AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
Base properties (A-level only): Amines are weak bases.
A base accepts a proton, and in an amine it is the lone pair on the nitrogen atom that does the accepting.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 1 of 2
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A base accepts a proton, and in an amine it is the lone pair on the nitrogen atom that does the accepting. In water, ethylamine sets up the equilibrium CH₃CH₂NH₂ + H₂O ⇌ CH₃CH₂NH₃⁺ + OH⁻, and that equilibrium lies well to the left, so only a small fraction of the molecules are protonated at any moment. That partial reaction is what 'weak' means; it is nothing to do with concentration, so a concentrated solution of a weak base is still a weak base. With acids the reaction is complete and a salt forms, so ethylamine and hydrochloric acid give ethylammonium chloride, an ionic solid. A weak base titrated against a weak acid gives no steep section on the pH curve, so no indicator will work.
What earns the marks
- The lone pair on the nitrogen atom accepting a proton.
- An equilibrium equation with water in which the position of equilibrium lies to the left.
- The salt formed with an acid, named and written with the correct charges.
- Stating that a weak acid with a weak base gives no steep section on the pH curve, so an indicator cannot be used.
Common mistakes and how to put them right
Saying an amine releases hydroxide ions of its own, as sodium hydroxide does.
Placing the lone pair on a carbon atom rather than the nitrogen atom.
Writing the salt as a neutral molecule with a dative arrow and no charges.
Confusing weak with dilute when explaining what 'weak base' means.
Exam tips
- Use ⇌ for the reaction of a weak base with water and → for its reaction with a strong acid.
- If a titration question involves an amine and a carboxylic acid, the answer is almost always about the missing vertical section of the pH curve.
Try the lesson
The start of the Base properties (A-level only): Amines are weak bases. lesson, made from checked teaching notes for this specification point.
Have a go first
Which statement explains why an amine such as methylamine acts as a base when it is added to water?
Listen
A base accepts a proton, and in an amine it is the lone pair on the nitrogen atom that does the accepting. In water, ethylamine sets up the equilibrium CH₃CH₂NH₂ + H₂O ⇌ CH₃CH₂NH₃⁺ + OH⁻, and that equilibrium lies well to the left, so only a small fraction of the molecules are protonated at any moment. That partial reaction is what 'weak' means; it is nothing to do with concentration, so a concentrated solution of a weak base is still a weak base. With acids the reaction is complete and a salt forms, so ethylamine and hydrochloric acid give ethylammonium chloride, an ionic solid. A weak base titrated against a weak acid gives no steep section on the pH curve, so no indicator will work. A strong answer earns marks for these points. the lone pair on the nitrogen atom accepting a proton. an equilibrium equation with water in which the position of equilibrium lies to the left. the salt formed with an acid, named and written with the correct charges. stating that a weak acid with a weak base gives no steep section on the pH curve, so an indicator cannot be used. Exam tip: Use ⇌ for the reaction of a weak base with water and → for its reaction with a strong acid.
Key words
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proton acceptor
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lone pair on nitrogen
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position of equilibrium
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salt formation
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See every definition
- proton acceptor
- A base accepts a proton, and in an amine it is the lone pair on the nitrogen atom that does the accepting.
- lone pair on nitrogen
- In an amine it is this that accepts the proton, and it must not be placed on a carbon atom rather than the nitrogen atom.
- position of equilibrium
- In water, ethylamine sets up an equilibrium with water, and that equilibrium lies well to the left, so only a small fraction of the molecules are protonated at any moment.
- salt formation
- With acids the reaction is complete and a salt forms, so ethylamine and hydrochloric acid give ethylammonium chloride, an ionic solid.
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