Question

In which of the following sets the order of boiling points is incorrect?
A. ${\text{Xe}}\,{\text{ > }}\,{\text{Ar}}\,{\text{ > }}\,\,{\text{Ne}}$
B. ${\text{HF}}\,{\text{ > }}\,{\text{HCl}}\,{\text{ > }}\,\,{\text{HBr}}$
C. ${{\text{H}}_2}{\text{O}}\,{\text{ > }}\,{{\text{H}}_2}{\text{S}}\,{\text{ > }}\,{{\text{H}}_2}{\text{Se}}$
D. ${{\text{C}}_{\text{3}}}{{\text{H}}_{{\text{6}}\,}}{\text{ > }}\,{{\text{C}}_{\text{2}}}{{\text{H}}_{\text{5}}}\,{\text{ > }}\,\,{\text{C}}{{\text{H}}_{\text{4}}}$

Answer 1

In the case of non-polar compounds, the boiling point depends upon the strength of Van der waals forces. As the strength of Van der waals forces increases the boiling point increases.

Complete step by step answer:
The temperature at which the vapour pressure of the liquid equals the vapour pressure of the surrounding then the liquid converts into vapour is known as boiling point.
As the strength of van der Waals forces increases, the bond of molecules becomes strong, so it requires more energy to break the bonding hence high energy for melting, so the boiling point increases on increasing the strength of van der Waals forces.
$\,{\text{Boiling}}\,{\text{point}} \propto {\text{strength}}\,{\text{Van der waals forces}}$
On going down in a group, the strength of van der Waals forces increases.
As the number of electrons increases the van der forces increase and down in a group extra shell is added, so the number of electrons increases down in a group.
Neon, argon, and xenon are the elements of the same group. Neon is of the second period, argon is of the third period and xenon is of fifth-period element.so, the increasing order of strength of van der Waals forces is as follows:
${\text{Xe}}\,{\text{ > }}\,{\text{Ar}}\,{\text{ > }}\,\,{\text{Ne}}$
The order of boiling point is correct in option (A) so, option (A) is incorrect.
Fluorine, chlorine, and bromine are elements of the same group. Fluorine is of the second period, chlorine is of the third period and bromine is of fourth-period element.so, the increasing order of strength of van der Waals forces is as follows:
${\text{HF}}\,{\text{ < }}\,{\text{HCl}}\, < \,\,{\text{HBr}}$
But in hydrogen fluoride, the hydrogen bonding is present with van der Waals forces which increase the strength of bonds in hydrogen fluoride hence boiling point.
So, the correct order of strength of forces in hydrogen halide or boiling point is as follows:
${\text{HF}}\,{\text{ > }}\,\,{\text{HBr}}\, > \,{\text{HCl}}\,$
The order of boiling point is incorrect in option (B) so, option (B) is correct.
Oxygen, sulphur, and selenium are elements of the same group. Oxygen is of the second period, sulphur is of the third period and selenium is of fourth-period element.so, the increasing order of strength of Van der Waals forces is as follows:
${{\text{H}}_2}{\text{O}}\,{\text{ > }}\,{{\text{H}}_2}{\text{S}}\,{\text{ > }}\,{{\text{H}}_2}{\text{Se}}$
The order of boiling point is correct in option (C) so, option (C) is incorrect.
In hydrocarbons, as the molecular weight increases the strength of van der Waals forces increases.
On increasing the molecular weight the surface area increases which increases the interaction.
The increasing order of molecular weight of hydrocarbon hence the point is as follows:
${{\text{C}}_{\text{3}}}{{\text{H}}_{{\text{6}}\,}}{\text{ > }}\,{{\text{C}}_{\text{2}}}{{\text{H}}_{\text{5}}}\,{\text{ > }}\,\,{\text{C}}{{\text{H}}_{\text{4}}}$
The order of boiling point is correct in option (D) so, option (D) is incorrect.

Therefore the option (B) ${\text{HF}}\,{\text{ > }}\,{\text{HCl}}\,{\text{ > }}\,\,{\text{HBr}}$ is correct.

Note: In the case of polar compound boiling point depends upon the covalent character in an ionic compound. The boiling point is inversely proportional to the covalent character. As the Covalent character increases the boiling point decreases.