A system’s properties change when it moves from one state to another. This transformation or process of changing state is inevitably accompanied by heat and work exchange as governed by the first law. When the work and heat effects associated with the process are such that it is feasible to restore the system to its original state, then the process is reversible. Effectively, if it is possible to move the system from its initial state to a final state, then reverse the process ending up at the initial state while restoring both the system and the surroundings exactly to their original conditions, then both the forward and reverse steps are considered to be reversible [4]. However, real processes are invariably accompanied by some dissipative phenomena, such as friction, with the result that it is never possible to restore both system and the surroundings to their original conditions. Real processes are thus irreversible by nature. The concept of reversibility and reversible process is, however, extremely important in thermodynamics, as a reversible process represents a limiting condition that provides a bound for a real or actual process. Changes occurring in a reversible process can be computed from the thermodynamic properties and used to estimate the actual changes that can be expected in a real process.
Certain system properties and changes in these properties can be determined from the knowledge of the initial and final states of the systems. Such properties or functions do not require any information about the actual process of transformation of the system. Such properties are variously referred to as state properties, state variables, state functions, and so on [4]. Temperature, pressure, entropy, and internal energy are some examples of a state property.
Conversely, some of the properties or variables depend not only on the initial and final states of the system but also on the path traversed by the system while undergoing the transformation. Such properties are called path functions, path variables, and so on. Work done by the system or on the system is an example of a path function. Thermodynamic processes are often represented on a pressure-volume (P-V) diagram, which shows the system pressure as a function of its volume. System work is represented by the area under the curve representing the system transformation on the P-V diagram, as shown in Figure 8.1 [5].
Figure 8.1 Representation of work done in a reversible process on a P–V diagram.
Source: Matsoukas, T., Fundamentals of Chemical Engineering Thermodynamics with Applications to Chemical Processes, Prentice Hall, Upper Saddle River, New Jersey, 2013.
Figure 8.2 shows two alternative paths followed by the system while undergoing transformation from state 1 to state 2. It can be clearly seen that the areas under the curve bound by volumes represented by points 4 and 6 are different when a direct path is followed from 1 to 2, as opposed to the path followed by first going from 1 to 3 and then from 3 to 2. As previously mentioned, these areas represent the system work, which is a path function. Other thermodynamic property changes (ΔU, ΔS, etc.) do not depend on which path is followed between 1 and 2. They are state functions and depend only on the initial and final states.
Figure 8.2 Work, a path-dependent function, as illustrated by areas under the curves 1-2 and 1-3-2.
Source: Kyle, B. G., Chemical and Process Thermodynamics, Third Edition, Prentice Hall, Upper Saddle River, New Jersey, 1999.
Leave a Reply