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Did you know that Thermodynamics is a compound of two Greek words, therme (“heat”) and dunamis (“power”)? It is the science that speaks of the power or energy contained in heat, and its conversion to other forms of energy. The term “energy” is itself derived from the Greek word energeia (“working”), and is normally defined as “the capacity to do work.”

From the education perspective, thermodynamics is one of those chapters that have the unique privilege of featuring in both the Physics and Chemistry textbooks of CBSE higher secondary curriculum. It is present as Chapter 6 in Chemistry and Chapter 12 in Physics Class 11th NCERT textbooks.

Concept of Thermodynamics

Thermodynamics is a physical science describing how systems change when they interact with each other or their surroundings. These interactions occur through transfer of energy and can be studied either at the macroscopic scale, through changes in temperature, pressure, and volume, or at the microscale, by analyzing the collective motion of their particles using statistical methods.

The field was firstly developed due to the need of increased steam engine efficiency in the early 19th century. The famous French physicist Nicolas Léonard Sadi Carnot (1824), often described as the “Father of thermodynamics,” was the first to study heat engines through a scientific scope and laid the foundations for the second law of thermodynamics through his well-known Carnot cycle.


A quantity or matter which is under the study of Thermodynamics is called a system.

There are three mains types of system: open system, closed system and isolated system. All these have been described below:

1) Open system: The system in which the transfer of mass as well as energy can take place across its boundary is called as an open system. Our previous example of engine is an open system. In this case we provide fuel to engine and it produces power which is given out, thus there is exchange of mass as well as energy. The engine also emits heat which is exchanged with the surroundings. The other example of open system is boiling water in an open vessel, where transfer of heat as well as mass in the form of steam takes place between the vessel and surrounding.

2) Closed system: The system in which the transfer of energy takes place across its boundary with the surrounding, but no transfer of mass takes place is called as closed system. The closed system is fixed mass system. The fluid like air or gas being compressed in the piston and cylinder arrangement is an example of the closed system. In this case the mass of the gas remains constant but it can get heated or cooled. Another example is the water being heated in the closed vessel, where water will get heated but its mass will remain same.

3) Isolated system: The system in which neither the transfer of mass nor that of energy takes place across its boundary with the surroundings is called as isolated system. For example if the piston and cylinder arrangement in which the fluid like air or gas is being compressed or expanded is insulated it becomes isolated system. Here there will neither transfer of mass nor that of energy. Similarly hot water, coffee or tea kept in the thermos flask is closed system. However, if we pour this fluid in a cup, it becomes an open system.

Image result for open system closed system isolated system

Control Volume

The control volume is an arbitrarily selected zone that surrounds the device under consideration.  the surface of this control volume is referred to as a control surface.

  • Mass, as well as heat and work (and momentum), can flow across the control volume
  • A control volume is specified  when an analysis is to  be made that involves a flow of mass
  • The control volume is separated from the surrounding by a control surface, which is analogous to the boundary of a system; however, the mass transfer may occur across the control surface.
  • The control volume may move in space and may have its volume change with time.
  • it is not necessary that the volume of a control volume by fixed, although in many cases a stationary control volume can be used.


The state or condition in which a system exists is specified by specifying its properties.  Properties can be classified into two groups as

  1.  Relevant properties: which are associated with energy and its transformation
  2. Irrelevant properties: which are not associated with energy and its transformation. Colour, odour, taste are irrelevant properties.

In thermodynamics one deals with relevant properties only.

  • A property is any characteristic (which can be quantitatively evaluated) that can be used to describe the state of a system.

The Four Laws of Thermodynamics

The fundamental principles of thermodynamics were originally expressed in three laws. Later, it was determined that a more fundamental law had been neglected, apparently because it had seemed so obvious that it did not need to be stated explicitly. To form a complete set of rules, scientists decided this most fundamental law needed to be included. The problem, though, was that the first three laws had already been established and were well-known by their assigned numbers. When faced with the prospect of renumbering the existing laws, which would cause considerable confusion, or placing the pre-eminent law at the end of the list, which would make no logical sense, a British physicist, Ralph H. Fowler, came up with an alternative that solved the dilemma: he called the new law the “Zeroth Law.” In brief, these laws are:

The Zeroth Law

This states that if two bodies are in thermal equilibrium with some third body, then they are also in equilibrium with each other. This establishes temperature as a fundamental and measurable property of matter.

