Entropy and enthalpy

Thank you, do you have the contact information for yjqin1 (number 149618)? :)

There is less entropy contact; simply put, it refers to the level of disorder. Enthalpy, as I understand it, is heat~ These two concepts are fundamental in thermodynamics; I would appreciate it if @yjqin1 could give some guidance, as his understanding is likely more accurate~

This post was last edited by colinzh on 2015-9-1 at 13:23. Entropy, along with enthalpy, heat, and work, are all extensive properties of matter; that is, they can be added together. Neither of them is a fundamental physical quantity; like the Gibbs function and the Helmholtz function, they are parameters created to better reflect the state of matter and thermodynamic change processes. Before discussing entropy, it is necessary to talk about enthalpy first; enthalpy represents the amount of heat that a substance contains, and its unit is the same as that of heat. Entropy is a measure of the degree of disorder in a substance. In chemical engineering, entropy generally refers to thermal entropy, which denotes the change in entropy resulting from differences in thermodynamic properties; its unit is a derived unit based on heat. Both entropy and enthalpy have values only in the context of a process. In practice, once a standard state is defined, entropy and enthalpy can also possess specific values, just like mass, to indicate the state of a substance. In chemical engineering, the energy balance in heat exchange processes is essentially a enthalpy balance, and the law that defines the standard state is the zeroth law of thermodynamics. Given other conditions remain constant, entropy can reach equilibrium only in reversible processes; otherwise, all processes are processes of entropy increase, and there are no processes in which entropy decreases – this is the third law of thermodynamics.

Colinzh’s answer is very professional； Enthalpy is a parameter that indicates the amount of heat; the greater the heat, the higher the enthalpy ； Entropy, in simple terms, refers to the degree of disorder of heat within a system. Among them, the more uniform the system is, the higher the entropy value. Therefore, in an isolated system, heat continuously spreads out to achieve uniformity; as a result, the entropy in such a system either remains constant or keeps increasing. This is the principle of entropy increase (the third law of thermodynamics). It is explained in detail in \"Engineering Thermodynamics,\" but it is not easy to understand

As the people above mentioned, entropy can be considered as the degree of disorder among molecules, or the extent of molecular disorder. As can be seen from the formula, enthalpy is the sum of internal energy and kinetic energy. For fluids within a control volume, enthalpy is convenient to use, as there is no need to calculate the kinetic energy separately.

Enthalpy can be considered as the sum of internal energy and the energy between molecules, while entropy can be regarded as the degree of disorder or randomness among molecules

Entropy is a quantity that indicates the degree of disorder in a system; the higher its value, the more disordered the system is； Enthalpy is the sum of the thermodynamic energy of a system and the energy between its molecules

Please, the original poster, delete my extra replies. I’m speechless about my broken computer

The answer on the 5th floor was very good; I’ll just add some of my personal experience. For a parameter, if you don’t understand what it means, look at its unit. For example, the unit of enthalpy is kJ/kg, which indicates how much energy is contained in each kilogram of substance. The unit of entropy is kJ/°C, and its physical meaning is the gradient of energy with respect to temperature

Enthalpy is a state function with the units of energy, but it does not have a precise physical meaning. Its meaning is defined by H=U+PV, and it should not be mistaken for \"the heat contained in the system.\" (As stated in the physics textbook) Professor Zhu Wentao mentioned in his physics video that it’s hard to explain what P*V means.