Thermodynamic Processes: Isothermal, Adiabatic, Isobaric

Thermodynamic Processes: The term Thermodynamics is made up of two words thermo means heat, and dynamics means motion leading to mechanical work. Thus, thermodynamics implies that the branch of science deals with converting heat into mechanical work and vice versa. It is important to note that Thermodynamics is concerned only with energy changes accompanying a given process (physical or chemical) and not with the body’s total energy.

A system undergoes a thermodynamic process when some sort of energetic change occurs within the system, usually associated with changes in pressure, volume, internal energy, temperature, or heat transfer of any kind. Most thermodynamic processes proceed naturally from one direction to another. In other words, they have a favourite direction. In this article, we will discuss various thermodynamic systems and their applications. Read on to find out more!

Types of Thermodynamics Systems

The thermodynamic systems can be divided into three types depending upon the nature of the boundary.

• The boundary may be closed and insulated so that neither matter nor heat (energy) can be exchanged between the system and surroundings.
• The boundary may be closed but not insulated, and thus only energy can be exchanged and not matter.
• The boundary may be open when both matter and energy can be exchanged between the system and the surroundings.

The three respective systems are known as isolated, closed, and open. These are described below:

(a) Isolated System: A system that can exchange neither energy nor matter with its surroundings is called an isolated system. Consider a system consisting of water in contact with its vapour in a closed and insulated vessel. Since the vessel is closed, no water or its vapours(matter) can leave or enter the vessel. Further, since the vessel is insulated too, it can neither lose nor gain heat (energy) with its surroundings.

(b) Closed System: A system that can exchange energy but not matter with its surroundings is called a closed system. Consider a system consisting of water in contact with its vapour in a closed (but not insulated) vessel. Since the vessel is closed, no exchange of matter between the system and the surroundings can occur, but since the vessel has conducting walls, the system inside the vessel can exchange (gain or lose) energy with its surroundings.

(c) Open System: A system that can exchange both matters, as well as energy with its surroundings is called an open system. Consider evaporation of water in an open beaker. The water vapour (matter) will escape into the surroundings while the heat (energy) required for the evaporation will be absorbed by the system from the surroundings.

Other examples of an open system are the reaction between zinc granules and dilute hydrochloric acid in an open beaker. The hydrogen (matter) evolved gas escapes from the system to the surroundings; simultaneously, the heat evolved in the reaction is also transferred to the surroundings by radiation.

Thermodynamics Processes

The operation by which a thermodynamics system changes from one state to another is called a thermodynamic process. Heating, cooling, expansion, compression, fusion, vaporization, etc., are some examples of a thermodynamic process.

A thermodynamic process is always accompanied by a change in energy, although a change of matter may also occur in the case of an open system. Depending upon the condition of change, five different types of thermodynamics systems have been recognized.

Example 1: Two metals (A and B) are in thermal contact and thermal equilibrium. Metal A is heated to an infinite amount so that heat flows through it to metal B. This process can be reversed by cooling an infinitesimal amount, at which point heat will begin to flow from B to A until they are once again in thermal equilibrium.

Example 2: A gas is expanded slowly and steadily in a reversible process. By increasing the pressure by an infinitesimal amount, the same gas can slowly and adiabatically compress back to the initial state.

It should be noted that these are just some idealised examples. For practical purposes, a system that is in thermal equilibrium ceases to be in thermal equilibrium when one of these changes is brought about. Thus the process is not actually completely reversible. It is an ideal model for how such a situation would come about, although with careful control of experimental conditions a process can be carried out that is extremely close to being completely reversible.

Types of Processes

The thermodynamics process can be carried out in different ways and under different conditions. The process can be classified as follows:

(a) Isothermal process: All the heat enters or leaves the system, yet the system’s temperature remains constant throughout the process, which is called the isothermal process. Suppose a chemical reaction is taking place in a closed but not insulated vessel of the type. If the process is exothermic, the evolved gas is given out by the system to the surroundings instantaneously, and thus the temperature of the system does not rise at all at any stage of the process. If, on the other hand, the reaction is endothermic, the required amount of heat is absorbed instantaneously by the system from the surroundings. Thus, again the temperature of the system does not fall at any stage of the process.