Systems are in thermal equilibrium if they do not transfer heat, even though they are in a position to do so, based on other factors. For example, food that’s been in the refrigerator overnight is in thermal equilibrium with the air in the refrigerator: heat no longer flows from one source (the food) to the other source (the air) or back.

What the Zeroth Law of Thermodynamics means is that temperature is something worth measuring, because it indicates whether heat will move between objects. This will be true regardless of how the objects interact. Even if two objects don’t touch, heat may still flow between them, such as by radiation (as from a heat lamp). However, according to the Zeroth Law of Thermodynamics, if the systems are in thermal equilibrium, no heat flow will take place.

The First Law

This law states that the total increase in the energy of a system is equal to the increase in thermal energy plus the work done on the system. This states that heat is a form of energy and is, therefore, subject to the principle of conservation. Work done on, or by a gas, we have found that the amount of work depends not only on the initial and final states of the gas but also on the process, or path which produces the final state. Similarly, the amount of heat transferred into, or from a gas also depends on the initial and final states and the process which produces the final state. Many observations of real gases have shown that the difference of the heat flow into the gas and the work done by the gas depends only on the initial and final states of the gas and does not depend on the process or path which produces the final state. This suggests the existence of an additional variable, called the internal energy of the gas, which depends only on the state of the gas and not on any process. The internal energy is a state variable, just like the temperature or the pressure. The first law of thermodynamics defines the internal energy (E) as equal to the difference of the heat transfer (Q) into a system and the work (W) done by the system.

E2 – E1 = Q – W
We have emphasized the words “into” and “by” in the definition. Heat removed from a system would be assigned a negative sign in the equation. Similarly, work done on the system is assigned a negative sign.

The internal energy is just a form of energy like the potential energy of an object at some height above the earth, or the kinetic energy of an object in motion. In the same way that potential energy can be converted to kinetic energy while conserving the total energy of the system, the internal energy of a thermodynamic system can be converted to either kinetic or potential energy. Like potential energy, the internal energy can be stored in the system. Notice, however, that heat and work can not be stored or conserved independently since they depend on the process. The first law of thermodynamics allows for many possible states of a system to exist, but only certain states are found to exist in nature.

The Second Law

It states that heat energy cannot be transferred from a body at a lower temperature to a body at a higher temperature without the addition of energy. This is why it costs money to run an air conditioner.

Heat Engines: It is impossible to extract an amount of heat QH from a hot reservoir and use it all to do work W . Some amount of heat QC must be exhausted to a cold reservoir. This precludes a perfect heat engine.

This is sometimes called the “first form” of the second law, and is referred to as the Kelvin-Planck statement of the second law.

Refrigerator: It is not possible for heat to flow from a colder body to a warmer body without any work having been done to accomplish this flow. Energy will not flow spontaneously from a low temperature object to a higher temperature object. This precludes a perfect refrigerator. The statements about refrigerators apply to air conditioners and heat pumps, which embody the same principles.

This is the “second form” or Clausius statement of the second law.

It is important to note that when it is stated that energy will not spontaneously flow from a cold object to a hot object, that statement is referring to net transfer of energy. Energy can transfer from the cold object to the hot object either by transfer of energetic particles or electromagnetic radiation, but the net transfer will be from the hot object to the cold object in any spontaneous process. Work is required to transfer net energy to the hot object.

Entropy: In any cyclic process the entropy will either increase or remain the same.

Entropy: a state variable whose change is defined for a reversible process at T where Q is the heat absorbed.
Entropy: a measure of the amount of energy which is unavailable to do work.
Entropy: a measure of the disorder of a system.
Entropy: a measure of the multiplicity of a system.

Since entropy gives information about the evolution of an isolated system with time, it is said to give us the direction of “time’s arrow” . If snapshots of a system at two different times shows one state which is more disordered, then it could be implied that this state came later in time. For an isolated system, the natural course of events takes the system to a more disordered (higher entropy) state.

