Under certain conditions, the nature of the system no longer changes with time, and its state is certain. A series of physical quantities representing the state of a system are called state functions. Sometimes it is also called thermodynamic potential, but "thermodynamic potential" refers to four thermodynamic functions with energy dimensions, such as internal energy, enthalpy, Gibbs free energy and Helmholtz free energy.

The state function characterizes and determines the macroscopic properties of the system state. The state function has a definite value only for the equilibrium system, but not for the non-equilibrium system. When solving various thermodynamic functions, it is usually necessary to do path integration. If the integration result has nothing to do with the path, the function is called a state function, otherwise it is called a non-state function.

If the attribute A of the system is defined, in the state 1, A has the value A1; In state 2, there is a value A2. No matter how to realize the transition from 1 to 2, the difference between the two states dA≡A2-A 1 is constant, so a is called a state function. Such as temperature, pressure, volume, density, energy, shape, etc. , and thermodynamic functions, such as u (internal energy), h (enthalpy), g (Gibbs function), f (free energy), s (entropy), etc. , can be defined as the nature of the system independent of path, but work and heat can't, because work and heat can't be related to the specific state of the system.

introduce

The comprehensive performance of all macroscopic properties (chemical properties and physical properties) of the system is the state. That is to say, thermodynamics uses the macroscopic properties of the system to determine its state. Therefore, when the macro nature of the system is determined, the system should have a certain state. On the other hand, after the state of the system is determined, various macro attributes will have certain values. Therefore, the macroscopic property of the system should be a single-valued function of its state. Therefore, thermodynamics calls various macroscopic properties state functions. These macroscopic properties are determined with the determination of the state and change with the change of the state.

The state function is an attribute determined by the state of the system. When the state is fixed, the value of the state function is also fixed. If the state changes, the change value of the corresponding state function is only related to the initial state and the final state of the system, and has nothing to do with the specific process experienced between the initial state and the final state. Temperature, pressure, volume and internal energy are all state functions. For example, the system changes from1.01325x0/0pa273k to 3.03975x010pa298k, the pressure changes by 2.02650x010pa, and the temperature changes by 25K, regardless of the specific process. The differential of the state function must be fully differential.

trait

1, the change value of the state function only depends on the initial state and final state of the system, and has nothing to do with the intermediate change process; Not all state functions are independent, but some are interrelated and restricted. For example, for a common temperature-volume thermodynamic system, only three of the four are independent, and the equation of state f(p, V)=0 is often related between p and v (such as PV in an ideal gas).

2. The small change dX of the state function is completely differential. The integration of total differential has nothing to do with the integration path. Using these two characteristics, we can judge whether a function is a state function.

3. It is single-valued.

4. The set of state functions (sum, difference, product and quotient) is also a state function.