Which property is NOT considered a state function?

The concept of state functions is essential in thermodynamics. A state function is a property whose value depends only on the state of the system, not on the path taken to reach that state. Internal energy, entropy, and enthalpy are all state functions; their values depend solely on the current state of the system, such as pressure, volume, and temperature.

In contrast, the reaction rate is not a state function. It is dependent on how quickly a reaction occurs, which is influenced by various factors such as concentration, temperature, and the presence of catalysts. The reaction rate is a measure of change over time rather than a property defined by the state of the system at a particular moment. Thus, its value can vary widely depending on conditions, making it distinct from the other properties listed.