Question

The following alkane molecules all have the molecular formula of C5H12-

Their structures and boiling points are shown below.
A
B
C
H3C
isomer of C,H12
H₂
H₂
C.
H3C
H₂
CH3
T
CH
H3C
C
H₂
CH3
CH3
CH3
boiling point /°C
36
H3C
CH3
Explain the trend in boiling points shown by A, B and C in the table.
28
9

Answer 1

The surface area of a molecule influences its boiling point by affecting the strength of intermolecular forces; larger surface areas generally result in stronger forces, leading to higher boiling points. Although each is the same molecule, A is more spread out with a larger surface area, so the boiling point is higher, because the larger surface area caused higher intermolecular forces.

Answer 2

The trend in boiling points shown by A, B, and C in the table is due to the difference in surface area of the molecules. Isomer B has the smallest surface area and the lowest boiling point, while isomer A has the largest surface area and the highest boiling point. Isomer C has an intermediate surface area and an intermediate boiling point.

The trend in boiling points shown by A, B, and C in the table is as follows:

Isomer Boiling point (°C)

A 36

B 28

C 9

There are two main factors that affect the boiling point of a liquid: the strength of the intermolecular forces between the molecules and the surface area of the molecules.

Intermolecular forces are the attractive forces that hold molecules together. The stronger the intermolecular forces, the higher the boiling point. There are four main types of intermolecular forces: hydrogen bonding, dipole-dipole interactions, London dispersion forces, and ion-dipole interactions.

Surface area is the amount of space that a molecule takes up. The larger the surface area, the more contact the molecules have with each other and the stronger the intermolecular forces.

In the case of A, B, and C, all three isomers have the same molecular formula (C5H12), so they have the same intermolecular forces. However, isomer B has the smallest surface area because it is a branched alkane. Branched alkanes have less surface area than straight-chain alkanes because the branches prevent the molecules from packing together as tightly.

As a result of its smaller surface area, isomer B has weaker intermolecular forces than isomers A and C. This is why isomer B has the lowest boiling point.

Isomer A has the largest surface area because it is a straight-chain alkane. Straight-chain alkanes have the largest surface area because the molecules can pack together tightly. As a result of its larger surface area, isomer A has stronger intermolecular forces than isomers B and C. This is why isomer A has the highest boiling point.

Isomer C has a surface area that is in between the surface areas of isomers A and B. As a result of its intermediate surface area, isomer C has intermediate intermolecular forces. This is why isomer C has an intermediate boiling point.