What is Entropy

Thermodynamics is an important concept that helps understand entropy and how it works. So, what is Entropy? It is a thermodynamic physical property applied across different aspects of physics and chemistry. The state of randomness or disorder in the molecular system is entropy. Rudolf Clausius first introduced the concept of entropy in 1850. It is majorly explained and applied as a property of the classical thermodynamic system. The disorder in the system is known as entropy, and the greater the disorder, the greater is the entropy. It is a heat-generating process. Entropy’s meaning is central to the second and third law of thermodynamics. The topics discusses entropy and how it is applied in the system. 

Entropy? Meaning and calculation 

• The entropy is a measurement of randomness in the system. Higher is the randomness in the system, higher is the generation of heat. 

• Entropy is a reversible process. The units of entropy are units of Joules per Kelvin (J.K-1) or kg.m2.s-2.K-1. 

• A highly organised system has low entropy, whereas a highly disorganised system has high entropy. 

• The entropy is measured based on structural irregularity. The solids have the lowest entropy as solids have a regular arrangement. The gases have higher entropy values due to their disorganised arrangement.  

Entropy is denoted by S, and the change in entropy is denoted ΔS. The total entropy is any kind of spontaneous process or system which can be shown as:

The temperature is inversely proportional to entropy. The change in entropy is defined as the change in the heat (ΔQ) and is reversibly proportionate to the absolute temperature. During the entropy change, the formula of entropy is 

∆S = ΔQ/T

Where ∆S = change in entropy

ΔQ = amount of heat emitted or absorbed isothermally 

T = temperature 

When the system reaches a maximum at equilibrium, the change in entropy becomes zero. This is represented by: 

ΔStotal = ΔSsystem + ΔSsurroundig

In the thermodynamics and physical chemistry domain, the entropy is related to the internal energy of the system by the following equation:

dU = TdS – pdV

where

dU = change in the internal energy

T = absolute temperature 

dS = change in entropy

p = external pressure 

dV = change in volume 

What is entropy according to the First Law of Thermodynamics?

According to the First Law of Thermodynamics, energy is neither created nor destroyed, but energy is conserved. The change in the system’s internal energy is equal to the sum of heat gained or lost by the system. Hence according to the first law of thermodynamics:

• There is an increase in entropy with change in the state of the matter. This means the solid state has the lowest entropy, whereas the gaseous state has the highest. 

What is entropy according to the Second Law of Thermodynamics?

Thermodynamics is the study of heat change or heat transfer in a system. It helps in studying the transfer of heat from one body to another. According to the second law of thermodynamics, the total entropy of a closed system cannot decrease. The entropy of one system is decreased by raising the entropy of another system. According to the second law of thermodynamics, all spontaneous processes require a net increase in entropy of the system, and hence it continuously increases the entropy. Hence, it can be stated as:

“In any spontaneous process, the rate of entropy in the universe always increases”. Hence, the entropy of the universe is always increasing. It is represented by 

ΔSuniverse = ΔSsystem + ΔSsurrounding (Always>0)

ΔSsystem = 0 for a system in equilibrium

ΔSsystem≠0 for an irreversible process

So, for a spontaneous and irreversible system, the value ΔS>0.

What is entropy according to the Third Law of Thermodynamics?

As the absolute temperature approaches zero, the entropy of the pure crystalline solid approaches zero. The system’s randomness or entropy is minimum when the absolute temperature is zero. The system’s heat reduces with the decrease in temperature, and the absolute temperature is the lowest. The third law of thermodynamics is based on the fact that the system’s entropy is perfectly ordered at absolute zero temperature. 

Entropy and Examples of Entropy 

What is Entropy? Examples of entropy help in understanding its applications. Here are some examples of entropy and its application in everyday life:

• When the hot and cold water is mixed, it leads to an increase in entropy. The increase in the entropy leads to degradation of the energy from higher to lower levels with no work done. 

• Dissolving the salt in the water increases entropy because of the breakdown of the salt crystals. The water splits the sodium and chlorine atoms. This helps with the free movement of ions in water molecules, increasing entropy. 

• The cooling of food items, especially the conversion of water into ice, is an example of a decrease in entropy.

Conclusion

Entropy is associated with various day to day activities and hence is the change in the state of energy accompanied by heat. So, what is entropy thermodynamically? It is represented by the first, second, and third law of thermodynamics which helps in a change in heat transfer. When the randomness in the system is high, it is marked by the high rate of entropy, whereas the lower is the randomness of molecules in the system; it has low entropy.