# Classical Thermodynamics, Laws and Basic Concepts

The study of thermodynamics is the branch of physics that studies and describes the thermodynamic transformations induced by heat and work in a thermodynamic system. These transformations are the result of processes that involve changes in the state variables of temperature and energy at the macroscopic level.

Classical thermodynamics is based on the concept of a macroscopic system, that is, a portion of mass physically or conceptually separated from the external environment, often assumed for convenience to be undisturbed by energy exchange with the system.

The state of a macroscopic system that is in equilibrium conditions is specified by quantities called thermodynamic variables or state functions such as temperature, pressure, volume, and chemical composition. The main notations in chemical thermodynamics have been established by the International Union of Pure and Applied Chemistry.

However, there is a branch of thermodynamics, called non-equilibrium thermodynamics, which studies thermodynamic processes characterized by the inability to achieve stable equilibrium conditions.

## Classical Laws of Thermodynamics

The principles of thermodynamics were enunciated during the 19th century and regulate thermodynamic transformations, their progress, their limits. They are real, unproven and unprovable axioms, based on experience, on which the entire theory of thermodynamics is based.

We can distinguish three basic principles, plus a "zero" principle that defines the temperature and that is implicit in the other three.

### Zero Law of Thermodynamics

When two interacting systems are in thermal equilibrium, they share some properties, which can be measured, giving them a precise numerical value. As a result, when two systems are in thermal equilibrium with a third, they are in equilibrium with each other, and the shared property is temperature.

The zero principle of thermodynamics simply says that if a body "A" is in thermal equilibrium with a body "B" and "B" is in thermal equilibrium with a body "C", then "A" and "C" are in thermal equilibrium equilibrium between them.

This principle explains the fact that two bodies at different temperatures, between which heat is exchanged (even if this concept is not present in the zero principle) end up reaching the same temperature.

### First Law of Thermodynamics

The first law of classical thermodynamics, also known as the principle of conservation of energy, states that the total energy of an isolated system is conserved. In other words, energy cannot be created or destroyed, it can only be transformed from one form to another.

The mathematical formulation of the first law of thermodynamics is:

ΔU = Q - W

where ΔU represents the change in the internal energy of the system, Q is the heat transferred to the system from the surroundings, and W is the work done by the system on the surroundings.

This equation indicates that any change in the internal energy of a system is due to heat transfer and work done. If Q is positive, it means that heat is being supplied to the system, while if it is negative, then the system is releasing heat to the surroundings. Similarly, if W is positive, it indicates that the system does work on the environment, and if it is negative, the environment does work on the system.

### Second Law of Thermodynamics

There are several statements of the second principle, all equivalent, and each of the formulations emphasizes a particular aspect. It states that "it is impossible to carry out a cyclical machine whose only result is the transfer of heat from a cold to a warm body" (Clausius statement) or, equivalently, that "it is impossible to carry out a transformation whose result is only that of converting heat extracted from a single source into mechanical work" (Kelvin statement).

This last limitation denies the possibility of carrying out the so-called perpetual motion of the second kind. L ' total entropy of an isolated system remains unchanged when a reversible transformation takes place and increases when an irreversible transformation takes place.

### Third Law of Thermodynamics

The third law of classical thermodynamics states that it is impossible to reach absolute zero (0 Kelvin) through a finite number of thermodynamic transformations. This law was formulated by Walther Nernst in 1906.

In more precise terms, the third law states that the entropy of a perfectly crystalline, pure system is zero when the temperature reaches absolute zero. Entropy is a measure of the disorder or randomness of a system, and the third law states that as the temperature approaches absolute zero, the entropy of the system also approaches zero.

## Applications of Classical Thermodynamics

Classical thermodynamics has a wide range of practical applications. Here are some of the areas where classical thermodynamics is widely used:

1. Power Engineering: Classical thermodynamics is essential for the design and optimization of power generation systems, such as power plants, gas turbines, internal combustion engines, and renewable energy systems. It helps to understand energy efficiency, thermodynamic cycles and heat transfer in these systems.

2. Chemical Engineering: Classical thermodynamics is crucial to the design and operation of chemical processes, including chemical production, petroleum refining, materials synthesis, and food production. It allows the analysis of chemical equilibria, heat transfer calculations and process optimization.

3. Refrigeration and air conditioning: Classical thermodynamics is essential to understand refrigeration cycles and air conditioning systems. Helps in the design of refrigeration systems, the selection of refrigerants and the calculation of cooling capacity.

4. Materials Science: Classical thermodynamics is used to study the properties of materials in different thermodynamic states, such as the solid, liquid, and gas phase. It helps predict phase stability, phase transitions, and equilibrium properties such as vapor pressure and solubility.

5. Study of chemical equilibrium: Classical thermodynamics is fundamental to understanding chemical equilibrium and the behavior of chemical reactions. It allows to determine if a reaction is spontaneous or not, and provides information on the thermodynamic performance of chemical processes.

6. Atmospheric and Climate Research: Classical thermodynamics is applied in the study of the atmosphere, climate, and weather phenomena. It helps to understand the processes of heat transfer in the atmosphere, cloud formation, and solar radiation.

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Published: March 7, 2018
Last review: July 15, 2023