Thermodynamics is the study of the changes in energy associated with the change in temperature and heat. It also deals with the work done for the conversion of energy from one form to another. Three laws govern the science of thermodynamics and here we will discuss the second law of thermodynamics. The second law of thermodynamics talks about the concept of entropy and tells that the entropy of the universe is always increasing. By this law, the entropy of the universe can never be negative. So, let's understand this concept of entropy and the change in entropy.


Entropy 

Entropy is the measure of disorder or randomness. This randomness could be in regards to the entire universe or a simple chemical reaction or something as simple as the heat exchange and heat transfer. The term disorder denotes the irregularity or lack of uniformity of a thermodynamic system.


The entropy is denoted by ‘S’ and it is an extensive property because the value of entropy or entropy change is dependent on the substance present in a thermodynamic system. Entropy is an interesting concept as it challenges the belief of complete heat transfer. It helps redefine the second law of thermodynamics. 


Entropy relates to spontaneity i.e.; the more is the spontaneity in a thermodynamic process, the higher is its entropy or the degree of disorder. In simpler words, entropy gives us an idea about that portion of energy that does not convert into work done and adds to the disorder of the system instead. Since energy gives the ability to get work done, it is practically impossible for all the energy to be used in doing work. Entropy gives us a measure of that.


As is clear from the law of thermodynamics that energy can neither be created nor destroyed but can be converted from one form to another, it is not possible to signify entropy at a single point, and hence, it can be measured as a change. That is why we calculate the entropy change.


Entropy Change 

Entropy change can be defined as the change in the state of disorder of a thermodynamic system that is associated with the conversion of heat or enthalpy into work. A system with a great degree of disorderliness has more entropy.


Entropy is a factor of state function i.e., its value does not dependent on the pathway of the thermodynamic process and it acts as the determinant of only the initial and final state of the system. In the rule of chemical reactions, the changes in entropy occur as a result of the rearrangement of atoms and molecules that change the initial order of the system. This may either lead to an increase or a decrease in the randomness of the system and hence, will lead to an increase or a decrease in the entropy respectively.


Change in Entropy Formula Thermodynamics

The entropy change of a thermodynamic system is represented as ΔS. We can calculate the entropy change of a chemical reaction or a system by using the change in entropy formula:

ΔS=(Q/T)rev

Where, 

Q is the heat transfer to or from the thermodynamic system

T is the absolute temperature. 

The SI unit of entropy change is J/Kmol


Example: 

The entropy of vaporization of water can be calculated by dividing the heat of vaporization with the boiling point i.e., 1000C or 373 K.

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Did you know?

The physicist Clausius discovered the concept of entropy with the help of a steam engine and he coined the term entropy because it sounded similar to the word energy.

The formula for entropy changes of the universe can be denoted through the following change in the entropy equation: 

ΔSuniverse = ΔSsystem+ ΔSsurrounding

This change in entropy formula provides an idea about the spontaneity of a process or a chemical reaction. 

For a spontaneous process, there is an increase in entropy leading to ΔStotal to be greater than zero. 

Now let’s discuss further how does the change in entropy varies with different processes and conditions:


Entropy Change with Temperature:

Considering the formula for entropy change it is clear to conclude that the change in entropy is increased when heat transfer occurs at a lower temperature and the entropy change is more for the same at a higher temperature. 


Entropy Change in a Reversible Process:

In conceptual terms, the entropy change definition applies to a reversible process. Thus, the change in entropy of the reversible process is the same as described above. 


Entropy Change in an Irreversible Process:

From a practical point of view, no process can be irreversible process is considered. As discussed above, the entropy is dependent only on the initial and final state of the system irrespective of the pathway of the thermodynamic process.

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Thus, the change in entropy is the same for an irreversible and a reversible process as it is independent of the pathway. This concept is also used in determining the entropy change for an ideal gas as it is an irreversible non-quasi static process.


Characteristics of Entropy:

The important characteristics of the entropy of a thermodynamic system are as follows:

  • Entropy denotes the tendency of the universe to move towards disorder or randomness.

  • Entropy can be denoted as a function of the enthalpy or heat that can be converted into work. 

  • Entropy depends on the mass of a thermodynamical system. It does not depend on the path of heat exchange or heat conversion and that is why it is an extensive property.

  • The entropy of the universe keeps increasing

  • The change in entropy for the adiabatic process is zero hence, it has constant entropy.

FAQs (Frequently Asked Questions)

1. What are Entropy and Entropy Change?

Answer: Entropy represents the total degree of disorder or non-uniformity present in a thermodynamic system. Entropy denotes the heat energy that could not be converted into the work of a system. It is an important concept in the laws of thermodynamics as entropy entails that the universe is always shifting to a higher degree of disorderliness with time. The entropy of the universe is always increasing. Some degree of disorder occurs during a chemical reaction where two chemicals interact and there is exchange or rearrangement of atoms, molecules, or chemical bonds. This rearrangement contributes to the increase in disorder and hence increases the entropy. Entropy change refers to the difference in the initial and final state of a system after heat transfer.

2. What are the Properties of Entropy?

Answer: Some of the properties of entropy are:

  • It is an extensive property i.e., it only depends on the mass of a system

  • The entropy of the universe is always increasing

  • The entropy can never be zero

  • The entropy of an adiabatic thermodynamic system remains constant

  • Change in entropy is inversely proportional to the temperature i.e. if the temperature is increased then the change in entropy will be of smaller magnitude whereas when the temperature is reduced there is more change in entropy of the system

  • Change in entropy for a cyclic process is zero as the state does not change

  • Change in total entropy for reversible process ΔStotal = 0 hence, ΔSsystem = - ΔSsurrounding

  • Change in total entropy for an irreversible or spontaneous process is more than 0.