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  Entropy for reversible process

Entropy for reversible process:

ΔS system = - ΔS surroundings

∴ ΔS universe = ΔS system + ΔS surroundings = 0.

For an irreversible (i.e., spontaneous) process:

ΔS universe > 0 (i.e., positive value)

Entropy change in a physical (i.e., phase) transformation can be found out mainly for evaporation of liquids at the boiling points, by using Trouton’s rule. According to this rule, the heat of vaporization (ΔHvap) in calories per mole divided by the boiling point of the liquid in Kelvin is a constant equivalent to 21 cal deg-1mole-1 and is acknowledged as the entropy of vaporization.

ΔS vap  = (ΔH vap)/Tb
= 21 cal deg-1mole-1

Here,
ΔH vap = Enthalpy change of vaporization = Latent heat of vaporization.

This equation is useful for estimating the molar heat of vaporization of a liquid of identified boiling point. Substances which deviate from this rule are as follows:

1) Low boiling liquids like hydrogen and Helium that boil only a little above 0K.

2) Polar substances such as alcohol, water, that form hydrogen bonded liquids and display very high boiling points and also high ΔH vap.

3) Liquids like acetic acid whose molecules are partially related in the vapor phase and possess very low entropy vaporization that is very much less than 21 cals/mol/deg.

Those liquids which follow Troutons rule are said to behave ideally.

 

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