NSC Physical Sciences Organic Compounds and Macromolecules: Explain the increase in boiling

Question

Explain the increase in boiling points of the alkanes, as indicated in the table.

The boiling points of straight-chain alkanes increase as the number of carbon atoms increases. This trend can be attributed to several factors:

1. **Molecular Structure**: As the chain length of the alkane increases, the molecular size also increases. Longer chains have a greater surface area, allowing for more significant interactions between molecules.

2. **Intermolecular Forces**: The primary forces acting between the alkane molecules are London dispersion forces, which arise due to temporary dipoles formed as electrons move around the nucleus. With more carbon atoms in the chain, there are more electrons, leading to stronger induced dipoles, and consequently, stronger intermolecular forces.

3. **Energy Requirement**: More energy is required to overcome/break these intermolecular forces as the boiling point increases. Therefore, the increase in the boiling points of alkanes can be directly correlated with the increase in the number of carbon atoms, resulting in stronger intermolecular interactions that require more energy to break.

Explain the difference between the boiling points of an alkane and an alcohol, each having THREE carbon atoms per molecule, by referring to the TYPE of intermolecular forces.

The boiling points of alcohols are generally higher than those of alkanes due to the presence of hydrogen bonding in alcohols. In contrast, alkanes primarily exhibit London dispersion forces, which are comparatively weaker.

- **Intermolecular Forces in Alkanes**: Alkanes only have London dispersion forces, which depend on the size and shape of the molecules.

- **Intermolecular Forces in Alcohols**: Alcohols possess hydrogen bonding due to the hydroxyl (-OH) group. This strong attraction between molecules significantly increases the boiling point relative to alkanes.

Does the vapour pressure of the alcohols INCREASE or DECREASE with an increase in the number of carbon atoms?

The vapour pressure of alcohols decreases as the number of carbon atoms increases. This is due to the stronger hydrogen bonding that becomes more prominent with larger alcohols, which stabilizes their liquid phase and reduces the tendency for molecules to escape into the vapour phase.

How will the boiling point of 2-methylpropane compare to that of its chain isomer?

The boiling point of 2-methylpropane will be LOWER THAN that of its straight-chain isomer, n-butane. The branched structure of 2-methylpropane results in a smaller surface area, leading to weaker London dispersion forces compared to the more linear structure of n-butane, which facilitates stronger intermolecular attractions.

Explain the increase in boiling points of the alkanes, as indicated in the table - NSC Physical Sciences - Question 3 - 2018 - Paper 2

Worked Solution & Example Answer:Explain the increase in boiling points of the alkanes, as indicated in the table - NSC Physical Sciences - Question 3 - 2018 - Paper 2

Explain the increase in boiling points of the alkanes, as indicated in the table.

Explain the difference between the boiling points of an alkane and an alcohol, each having THREE carbon atoms per molecule, by referring to the TYPE of intermolecular forces.

Does the vapour pressure of the alcohols INCREASE or DECREASE with an increase in the number of carbon atoms?

How will the boiling point of 2-methylpropane compare to that of its chain isomer?

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