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Questions and Answers
What is the primary purpose of determining the melting point of a compound?
What is the characteristic of a pure crystalline compound's melting point?
What happens to the melting point of a compound when impurities are present?
What can be estimated by comparing the melting point of a material with that of a pure sample?
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Which of the following is a factor that affects the melting point of a compound?
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What can be concluded if two pure samples have the same melting point?
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What is the effect of intermolecular forces on the melting point of a compound?
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What is the characteristic of the melting point range of an impure substance?
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Which type of intermolecular interaction is responsible for the high melting point of butyric acid?
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What is the primary purpose of the laboratory exercise described in the content?
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Which of the following compounds is expected to have the lowest melting point?
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What is the purpose of sealing one end of the capillary tube in the capillary method of measuring melting point?
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Why does sodium butanoate have a higher melting point than butyric acid?
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What is the role of the porous plate in the capillary method of measuring melting point?
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Which type of intermolecular interaction is responsible for the higher melting point of methyl propionate compared to butane?
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What is the advantage of using the capillary method to measure melting point?
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Study Notes
Melting Point
- The melting point is a crucial physical property of a compound that can be used to identify a substance and indicate its purity.
- A pure crystalline compound typically has a sharp melting point and melts completely over a narrow temperature range of not more than 0.5-1°C.
- The presence of impurities can produce a depression in the freezing points and increase the width of the melting point range.
- A melting point range of > 5°C usually indicates that the substance is impure.
Factors Affecting Melting Point
- The size of the molecule affects the melting point, with smaller molecules typically having lower melting points.
- The force of attraction between molecules also impacts the melting point, with stronger intermolecular interactions resulting in higher melting points.
Examples of Melting Point
- Ethanol has a melting point of -114.1°C, while ethyl cellulose has a melting point of 151°C due to its larger molecular size.
- Isomers n-butanol and t-butanol have the same molecular formula but differ in their structure, and thus may have different melting points.
Intermolecular Interactions
- Ionic compounds typically have high melting points due to strong electrostatic forces.
- Organic compounds with polarity or hydrogen bonding tend to have higher melting points.
- Examples of intermolecular interactions include:
- Butane, which has a low melting point due to weak Van der Waals force of attraction.
- Methyl propionate, which has a higher melting point due to dipole-dipole interaction.
- Butyric acid, which has a higher melting point due to hydrogen bonding.
- Sodium butanoate, which has a higher melting point due to strong ionic interaction.
Determining Melting Point
- The most common method of determining melting point is the capillary method.
- The capillary method involves:
- Powdering the crystalline substance.
- Filling a capillary tube with the substance.
- Attaching the capillary tube to a thermometer.
- Heating the sample until it reaches melting point, then recording the melting point.
Aim of the Experiment
- The purpose of the laboratory exercise is to identify and assess the purity of an unknown solid organic compound by determining its melting point and comparing it to a list of possible compounds.
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Description
Discover the importance of melting point in identifying substances and its relation to purity. Learn how impurities affect the melting point range.
