Question

Why is change in entropy zero for reversible process and non zero for irreversible?

Answer 1

In a reversible process the change in entropy is zero. The reason is in a reversible process the products are converted back to the reactant again that means the original state of matter is achieved. If there is no change in state then the entropy is zero. Because entropy is a measure of randomness. The entropy of various state are gases > liquid > solid.

Whereas in an irreversible process there is definitely there is change in state or composition occur and due to which the entropy value changes. So change in entropy value is nonzero.