Q12.6 P - Chemistry: Molecular Nature Of Matter And Change

Question

Which forces are intramolecular and which intermolecular?

(a) Those allowing fog to form on a cool, humid evening

(b) Those allowing water to form when  ${\mathbf{H}}_{\mathbf{2}}$ is sparked

(c) Those allowing liquid benzene to crystallize when cooled

(d) Those responsible for the low boiling point of hexane

Step-by-Step Solution

Answer

1. On a chilly, humid evening, intermolecular forces cause fog to form.
2. When ${\mathbf{H}}_{\mathbf{2}}$ is ignited, intramolecular forces enable water to form.
3. When benzene is cooled, intermolecular forces cause it to crystallize.
4. Intermolecular forces are to blame for hexane's low boiling point.

Step 1: Fog

Fog is a type of aerosol consisting of liquid droplets and tiny crystals suspended in the air.

Step 2: Intramolecular forces

The H-bonding in the water vapors which are surrounded by a solid surface on a cool, humid evening allows the formation of fog.

 

Step 3: Formation of water

 ${\mathbf{H}}_{\mathbf{2}}$ gas when allowed to oxidize in the limited supply of oxygen forms water with the release of a large amount of energy.

Step 4: Intramolecular forces

When hydrogen is sparked in the presence of oxygen, the H atoms thus formed after electrolysis makes a bond with the O atoms present forming a water molecule.

As these interactions occur within atoms, the forces present are intramolecular in nature.

Step 5: Intermolecular forces

The liquid benzene has weak intermolecular forces within the molecules. When cooled, the liquid benzene gets condensed and the molecular energy of liquid benzene gets transformed into vibrational energy.

Step 6: Energy exchange

The energy released is compensated with the increase in the intermolecular force of attraction.

Step 7: Intermolecular forces

Weak intermolecular forces of attraction within the hexane molecules are responsible for the low boiling point of hexane.

Step 8: London dispersion forces

Hexane molecules consist of long chains and these chains interact with each other through weak London dispersion forces.