What Are Thermodynamic Properties?

The thermodynamic properties of an object refer to the law of change between pressure P, volume V, temperature T, composition, and other thermodynamic functions when the substance is in equilibrium.

Generally, the pressure P, volume V, temperature T, internal energy U, enthalpy H, and entropy S of an object are collectively referred to as the thermodynamic properties of the object. ^[1]

The ratio of pressure to the area of an object is called pressure. Pressure is used to compare the effects of pressure. The larger the pressure, the more obvious the effect of pressure. The formula for calculating pressure is: p = F / S, the unit of pressure is Pascal, and the symbol is Pa.

The methods of increasing the pressure are: increasing the pressure under the condition of constant force area or decreasing the pressure area under the condition of constant pressure. The methods of reducing the pressure are: reducing the pressure when the force area is constant or increasing the force area when the pressure is constant.

The liquid has pressure on the side wall and bottom of the container, and the pressure increases with the depth of the liquid.

The characteristics of liquid internal pressure are: liquid has pressure in all directions from the inside; pressure increases with depth; at the same depth, the pressure of liquid in all directions is equal; liquid pressure is also related to the density of the liquid, the greater the density of the liquid , The greater the pressure. The internal pressure of a liquid can be measured with a pressure gauge.

Temperature (temperature) indicates how cold or hot an object is

Internal energy is the randomness of the molecules that make up an object

An important state parameter that characterizes the energy of the material system in thermodynamics, which is usually represented by the symbol H. The physical meaning of enthalpy is an energy in which the thermodynamic energy of the system is added to the energy of PV.

Enthalpy

Entropy, one of the parameters that characterizes the state of matter in thermodynamics, is represented by the symbol S, and its physical meaning is a measure of the degree of chaos in the system. ^[3]

T. Clausius put forward the concept of entropy in 1854. In 1923, Chinese physicist Professor Hu Gangfu first translated entropie into "entropy" according to the meaning of thermal temperature quotient. A. Einstein once summarized the status of entropy theory in science as "the first law of entropy theory for the whole science". Charles Percy Snow wrote in his book Two Cultural and Scientific Revolutions: "A humanist who knows nothing about thermodynamics and a scientist who knows nothing about Shakespeare Oops. Not long after the law of entropy was established, JC Maxwell put forward a famous paradox to try to prove that an isolation system will automatically change from thermal equilibrium to imbalance. In fact, the system inputs energy and information into the so-called "isolation system" through the work of the Maxwell demon. This system is actually a "self-organizing system".

The second law of thermodynamics, based on the principle of entropy, has been regarded as a fallen source in history. American historian H. Adams (1850-1901) said: "This principle only means that the volume of ruins is constantly increasing." Some people even think that this law indicates that the race will change from bad to worse and eventually die out. The second law of thermodynamics was the worst law in society at that time. Society is essentially different from a thermodynamic isolation system, but rather a "self-organizing system."

Thermodynamic property state function

Entropy S is a state function, with a sum (capacity) property, and is a widely-measured non-conserved quantity, because the heat in its definition is proportional to the quantity of matter, but the determined state has a certain amount. The amount of change S is only determined by the system's permanent state and has nothing to do with whether the process is reversible or not. Since the change in the entropy of the system is equal to the sum of the thermal temperature quotient Q / T of the reversible process, the entropy change of the system can only be obtained through the reversible process. The reversible change of the isolated system or the adiabatic reversible change process S = 0. ^[4]

Thermodynamic properties

Entropy is a macroscopic quantity, which is a property manifested collectively by a large number of microscopic ions constituting the system. It includes the entropy contributed by the translation, vibration, rotation of electrons, and the motion of nuclear spins. It is meaningless to talk about the entropy of individual microscopic particles.

Absolute value of thermodynamic properties

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 calorimetry. The absolute value of entropy can also be calculated from the microstructure data of the molecule using statistical thermodynamics, called statistical entropy or spectral entropy.

Entropy was originally a material state parameter reflecting the irreversibility of spontaneous processes, which is derived from the second law of thermodynamics. The second law of thermodynamics is a rule summarized based on a large number of observations: in an isolated system, there is no energy exchange between the system and the environment, and the system always spontaneously changes in the direction of increasing chaos, always increasing the entropy value of the entire system This is the principle of entropy increase. Friction irreversibly transforms part of the mechanical energy into heat, which increases the entropy. Therefore, the entire universe can be regarded as an isolated system, which has evolved in the direction of increasing entropy.

Consider from a spontaneous process: the heat Q is transferred from the high temperature (T1) object to the low temperature (T2) object, the entropy of the high temperature object decreases dS1 = dQ / T1, the entropy of the low temperature object increases dS2 = dQ / T2, Looking at the objects together as a system, the change in entropy is dS = dS2-ds1> 0, that is, the entropy is increased.