Question

Which of the following is not a state function?
(A) $U + PV$
(B) $q + w$
(C) $\dfrac{{{q_{rev}}}}{T}$
(D) $q$

Answer 1

For a state function, the value does not change with the path followed. If you go from one state to another through two different paths, then these two different paths have the same values of path function.

Complete step by step answer:
You can define the thermodynamic state of the system by specifying the values of few thermodynamic variables. These thermodynamic variables are the measurable quantities and are also called state variables.
To completely define a system, you can use four variables pressure, temperature, volume and composition. For a definite state of a system, these four variables have definite values.
For another definite state of the system, these four variables have definite values that are different from earlier values.
A state function has the same value between two fixed states irrespective of the path followed to change the system from one particular state to another state.
When you integrate the differential of a state function, over a cyclic path, you will get zero value.
Thus, when you sum the changes in state function in a cyclic process, you get zero value.
In other words summation of change in state function in a cyclic process is equal to zero.
Enthalpy is represented by $U + PV$ . It is a state function as its value depends on the state of the system and is independent of the path.
Internal energy is represented by $q + w$ . It is a state function as its value depends on the state of the system and is independent of the path.
Entropy is represented by $\dfrac{{{q_{rev}}}}{T}$ . It is a state function as its value depends on the state of the system and is independent of the path.
Heat $q$ is not a state function. It is a path system. Its value does not only depend on the state of the system and is dependent on the path.

Hence, the correct option is the option (D).

Note: For a path function, the value changes with the path followed. If you go from one state to another state through two different paths, then these two different paths have different values of path function.