The relationship between boiling point and the number of carbon atoms in straight chain molecules of aldehydes, alkanes and primary alcohols is investigated - NSC Physical Sciences - Question 3 - 2020 - Paper 2

The structural formula of the functional group of aldehydes is:

  O
  ||
  C - H

This represents the carbonyl group (C=O) at the end of the carbon chain.

The trend observed in the graph indicates that as the number of carbon atoms in a molecule increases, the boiling point rises. This can be attributed to several factors:

1. Increase in Molecular Size: More carbon atoms lead to larger molecular size, increasing the molecular mass. Larger molecules generally have higher boiling points because they have greater surface area, which enhances the van der Waals forces between them.

2. Strength of Intermolecular Forces: The strength of the intermolecular forces, notably London dispersion forces, also increases with molecular size. Larger molecules have more electrons, which allows for larger instantaneous dipoles, resulting in stronger London forces.

3. Energy Requirements: More energy is required to overcome these enhanced intermolecular forces to change the state from liquid to gas, thereby resulting in higher boiling points.

The curve that represents compounds with London forces only is curve C. This is because alkanes, which primarily exhibit London forces, show a lower boiling point compared to aldehydes and alcohols.

The curve that represents the aldehydes is curve B. Aldehydes possess both London forces and dipole-dipole interactions due to the presence of the carbonyl group (C=O), leading to higher boiling points than hydrocarbons (alkanes) but lower than alcohols which exhibit hydrogen bonding.

The compound with a boiling point of 373 K is Butanal. This is inferred from the graph where the corresponding curve aligns with this boiling point.

The IUPAC name of the compound containing five carbon atoms with the lowest vapor pressure is Pentanol. Alcohols have lower vapor pressures than aldehydes or alkanes due to their ability to form hydrogen bonds.