The second law of thermodynamics enables us to divide all processes into two classes:

1. Reversible or ideal process

2. Irreversible or natural process

A reversible process is one which is performed in such a way that at the conclusion of the process, both the system and the surroundings may be restored to their initial states, without producing any changes in the rest of the universe. Throughout the entire reversible process, the system is in thermodynamic equilibrium with its surroundings. Since it would take an infinite amount of time for the reversible process to finish, perfectly reversible processes are impossible. However, if the system undergoing the changes responds much faster than the applied change, the deviation from reversibility may be negligible. In a reversible cycle, a cyclical reversible process, the system and its surroundings will be returned to their original states if one half cycle is followed by the other half cycle. Reversibility means the reaction operates continuously at equilibrium. In an ideal thermodynamically reversible process, the energy from work performed by or on the system would be maximized, and that from heat would be zero. However, heat cannot fully be converted to work and will always be lost to some degree (to the surroundings). The phenomenon of maximized work and minimized heat can be visualized on a pressure-volume curve, as the area beneath the equilibrium curve, representing work done. In order to maximize work, one must follow the equilibrium curve precisely.

A reversible process is carried out infinitely slowly with an infinitesimal gradient, so that every state passed through by the system is an equilibrium state.

Causes of Irreversibility: Lack of equilibrium during the process or involvement of dissipative effects. Irreversible processes are a result of straying away from the curve, therefore decreasing the amount of overall work done; an irreversible process can be described as a thermodynamic process that departs from equilibrium. When described in terms of pressure and volume, it occurs when the pressure (or the volume) of a system changes so dramatically and instantaneously that the volume (or the pressure) does not have time to reach equilibrium. A classic example of irreversibility is allowing a certain volume of gas to be released into a vacuum. By releasing pressure on a sample and thus allowing it to occupy a large space, the system and surroundings are not in equilibrium during the expansion process and there is little work done. However, significant work will be required, with a corresponding amount of energy dissipated as heat flow to the environment, in order to reverse the process (compressing the gas back to its original volume and temperature).

The Third Law

This law says that the entropy of a pure crystal at absolute zero is zero. As explained above, entropy is sometimes called “waste energy,” i.e., the energy that is unable to do work. Since there is no heat energy whatsoever at absolute zero, there can be no waste energy. Entropy is also a measure of the disorder in a system. And while a perfect crystal is by definition perfectly ordered, any positive value of temperature means there is motion within the crystal, which causes the disorder. For these reasons, there can be no physical system with lower entropy, so entropy always has a positive value.

The third law of thermodynamics has evolved from the Nernst theorem – the analysis of an entropy change in a reacting system at temperatures approaching absolute zero , which was first proposed by Nernst and followed by a discussion between him, Einstein and Planck.

Planck formulation: When temperature falls to absolute zero, the entropy of any pure crystalline substance tends to a universal constant (which can be taken to be zero) S → 0 as T → 0.
Entropy selected according to S = 0 at T = 0 is called absolute. If S depends on x (where x may represent any independent thermodynamic parameter such as volume or extent of a chemical reaction), then x is presumed to remain finite in. The Planck formulation unifies other formulationsgiven below into a single statement but has a qualifier “pure crystalline substance”, which confines application of the law to specific substances. This is not consistent with understanding the laws of thermodynamics as being the most fundamental and universally applicable principles of nature. This formulation does not comment on entropy of other substances at T = 0 and thus
is not universally applicable. The Planck formulation, in fact, necessitates validity of two statements of unequal universality: the Einstein statement and the Nernst theorem.

Interesting Applications of Thermodynamics

  1. All vehicular engines work on the second law of thermodynamics and are an extension of Carnot’s cycle (maximum possible efficiency in a cyclic process). This is the case irrespective of the type of the engine used – Petrol/Diesel/LPG.
  2. Refrigeration Systems (heat pumps) work on the second law again.
  3. Chemical reactions – like digestion of food in your body to an intense cardiac exercise are all examples of thermodynamic processes.
  4. All processes that involve heat transfer, form one of the three fundamental processes of Conduction, Convection, and Radiation and studying them in devices like radiators, condensers, air conditioners, etc.
  5. Power plants run on the principles of thermodynamics – from nuclear energy to thermal energy everything is fundamentally based on thermodynamic principles.
  6. Pressure cookers, water boiling, microwave oven heating are all essentially thermodynamic processes.

