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Energy Changes During Transitions or Phase Changes

Energy Changes During Transitions or Phase Changes

 The three states of matter – solid, liquid and gas differ from one another in the arrangement of their constituent particles.
 The magnitudes of intermolecular forces acting between the particles in these states are also different.

 It is a common observation that when a solid is converted into the liquid state, energy is to be supplied. This energy is spent in breaking the intermolecular forces in the solid which are of high magnitude.

 Whenever there is a change in the state of matter (solid → liquid or liquid → gas), the process is called phase change or transition. It is also accompanied by the change in enthalpy or heat content of the system.

(1) Heat of Fusion

 Heat of Fusion is defined as: the heat change (or enthalpy change) when one mole of a solid substance is converted into the liquid state at its melting point.

 As an example, we can take the melting of one mole of ice at its melting point, 0ºC or 273 K. The process can be represented as:

and is accompanied by the absorption of 1.43 kcal of heat. From the values of fusion of various substances we can compare their magnitudes of intermolecular forces.

 Greater the heat of fusion of a substance higher the magnitude of intermolecular forces.

(2) Heat of Vaporization

 The heat of vaporization is defined as: the heat change (or enthalpy change) when one mole of liquid is converted into vapour or gaseous state at its boiling point.

 For example, when one mole of water is converted into steam at 100ºC or 373 K, the heat absorbed is 9.71 kcal which is the heat of vaporization of water. The change can be represented as:

 The heats of vaporization of ethyl alcohol (C2H5OH) and benzene (C6H6) are 7.29 kcal mol-1 and 7.36 kcal mol–1 respectively. The values of heats of vaporization can also be used for the comparison of the magnitude of intermolecular forces of attraction in liquids.

(3) Heat of Sublimation

 Sublimation is a process when a solid changes directly into gaseous state without changing into liquid state. It occurs at a temperature below the melting point of the solid.

 Heat of sublimation is defined as: the heat change (or enthalpy change) when one mole of a solid is directly converted into the gaseous state at a temperature below its melting point.

 For example, the heat of sublimation of iodine is 14.92 kcal mol–1. It can be represented as:

(4) Heat of Transition

 The heat of transition is defined as: the change in enthalpy which occurs when one mole of an element changes from one allotropic form to another.

 For example, the transition of diamond into amorphous carbon may be represented as:

where – 0.016 kcal and – 1.028 kcal are heats of transition of monoclinic sulphur to rhombic sulphur and white phosphorus to red phosphorus respectively.

Reference: Essentials of Physical Chemistry /Arun Bahl, B.S Bahl and G.D. Tuli / multicolour edition.

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