Question

What is the definition of entropy?

Answer 1

Entropy by definition is the degree of randomness or disorder (chaos) in a system.

Explanation:

Entropy by definition is the degree of randomness or disorder (chaos) in a system.

Here is a complete lessons about entropy, I hope you find it helpful.
Thermodynamics | Spontaneous Process & Entropy.

Answer 2

Entropy (`S`) is a measure of the number of ways the microstates in a system can arrange themselves to form a single observable macrostate.

That is, it is a quantity that describes the number of microscopic arrangement possibilities for a given system.

STATISTICAL MECHANICS DEFINITION

`\mathbf(S = k_BlnOmega)`

where:

• `Omega` is the number of microstates that collectively generate the same macrostate (observable).
• `k_B = 1.3806xx10^(-23) "J/K"` is the Boltzmann constant.

Take an ensemble (loosely-speaking, a group) of molecules that can be arranged in multiple ways.

The more ways you can arrange them, the more "disordered" they are. This corresponds with a greater `Omega` giving a greater entropy `S`.

THERMODYNAMICS DEFINITION

A consistent thermodynamic definition of entropy is also:

`\mathbf(DeltaS >= (q)/T)`

(where `q_"rev"` is reversible, i.e. efficient, heat flow, `q_"irr"` is irreversible, inefficient heat flow, and `q_"irr" < q_"rev"`. Both `q_"irr"` and `q_"rev"` are contained in `q`.)

So another way you can think about it is that for a given temperature:

The more the heat that you put into the system affects the microscopic arrangement of molecules, the more "disordered" the system is.

GENERAL CHEMISTRY DEFINITION

This "disorder" is a definition of entropy you were introduced to in general chemistry, and is generalized to be greater for gases than for liquids, for instance.

Gases are more freely-moving than liquids, so gases can assume more microstates than the liquid phase of the same substance can. Thus, gases are more "disordered", and they have a higher entropy.