Question

(a) What is meant by the term state function? (b) Give an example of a quantity that is a state function and one that is not. (c) Is work a state function? Why or why not?

Short answer

(a) A state function is a property of a system that depends only on its current state and is independent of the path taken to reach that state. (b) Internal energy is an example of a state function, while heat is an example of a non-state function. (c) Work is not a state function, as it depends on the specific process the system undergoes and the path taken, not just the system's initial and final states.

Step-by-step solution

(a) Understanding the Concept of a State Function

A state function, often called a state variable or state quantity, is a property of a system that depends only on the current state of the system, i.e., it's independent of the path taken to get to that state. In thermodynamics, these are usually macroscopic properties that describe the system's overall state and can be determined immediately from the current conditions of the system. Contrast this with path functions, which do depend on the processes or paths taken between states.

(b) Examples of State and Non-State Functions

An example of a state function is internal energy. In thermodynamics, internal energy (often denoted by \( U \)) is the total energy contained by a thermodynamic system. It is the energy necessary to create the system, and does not include the kinetic energy of motion of the system as a whole, nor the potential energy of the system as a whole due to external force fields. It also does not include the energy of any non-material 'external' force fields, such as gravitational fields, magnetic fields, etc. An example of a non-state function, or path function, is heat. Heat transfer into or out of a system, unlike internal energy, does depend on the path taken during a process and the process's history.

(c) Is Work a State Function and Why?

Work is not a state function. This is because it is a process-dependent quantity. The amount of work done on or by a system depends on the specific process the system undergoes, not just the system's initial and final states. For example, there can be different amounts of work done in compression or expansion processes, even if the initial and final states of the system are the same. This shows that work, unlike state functions, depends on the path taken and is therefore not a state function.