Standard conditions and standard condition. Elements of chemical thermodynamics and kinetics

In systems with chemical processes, the main reason for changes in thermodynamic parameters is chemical reactions. Since chemical transformations are very diverse, the problem of choosing the origin of energy quantities, for example thermodynamic potentials, arises. For this purpose, concepts such as standard states And standard conditions. Considering that in chemical reactions elements do not rotate into each other, the totality of all chemical elements in the form of simple substances that are in more stable forms at 25°C is taken as the zero reference. This selected a collection of simple substances forms the basis for thermodynamic calculations, and each simple substance included in the basis is a basic substance. To perform thermodynamic calculations, the parameters of the substance in the standard state are used.

The standard state is selected according to the IUPAC recommendation as follows:

1) the temperature of the substance in the standard state is equal to the temperature of the system: T = T(system);

2) the pressure above the substance or the pressure of the gaseous substance in the standard state (P 0) is equal to 1 bar:

P = P 0 = 1 bar

(1 bar = 10 5 Pa). (Pascals have been recommended for use by IUPAC since 1982.) Previously, one atmosphere (1 atm = 101325 Pa) was used as the standard state. Although the differences in units are small, it is recommended to pay attention to the pressure units.

3) For liquid and solid substances, real states are taken at P 0 = 1 bar and temperature T. (Sometimes substances in hypothetical states are also introduced into consideration - for example, water in the form of a gas at a pressure of 1 bar at a temperature below 100 ° C or in the form ice at 25 °C.)

Thermodynamic quantities that characterize substances in the standard state are called standard, and a superscript is used to designate them, for example.

The temperature used is 298.15 K (25°C).

The standard enthalpy and Gibbs energy of formation of substance A is usually understood as the change in enthalpy and Gibbs energy in the reaction:

Where are the basic substances.

The index f comes from the English word formation. It is used to denote the reaction of formation of a substance from simple basic substances. If substances and A are under standard conditions, then we speak of standard enthalpy, standard entropy and standard Gibbs energy of formation under standard conditions:

For basic substances in any standard states, =0 kJ/mol and =0 kJ/mol are taken. Typically, the calculation of thermodynamic parameters at any temperature is based on the use of standard states at standard conditions, for which kJ/mol and kJ/mol are also assumed. The tables usually give the values ​​of standard enthalpies of formation of compounds from simple basic substances under standard conditions with designations per one mole of substance A formed.

For a long time, physicists and representatives of other sciences had a way of describing what they observed in the course of their experiments. The lack of a common opinion and the presence of a large number of terms taken out of thin air led to confusion and misunderstandings among colleagues. Over time, each branch of physics acquired its own established definitions and units of measurement. This is how thermodynamic parameters emerged that 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 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.

There are intensive and extensive parameters. 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 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 simply a property. It all depends on what content the scientist uses it in. But in some cases, there are standardized recommendations that the drafters of documents, textbooks or orders must adhere to.

Classification

There are several classifications of thermodynamic parameters. So, based on the first point, 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 of the substance was taken for the reaction, since they level out during interaction.

Based on the conditions under which the substances that make up the system are located, quantities can be divided into those that describe phase reactions and chemical reactions. In addition, the reactants must be taken into account. They can be:

• thermomechanical;
• thermophysical;
• thermochemical.

In addition, any thermodynamic system performs a specific function, so parameters can characterize the work or heat obtained as a result of a reaction, and also allow one to calculate the energy required to transfer the mass of particles.

State Variables

The state of any system, including a thermodynamic one, can be determined by a combination of its properties or characteristics. All variables that are completely determined only at a specific moment in time and do not depend on how exactly the system came to this state are called thermodynamic parameters (variables) of the state or functions of the state.

A system is considered stationary if the variable functions do not change over time. One option 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 parameters of the state. The sequence in which the states of a system continuously transform into each other is called the “process path.”

Unfortunately, confusion with terms still exists, since the same variable can be either independent or the result of the addition of 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 quantity that characterizes the amount of kinetic energy per unit of particles in a thermodynamic system in a state of equilibrium.

If we approach the definition of the concept from the point of view of thermodynamics, then temperature is a quantity 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 energy is given off by the hotter body and absorbed by the colder one.

There are thermodynamic systems in which, when energy is added, disorder (entropy) does not increase, but, on the contrary, 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 continuously move chaotically and collide with each other and with the vessel in which they are located. It is these impacts that cause the pressure of the substance on the walls of the vessel or on the body that is placed in the gas. The force is distributed equally in all directions precisely because of the unpredictable movement of molecules. To increase the pressure, it is necessary to 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 kinetic energy caused by the movement of molecules of a substance, as well as potential energy that appears when molecules interact with each other.

This parameter is unambiguous. That is, the value of internal energy is constant every time the system finds itself in the desired state, regardless of how it (the state) was achieved.

It is impossible to change internal energy. It consists of the heat generated by the system and the work 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 states that entropy does not decrease. Another formulation postulates that energy never transfers from a body at a lower temperature to a body at a higher temperature. 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 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 draw the following conclusion: if the temperature of the bodies that make up the system tends to zero, then the entropy will be zero.

Entropy as a thermodynamic parameter of the state of a gas is used as an indication of the measure of disorder, chaotic movement of particles. It is used to determine the distribution of molecules in a certain area 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 a state of 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, the formulation “energy of the expanded system” is used instead of enthalpy. To make it easier to explain this value, you can imagine a vessel filled with gas, which is uniformly compressed by a piston (for example, an internal combustion engine). In this case, 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 to the required state. Changing this parameter depends only on the initial and final state of the system, and the path by 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 equivalent to the total chemical energy of the system. It shows what changes will occur during chemical reactions and whether substances will interact at all.

Changing the amount of energy and temperature of a system during a reaction affects concepts such 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 amounts of substance remain unchanged.

Helmholtz energy

Helmholtz energy (according to other sources - simply free energy) represents the potential amount of energy that will be lost by a system when interacting with bodies outside of 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 transition 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 number of particles, also do not occur.

STANDARD STATE in thermochemistry is the state of a substance in which it is found at a temperature of 298.15 K and a pressure of 101.325 kPa (760 mm Hg).

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"STANDARD CONDITION" in books

Oilo standard