Trimethylamine, (CH₃)₃N, is an amine found in decaying fish. When trimethylamine dissolves in water, the solution formed turns red litmus paper blue. Explain why the solution formed is alkaline, and state what type of reaction occurs when trimethylamine reacts with hydrochloric acid.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Trimethylamine, (CH₃)₃N, is an amine responsible for the smell of rotting fish. It is a colourless gas at room temperature that dissolves readily in water.
Model answer (5 marks)
Trimethylamine contains a nitrogen atom with a lone pair of electrons.
The lone pair accepts a proton (H⁺) from water.
This proton transfer produces hydroxide ions (OH⁻) in solution.
The excess hydroxide ions make the solution alkaline.
When trimethylamine reacts with hydrochloric acid it undergoes a neutralisation reaction, forming the trimethylammonium chloride salt.
The lone pair accepts a proton (H⁺) from water.
This proton transfer produces hydroxide ions (OH⁻) in solution.
The excess hydroxide ions make the solution alkaline.
When trimethylamine reacts with hydrochloric acid it undergoes a neutralisation reaction, forming the trimethylammonium chloride salt.
Examiner tips
- Use the exact phrase "lone pair of electrons" and "accepts a proton" to match the scheme. Include the word "hydroxide ions" and state they are in excess to justify alkalinity. Mention the reaction type explicitly as "neutralisation reaction". Keep each point brief and separate with line breaks or numbering.
Common mistakes
- Saying the amine is a base without mentioning the lone pair. Claiming the solution is acidic instead of alkaline. Calling the reaction a "synthesis" or "decomposition" rather than a neutralisation.
Mark scheme (5 marks)
- Amines contain a nitrogen atom with a lone pair of electrons
- The lone pair accepts a proton (H⁺) from water
- This produces hydroxide ions (OH⁻) in solution
- Hydroxide ions (OH⁻) in excess make the solution alkaline
- The reaction between trimethylamine and hydrochloric acid is a neutralisation reaction
Key terms in this question
Related
- All Eduqas A-Level Chemistry revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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