An important section of physics is thermodynamics. She studies various transitions between system states. From the mathematical point of view, it is easiest to investigate processes in which one of the thermodynamic parameters is fixed. In this article, we will consider in detail the question that this is an isothermal process.
Thermodynamic system
A system in thermodynamics is usually called a set of its constituent elements, which can exchange energy with each other. As a rule, these elements are atoms and molecules. The system is in thermodynamic equilibrium when all its macroscopic characteristics do not change in time.
Systems are open, closed and isolated. In open systems, energy and matter exchange with the environment; in closed systems, only energy exchange with the environment is possible. Finally, isolated systems are entirely left to their own devices. Their energy and amount of matter are conserved over time.
Note that the concept of a thermodynamic system is not associated with the aggregate state of matter. So, water and water vapor in a closed vessel or ice floating in a glass of water are examples of thermodynamic systems in which two aggregate states of the same chemical substance are in equilibrium at the same time.
Thermodynamic parameters
In order to be able to describe the state of the thermodynamic system and its change, parameters such as pressure P, temperature T, and volume V are used in physics. All of them are experimentally measured quantities. So, the pressure is determined by a barometer (unit of measurement is Pascal), temperature is determined by a thermometer (unit of measurement is Kelvin), the volume is determined based on the shape and linear dimensions of the object (unit of measurement is cubic meters).
The considered values ββ(P, T, V) mankind has measured since ancient times, using various empirical methods and methods of comparison (hot - cold, large-small). Only with the advent of modern times, many European scientists began to approach the issue of measuring macroscopic parameters from a scientific point of view. The development of the molecular-kinetic theory of gases and liquids in the eighteenth and nineteenth centuries made it possible to connect macroscopic thermodynamic characteristics with the microscopic parameters of the system.
What is an isothermal process?
Now it's time to answer the main question of the article. To answer the question that this is an isothermal process is not very difficult. This is the name of any transition between two states of a system in which the absolute temperature remains constant and does not change. For example, if you slowly increase the volume of gas in the cylinder by moving the piston, then this thermal process will be isothermal (heat exchange with the environment during a slow expansion of the gas will keep its temperature constant).
The above example is not the only one. Considering that this is an isothermal process, the following important examples should be noted that describe the change in the state of the system at a constant temperature:
- isothermal expansion and compression of an ideal gas;
- mixing ideal gases at a constant temperature;
- transitions between aggregate states.
We consider each of them in more detail in the article.
Boyle-Marriott Law
In the second half of the 17th century, the Englishman Robert Boyle and the Frenchman Edm Mariotte independently established experimentally the following relation for gas systems:
P * V = const.
In the experiments they used different gases. In this case, the product of pressure on the volume always remained constant. This equation is called the Boyle-Mariotte law. Graphically, the function P (V) is a hyperbolic dependence.
We just examined the isothermal process in an ideal gas. It is not difficult to understand the expression above if we recall what the universal equation of state for a gas system looks like:
P * V = n * R * T
Since T = const, n is the amount of substance that also does not change, since the system is closed, R is the universal gas constant, the right side of the universal equation for the process in question is constant, which means that the left side also does not change.
Avogadro principle
In 1810, experimenting with different gases, Amedeo Avogadro came to an amazing conclusion. It turned out that the volume occupied by the gas is a function of only the amount of substance at a certain pressure and temperature. It doesnβt matter what molecules and atoms the gas system consists of, 1 mole of any gas always occupies the same volume if P = const and T = const.
It is not difficult to understand the stated principle if we again turn to the universal equation for an ideal gas. We have:
V / n = R * T / P = const at P = const, T = const
The considered process is isobaric-isothermal.
The Avogadro principle underlies Dalton's law. If two ideal gases are mixed at a constant temperature in a vessel with a certain fixed volume, then the absolute pressure in the system will be equal to the sum of the partial pressures of each component. In turn, the partial pressure of a component is directly proportional to the atomic fraction of its particles in the mixture:
P = β i = 1 m (P i ) = R * T / V * β i = 1 m (n i )
Here m is the number of components in the mixture. The gas mixing process is isochoric-isothermal.
The transition between the aggregative states of matter
We are talking about the processes of melting a solid or crystallization of a liquid, the processes of boiling a liquid or condensation of a gas. All these transitions occur either with the release or absorption of heat. So, as a result of crystallization, liquid molecules lose energy, releasing it into the environment. The latter leads to a decrease in the kinetic energy of molecules and the formation of a certain structure (amorphous, crystalline) from them. On the contrary, melting is an endothermic process, that is, it goes with the absorption of heat. This heat is spent on the destruction of chemical bonds between molecules and atoms in a solid.
Any transition between the aggregate states of matter is an isothermal process, that is, it occurs at a constant temperature. For example, everyone knows that pure ice melts at 0 o C, but at 0 o C water also crystallizes.
Determination of the thermal effect of the transition between aggregate states
All thermodynamic calculations of crystallization, melting, condensation and boiling processes are carried out using the following formula:
Q = Ξ» * m
Here Q is the amount of heat that is released or absorbed as a result of the conversion of m kilograms of matter from one state of aggregation to another. The value Ξ» is a constant of the thermal process, the name of which depends on the specific transformation. For example, in the case of melting, it is called the specific heat of fusion. Measured Ξ» in J / kg.