Characteristics Of Entropy

Entropy is an intrinsic property of a substance and is not affected by the external position of a system or its motion relative to other systems. It is a measure of the randomness of the system. Thus, the higher the randomness of the system, the higher the entropy of the system and the lower the randomness of the system, the lower the entropy of that system.

Entropy 

"Entropy is defined as a measure of the system's thermal energy that is incapable of doing work". It is a state function. 

For Example

Heat is a form of energy that is incapable of doing work. Therefore, the more heat added to the system (or tranfer from the system), the higher (or the lower) will be its entropy. 

The formula for change in entropy is given by the equation; 

                   ➩ ∆S = ∆Q/T

For a reversible process, the change in entropy is given as;

The Total Change In Entropy

Entropy change is zero at equilibrium condition or whenever the process becomes reversible;

                  ➩ [ (∆S)total ≽ 0 ]

This mathematical statement of the second law confirms that every process proceeds in such a direction that the total entropy change associated with it is always positive and the limiting value of zero is achieved only by a reversible process.

Characteristics of Entropy

The existence of entropy is due to the second law from which it arises in the same way that internal energy arises from the first law. Let us look at the characteristics of entropy;

(1) Entropy is usually measured in per mol. Therefore its unit becomes;

                   " J/K-mol "

(2) Since, entropy is a state function therefore the change in entropy is the same for both reversible and irreversible processes. And also for a complete cycle, the total change in entropy is zero.

          ➩∆SAB(reversible) ＝ ∆SAB(irreversible)

While for reversible process;

                  ➩ (∆S)AB ＝ – ∆(∆S)BA

            And;

                 ➩ ∆S(cycle) ＝ ∳(dQ/T) ＝ 0

(3) Entropy is a state function, which means that it does not depend on the process path taken from the initial state to the final state, but on the initial and final state itself. Thus,

                ➩ { ∆S = S(final) - S(initial) }

Whereas in case of cyclic process, the change in entropy is zero. 

                ➩ { ∆S(cyclic process) = 0 }

(4) Entropy is an extensive properties which means that it's value does not depend on the amount/mass of substance whereas specific entropy is an intensive property that is why it's value depends only on the amount/mass of substance.

(5) FOR AN ISOLATED SYSTEM

The entropy of an isolated system always increases as the randomness or disorder within that system increases continuously. Therefore;

           ➩ ∆S(isolated system) > 0

Since, in case of an isolated system;

         ∵ ∆Q =0 ➩ ∆S =∆Q/T ➩ ∆S=0 

But this does not happen because within an isolated system, the randomness is always increasing.

(6) FOR A NON-ISOLATED SYSTEM 

where the system and the surroundings interact with each other, if the entropy of the system decreases then the entropy of the surrounding increases. Or if the entropy of the system increases, the entropy of the surrounding decreases. Thus;

      (∆S)sys >0 ,then (∆S)surr <0

      (∆S)sys <0 ,then (∆S)surr >0

And overall;

                 ➩ (∆S)total > 0

(7) The entropy of the universe always increases. And the change in entropy will be zero when the process is "Adiabatic Reversible Process".

For adiabatic reversible process;

(8) FOR AN ISOTHERMAL REVERSIBLE PROCESS;

( no temperature difference between system and surrounding)

In this case where the system and its surroundings have the same temperature, the total change in entropy is zero. While on the other hand, the change in entropy of the system and its surroundings is not zero. Thus,

                      ➩ (∆S)total ＝ 0

            ➩ (∆S) system = (∆S) surroundings ≠ 0

(9) The entropy of a process also tells us about the exothermic and endothermic behavior of a process. It tells us that,

  ∆S(system)<0 &             ➝ "Exothermic" 

       ∆S(surrounding)>0

  ∆S(system)<0 &             ➝ "Endothermic" 

      ∆S(surrounding)>0

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