# Measurement of Entropy Change

For a reversible change at constant temperature the change in entropy is equal to heat absorbed or evolved divided by the constant temperature in Kelvin.

Thus,

The unit of entropy in . The value of AS is positive if heat is absorbed and negative if heat is evolved. The entropy change in melting a solid can be calculated if enthalpy of fusion is known.

Similarly, the entropy change for vaporisation of a liquid into its vapour at its boiling point can be calculated if enthalpy of vapourisation is known.

Similarly the entropy of sublimation is given as

It is important to note that, the entropy change for a reaction carried out reversibly is different from a reaction carried out irreversibly. In other words, for an irreversible spontaneous change,

For a reversible change,

*Entropy changes for Ideal gases:*

**(A)** For change of state: (Initial to final)

**(B)** For isothermal process:

**(C)** For Isochoric process:

(D) For Isobaric process:

*Some examples of increase or decrease of entropy:*

On stretching a rubber band the entropy decreases as the coiled macro molecule become un-coiled i.e. randomness in structure decreases.

* Increase of entropy losses:*

On boiling an egg entropy increases as due to denaturation the helical structure of protein become more complicated and random coiled structure.

In a solution of is very high due to extensive H-bonding.

**Trouton’s rule:** According to .it of most of the liquids is / mole K at normal B.P.

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