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 comparing the melting points of these compounds is the molecular structure, specifically the length of the carbon chain and the presence of branching.

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

• Structure: Pentan-2-one has a longer carbon chain compared to 3-methylbutanone, which is more branched and compact. The linear structure of Pentan-2-one results in greater surface area, allowing for stronger van der Waals forces compared to the more compact and branched structure of 3-methylbutanone.

• Intermolecular Forces: Pentan-2-one exhibits stronger intermolecular forces due to its longer chain length, leading to a higher melting point.

• Energy: Less energy is required to overcome the weaker intermolecular forces present in 3-methylbutanone, resulting in its lower melting point. Therefore, Pentan-2-one has a higher melting point than 3-methylbutanone.

Vapour pressure is defined as the pressure exerted by a vapor in thermodynamic equilibrium with its liquid (or solid) phase at a given temperature in a closed system.

The investigation shows that as the carbon chain length of the straight-chain primary alcohols increases, the vapour pressure decreases. This indicates that longer carbon chains lead to stronger intermolecular forces, resulting in lower vapour pressures.

The alcohol with the HIGHEST boiling point in the table is 1-decanol (C₁₀H₂₁OH), which can also be referred to by its systematic IUPAC name.

Upon repeating the investigation at 320 K, the vapour pressure of each alcohol is expected to INCREASE. Higher temperatures provide more energy to the molecules, allowing more molecules to escape into the vapor phase, thereby increasing the vapour pressure.