The melting points of some organic compounds are given in the table below - NSC Physical Sciences - Question 3 - 2022 - Paper 2

The controlled variable for the comparison of the melting points of compounds A, B, and C is the structural characteristics of the compounds, such as chain length and functional groups, which should remain constant to ensure a fair comparison.

The melting points of compounds C (Pentan-2-one) and D (3-methylbutanone) differ primarily due to their molecular structures.

• Structure: Pentan-2-one has a longer carbon chain compared to 3-methylbutanone, which is more branched and compact. This affects how the molecules pack together in the solid state.
• Intermolecular Forces: Pentan-2-one experiences stronger intermolecular forces due to a larger surface area compared to the more spherical 3-methylbutanone.
• Energy Requirement: As a result, Pentan-2-one requires more energy to overcome these intermolecular forces, leading to a higher melting point compared to 3-methylbutanone, which has weaker forces and a lower melting point.

Vapour pressure is defined as the pressure exerted by a vapor in equilibrium with its liquid in a closed system. It represents the tendency of the liquid to evaporate and is influenced by temperature and the nature of the liquid.

The investigation reveals that as the carbon chain length of the alcohol increases, the vapour pressure decreases. This indicates that larger molecules with more extensive intermolecular forces tend to have lower vapour pressures.

The alcohol with the highest boiling point in the table is C₆H₁₃OH, which is known as hexan-1-ol.

When the investigation is repeated at 320 K, it is expected that the vapour pressure of each compound will INCREASE. As temperature rises, the kinetic energy of the molecules increases, leading to a greater tendency for molecules to escape from the liquid phase into the vapor phase.