Base properties (A-level only) — AQA A-Level Chemistry
Test yourself on Base properties (A-level only) with AQA A-Level practice questions.
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Base properties (A-level only) explained
A base accepts a proton, and in an amine it is the lone pair on the nitrogen atom that does the accepting.
Read the full explanation
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.
The difference in base strength between ammonia, primary aliphatic and primary aromatic amines.
Base strength here comes down to one question: how available is the lone pair on the nitrogen atom? Alkyl groups release electron density towards the nitrogen by a positive inductive effect, so in ethylamine the lone pair is pushed out and is more available to a proton than in ammonia, and the cation formed is better stabilised. Ethylamine is therefore the stronger base. In phenylamine the nitrogen's lone pair lies next to the delocalised ring and overlaps with it, so part of the electron density is drawn into the ring and spread over the whole system. The pair is much less available, so phenylamine is a far weaker base than ammonia. The order is primary aliphatic amine, then ammonia, then primary aromatic amine.
Your focus
- Write the equilibrium equation for a named amine in water and state where the equilibrium lies.
- Explain the difference between a weak base and a dilute base using an amine as the example.
- Explain why a titration of a carboxylic acid with an amine cannot be followed using an indicator.
Show all 6 objectives
- Name the effect the benzene ring has on the nitrogen lone pair in phenylamine and the opposite effect the alkyl group has in ethylamine, and use the two to place each amine either side of ammonia.
- Explain the position of each using the availability of the nitrogen lone pair.
- Predict which of two given amines is the stronger base and justify the prediction.
Base properties (A-level only) exam tips
Marking Points
- one mark for the lone pair on the nitrogen atom accepting a proton
- one mark for an equilibrium equation with water in which the position of equilibrium lies to the left
- one mark for the salt formed with an acid, named and written with the correct charges
- one mark for stating that a weak acid with a weak base gives no steep section on the pH curve, so an indicator cannot be used
- one mark for explaining base strength in terms of the availability of the lone pair on the nitrogen atom
- one mark for alkyl groups releasing electrons by a positive inductive effect, making the lone pair more available
- one mark for the lone pair in phenylamine being delocalised into the benzene ring
- one mark for the order ethylamine, then ammonia, then phenylamine
Examiner 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.
- 💡Name the effect you are using: 'positive inductive effect' or 'delocalised into the ring'. Saying electrons are 'pushed' rarely scores on its own.
- 💡Answer in comparisons. Every mark here is for one species measured against another.
Common Mistakes
- 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
- explaining the trend by the size of the groups rather than the electron density at the nitrogen
- saying the ring withdraws the lone pair by an inductive effect rather than by delocalisation
- ranking phenylamine as the strongest because the ring is large
- discussing the stability of the amine molecule rather than the availability of its lone pair