AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
An acid is a proton donor.
Full specification wording
Brønsted–Lowry acid–base equilibria in aqueous solution (A-level only): An acid is a proton donor.
Brønsted and Lowry defined an acid as a proton donor.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 1 of 3
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Brønsted and Lowry defined an acid as a proton donor. A proton here means H⁺, a hydrogen atom that has lost its only electron, so a bare nucleus. In water that proton is never free: it bonds to a lone pair on a water molecule, which is why HCl + H₂O → H₃O⁺ + Cl⁻ is the honest equation and [H⁺] is a shorthand for [H₃O⁺]. What remains after the proton has gone is the conjugate base, Cl⁻ here. The definition says nothing about how much of the acid donates. A strong acid such as HNO₃ gives up essentially every proton; a weak acid such as CH₃COOH gives up only a small fraction and sits at an equilibrium. Strength is about that fraction; concentration is about how many moles are in a cubic decimetre.
What earns the marks
- Defining an acid as a proton donor or H⁺ donor.
- Identifying the species that loses the proton in a given equation.
- Naming the conjugate base as the particle remaining after the proton has been donated.
- Writing H₃O⁺ rather than H⁺ when the acid reacts with water.
Common mistakes and how to put them right
Defining an acid as a substance that produces hydrogen ions in solution, which is the Arrhenius idea rather than Brønsted–Lowry.
Calling an acid a 'hydrogen donor', which is refused because a hydrogen atom carries an electron.
Confusing strength with concentration, so concentrated ethanoic acid is described as a strong acid.
Giving the conjugate base of H₂SO₄ as SO₄²⁻ rather than HSO₄⁻.
Exam tips
- Write the word 'proton' in the definition; 'hydrogen ion' is usually accepted but 'hydrogen' alone is not.
- To find a conjugate base, remove exactly one H⁺ from the acid and reduce the charge by one.
Try the lesson
The start of the Brønsted–Lowry acid–base equilibria in aqueous solution (A-level only): An acid is a proton donor. lesson, made from checked teaching notes for this specification point.
Have a go first
In the Brønsted–Lowry model, which particle is transferred when an acid acts as a proton donor?
Listen
Brønsted and Lowry defined an acid as a proton donor. A proton here means H⁺, a hydrogen atom that has lost its only electron, so a bare nucleus. In water that proton is never free: it bonds to a lone pair on a water molecule, which is why HCl + H₂O → H₃O⁺ + Cl⁻ is the honest equation and [H⁺] is a shorthand for [H₃O⁺]. What remains after the proton has gone is the conjugate base, Cl⁻ here. The definition says nothing about how much of the acid donates. A strong acid such as HNO₃ gives up essentially every proton; a weak acid such as CH₃COOH gives up only a small fraction and sits at an equilibrium. Strength is about that fraction; concentration is about how many moles are in a cubic decimetre. A strong answer earns marks for these points. defining an acid as a proton donor or H⁺ donor. identifying the species that loses the proton in a given equation. naming the conjugate base as the particle remaining after the proton has been donated. writing H₃O⁺ rather than H⁺ when the acid reacts with water. Exam tip: Write the word 'proton' in the definition; 'hydrogen ion' is usually accepted but 'hydrogen' alone is not.
See it
Key words
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Brønsted–Lowry acid
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conjugate base
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strength versus concentration
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See every definition
- Brønsted–Lowry acid
- A proton donor, where a proton means H⁺, a hydrogen atom that has lost its only electron.
- conjugate base
- The particle remaining after the proton has been donated.
- strength versus concentration
- Strength is about the fraction of protons given up, while concentration is about how many moles are in a cubic decimetre.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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