Question

Rationalize the difference in boiling points for each of the following pairs of substances:

Short answer

1. The n-pentane contains long chains, increasing the London Forces between the molecules. So, more energy is required to overcome these forces, which is reflected in a higher boiling point compared to neopentane which does not contain long chains.
2. The increasing polarity increases the boiling point of the molecule. In HF and HCI, HF has the greater polarizability due to the presence of higher electronegative F atoms. This leads to the formation of hydrogen bonds between HF molecules.
   Thus, due to the presence of hydrogen bonding, HF has a higher boiling point than HCI.
3. LiCl is an ionic compound, while HCI is a covalent compound. Ionic forces are much stronger than covalent forces. This leads to an increase in the boiling point of the molecule.
   Thus, due to the presence of strong ionic forces, LiCl has a higher boiling point than HCI.
4. n-hexane is a larger molecule than pentane. Due to the increasing number of bonds in the n-hexane, we must give some excess energy to boil it. Due to the increase of the London dispersion forces, the boiling point of the molecule. Thus, due to the presence of strong London dispersion forces, n-hexane has a higher point than n-pentane.

Step-by-step solution

Intermolecular force

Intramolecular forces are well-known forces that work between the molecules as they hold atoms of the molecule together within a molecule.Intermolecular forces are forces that exist between molecules of any compound, such as in water molecules.

Hydrogen bond

Hydrogen bonding acts between the molecules, and it is known as a weak bond that usually acts in the presence of hydrogen in molecules.

A hydrogen bond will be the electromagnetic attractive interaction between polar molecules, where hydrogen will be a highly electronegative atom, like nitrogen, oxygen or fluorine.

Explanation

1. The n-pentane contains long chains, increasing the London Forces between the molecules. So more energy is required to overcome these forces, which is reflected in a higher boiling point compared to neopentane which does not contain long chains.
2. The increasing polarity increases the boiling point of the molecule. In HF and HCI, HF has the greater polarizability due to the presence of higher electronegative F atoms. This leads to the formation of hydrogen bonds between HF molecules.
   Thus, due to the presence of hydrogen bonding, HF has a higher boiling point than HCI.
3. LiCl is an ionic compound, while HCI is a covalent compound. Ionic forces are much stronger than covalent forces. This leads to an increase in the boiling point of the molecule.
   Thus, due to the presence of strong ionic forces, LiCl has a higher boiling point than HCI.
4. n-hexane is a larger molecule than pentane. Due to the increasing number of bonds in the n-hexane, we must give some excess energy to boil it. Due to the increase of the London dispersion forces, the boiling point of the molecule. Thus, due to strong London dispersion forces, n-hexane has a higher point than n-pentane.