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Entropy Change

What is Entropy?

The decrease in enthalpy may contribute to the spontaneity of a process but it cannot explain spontaneity in all the cases. Since we acknowledge many spontaneous endothermic reactions around us. For example, an isothermal reversible expansion of an ideal gas, where change in enthalpy,

ΔH= 0

Hence, we define a new state function to explain the spontaneity of a process. This state function is named entropy. Entropy is generally defined as the degree of randomness of a macroscopic system.

Entropy Change

Internal interactions between various subsystems giving multiple entropy changes.

Table of Contents

What is Entropy Change?

Since entropy is a state function, the entropy change of a system depends only on the initial and final state irrespective of the path taken. Hence, change in entropy does not differ with the nature of the processes either reversible or irreversible.

Thus, the greater the disorderliness in an isolated system, the higher is the entropy. In a chemical reaction, the change in entropy can also be attributed to the rearrangement of atoms or ions from one pattern to another. In the products, if the molecules are very much disordered in comparison to the reactants, there will be a resultant increase in entropy during the reaction. Thus, the change in entropy accompanying a chemical reaction can be estimated qualitatively by considering the disorderliness of the structures of the species involved in the reaction. For example, the crystalline solid state generally exhibits lower entropy in comparison to other solids.

The addition of heat to a system increases the randomness in the system due to an increase in molecular motions. Generally, a system at a higher temperature has greater randomness than at a lower temperature. Thus, temperature too helps in the measurement of the randomness of particles in a system. Heat added to a system at a lower temperature causes greater randomness than in comparison to when heat is added to it at a higher temperature. Thus, the entropy change is inversely proportional to the temperature of the system.

Expression for entropy change

The general expression for entropy change can be given by:

\(\begin{array}{l}ΔS\end{array} \)
\(\begin{array}{l}\frac{q_rev}{T}\end{array} \)


\(\begin{array}{l}q\end{array} \)
= heat
\(\begin{array}{l}T\end{array} \)
= temperature

For a spontaneous process, entropy change for the system and the surrounding must be greater than zero, that is

\(\begin{array}{l}ΔS_{total}~\gt~0\end{array} \)
. The general expression can be given as:

\(\begin{array}{l}ΔS_{total}\end{array} \)
\(\begin{array}{l}ΔS_{sys}~+~ΔS_{surr}~\gt~0\end{array} \)

Frequently Asked Questions – FAQs

What is Entropy?

Generally, entropy is defined as a measure of randomness or disorder of a system.

Can entropy ever decrease?

It just says that the total entropy of the universe can never decrease. Entropy can decrease somewhere, provided it increases somewhere else by at least as much. The entropy of a system decreases only when it interacts with some other system whose entropy increases in the process. That is the law.

Can entropy be infinite?

Since no finite system can have an infinite number of microstates, it’s impossible for the entropy of the system to be infinite. In fact, entropy tends toward finite maximum values as a system approaches equilibrium.

Can entropy be negative?

So if entropy is the amount of disorder, negative entropy means something has less disorder or more order. The shirt is now less disordered and in a state of negative entropy, but you are more disordered and thus the system as a whole is in a state of either zero entropy or positive entropy.

What causes entropy?

Several factors affect the amount of entropy in a system. If you increase temperature, you increase entropy.
More energy put into a system excites the molecules and the amount of random activity.
As the gas expands in a system, entropy increases.

For more information on the entropy formula and the effect of entropy on the spontaneity of a process, download BYJU’S – The Learning App.

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