₂NO₃⁺ + HSO₄⁻.
Explain, in terms of proton transfer, why Reaction 1 is an acid–base reaction, identifying the acid and the base.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).