Question

Which substance in each of the following pairs would you expect to have the higher boiling point? Explain why.

Answer 1

Explanation:

Boiling point is defined as the temperature when the vapor pressure of liquid becomes equal to the atmospheric pressure surrounding the liquid. And, during this temperature liquid state of substance changes into vapor state.

More stronger is the presence of intermolecular forces within the molecules of a compound more heat is required by it to break the bond. Hence, boiling point will also increase.

(a)  As both Ne and Xe are noble gases and has intermolecular dispersion forces. But as the atomic size of Xe is more than the atomic size of Ne and we know that dispersion forces increases with increase in size.

Hence, boiling point of Xe will be more than the boiling point of Ne.

(b)   Both $CO_{2}$ and $CS_{2}$ are non-polar in nature with intermolecular dispersion forces. So, more is the molecular weight of a compound more heat will be required by it in order to break the bonds.

As molecular weight of $CS_{2}$ is more than the molecular weight of $CO_{2}$. Hence, $CS_{2}$  will have high boiling point.

(c)  Molecular weight of $CH_{4}$ is 16 g/mol and molecular weight of $Cl_{2}$ is 35 g/mol.

Hence, $Cl_{2}$ will have high boiling point due to the presence of high molecular weight as compared to $CH_{4}$ and more strong dispersion forces.

(d) $F_{2}$ is a covalent compound so, it will also be non-polar in nature. Hence, it has weak intermolecular dispersion forces. On the other hand, LiF is an ionic compound and it has strong intermolecular forces of attraction due to the presence of opposite charges on the combining atoms.

Hence, LiF will have high boiling point as compared to $F_{2}$.

(e)  Both $NH_{3}$ and $PH_{3}$ are polar molecules in which dipole-dipole forces does exist. Since, nitrogen atom is more electronegative than phosphorous atom so, it will have stronger hydrogen bonding and strong intermolecular forces.

Hence, $NH_{3}$ will have high boiling point as compared to $PH_{3}$.