Isothermal processes are often carried out by placing the system in a thermostat (a constant temperature bath). For an isothermal process, change in temperature \(\left( {{\rm{dT}}} \right) = 0.\)

(b) Adiabatic process: A process during which no heat enters or leaves the system during any step of the process is known as the adiabatic process. In this process, since no heat enters or leaves the system, the temperature will decrease or increase when the reaction is endothermic and exothermic, respectively. Such reactions (processes) are often carried out in closed insulated containers such as thermos bottles. For an adiabatic process, change in heat \(\left( {{\rm{dQ}}} \right) = 0.\)

(c) Isobaric process: A process during which pressure of the system remains constant throughout the reaction is known as the isobaric process. For example, the heating of water to its boiling point and its vaporization occurs at the same atmospheric pressure.
For an isobaric process, \({\rm{dP = 0}}{\rm{.}}\)

(d) Isochoric process: A process during which the system’s volume remains constant throughout the reaction is known as the isochoric process. The heating of a substance in a non-expanding chamber is an example of the isochoric process.
For an isochoric process, \({\rm{dV = 0}}{\rm{.}}\)

(e) Cyclic process: When a system returns to its original state after completing a series of changes. Then it is said that a cycle is completed. This process is known as a cycle or cyclic process.
For a cyclic process \({\rm{dU = 0,}}\,{\rm{dH = 0,}}\,{\rm{dP = 0,}}\,{\rm{dV = 0,}}\,{\rm{dT = 0}}{\rm{.}}\)

Reversible and Irreversible Process

When a thermodynamic process occurs so that the properties of the system remain practically uniform, the process is called a reversible process. A reversible process can be conceived to proceed very slowly through a succession of infinitesimal steps, and its direction can be reversed at any point by making a small change in a variable like temperature, pressure, etc. The concept of reversibility will be understood from the following experiment.

To understand the reversible process, imagine a gas confined within a cylinder provided with a frictionless piston upon which is piled some very fine sand. Suppose the pressure P exerted by the gas on the piston is equal to the combined pressure exerted by the piston’s weight, the pile of sand and the atmospheric pressure.

Thus, under these conditions, the piston will allow neither more downward nor upward, and, consequently, there will be no change in the volume of the gas. When one grain of sand is removed, the pressure on the piston is lowered by an infinitesimally small amount \({\rm{dP}}\) pressure on the piston, \({\rm{P – dP,}}\) is infinitesimally smaller than the pressure of the gas p. Hence the piston will move up, and the gas will expand by an infinitesimally small amount. Suppose the particle of sand is replaced; that is, the pressure of the cylinder is increased by the same infinitesimally small amount \(\left( {{\rm{dP}}} \right).\) In that case, the gas will return to the original volume.

By the continued removal of the particles of sand, the gas can be allowed to undergo finite expansion. Still, each step in this expansion is an infinitesimal one and can be reversed by an infinitesimally change in the external conditions. At all steps, the equilibrium is restored immediately.

However, suppose the pressure on the piston (external pressure) is decreased very much suddenly. In that case, it moves upward rapidly in a single operation, and the gas in the cylinder will quickly expand. The expansion of the gas, in this case, is said to take place irreversibly.

Thus, the reversible and irreversible process may be defined as below.
A reversible process is carried out infinitesimally slowly so that all changes occurring in the direct process can be exactly reversed. The system remains almost in a state of equilibrium at all times. Alternatively, a reversible process may be defined as that which is carried out in stages. The driving force at every stage is only infinitesimally greater than the opposing force and which can be reversed by increasing the opposing force by an infinitesimally amount.

A reversible process will take infinite time and thus cannot be realized in practice. Therefore, a reversible process is only imaginary and theoretical. However, some processes can be made to approach reversibility, and the concept of the reversible thermodynamics process is of great importance in the study of thermodynamics.

An irreversible process is not carried out in infinitesimally slowly steps (instead, it is carried out in a single step) and cannot be carried in the reverse order. All the spontaneous processes occurring around us, like the expansion of gases, flow of heat from hot bodies to colder ones, etc., are irreversible.

The Carnot Cycle

In 1924, French engineer Sadi Carnot created an idealised, hypothetical engine with the maximum possible efficiency in line with the second law of thermodynamics. He arrived at the following equation for his efficiency:

\(e_{carnot}=\frac{(t_h-t_c)}{t_h}\)

Summary

Thermodynamics is an important branch of both chemistry and physics. In this article, we learnt how the property of thermodynamics transforms heat into work-the different types of thermodynamic processes like adiabatic, isothermal, isochoric, isobaric process and cyclic processes.

FAQs

Q.1. What is an example of the thermodynamic process?
Ans: An example of a thermodynamic process is increasing the pressure of gas while maintaining a constant temperature. Heating, cooling, expansion, compression, fusion, vaporization, etc., are some examples of a thermodynamic process. 

Q.2. What are the four thermodynamic processes?
Ans: The four types of thermodynamic processes are isothermal process, isobaric process, adiabatic process, isochoric process.

Q.3. How many thermodynamic processes are there?
Ans: Depending upon the condition of change, different types of thermodynamics systems have been recognized. They are the isothermal process, the isobaric process, adiabatic process, isochoric process, reversible process, irreversible process, and cyclic process.

Q.4. What is the cycle of thermodynamic processes?
Ans: When a system returns to its original state after completing a series of changes. Then it is said that a cycle is completed. This process is known as a cycle or cyclic process. 

Q.5. In which thermodynamic process work done is maximum?
Ans: A process during which no heat enters or leaves the system during any step of the process is known as the adiabatic process. The work done in the adiabatic process is maximum.

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