The key difference between Adiabatic and isentropic processes is that adiabatic processes can be either reversible or irreversible, while an isentropic process is a reversible process.
In chemistry, we divide the universe into two parts. The part we are interested in is a system, and the rest is the surrounding. A system can be an organism, a reaction vessel or even a single cell. We can distinguish the systems by the kind of interactions they have or by the types of exchanges that take place. Sometimes, matter and energy exchange through the system boundaries. The exchanged energy can take several forms such as light energy, heat energy, sound energy, etc. If the energy of a system changes because of a temperature difference, we say there has been a flow of heat. However, some processes involve temperature variations but no heat flow; these are known as adiabatic processes. An isentropic process is a type of adiabatic process.
CONTENTS
1. Overview and Key Difference
2. What are Adiabatic Processes
3. What are Isentropic Processes
4. Side by Side Comparison – Adiabatic vs Isentropic Processes in Tabular Form
5. Summary
What are Adiabatic Processes?
Adiabatic change is a change in which no heat is transferred into or out of the system. Heat transfer can be mainly stopped in two ways. One is by using a thermally insulated boundary so that no heat can enter or exit. For example, a reaction that occurs in a Dewar flask is adiabatic. The other method an adiabatic process can take place is when a process takes place very rapidly; thus, there is no time left to transfer heat in and out.
In thermodynamics, we show the adiabatic changes by dQ=0. In these instances, there is a relationship between the pressure and the temperature. Therefore, the system undergoes changes due to pressure in adiabatic conditions. This is what happens in cloud formation and large scale convectional currents. At higher altitudes, there is lower atmospheric pressure. When air heats up, it tends to go up. Because the outside air pressure is low, the rising air parcel will try to expand. When expanding, the air molecules do work, and this will affect their temperature. This is why the temperature reduces when rising up.
Figure 01: Adiabatic Process in a Graph
According to thermodynamics, the energy in the parcel remains constant, but it can be converted to do the expansion work or to maintain its temperature. There is no heat exchange with the outside. This same phenomenon applies to air compression, too (e.g., a piston). In that situation, when the air parcel compresses, temperature increases. These processes are called adiabatic heating and cooling.
What are Isentropic Processes?
Spontaneous processes increase the entropy of the universe. When this happens, either system entropy or the surrounding entropy may increase. An isentropic process happens when the system entropy remains constant.
Figure 02: An Isentropic Process
A reversible adiabatic process is an example of an isentropic process. Moreover, the constant parameters in an isentropic process are entropy, equilibrium and heat energy.
What is the Difference Between Adiabatic and Isentropic Processes?
An adiabatic process is a process in which no heat transfer takes place, while an isentropic process is an idealized thermodynamic process that is both adiabatic and reversible. Hence, the key difference between adiabatic and isentropic processes is that adiabatic processes can be either reversible or irreversible while isentropic processes are reversible. Furthermore, an adiabatic process occurs without any heat transfer between the system and surrounding while an isentropic process occurs with no irreversibility and no heat transfers.
Summary – Adiabatic vs Isentropic Processes
An adiabatic process is a process where no heat transfer takes place. An isentropic process is an idealized thermodynamic process that is both adiabatic and reversible. Hence, the key difference between adiabatic and isentropic processes is that adiabatic processes can be either reversible or irreversible, while isentropic processes are reversible.
Reference:
1. “The Laws of Thermodynamics I.” Thermodynamics and Introductory Statistical Mechanics, 2005, pp. 14–31., doi:10.1002/047168175x.ch3.
Image Courtesy:
1. “Adiabatic” (CC BY-SA 3.0) via Commons Wikimedia
2. “Isentropic” By Tyler.neysmith – Own work (CC BY-SA 3.0) via Commons Wikimedia