Chapter 11, Problem 11.69QP

Interpretation Introduction

Interpretation:

• The relationship between intermolecular force in a liquid and its boiling point and critical temperature has to be outlined.
• The reason for the greater critical temperature of water has to be outlined.

Concept Introduction:

• Intermolecular force

Intermolecular force refers to the attractive forces between the molecules of a    substance. It is the force which holds the molecules together. Many physical properties of the substance such as – melting point, boiling point, surface tension, viscosity etc., are influenced by the strength of intermolecular force present in the substance.

• The three types of intermolecular forces are – London dispersion force, dipole-dipole force and Hydrogen bonding.  They are collectively known as Van der Waals forces.
• London dispersion forces exist in all types of molecules. This is the force responsible for the condensation of non-polar compounds into liquids or solids under low temperature.
• Dipole-dipole forces exist in polar covalent compounds. Hydrogen bonding exists in polar covalent compounds containing Fluorine, Oxygen or Nitrogen directly bonded to Hydrogen.
• The strength of intermolecular forces is,

$\text{London}\text{dispersion}\text{forces}\text{<}\text{Dipole-dipole}\text{forces}\text{<}\text{Hydrogen}\text{bonding}$

• Boiling point

The temperature at which the vapor pressure of liquid becomes equal to atmospheric pressure is boiling point of the liquid.  During boiling the molecules in liquid phase partly evaporates to vapor phase. The molecules in vapor phase and that of the liquid phase remain in equilibrium with each other.

• Critical temperature

Critical temperature is defined as the temperature above which a gas cannot be liquefied irrespective of the external pressure.