For a long time, there was a way among physicists and representatives of other sciences to describe what they observe in the course of their experiments. The lack of consensus and the presence of a large number of terms taken "from the ceiling" led to confusion and misunderstanding among colleagues. Over time, each branch of physics acquired its own well-established definitions and units of measurement. So there were thermodynamic parameters that explain most of the macroscopic changes in the system.
Definition
State parameters, or thermodynamic parameters, are a series of physical quantities that, collectively and individually, can characterize the observed system. These include concepts such as:
- temperature and pressure;
- concentration, magnetic induction;
- entropy;
- enthalpy;
- Gibbs and Helmholtz energies and many others.
Intensive and extensive parameters are distinguished. Extensive are those that are directly dependent on the mass of the thermodynamic system, and intensive are those that are determined by other criteria. Not all parameters are equally independent, therefore, in order to calculate the equilibrium state of the system, it is necessary to determine several parameters at once.
In addition, there are some terminological differences among physicists. The same physical characteristic of different authors can be called either a process, or a coordinate, or a quantity, or a parameter, or simply a property. It all depends on the content in which the scientist uses it. But in some cases there are standardized recommendations that compilers of documents, textbooks or orders must adhere to.
Classification
There are several classifications of thermodynamic parameters. So, based on the first paragraph, it is already known that all quantities can be divided into:
- extensive (additive) - such substances obey the law of addition, that is, their value depends on the amount of ingredients;
- intense - they do not depend on how much substance was taken for the reaction, since they are equalized during the interaction.
Based on the conditions under which the substances that make up the system are located, the values can be divided into those that describe phase reactions and chemical reactions. In addition, the properties of the substances that enter into the reaction must be taken into account. They can be:
- thermomechanical;
- thermophysical;
- thermochemical.
In addition, any thermodynamic system performs a certain function, so the parameters can characterize the work or heat obtained as a result of the reaction, and also allow you to calculate the energy needed to transfer the mass of particles.
State variables
The state of any system, including thermodynamic, can be determined by a combination of its properties or characteristics. All variables that are fully determined only at a particular moment in time and do not depend on how exactly the system entered this state are called thermodynamic parameters (variables) of the state or state functions.
A system is considered stationary if the variable functions do not change over time. One of the options for a stationary state is thermodynamic equilibrium. Any, even the smallest change in the system is already a process, and in it there can be from one to several variable thermodynamic parameters of the state. The sequence in which the states of the system continuously transition into each other is called the "process path".
Unfortunately, confusion with the terms still exists, since the same variable can be both independent and the result of the addition of several functions of the system. Therefore, terms such as “state function”, “state parameter”, “state variable” can be considered in the form of synonyms.
Temperature
One of the independent state parameters of a thermodynamic system is temperature. It is a quantity that characterizes the amount of kinetic energy per unit particle in a thermodynamic system in equilibrium.
If we approach the definition of a concept from the point of view of thermodynamics, then the temperature is a value inversely proportional to the change in entropy after adding heat (energy) to the system. When the system is in equilibrium, the temperature value is the same for all its “participants”. If there is a temperature difference, then the energy is given away by a warmer body and absorbed by a colder one.
There are thermodynamic systems in which, with the addition of energy, disorder (entropy) does not increase, but rather decreases. In addition, if such a system interacts with a body whose temperature is greater than its own, then it will give its kinetic energy to this body, and not vice versa (based on the laws of thermodynamics).
Pressure
Pressure is a quantity characterizing the force acting on the body perpendicular to its surface. In order to calculate this parameter, it is necessary to divide the entire amount of force by the area of the object. The units of measure for this force will be Pascal.
In the case of thermodynamic parameters, the gas occupies the entire volume accessible to it, and, in addition, the molecules and its constituents move randomly randomly and collide with each other and with the vessel in which they are located. It is these shocks that determine the pressure of the substance on the walls of the vessel or on the body, which is placed in the gas. The force spreads in all directions in the same way precisely because of the unpredictable movement of molecules. To increase pressure, it is necessary to increase the temperature of the system, and vice versa.
Internal energy
The main thermodynamic parameters, depending on the mass of the system, include internal energy. It consists of the kinetic energy due to the movement of the molecules of the substance, as well as the potential energy that appears when the molecules interact with each other.
This parameter is unique. That is, the value of internal energy is constantly whenever the system is in the desired state, regardless of the way in which it (the state) was achieved.
It is impossible to change the internal energy. It consists of the heat generated by the system and the work that it produces. For some processes, other parameters are taken into account, such as temperature, entropy, pressure, potential, and number of molecules.
Entropy
The second law of thermodynamics says that the entropy of an isolated system does not decrease. Another formulation postulates that energy never transfers from a body with a lower temperature to a warmer one. This, in turn, denies the possibility of creating a perpetual motion machine, since it is impossible to transfer all the energy available in the body to work.
The very concept of “entropy” was introduced into everyday life in the mid-19th century. Then it was perceived as a change in the amount of heat to the temperature of the system. But such a definition is only suitable for processes that are constantly in a state of equilibrium. From this we can deduce the following conclusion: if the temperature of the bodies that make up the system tends to zero, then the entropy will be equal to zero.
Entropy as a thermodynamic parameter of a gas state is used as an indication of a measure of randomness, randomness of particle motion. It is used to determine the distribution of molecules in a certain region and vessel, or to calculate the electromagnetic force of interaction between ions of a substance.
Enthalpy
Enthalpy is energy that can be converted into heat (or work) at constant pressure. This is the potential of a system that is in equilibrium if the researcher knows the level of entropy, the number of molecules, and pressure.
If the thermodynamic parameter of an ideal gas is indicated, instead of the enthalpy, the phrase "energy of the extended system" is used. In order to make it easier to explain this value, you can imagine a vessel filled with gas, which is evenly compressed by a piston (for example, an internal combustion engine). In this case, the enthalpy will be equal not only to the internal energy of the substance, but also to the work that needs to be done to bring the system to the required state. Changing this parameter depends only on the initial and final state of the system, and the way in which it is obtained does not play a role.
Gibbs Energy
Thermodynamic parameters and processes, for the most part, are associated with the energy potential of the substances that make up the system. Thus, Gibbs energy is the equivalent of the total chemical energy of the system. It shows what changes will occur during chemical reactions and whether substances will interact at all.
Changes in the amount of energy and temperature of the system during the course of the reaction affect concepts such as enthalpy and entropy. The difference between these two parameters will be called the Gibbs energy or the isobaric-isothermal potential.
The minimum value of this energy is observed if the system is in equilibrium, and its pressure, temperature and quantities of matter remain unchanged.
Helmholtz Energy
Helmholtz energy (according to other sources - just free energy) is the potential amount of energy that will be lost by the system when interacting with bodies that are not included in it.
The concept of Helmholtz free energy is often used to determine what maximum work the system can do, that is, how much heat is released when substances transfer from one state to another.
If the system is in a state of thermodynamic equilibrium (that is, it does not do any work), then the level of free energy is at a minimum. This means that changes in other parameters, such as temperature, pressure, and the number of particles, also do not occur.