The thermodynamic processes are physical or chemical phenomena involving heat flow (energy) or work between a system and its surroundings. When talking about heat, the image of fire rationally comes to mind, which is the quintessential manifestation of a process that releases a lot of thermal energy.
The system can be both macroscopic (a train, a rocket, a volcano) and microscopic (atoms, bacteria, molecules, quantum dots, etc.). This is separated from the rest of the universe to consider the heat or work that enters or leaves it.
However, not only does the heat flow exist, but the systems can also generate changes in some variable in their environment as a response to the phenomenon considered. According to thermodynamic laws, there must be a trade-off between response and heat so that matter and energy are always conserved.
The above is valid for macroscopic and microscopic systems. The difference between the first and last are the variables that are considered to define their energy states (in essence, the initial and the final).
However, thermodynamic models seek to connect both worlds by controlling variables such as pressure, volume and temperature of the systems, keeping some of these constants to study the effect of the others.
The first model that allows this approximation is that of ideal gases (PV = nRT), where n is the number of moles, which when divided by the volume V gives the molar volume.
Then, expressing the changes between system-around as a function of these variables, others can be defined, such as work (PV = W), essential for machines and industrial processes.
On the other hand, for chemical phenomena, other types of thermodynamic variables are of greater interest. These are directly related to the release or absorption of energy, and depend on the intrinsic nature of the molecules: the formation and types of bonds.
Systems and phenomena in thermodynamic processes
In the upper image the three types of systems are represented: closed, open and adiabatic.
In the closed system there is no transfer of matter between it and its surroundings, so that no matter can enter or leave; however, energy can cross the boundaries of the box. In other words: phenomenon F can release or absorb energy, thus modifying what is beyond the box.
On the other hand, in the open system the horizons of the system have their dotted lines, which means that both energy and matter can come and go between it and the surroundings.
Finally, in an isolated system the exchange of matter and energy between it and the surroundings is zero; for this reason, in the image the third box is enclosed in a bubble. It is necessary to clarify that the surroundings can be the rest of the universe, and that the study is the one that defines how far to consider the scope of the system.
Physical and chemical phenomena
What specifically is phenomenon F? Indicated by the letter F and within a yellow circle, the phenomenon is a change that takes place and can be the physical modification of matter, or its transformation.
What is the difference? Succinctly: the first does not break or create new links, while the second does.
Thus, a thermodynamic process can be considered according to whether the phenomenon is physical or chemical. However, both have in common a change in some molecular or atomic property.
Examples of physical phenomena
Heating water in a pot causes an increase in collisions between its molecules, to the point where the pressure of its vapor equals atmospheric pressure, and then the phase change from liquid to gas occurs. In other words: the water evaporates.
Here the water molecules are not breaking any of their bonds, but they are undergoing energetic changes; or what is the same, the internal energy U of the water is modified.
What are the thermodynamic variables for this case? The atmospheric pressure P ex , the temperature product of the combustion of the cooking gas and the volume of the water.
The atmospheric pressure is constant, but the temperature of the water is not, since it heats up; nor the volume, because its molecules expand in space. This is an example of a physical phenomenon within an isobaric process; that is, a thermodynamic system at constant pressure.
What if you put the water with some beans in a pressure cooker? In this case, the volume remains constant (as long as the pressure is not released when the beans are cooked), but the pressure and temperature change.
This is because the gas produced cannot escape and bounces off the walls of the pot and the surface of the liquid. We speak then of another physical phenomenon but within an isochoric process.
Examples of chemical phenomena
It was mentioned that there are thermodynamic variables inherent to microscopic factors, such as molecular or atomic structure. What are these variables? Enthalpy (H), entropy (S), internal energy (U), and Gibbs free energy (S).
These intrinsic variables of matter are defined and expressed in terms of macroscopic thermodynamic variables (P, T and V), according to the selected mathematical model (generally that of ideal gases). Thanks to this thermodynamic studies can be carried out on chemical phenomena.
For example, you want to study a chemical reaction of the type A + B => C, but the reaction only occurs at a temperature of 70 ºC. Furthermore, at temperatures above 100 ºC, instead of C being produced, D.
Under these conditions, the reactor (the assembly where the reaction takes place) must guarantee a constant temperature around 70 ºC, so the process is isothermal.
Types and examples of thermodynamic processes
They are those in which there is no net transfer between the system and its surroundings. This in the long term is guaranteed by an isolated system (the box inside the bubble).
An example of this are calorimeters, which determine the amount of heat released or absorbed from a chemical reaction (combustion, dissolution, oxidation, etc.).
Within the physical phenomena is the movement generated by the hot gas due to the pressure exerted on the pistons. Likewise, when an air current exerts pressure on a terrestrial surface, its temperature increases as it is forced to expand.
On the other hand, if the other surface is gaseous and has a lower density, its temperature will decrease when it feels a higher pressure, forcing its particles to condense.
Adiabatic processes are ideal for many industrial processes, where lower heat loss means lower performance which is reflected in costs. To consider it as such, the heat flow must be zero or the amount of heat entering the system must be equal to that entering the system.
Isothermal processes are all those in which the temperature of the system remains constant. It does this by doing work, so that the other variables (P and V) vary over time.
Examples of this type of thermodynamic process are innumerable. In essence, much of cell activity takes place at constant temperature (the exchange of ions and water across cell membranes). Within chemical reactions , all those that establish thermal equilibria are considered isothermal processes.
Human metabolism manages to maintain a constant body temperature (approximately 37ºC) through a wide series of chemical reactions. This is achieved thanks to the energy obtained from food.
Phase changes are also isothermal processes. For example, when a liquid freezes it releases heat, preventing the temperature from continuing to decrease until it is completely in the solid phase. Once this happens, the temperature can continue to decrease, because the solid no longer releases energy.
In those systems that involve ideal gases, the change in internal energy U is zero, so all the heat is used to do work.
In these processes the pressure in the system remains constant, varying its volume and temperature. In general, they can occur in systems open to the atmosphere , or in closed systems whose boundaries can be deformed by the increase in volume, in a way that counteracts the increase in pressure.
In cylinders inside engines, when the gas is heated, it pushes the piston, which changes the volume of the system.
If this were not the case, the pressure would increase, since the system has no way of reducing the collisions of the gaseous species on the cylinder walls.
In isochoric processes the volume remains constant. It can also be considered as those in which the system does not generate any work (W = 0).
Basically, they are physical or chemical phenomena that are studied inside any container, whether with agitation or not.
Examples of these processes are the cooking of food, the preparation of coffee, the cooling of an ice cream bottle, the crystallization of sugar, the dissolution of a poorly soluble precipitate, an ion exchange chromatography, among others.
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