Question

Can entropy change be zero?

Answer 1

Entropy tells the disorder or randomness in the system, and it is caused when the molecules move in a zig-zag motion. It is denoted by $\Delta S$, and it is defined for both the reversible and irreversible process.

Complete step-by-step answer: There are two types of processes, i.e., reversible and irreversible. A reversible process is a process in which the process starts from step 1 to step 2 then it returns to step 1. Irreversible processes are those in which the process starts from 1 to step 2 but doesn't return back to step 1.
Entropy tells the disorder or randomness in the system, and it is caused when the molecules move in a zig-zag motion. It is denoted by $\Delta S$, and it is defined for both the reversible and irreversible process.
According to the second law of thermodynamics, the entropy of the system, i.e., the disorder can never be negative it can be either positive or zero.
For a reversible process, the change in entropy is zero because of the increase in entropy when the process moves forwards, and then it decreases when it moves backward.
For an irreversible process, the change in entropy will always be positive because when the reactant changes to the product there is always an increase in the randomness.
So, the entropy change can be zero for the reversible process.

Note: Mathematically we can calculate the change in entropy by taking the ratio of change in heat of the system by the temperature of the system. It is given below:
$\Delta S=\frac{\Delta q}{T}$