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Difference between Reversible and Irreversible Processes

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  • Last Updated : 09 Mar, 2022
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The entire study of thermodynamics is rested upon three conceptions called the first, second, and third laws of thermodynamics. These laws of thermodynamics appertain solely where the system is in equilibrium or moves from one equilibrium state to another equilibrium state. The laws have been arrived at purely based on human experience and there’s no theoretical evidence for any of these laws. Still, the validity of these laws is supported by the fact that insignificancy contrary to these laws has been planted consequently far and nothing contrary is anticipated.  

Significance of Thermodynamics –

  • It helps us to prognosticate whether any contributed chemical reaction can come about under the contributed set of conditions
  • It helps in prognosticating the termination of reaction before the equilibrium is achieved.
  • It helps to conclude some consequential laws like the Law of chemical equilibrium, Distribution law, etc.

First Law of Thermodynamics

The first law of thermodynamics is solely the law of conservancy of energy which states that-

Energy can neither be created nor destroyed although it may be proselytized from one form to another. This implies that the whole energy of the universe ( i.e., the system and the surroundings) remains constant, although it may experience transfiguration from one form to the other.  

In simple words, it can be stated that the energy of an isolated system is constant.

Second Law of Thermodynamics

Restriction of 1st law of thermodynamics existed that it doesn’t prognosticate the spontaneity of a process. The aftereffect which helps to prognosticate spontaneity of a process is called the second law. It states that in any spontaneous process, the entropy of the universe (system and surroundings) always increases. Some other definitions are All spontaneous processes are thermodynamically irreversible or without the help of an extraneous agency, a spontaneous process can not be reversed. To conclude the most ordinary statement of the second law, let us readdress the succeeding spontaneous processes.

  • Cooling down of a cup of tea
  • Spreading of a drop of ink in water
  • The flow of water down a hill.
  • Mixing of two gases

The second law of thermodynamics can be described as follows-

All spontaneous processes (or naturally occurring processes) are thermodynamically irreversible.

The aforesaid procedures can, still, be convinced about in the reverse direction by operating some accidental agency, For example, water can be fabricated to advance uphill with an electric motor, tea can be framed hot again by heating. Hence, the second law can be outlined alternatively as follows –

Without the help of an external agency, a spontaneous process cannot be reversed, example heat cannot by itself flow from a colder to a hotter body. 

Third Law of Thermodynamics

It is the fluently understood adherence that the entropy of an absolute substance increases with an increase in temperature because molecular motion (i.e., translational, vibrational, and rotational) increases with an increase in temperature. Again, entropy decreases with a drop in temperature. Nernst formulated a consequent observation about the entropy of impeccable crystalline substances at absolute zero and put forward an observational concept known as the third law of thermodynamics,

The entropy of perfectly crystalline solid approaches zeroes the temperature approaches absolute zero.

In dissimilar terms, 

The entropy of all ideally crystalline solids may be taken as zero at the absolute zero of temperature.  

Reversible Process

The process in which the system and the surroundings can be rebuilt from the final state to the original state without any change in the thermodynamic properties of the universe is called a reversible process.

Reversible Process

Let us assume that the system has experienced a modification from state A to state B. However, and there is no change in the universe, the process is expressed as a reversible process if the system can be restored from state B to state A. The reversible process can be completely reversed and there is no track left to demonstrate that the system had experienced a thermodynamic change. For the system to sustain the reversible change, it must be infinitely sluggish.

All the state fluctuations that occur during the reversible process are in thermodynamic equilibrium with each other. So there are two consequent contingencies to doing the reversible process. Basically, the process must take place in an infinitely short path and second all the original and final states of the system must be in equilibrium with each other.

However, if the heat content of the system remains constant during the reversible process. It is an adiabatic process, this process is also a isotropic process, i.e. the entropy of the system remains constant.

