Before discussing what ideal gas pressure is, the content of the concept of “ideal gas” should be clarified. And this concept characterizes a mathematical model, a universal formula, assuming that the distribution of potential and kinetic energy of interacting molecules is such that the value of potential energy can be neglected. The chemical-physical meaning consists in the fact that the absolute elasticity of the walls of the vessels in which the gas is located is assumed, and in addition, the magnitude of the attractive forces of the molecules, the impulses of their impact on the walls of the vessel and against each other are recognized as insignificant.
Such an understanding of the essence of an ideal gas finds wide application in the field of solving the problems of thermodynamics of gases.
In the physical sense, varieties of an ideal gas are distinguished: a classical one, whose properties are determined by the classical laws of mechanics, and a quantum one, whose character is derived from the principles of quantum mechanics.
The first to derive a general equation was the great French physicist Benoit Clapeyron. He also developed the basic principles of the ideal gas theory, which form the basis of all modern theories that study various gases.
The starting point of this teaching is the conclusion that the pressure of an ideal gas is unchanged with a linear dependence of its volume on temperature. In this case, some conditional assumptions must be taken into account:
- the diameter of the ideal gas molecule is small to the extent that it can be neglected;
- the momentum between the molecules can be transmitted only during collisions, thus, the force of attraction between them can also be neglected;
- the total value of the energy of the gas molecules is recognized as a constant, in the absence of heat transfer and work performed on this gas. In this case, the pressure of an ideal gas depends on the sum of the momenta that are created when the molecules collide with the walls of the vessel.
During the existence of the doctrine, many scientists were studying the physicochemical nature of gases, and many of them had different approaches. This led to the fact that in physical theory they consider the classification of an ideal gas from the point of view of the laws that this or that physicist put in the basis of his research - Fermi gas, Bose gas and others. So, for example, according to the equivalent approach, the gas in question simultaneously satisfies the laws of Boyle-Mariotte and Gay-Lussac: pV = bT, where p is pressure, T is absolute temperature. Mendeleev’s formula gives a more extensive idea of the properties: pV = m / M x RT, where are indicated: R - gas constant, M - molar mass, m - mass.
One of the earliest and most developed teachings on the properties of gases was the description of such a property as the pressure of an ideal gas. But there were some flaws in this concept related to the one-sided approach to research. So, even by measuring the pressure, we will not be able to find out the parameters of the average kinetic energy of each of the individual molecules, as well as the concentration of these molecules in the vessel. Therefore, some parameter is still needed, with the help of which it is possible to solve the arisen problem. The temperature was proposed by physicists as such a quantity. This scalar quantity in thermodynamics gives an idea of the thermal state of the system and what its dynamics are. But in gas theory, temperature is also important as a molecular kinetic parameter, because it describes the behavior of gas molecules in a vessel and also reflects their average kinetic energy. This value is called the Boltzmann constant.
In order to avoid entering into the complexity of higher mathematics when searching for a pressure formula , it is necessary to artificially introduce some simplifications:
- the form of molecules can be represented as a ball;
- the distance between the molecules is infinitely large, excluding the action of attractive forces;
- the speed of movement of the molecules is set at an average level;
- imagine the walls of the vessel are absolutely elastic.
From this we can derive a formula in which the pressure of an ideal gas will be the quotient of dividing the magnitude of the force acting perpendicular to the vessel wall and the surface area on which this force acts: p = F / S.
In those cases when our simplifications do not work to establish how the pressure of an ideal gas changes, additional values will need to be entered into this simple formula.