Thermodynamic

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Thermodynamics

Thermodynamics (from the Greek thermós, "heat" and dynamos, "power, force") refers to the branch of physics that studies the mechanical actions of heat and other similar forms of energy.

Its study approaches objects as real macroscopic systems, using the scientific method and deductive reasoning, paying attention to extensive variables such as entropy, internal energy, or volume; as well as non-extensive variables such as temperature, pressure, or chemical potential, among other types of magnitudes.

However, thermodynamics does not offer an interpretation of the magnitudes it studies, and its objects of study are always systems in a state of equilibrium, that is, those whose characteristics are determined by internal elements rather than by external forces acting on them.

For this reason, he believes that energy can only be exchanged from one system to another in the form of heat or work.

The formal study of thermodynamics began thanks to Otto von Guericke in 1650, a German physicist and jurist who designed and built the first vacuum pump, refuting Aristotle's maxim that "nature abhors a vacuum" with his applications.

After this invention, scientists Robert Boyle and Robert Hooke perfected their systems and observed the correlation between pressure, temperature, and volume. Thus, the principles of thermodynamics were born.

Thermodynamic System

A thermodynamic system is understood as a part of the universe that, for the purposes of study, is conceptually isolated from the rest and attempted to be understood autonomously.

It takes note of the ways in which energy changes or is preserved and, at the same time, of its exchanges of matter and/or energy with the environment or with other similar systems (if any). It is, therefore, a method of studying thermodynamics.

The main criterion for classifying these systems is based on their degree of isolation from the environment, thus distinguishing between:

Open Systems: Those that freely exchange energy and matter with their environment, as most known systems do in everyday life. For example: a car. You supply fuel, and it returns gases and heat to the environment.

Closed systems: Those that exchange energy with their surroundings, but not matter. This is what happens with a closed container, such as a can, whose contents remain unchanged but lose heat over time, dissipating it into the surrounding air.

Insulated systems: Those that, to a certain extent, do not exchange energy or matter with the environment. Perfectly insulated systems do not exist, of course, but they do exist to a certain degree: a thermos containing hot water will maintain its temperature for a while, long enough to remain insulated.

Laws of Thermodynamics

The laws of thermodynamics (or principles of thermodynamics) describe the behavior of three fundamental physical quantities—temperature, energy, and entropy—that characterize thermodynamic systems. The term "thermodynamics" comes from the Greek words "thermos," meaning "heat," and "dynamos," meaning "force."

Mathematically, these principles are described by a set of equations that explain the behavior of thermodynamic systems, defined as any object of study (from a molecule or a human being to the atmosphere or boiling water in a saucepan).

There are four laws of thermodynamics, and they are crucial to understanding the physical laws of the universe and the impossibility of certain phenomena such as perpetual motion.

First Law

The first law is called the Law of Conservation of Energy because it dictates that in any physical system isolated from its surroundings, the total amount of energy will always be the same, even though it can be transformed from one form of energy to another. In other words, energy cannot be created or destroyed, only transformed.

Thus, when a given amount of heat (Q) is supplied to a physical system, its total amount of energy can be calculated as the heat supplied minus the work (W) done by the system on its surroundings. Expressed in a formula: ΔU = Q – W.

As an example of this law, let's imagine an airplane engine. It is a thermodynamic system consisting of fuel that, when reacting chemically during the combustion process, releases heat and performs work (causing the airplane to move). So, if we could measure the amount of work done and the amount of heat released, we could calculate the total energy of the system and conclude that the energy in the engine remained constant during the flight: no energy was created or destroyed, but rather it was changed from chemical energy to heat energy and kinetic energy (motion, i.e., work).

Second Law

The second law, also called the "Law of Entropy," can be summarized as the amount of entropy in the universe tends to increase over time. This means that the degree of disorder in systems increases until they reach a point of equilibrium, which is the state of greatest disorder in the system.

This law introduces a fundamental concept in physics: the concept of entropy (represented by the letter S), which in the case of physical systems represents the degree of disorder. It turns out that in every physical process in which energy is transformed, a certain amount of energy is unusable; that is, it cannot perform work.

If it cannot perform work, in most cases that energy is heat. The heat released by the system increases the disorder of the system, its entropy. Entropy is a measure of the disorder of a system.

