# 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 as entropy. Entropy is generally defined as the degree of randomness of a macroscopic system.

Internal interactions between various subsystems giving multiple entropy changes.

### What is Entropy Change?

Since entropy is a state function, the entropy change of a system depends only on 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, 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 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.

Addition of heat to a system increases the randomness in the system due to increase in molecular motions. Generally, a system at a higher temperature has greater randomness than at lower temperature. Thus, temperature too helps in the measurement of randomness of particles in a system. Heat added to a system at lower temperature causes greater randomness than in comparison to when heat is added to it at a higher temperature. Thus, 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:

$ΔS$ = $\frac{q_rev}{T}$

Where, $q$ = heat
$T$ = temperature

For a spontaneous process, entropy change for the system and the surrounding must be greater than zero, that is $ΔS_{total}~\gt~0$. The general expression can be given as:

$ΔS_{total}$ = $ΔS_{sys}~+~ΔS_{surr}~\gt~0$