State function refers to the macroscopic properties characterizing the characteristics of the system, most of which refer to the energydimensionOfThermodynamic function(such as internal energyenthalpy、Gibbs free energyHelmholtzfree energy）。It is mainly used in the engineering field.
The state function only appliesEquilibrium stateThere are definite values for the system of.In addition, the state functions are interrelated and mutually restricted.
Under certain conditions, the properties of the system no longer change with time, and its state is determined. A series of physical quantities representing the system state are called state functions.[1]Sometimes it is also called thermodynamic potential, but "thermodynamic potential" more often refers to internal energy, enthalpy, Gibbs free energy, Helmholtz free energy, etcdimensionOfThermodynamic function。
The state function characterizes and determines the macroscopic properties of the system state.The state function only has a definite value for the system in equilibrium, and has no definite value for the system in unbalanced state.When solving various thermodynamic functions, path integral is usually required. If the integration result is path independent, this function is called a state function, otherwise it is called a non-state function.
If a property A of the system is defined, in state 1, A has the value A1;In state 2, there is a value A2. Regardless of the way to achieve from 1 to 2, the difference between the two states of A dA ∨ A2-A1 is always true, then A is called a state function.For example: temperaturepressure, volume, density, energy, form, etc., andThermodynamic function: U (internal energy), H (enthalpy), G (Gibbs function), F (free energy), S (entropy) can be defined as a path independent property of the system, andWorkAnd heat is not, because work and heat can not be associated with the specific state of the system.[2]
introduce
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The comprehensive expression of all macroscopic properties (chemical properties and physical properties) of the system is the state.This means that,thermodynamicsThe macro nature of the system is used to determine its state.Therefore, when various macro properties of the system are determined, the system should have a certain state.On the contrary, when the state of the system is determined, various macro properties will have certain values.Therefore, the various macroscopic properties of the system should be single valued functions of its state.So thermodynamics calls all kinds of macroscopic properties state functions.These macroscopic properties are determined with the determination of the state and change with the change of the state.[3]
The state function is a property determined by the state of the system.When the state is certain, the value of the state function is also certain. If the state changes, the change value of the corresponding state function is only related to the initial and final states of the system, regardless of the specific process experienced between the initial and final states.Temperature, pressure, volumeinternal energyAnd so on are state functions.For example,systemFrom 1.01325 × 10 Pa 273K to 3.03975 × 10 Pa 298K, the pressure change is 2.02650 × 10 Pa, and the temperature change is 25K, which has nothing to do with the specific process of change.The differential of the state function must be total differential.
features
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1. The change value of the state function only depends on the initial and final states of the system, and has nothing to do with the intermediate change process;Not all state functions areindependentSome are mutualrelationFor example, for an ordinary temperature volume thermodynamic system, only three of the four variables p (pressure), V (volume), T (temperature) and n (amount of matter) areindependentThe equation of state f (p, V)=0 is often associated between p and V (such as in ideal gaspV=nRT）。
2. The slightly changed dX of the state function is the total differential.The integral of total differential is independent of the integral path.These two characteristics can be used to determine whether a function is a state function.
3. It has single value.
4. . of the state functionaggregate(sum, difference, product, quotient) is also a state function.[4]
classification
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According to its nature, state functions can be divided into two categories:
One is capacity property (also called breadth property).Under certain conditions, the amount of such properties is only proportional to the amount of substances contained in the system, which is additive.Such as mass, volume, internal energy, etc.
The other is the strength property, whose value is independent of the amount of matter in the system, does not have additivity, and only depends on the characteristics of the system itself.Such as density, temperature, etc.
Simple system
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Simple thermodynamic systems generally have the following state functions, and two of them can be selected as independent variables at will:[5]
Where T=temperature, S=entropy, P=pressure, V=volume
Two of T, S, P and V are selected as independent variables, and their differential expressions are:
dU = TdS - PdV
dF = - SdT - PdV
dH = TdS + VdP
dG = - SdT + VdP
By calculating the partial derivative (partial derivative) of the above differential expression, we can get the "McIntosh relation" between the partial derivatives of T, S, P, V variables.
