Entropy is an abstract physical quantity, which is derived by operation. Its physical meaning represents the disorder degree of the system. With the increase of disorder degree, entropy increases; On the contrary, entropy decreases.

For example, the melting of candles and ice cubes, the decomposition reaction in chemistry, the first process of ionization, and the mixing of ethanol and water are all processes of entropy increase. On the contrary, the process entropy decreases.

Properties of entropy

1, state function

Entropy s is a state function with additive (capacity) properties (that is, for system M, it can be divided into M 1 and M2, and then there is SM=SM 1+SM2), which is a non-conservative quantity with a width, because the heat in its definition is directly proportional to the quantity of matter, but the determined state has a definite quantity. Its variation δ s only depends on the constant state of the system, and has nothing to do with whether the process is reversible or not.

Since the change value of the system entropy is equal to the sum of the thermal Wen Shang δQ/T of the reversible process, the entropy change of the system can only be obtained through the reversible process. Reversible change process of isolated system or adiabatic reversible change process δ S = 0.

2. Macro quantity

Entropy is a macroscopic quantity, which is the property of a large number of microscopic ions that constitute the system. It includes the entropy contributed by molecular translation, vibration, rotation, electron motion and nuclear spin motion, and it is meaningless to talk about the entropy of a single microscopic particle.

3. Absolute value

The absolute value of entropy cannot be determined by the second law of thermodynamics. The absolute value of entropy can be determined by the third law according to calorimetric data, which is called prescribed entropy or calorimetric method. The absolute value of entropy can also be calculated from molecular microstructure data by statistical thermodynamics, which is called statistical entropy or spectral entropy.