Branches Of Thermodynamics

  • Classical Thermodynamics: The branch of thermodynamics that deals with states of equilibrium based on the laws of thermodynamics.
  • Statistical Mechanics: This theory provides definitions  and properties of macroscopic systems using the known properties of microscopic systems.
  • Chemical Thermodynamics: Chemical thermodynamics deals with the interrelation of energy and chemical reactions.
  • Equilibrium Thermodynamics: This theory looks at how the matter and energy transforms as the system attains equilibrium.

Fundamental Concepts of Thermodynamics

Thermodynamic equilibrium

The word equilibrium is nothing but a state of balance. An equilibrium state does not include any unbalanced potentials (or driving forces) within the system. So, any system that is within the limits of equilibrium doesn’t experience any changes when it is cut off from its surroundings. When it comes to two systems, it can be concluded that they are in a thermodynamic equilibrium when they are in chemical, mechanical and thermal equilibrium with each other.

A system can be termed as being in thermodynamic equilibrium if the conditions for the following three equilibrium are satisfied:

  1. Mechanical equilibrium (does not include any unbalanced forces): Two systems are in thermal equilibrium if their temperatures remain the same.
  2. Thermal equilibrium (there are no temperature variations): Two systems are believed to be in mechanical equilibrium if their pressures are alike.
  3. Chemical equilibrium: In chemical equilibrium, the chemical activities or concentrations of the reactants and products do not have any net change over time. Two systems are in diffusive equilibrium if their chemical potentials are same.

Intensive Property

An intensive property is a bulk property, meaning that it is a physical property of a system that does not depend on the system size or the amount of material in the system. Examples of intensive properties include temperature, T, refractive index, n, density, ρ, and hardness of an object, η. When a diamond is cut, the pieces maintain their intrinsic hardness (until the sample reduces to a few atoms thick), so hardness is independent of the size of the system. Similarly, melting points of liquids is an intrinsic property as it does not vary even if some part of the liquid is taken away.

Extensive Property

An extensive property is additive for subsystems. This means the system could be divided into any number of subsystems, and the extensive property measured for each subsystem; the value of the property for the system would be the sum of the property for each subsystem. For example, both the mass, m, and the volume, V, of a diamond are directly proportional to the amount that is left after cutting it from the raw mineral.

Hess’ Law

Hess’s Law is named after Russian Chemist and Doctor Germain Hess. Hess helped formulate the early principles of thermochemistry. His most famous paper, which was published in 1840, included his law on thermochemistry. Hess’s law is due to enthalpy being a state function, which allows us to calculate the overall change in enthalpy by simply summing up the changes for each step of the way, until product is formed. All steps have to proceed at the same temperature and the equations for the individual steps must balance out. The principle underlying Hess’s law does not just apply to Enthalpy and can be used to calculate other state functions like changes in Gibbs’ Energy and Entropy.

Hess’s Law states that no matter the multiple steps or intermediates in a reaction the total enthalpy change is equal to the sum of each individual reaction. It is also known as the conservation of energy law. So this means that we can determine the total enthalpy change of A+B+C=ABC by determining this actual reaction’s enthalpy change. Or we can determine the enthalpy change for A+B=AB and AB+C=ABC and then add these two together.

Another way to look at this is with a graph. Let’s say that it takes a lot of energy for A and B to form AB. But then once it combines with C into ABC it releases a lot of energy:

Change in energy graph
1. Heat transfer and work transfer are the energy interactions. A closed system and its surroundings can interact in two ways: by heat transfer and by work transfer. Thermodynamics studies how these interactions bring about property changes in a system.
2. The same effect in a closed system can be brought about either by transfer or by work transfer. whether heat transfer or work transfer has taken place depends on what constitutes the system.
3. Both heat transfer and work transfer are boundary phenomena. Both are observed at the boundaries of the system and both represent energy crossing the boundaries of the system.
4. Heat transfer is the energy interaction due to temperature difference only. All other interactions may be termed as work transfer.
5. Both heat and work are path functions and inexact differentials. The magnitude of heat transfer or work transfer depends upon the path the system follows during the change of state.