The formulation of this law establishes that the change in entropy (dS) will always be equal to or greater than the heat transfer (dQ) divided by the temperature (T) of the system. That is, dS ≥ dQ / T.

To understand this with an example, simply burn a given amount of matter and then collect the resulting ashes. When weighed, we will see that it is less matter than was present in its initial state: part of the matter was converted into heat in the form of gases that cannot perform work on the system and contribute to its disorder.

Third Law

The third law states that the entropy of a system brought to absolute zero will be a definite constant. In other words:

Upon reaching absolute zero (zero in Kelvin units), the processes of physical systems stop.

Upon reaching absolute zero (zero in Kelvin units), entropy has a constant minimum value.

It is difficult to reach the so-called absolute zero (-273.15 ° C) on a daily basis, but we can understand this law by analyzing what happens in a freezer: the food we place there will cool so much that the biochemical processes inside will slow down or even stop. This slows down its decomposition and makes it suitable for consumption for much longer.

Zeroth Law of Thermodynamics

The "zeroth law" is known by that name even though it was the last to be postulated. Also known as the Law of Thermal Equilibrium, this principle states that: "If two systems are independently in thermal equilibrium with a third system, they must also be in thermal equilibrium with each other." It can be logically expressed as follows: If A = C and B = C, then A = B.

This law allows us to compare the thermal energy of three different bodies, A, B, and C. If body A is in thermal equilibrium with body C (they have the same temperature), and B also has the same temperature as C, then A and B have the same temperature.

Another way to state this principle is to argue that when two bodies with different temperatures come into contact, they exchange heat until their temperatures equalize.

Everyday examples of this law are easy to find. When we enter cold or hot water, we will notice the temperature difference only for the first few minutes, since our body will then enter thermal equilibrium with the water and we will no longer notice the difference.

The same thing happens when we enter a hot or cold room: we will notice the temperature at first, but then we will stop noticing it. difference because we will enter into thermal equilibrium with it.

Chemical Thermodynamics

Chemical thermodynamics is a separate field of study, focused on the correlation between heat and work, and chemical reactions, all framed within the principles of thermodynamics.

That is, it involves applying the laws of thermodynamics, especially the first two, to the world of reactions between substances and compounds, to obtain the so-called "fundamental Gibbs equations," which govern how the chemical energy contained in different compounds changes and is transmitted, or how the degree of entropy of the universe increases each time a spontaneous reaction occurs.

Activity. Answer the following questions.

1. What does thermodynamics study?

2. What type of thermodynamic systems exchange energy and matter with their environment?

3. What type of thermodynamic systems exchange only energy with their environment, but not matter?

4. What type of thermodynamic systems do not exchange energy or matter with their surroundings?

5. What does the Zeroth Law of Thermodynamics tell us?

Once you click this button, the questions will be closed and you will not be able to change your answer.

6. What variables does thermodynamics study?

7. What is an open system according to thermodynamics?

8. What is an isolated system according to thermodynamics?

9. Why does thermodynamics only study systems in equilibrium?

10. What does the First Law of Thermodynamics establish?

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Answers to open questions:

6. Thermodynamics studies extensive variables such as entropy, internal energy, or volume; and non-extensive variables such as temperature, pressure, or chemical potential, among other types of quantities.

7. An open system is one that freely exchanges energy and matter with its environment, as occurs in the case of a car that receives fuel and emits gases and heat.

8. An isolated system is one that, to a certain extent, does not exchange energy or matter with its environment. An example is a thermos that contains hot water and maintains its temperature for a period of time.

9. Thermodynamics only studies systems in a state of equilibrium, that is, those whose characteristics are determined by internal elements and not so much by external forces acting on them.

10. he First Law of Thermodynamics states that the total amount of energy in a physical system isolated from its environment will always be the same, although it can be transformed from one form of energy to another.

1. Leskow, E. C. (2024f, octubre 24). Termodinámica - Concepto, leyes y sistema termodinámico. Concepto. https://concepto.de/termodinamica/

7. The Organic Chemistry Tutor. (2016a, julio 17). Thermochemistry Equations & Formulas - Lecture review & Practice Problems [Vídeo]. YouTube. https://www.youtube.com/watch?v=LsqKL3pBVMA