Reversible Process

Types of Reversible Process

There are two types of reversible process which are presented below-

  • Internally reversible process: A process is clarified to be Internally reversible if no irreversibility occurs within the system’s termination. In these processes, a system passes through a series of equilibrium states, and when the process is reversed, the system passes through similar equilibrium states, replacing its original state.
  • Externally Reversible Process: In an externally reversible process, no irreversibility occurs outside the boundaries of the system during the process. For example, if the surface of contact between the system and the force is at the same temperature, the heat transfer between the force and the system is also an externally reversible process.

Irreversible Process

Irreversible processes are correspondingly called innate processes because all the actions that take place in nature are irreversible processes. The natural process is due to the finite gradient between the two states of the system. For example, the temperature gradient between two bodies causes heat to flow between two bodies; This is actually the natural flow of heat.

Also, water flows from high level to low level, current overflows from high potential to low potential, etc. The original state of the system and the environment cannot be recreated from the final state in an irreversible process.

The coloured states of the system are not in equilibrium with each other on the path of modification from the original state to the final state during the irreversible process. During the irreversible process, the entropy of the system increases significantly and cannot be downgraded back to its most critical value.

Irreversible Process

Types of Irreversible Process

There are two types of irreversible processes which are presented below-

  • Internal irreversibility:  In this internal irreversibility, the disruptive effect is present within the working fluid.
  • External irreversibility: In this external irreversibility, the dissipators subsist outside the movable working fluid. Example mechanical friction due to an external source.

Difference between Reversible and Irreversible Process

Reversible Process

Irreversible Process

The process is carried out infinitesimally slowly, i.e., the difference between the driving force and the opposing force is very very small.This process is not carried out infinitesimally slowly but is carried out rapidly, i.e., the difference between the driving force and the opposing force is quite large. 
At any stage during the process, equilibrium is not disturbed.Equilibrium may exist only after the completion of the process.
It takes infinite time for completion.It takes a finite time for completion. 
It is only imaginary and cannot be achieved in actual practice.These processes actually occur in nature.
Work obtained in this process is maximum.Work obtained in this process is not the maximum.

Sample Questions

Question 1: Water decomposes by absorbing 286.2 kJ of electrical energy per mole. When H2 and O₂ combine to form one mole of H₂O, 286.2 kJ of heat is produced. Which law is proved? What statement of the law follows from it?

Answer:

Law of conservation of energy (or 1st law of thermodynamics) is proved here. According to which the energy can neither be created or destroyed, although it may be converted from one form to another.

Question 2: The water can be raised with the help of a pump to the water tank located at the top of the house. Then why is it not considered spontaneous?

Answer:

A spontaneous process should continue to happen on its own after initiation. But in the given case it is not so as long as the pump is working the water will go up.

Question 3: We are consuming a lot of electrical energy, solar energy etc. Therefore, do you conclude that the energy of the universe is continuously decreasing? Explain. Which other thermodynamic quantity is continuously increasing or decreasing?

Answer:

No, the energy of the universe remains constant (law of conservation of energy). The entropy of the universe is constantly increasing.

Question 4: Explain why the entropy of a pure crystalline substance is zero at 0K? Mention the law on which it is based. enforce this law?

Answer:

At 0 K, the constituent particles of a pure crystalline substance have an orderly arrangement and no disorder. Therefore, its entropy is taken to be zero. This statement is based on the third law of thermodynamics. The law is applied to find the absolute entropy of a substance in any given state at any given temperature.

Question 5: What are the differences between Reversible Processes and Irreversible Processes?

Answer:

Following are the differences between Reversible Processes and Irreversible Processes:

Reversible Process

Irreversible Process

The process is carried out infinitesimally slowly, i.e., the difference between the driving force and the opposing force is very very small.This process is not carried out infinitesimally slowly but is carried out rapidly, i.e., the difference between the driving force and the opposing force is quite large. 
At any stage during the process, equilibrium is not disturbed.Equilibrium may exist only after the completion of the process.
It takes infinite time for completion.It takes a finite time for completion. 
It is only imaginary and cannot be achieved in actual practice.These processes actually occur in nature.
Work obtained in this process is maximum.Work obtained in this process is not the maximum.

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