The entire theory of temperature is crucial to any kind of discussion involving thermodynamics. Let’s say, for example, a steel rod remains colder than a wooden rod at room temperature merely because of the fact that steel is better at keeping the heat away from the skin. Primarily, when two objects come into thermal contact, heat flows between them until the time they come into equilibrium with each other. The moment the heat flow stops, they come at the same temperature.

Basic Feature of Entropy

Entropy is defined as the quantitative measure of disorder or randomness in a system. The concept comes out of thermodynamics, which deals with the transfer of heat energy within a system. Instead of talking about some form of “absolute entropy,” physicists generally talk about the change in entropy that takes place in a specific thermodynamic process.

Calculating Entropy

In an isothermal process, the change in entropy (delta-S) is the change in heat (Q) divided by the absolute temperature (T):

In any reversible thermodynamic process, it can be represented in calculus as the integral from a process’s initial state to its final state of dQ/T.

In a more general sense, entropy is a measure of probability and the molecular disorder of a macroscopic system. In a system that can be described by variables, there are a certain number of configurations those variables may assume. If each configuration is equally probable, then the entropy is the natural logarithm of the number of configurations, multiplied by Boltzmann’s constant.

S = kB ln W

where S is entropy, kB is Boltzmann’s constant, ln is the natural logarithm and W represents the number of possible states. Boltzmann’s constant is equal to 1.38065 × 10−23 J/K.

Units of Entropy

Entropy is considered to be an extensive property of matter that is expressed in terms of energy divided by temperature. The SI units of entropy are J/K (joules/degrees Kelvin).

Triple Point

 Every Substance has triple point.

It is the point at which it exists in solid, liquid and vapour phase.

  • It is the point of intersection of sublimation line, vapourisation line and fusion line on P-T diagram where all three phases coexist in equilibrium.
  • on another property diagram ( P-V and T-V) diagrams, such conditions are represented by a line called triple line
  • The triple point is a point in P-T diagram. It is a line in P-v diagram and it is a triangle in U-v diagram
  • fusion line have negative (-ve) slope. It is due to the anomalous behaviour of water i.e, the melting point of ice decreases with increasing pressure.
  • Only three substances (water, Antimony and Bismuth have negative (-ve) slope for fusion line whereas all remaining substances have positive ( +ve) slope.
  • if any substance heated below its triple point pressure, solid becomes vapours and process is called sublimation.
  • If any substance is cooled below its triple point pressure, vapour becomes solid and the process is called sublimation.
  • If any substance is heated above its triple point pressure, solid becomes liquid and liquid becomes vapour.
  • Iso-thermal compression less than triple point temperature makes substance go from vapour to solid state.
  • Iso-thermal expansion less than triple point temperature makes substance go from solid to vapour state.
  • Iso-thermal compression greater than triple point temperature makes   substance go from vapour to liquid state
  • Iso-thermal expansion greater than triple point temperature makes substance go from liquid to vapour state.

Thermodynamics & Engineering Prospects

Heat and Thermodynamics form about 8% weightage in JEE Advanced, making it an essential topic for a better rank. Chemical Thermodynamics forms about a 5% weightage in JEE Advanced. Thus, the combined 13% can take your admission from IIT Mandi to IIT Kharagpur. Thermodynamics has a high weightage in both Physics and Chemistry in JEE.

A thermodynamics engineer applies the theory of thermodynamics to many mechanical systems. A bachelor’s degree in Mechanical, Electrical, Chemical, Aerospace, Civil (in turn Environmental) or even Biotechnology can land you this job. One can find work in chemical manufacturing companies, engine manufacturing, industrial manufacturing, aerospace manufacturing, power plants (Electrical power utility and distribution) and other industries in Electrical and Electronics engineering. Biomedical and petroleum engineering is an extension of the above fields and has huge financial turnovers. The basic concepts of Class 11th can branch out into many new topics that the students can explore.

Thus, thermodynamics plays an important role for engineering aspirants and provides them a new career option, which has a lot of scope in the future.

The science of thermodynamics has been developed over centuries, and its principles apply to nearly every device ever invented. Its importance in modern technology cannot be overstated.

Another important chapter with respect to JEE is Kinematics, which forms the base for Mechanics.

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