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  Characteristics of entropy

Characteristics of entropy ‘S’:

1) The word ‘S’ entropy is develop from the formulation of II law of thermodynamics as thermodynamic state function.

2) Entropy change ‘ΔS’ of a system beneath a procedure is stated as the constant equivalent to the ratio of the heat change associated a process at constant temperature to the temperature of the system beneath process. The process must be reversible at that temperature.
 
Heat, q is not a state function, however for a reversible process, Δq = (q2-q1) divided by temperature (T) of the process is a state function.

3) A spontaneous procedure is accompanied by raise in the ‘disorder’ or ‘randomness’ of the molecules comprising the system. Entropy rises in all spontaneous processes. Therefore entropy might be regarded as the measure of disorder or arbitrariness of the molecules of the system.

4) Whenever a system undergoes a physical or a chemical procedure, there takes place a change in the entropy of the system and as well in its surroundings. This total change in the entropy of the system and its surroundings is referred as the entropy change of the universe brought around by the procedure. For an isothermal process (i.e., T=constant), the entropy change of the universe during a reversible procedure is Zero.

The entropy of the universe rises in an irreversible process.

5) The energy of the universe stays constant however the entropy of the universe tends to maximum.

6) For a spontaneous procedure, at constant T, ΔS is positive (i.e., ΔS > 0).

ΔS is positive (i.e., ΔS > 0). For an equilibrium procedure, ΔS is zero.

For non spontaneous process,

ΔS is negative or (i.e., ΔS < 0).

7) Units of entropy:

The dimension of entropy is energy in terms of heat X temperature-1. The entropy is stated as calories per degree that is termed to as the entropy units (i.e., eu).  As entropy also depends on the amount of the substance, unit of entropy is calories per degree per mole or eu. Per mole.

cgs units of entropy is cal.K-1 symbolized as eu. The SI unit is JK-1 and symbolized as EU. 1 eu = 4.184 EU.

8) Entropy change is associated to enthalpy change as shown below:

For reversible and isothermal process,

ΔS = (Δq rev)/T

As ΔH is the heat absorbed or evolved in the process at constant T and pressure P. ΔS is also computed from ΔH as ΔS = ΔH/T, where T is the temperature of the process including ΔH, quantity of enthalpy change, at constant pressure.

 

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