What is a state function?

Question

What is a state function? I've been briefly introduced to the idea in an introductory module, and I'm confused by the whole idea, I understand the 'path dependence' from line integrals, but is it about the arguments that the function itself takes, I.E. it cannot be simply expressed in terms of the state variables, or is it about the form of the function, because they should generally be linked by their time dependence?

Answer 1

In my view, your confusion is understandable.

On the one hand, Wikipedia uses the term "state function" to describe a mathematical function (e.g., ideal gas equation) that describes the relationship between "state variables" (pressure, volume, temperature, mass, etc.) of a system in equilibrium. This is my preference.

Then the article goes on to call the state variables themselves "state quantities" or "state functions", tending to blur the difference between the function that relates the properties of a system at equilibrium with the variables (properties) themselves.

But when it does the latter it explicitly cites the examples of internal energy, enthalpy, and entropy, seemingly to draw a distinction between these properties of a system and other properties such as pressure, volume, temperature, etc.. And yet they are all system properties.

Rather than getting hung up on the inconsistencies, the important thing to remember is the change in properties (internal energy, enthalpy, entropy, temperature, pressure, volume, etc.) of a system between equilibrium states, and the mathematical function that relates the properties, is independent of the path between the states.

Hope this helps.

Answer 2

State function (or a function of state) is independent on the path, by which one arrives to this state. In other words, as its name says, it depends only on the state itself (more precisely on the state variables). Thus, e.g., if the state of a thermodynamic system is fully described by pressure and volume, any function that depends only on pressure and volume (such as the internal energy of an ideal gas) is a state function. On the other hand, heat or work are not state functions, since we can arrive to the same state by different paths, i.e., via transferring different amounts of heat and work.