The key difference between adiabatic and isothermal is that adiabatic means there is no heat exchange between the system and the surrounding while isothermal means there is no temperature change.
For the purpose of chemistry, the universe is divided into two parts. The part we are interested in is called a system, and the rest is called the surrounding. A system can be an organism, a reaction vessel or even a single cell. The systems are distinguished by the kind of interactions they have or by the types of exchanges that take place.
Systems can be classified into two as open systems and closed systems. Sometimes, matters and energy can be exchanged 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. Adiabatic and polytropic are two thermodynamic processes, which is related to heat transfer in systems.
CONTENTS
1. Overview and Key Difference
2. What is Adiabatic
3. What is Isothermal
4. Side by Side Comparison – Adiabatic vs Isothermal in Tabular Form
5. Summary
What is Adiabatic?
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 carried out in a Dewar flask is adiabatic. The other type of adiabatic process happens when a process takes place very rapidly; thus, there is no time left to transfer heat in and out.
In thermodynamics, adiabatic changes are shown by dQ=0. In these instances, there is a relationship between pressure and 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 is heated, 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. That is why the temperature reduces when rising up.
Figure 01: Adiabatic Process
According to thermodynamics, the energy in the parcel remains constant, but it can be converted to do the expansion work or maybe to maintain its temperature. There is no heat exchange with the outside. This same phenomenon can be applied 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 is Isothermal
Isothermal change is the one in which the system remains at a constant temperature. Therefore, dT=0. A process can be isothermal, if it happens very slowly and if the process is reversible. So that, the change occurs very slowly, there is enough time to adjust the temperature variations. Moreover, if a system can act like a heat sink, where it can maintain a constant temperature after absorbing heat, it is an isothermal system.
Figure 2: Isothermal Change
For an ideal has in isothermal conditions, the pressure can be given from the following equation.
P=nRT /V
Since work, W= PdV following equation can be derived.
W= nRT ln (Vf/Vi)
Therefore, at constant temperature the expansion or compression work happens while changing the system volume. Since there is no internal energy change in an isothermal process (dU=0), all the heat supplied is used to do work. This is what happens in a heat engine.
What is the Difference Between Adiabatic and Isothermal?
Adiabatic means there is no heat exchange between the system and the surrounding, therefore, the temperature will increase if it is a compression, or temperature will decrease in expansion. In contrast, isothermal means, there is no temperature change; thus, the temperature in a system is constant. This is acquired by changing the heat. In adiabatic dQ=0, but dT≠0. However, in isothermal changes dT=0 and dQ ≠0. So, this is the key difference between adiabatic and isothermal. Moreover, the adiabatic changes take place rapidly, whereas the isothermal changes take place very slowly.
Below info-graphic summarizes the differences between adiabatic and isothermal.
Summary – Adiabatic vs Isothermal
The key difference between adiabatic and isothermal is that adiabatic means there is no heat exchange between the system and the surrounding while isothermal means there is no temperature change.
Image Courtesy:
1. “Adiabatic” (CC BY-SA 3.0) via Commons Wikimedia
2. “Isothermal process” By Netheril96 – Own work (CC0) via Commons Wikimedia