NSC Physical Sciences Optical Phenomena and Properties of Materials: 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 is primarily due to the increase in chain length and molecular size, which leads to a higher molecular mass and increased surface area. As the molecular structure becomes larger, there is an increase in the strength of the London dispersion forces between the molecules. Consequently, more energy is required to overcome these intermolecular forces, resulting in higher boiling points.

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.

Alkanes have primarily London dispersion forces, while alcohols have hydrogen bonding due to the presence of the hydroxyl (-OH) group. The hydrogen bonds in alcohols are much stronger than the London dispersion forces in alkanes, leading to higher boiling points for alcohols compared to alkanes with the same number of carbon atoms.

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

The vapour pressure of alcohols generally DECREASES as the number of carbon atoms increases. This is due to the increase in intermolecular forces that require more energy to overcome, thereby reducing the tendency of 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 chain isomer. This is because branched alkanes have a smaller surface area compared to straight-chain alkanes, resulting in weaker London dispersion forces and lower boiling points.

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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