Energetics of Phase Changes

Phase Changes

Whenever matter undergoes a physical or’ a chemical change, it is always accompanied by an energy change in the form of heat. This change in energy is the quantitative measure of the difference in the strength of intermolecular forces. In case of physical change at constant pressure called enthalpy change.

Enthalpy change is of three types:

  1. Molar heat of fusion

It is the amount of heat absorbed by one mole of a solid when it melts into liquid form at its melting point at one atmospheric pressure.

  1. Molar heat of vaporization

The amount of heat absorbed when one mole of a liquid is changed to its vapour at its boiling point at one atmospheric pressure is called molar heat of vaporization.

  1. Molar heat of sublimation

The amount of heat absorbed when one mole of a solid sublimes to give vapour at a particular temperature at one atmospheric pressure is called molar heat of sublimation.

All these enthalpy changes are positive because all are endothermic processes.

Energy Changes and Intermolecular Attractions

When solid melts then atoms, molecules or ions undergo relatively small changes in intermolecular distances and the potential energy also undergoes a small change. But when liquid evaporates, then large changes in intermolecular distances and in potential energy take place. So molar heat of vaporization is greater that molar heat of fusion. The values of molar heat of sublimation, are even larger than molar heat of vaporization, because attractive forces in solids are stronger than those in liquids.

The values of molar heat of vaporization and molar heat of sublimation, tell us directly the energy needed to separate molecules from each other. So from these values, we can compare the strengths of intermolecular forces in different compounds.

lodine is solid

The AH, for H2O, NH3 and SO2 is reasonably high due to polarities of molecules. Molar heat of vaporization for iodine is the highest amongst its family members due to its greater polarizability. Similarly, hexane C6H14 has the highest molar heat of vaporization values amongst the hydrocarbon due to large size of molecules. Actually the London dispersion forces in 12 and C6H14 are sufficiently strong and are responsible for solid behavior of iodine.


Q: How physical state is changed by dynamic equilibrium?

Ans. Dynamic equilibrium is a situation when two opposing changes occur at equal rates. When their opposing effects are cancelled then the state of dynamic equilibrium exists. Being a chemist we should know that the concept of dynamic equilibrium is the fate or the ultimate goal of all the reversible chemical reactions and all the physical changes.

So, if we have a piece of ice floating on water at 0°C then all the three phases exist and if the temperature is constant then there is a dynamic equilibrium. At 0°C solid water (Ice) exists in dynamic equilibrium with liquid water.

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