For a long time, physicists and representatives of other sciences had a way of describing what they observe in the course of their experiments. The lack of consensus and the presence of a large number of terms taken "out of the blue" led to confusion and misunderstandings among colleagues. Over time, each branch of physics acquired its established definitions and units of measurement. This is how thermodynamic parameters appeared, which explain most of the macroscopic changes in the system.
Definition
State parameters, or thermodynamic parameters, are a number of physical quantities that together and each separately 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.
Select intensive and extensive parameters. Extensive are those that are directly dependent on the mass of the thermodynamic system, andintensive - which 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 disagreements among physicists. The same physical characteristic can be called by different authors either a process, or a coordinate, or a quantity, or a parameter, or even just a property. It all depends on the content in which the scientist uses it. But in some cases, there are standardized recommendations that drafters 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 number of ingredients;
- intense - they do not depend on how much of the substance was taken for the reaction, since they are aligned during the interaction.
Based on the conditions under which the substances that make up the system are located, the quantities can be divided into those that describe phase reactions and chemical reactions. In addition, the properties of the reactants must be taken into account. They can be:
- thermomechanical;
- thermophysical;
- thermochemical.
Besides this, any thermodynamic system performs a certain function, so the parameters cancharacterize the work or heat produced as a result of the reaction, and also allow you to calculate the energy required 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 completely determined only at a particular moment in time and do not depend on how exactly the system came to this state are called thermodynamic state parameters (variables) or state functions.
The system is considered stationary if the variable functions do not change over time. One version of the steady state is thermodynamic equilibrium. Any, even the smallest change in the system, is already a process, and it can contain from one to several variable thermodynamic state parameters. The sequence in which the states of the system continuously transition into each other is called the "process path".
Unfortunately, there is still confusion with the terms, since the same variable can be both independent and the result of adding several system functions. Therefore, terms such as "state function", "state parameter", "state variable" can be considered as synonyms.
Temperature
One of the independent parameters of the state of a thermodynamic system is temperature. It is a value that characterizes the amount of kinetic energy per unit of particles inthermodynamic system in equilibrium.
If we approach the definition of the 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 off by a hotter body and absorbed by a colder one.
There are thermodynamic systems in which when energy is added, disorder (entropy) does not increase, but rather decreases. In addition, if such a system interacts with a body whose temperature is higher than its own, then it will give up its kinetic energy to this body, and not vice versa (based on the laws of thermodynamics).
Pressure
Pressure is a quantity that characterizes the force acting on a 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 this force will be pascals.
In the case of thermodynamic parameters, the gas occupies the entire volume available to it, and, in addition, the molecules that make it up constantly move randomly and collide with each other and with the vessel in which they are located. It is these impacts that determine the pressure of the substance on the walls of the vessel or on the body that is placed in the gas. Force propagates equally in all directions precisely because of the unpredictablemolecular movements. To increase the pressure, you must increase the temperature of the system, and vice versa.
Internal energy
The main thermodynamic parameters that depend on the mass of the system include internal energy. It consists of the kinetic energy due to the movement of the molecules of a substance, as well as the potential energy that appears when the molecules interact with each other.
This parameter is unambiguous. That is, the value of internal energy is constant whenever the system is in the desired state, regardless of which way it (the state) was reached.
It is impossible to change the internal energy. It is the sum of the heat given off 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 the number of molecules.
Entropy
The second law of thermodynamics states that the entropy of an isolated system does not decrease. Another formulation postulates that energy never passes from a body with a lower temperature to a hotter one. This, in turn, denies the possibility of creating a perpetual motion machine, since it is impossible to transfer all the energy available to the body into work.
The very concept of "entropy" was introduced into use in the middle of the 19th century. Then it was perceived as a change in the amount of heat to the temperature of the system. But this definition only applies toprocesses that are constantly in equilibrium. From this we can draw the following conclusion: if the temperature of the bodies that make up the system tends to zero, then the entropy will also be equal to zero.
Entropy as a thermodynamic parameter of the gas state is used as an indication of the measure of randomness, randomness of particle motion. It is used to determine the distribution of molecules in a certain area and vessel, or to calculate the electromagnetic force of interaction between the ions of a substance.
Enthalpy
Enthalpy is the 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 entropy level, the number of molecules and the pressure.
If the thermodynamic parameter of an ideal gas is indicated, instead of enthalpy, the wording "energy of the extended system" is used. In order to make it easier to explain this value to ourselves, we can imagine a vessel filled with gas, which is uniformly 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 must be done to bring the system into the required state. Changing this parameter depends only on the initial and final state of the system, and the way in which it will be obtained does not matter.
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, the Gibbs energy is the equivalent of the total chemical energy of the system. It shows what changes will occur in the course of chemical reactions and whether substances will interact at all.
Change in the amount of energy and temperature of the system during the course of the reaction affects such concepts as enthalpy and entropy. The difference between these two parameters will be called the Gibbs energy or isobaric-isothermal potential.
The minimum value of this energy is observed if the system is in equilibrium, and its pressure, temperature and amount 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 a system can perform, that is, how much heat is released when substances change 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 changing other parameters, such as temperature,pressure, the number of particles also does not occur.
