What does every acid–base reaction involve?

Proton transfer from the acid to the base.

What is a conjugate pair?

An acid and its conjugate base (or a base and its conjugate acid), differing by one proton.

Identify the conjugate pairs in NH₃ + H₂O ⇌ NH₄⁺ + OH⁻.

NH₃ (base)/NH₄⁺ (conjugate acid) and H₂O (acid)/OH⁻ (conjugate base).

pH = −log₁₀[H⁺], with [H⁺] in mol dm⁻³.

What is a strong acid?

An acid that is fully dissociated in aqueous solution, e.g. HCl.

An acid that is only partially dissociated, so it sets up an equilibrium, e.g. CH₃COOH.

Calculate the pH of 0.0250 mol dm⁻³ HNO₃.

HNO₃ is strong, so [H⁺] = 0.0250 and pH = 1.60.

Why do 0.100 mol dm⁻³ HCl and ethanoic acid have different pH?

HCl is fully dissociated, ethanoic acid only partially, so its [H⁺] is lower and its pH higher.

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

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Define a Brønsted–Lowry acid.

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Define a Brønsted–Lowry acid.: A proton (H⁺) donor.
Define a Brønsted–Lowry base.: A proton (H⁺) acceptor.
What does every acid–base reaction involve?: Proton transfer from the acid to the base.
What is a conjugate pair?: An acid and its conjugate base (or a base and its conjugate acid), differing by one proton.
Name the conjugate base of H₂SO₄ and the conjugate acid of NH₃.: HSO₄⁻ and NH₄⁺.
Identify the conjugate pairs in NH₃ + H₂O ⇌ NH₄⁺ + OH⁻.: NH₃ (base)/NH₄⁺ (conjugate acid) and H₂O (acid)/OH⁻ (conjugate base).
Define pH.: pH = −log₁₀[H⁺], with [H⁺] in mol dm⁻³.
How do you calculate [H⁺] from pH?: [H⁺] = 10⁻ᵖᴴ.
What change in [H⁺] corresponds to a drop of 1 pH unit?: [H⁺] increases tenfold.
What is a strong acid?: An acid that is fully dissociated in aqueous solution, e.g. HCl.
What is a weak acid?: An acid that is only partially dissociated, so it sets up an equilibrium, e.g. CH₃COOH.
Calculate the pH of 0.0250 mol dm⁻³ HNO₃.: HNO₃ is strong, so [H⁺] = 0.0250 and pH = 1.60.
Why do 0.100 mol dm⁻³ HCl and ethanoic acid have different pH?: HCl is fully dissociated, ethanoic acid only partially, so its [H⁺] is lower and its pH higher.

Exam questions on Bronsted-Lowry acids and bases and pH

Ethanoic acid dissolves in water and reaches the equilibrium CH₃COOH + H₂O ⇌ CH₃COO⁻ + H₃O⁺.
Identify the two conjugate acid–base pairs in this equilibrium.2 marks
A swimming-pool technician lowers the pH of pool water by dosing it with dilute hydrochloric acid. Treat hydrochloric acid as fully dissociated, and assume all solutions are at 25 °C.
A technician dilutes 25.0 cm³ of 0.0800 mol dm⁻³ hydrochloric acid with water to a total volume of 1.00 dm³. Calculate the pH of the diluted solution.2 marks
A student compares two acid solutions at 25 °C, each of concentration 0.100 mol dm⁻³: hydrochloric acid and ethanoic acid. The pH of the ethanoic acid solution is measured as 2.88.
Calculate the pH of the hydrochloric acid and explain why the ethanoic acid has a higher pH at the same concentration.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).