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A chemist synthesised benzoic acid in the laboratory and purified it by recrystallisation - Leaving Cert Chemistry - Question b - 2010
Question b
A chemist synthesised benzoic acid in the laboratory and purified it by recrystallisation. The melting point of the product was measured before and after recrystalli... show full transcript
Worked Solution & Example Answer:A chemist synthesised benzoic acid in the laboratory and purified it by recrystallisation - Leaving Cert Chemistry - Question b - 2010
Step 1
Describe with the aid of a labelled diagram how the melting point of either the crude product or the recrystallised product could have been measured.
Answer
The melting point can be measured using a melting point apparatus or a simple setup involving a capillary tube.
- Setup: The sample is placed in a thin-walled capillary tube, about 2-3 mm in height.
- Temperature Measurement: The capillary tube is then attached to a thermometer and placed in a heating block or melting point apparatus.
- Heating: Heat is gradually applied, while the thermometer shows the temperature.
- Observation: The temperature is noted as the sample melts, indicating the melting point when the substance turns from solid to liquid.
- Diagram: [A labelled diagram would show the melting point tube in the heating block, thermometer readings, and a heat source.]
Step 2
Give two ways the chemist could have concluded from the melting point measurements that the recrystallised product was purer than the material first prepared.
Answer
- Higher Melting Point: The recrystallised product showed a higher melting point than the crude product, indicating higher purity.
- Sharper Melting Point Range: The melting point of the recrystallised product was sharper, suggesting that it melted over a narrower temperature range compared to the crude product, which reflects an increase in purity.
Step 3
Step 4