What happens in an acid–base reaction?

A proton is transferred from the acid to the base.

What is a conjugate acid–base pair?

Two species that differ by one H⁺: the acid and the base formed when it donates a proton.

pH = −log₁₀[H⁺], where [H⁺] is the hydrogen ion concentration in mol dm⁻³.

What is a strong acid?

An acid that is fully dissociated in aqueous solution.

An acid that is only partially dissociated in aqueous solution, so it is in equilibrium.

A weak acid is diluted 100 times. How does its pH change?

It rises by less than 2, because more of the acid dissociates as it is diluted.

Why is NH₄Cl solution acidic?

It is the salt of a strong acid and a weak base; NH₄⁺ donates protons to water.

Bronsted-Lowry acids and bases and pHEdexcel A-Level Chemistry: Flashcards

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Define a Brønsted–Lowry acid.

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Define a Brønsted–Lowry acid.: A proton donor.
Define a Brønsted–Lowry base.: A proton acceptor.
What happens in an acid–base reaction?: A proton is transferred from the acid to the base.
What is a conjugate acid–base pair?: Two species that differ by one H⁺: the acid and the base formed when it donates a proton.
Give the conjugate base of H₂SO₄ and the conjugate acid of NH₃.: HSO₄⁻ and NH₄⁺.
Define pH.: pH = −log₁₀[H⁺], where [H⁺] is the hydrogen ion concentration in mol dm⁻³.
How do you calculate [H⁺] from pH?: [H⁺] = $10^{-\mathrm{pH}}$ .
What is the pH of 0.0100 mol dm⁻³ HCl?: 2.00
What is [H⁺] when pH = 3.00?: 1.00 × 10⁻³ mol dm⁻³.
What is a strong acid?: An acid that is fully dissociated in aqueous solution.
What is a weak acid?: An acid that is only partially dissociated in aqueous solution, so it is in equilibrium.
A strong acid is diluted 100 times. How does its pH change?: It rises by exactly 2.
A weak acid is diluted 100 times. How does its pH change?: It rises by less than 2, because more of the acid dissociates as it is diluted.
Why is NH₄Cl solution acidic?: It is the salt of a strong acid and a weak base; NH₄⁺ donates protons to water.

Exam questions on Bronsted-Lowry acids and bases and pH

A student mixes aqueous ethanoic acid with aqueous ammonia: CH₃COOH(aq) + NH₃(aq) ⇌ CH₃COO⁻(aq) + NH₄⁺(aq).
Identify the two conjugate acid–base pairs in this equilibrium, stating which species in each pair is the acid and which is the base.2 marks
A school technician prepares solutions of strong monoprotic acids for a practical. All measurements are made at 298 K.
The technician dilutes 25.0 cm³ of 0.200 mol dm⁻³ hydrochloric acid to a final volume of 500 cm³. Calculate the pH of the diluted solution.2 marks
A student has equimolar 0.100 mol dm⁻³ solutions of nitric acid, a strong acid, and methanoic acid, a weak acid. At 298 K the pH of the nitric acid is 1.00 and the pH of the methanoic acid is 2.38.
Use the pH values to show that nitric acid is a strong acid and methanoic acid is a weak acid.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).