Question

Which one of the following lists the alkane in order of decreasing boiling point?
A) Octane, Methane, Butane, Ethane
B) Methane, Ethane, Propane, Butane
C) Hexane, Octane, Propane, Methane
D) Hexane, Pentane, Propane, Ethane

Answer 1

The boiling point of a compound depends on the ease with which vapour pressure of the liquid becomes equal to atmospheric pressure. Alkane molecules have van der Waals forces of attraction. Van der Waals forces of attraction depend on two factors:

1. Number of electrons surrounding the molecule which increases with an increase in the chain length.
2. The surface area or the size of the molecule

Complete step by step answer:
Alkanes are saturated hydrocarbon. They are less reactive and have little biological activity. The difference in the physical properties arises due to the shape and size of the molecule. It also depends on the intermolecular forces between hydrocarbons.
Alkanes have predictable properties. Let’s consider the physical properties of alkane.
The boiling point is defined as the temperature at which the pressure exerted by the surrounding on the liquid is equal to the vapour pressure of the liquid. At this temperature, the liquid is converted into vapor form without raising the temperature.
For a substance to be able to exist as a liquid or a solid there must be intermolecular forces of attraction between particles of the pure compound. All of these forces are electrostatic in nature, but the strength of these interactions varies considerably with the compound.
Alkanes have intermolecular van der Waals forces of attraction. Stronger intermolecular van der Waals forces give rise to the greater boiling point of alkanes. The van der Waals forces of attraction depend on two parameters:

1. The number of electrons surrounding the molecule, which increase with alkane’s molecular weight
2. The surface area or the size of molecular
   The boiling point has a linear relationship between the size and the molecular weight of the molecule. Straight chain alkanes have a higher boiling point than the branched boiling point. This is because straight-chain alkanes have a great van der Waals force of attraction in between the molecules.
   The boiling point of alkane increases with the increase in the number of carbon atoms in the chain. Thus straight-chain hydrocarbon has a higher boiling point and branched-chain alkanes have a low boiling point. This is because straight-chain alkanes have greater intermolecular attraction.

Here Hexane is a six-membered straight-chain, pentane has 5 carbons, propane has 3 carbons and ethane has 2 carbons. Thus increasing order of boiling point for the given alkanes is as follows,
$\text{ Hexane }>\text{ Pentane }>\text{ Propane }>\text{ Ethane }$

Hence, (D) is the correct option.

Note: Note that, as a rule of thumb, the boiling point rises 20 - 30 °C for each carbon added to the chain; this rule applies to other homologous series as well. For the same number of the carbon atoms of the corresponding alkene have a low boiling point compared to the alkane. Each carbon atom has a 2 electron less hydrocarbon thus has a low boiling point.