The main thermodynamic functions used in metallurgical calculations are the internal energy u, enthalpy H, entropy S, as well as their most important combinations: isobaric-isothermal G = H - TS and isochoric-isothermal F=U-TS potentials, reduced potential F \u003d -G / T.
According to the Nernst theorem for entropy the natural reference point is zero degrees on the Kelvin scale, at which the entropies of crystalline substances are equal to zero. Therefore, from a formal standpoint, in principle, one can always measure or calculate the absolute value of entropy and use it for quantitative thermodynamic estimates. That is, entropy does not introduce any difficulties into the practice of performing numerical thermodynamic calculations.
And here internal energy has no natural origin, and its absolute value simply does not exist. The same is true for all other thermodynamic functions or potentials, because they are linearly related to internal energy:
H = U + PV;
F = U - TS;
G = H - TS = U - TS + PV;
F= -G/T = S - H/T = S -(U+PV)/T.
Therefore, the values U, H, F, G And F thermodynamic system due to the uncertainty of the reference point can only be established up to constants. This fact does not lead to fundamental complications, because for solving all applied problems enough to knowchange quantities thermodynamic functions when changing temperature, pressure, volume, during the passage of phase and chemical transformations.
But in order to be able to carry out real calculations, it was necessary to adopt certain agreements (standards) on the unambiguous choice of certain constants and establish uniform rules for calculating the initial values of thermodynamic functions for all substances found in nature. Due to the linear dependence of the thermodynamic functions H, F, G, F from internal energy U This enough do for only one of these functions. was real unified origin of valuesenthalpy . Made it giving zero value to the enthalpies of certain substances in certain states under precisely specified physical conditions, which bear the name standard substances, standard conditions And standard states.
The following is the most common set of conventions under discussion as recommended by the International Commission on Thermodynamics of the International Union of Pure and Applied Chemistry (IUPAC). This set can be called thermodynamic standards, as practically established in the modern literature on chemical thermodynamics.
Standard Conditions
According to Nernst's theorem, for entropy, the natural reference point, or natural standard temperature, is zero degrees on the Kelvin scale, at which the entropies of substances are zero. In some reference books, published mainly in the USSR, the standard temperature is 0 K. Despite the great logic from the physical and mathematical points of view, this temperature is not widely used as a standard. This is due to the fact that at low temperatures the dependence of heat capacity on temperature is very complex, and it is not possible to use sufficiently simple polynomial approximations for it.
Standard physical conditions correspond to a pressure of 1 atm(1 physical atmosphere = 1.01325 bar)and temperature 298.15 K(25° WITH). It is believed that such conditions are most consistent with the actual physical conditions in chemical laboratories in which thermochemical measurements are carried out.
Standard Substances
In nature, all isolated, independent substances, called in thermodynamics individual , consist of pure elements of the table of D.I. Mendeleev, or are obtained by chemical reactions between them. That's why sufficient condition to establish a reference frame for thermodynamic quantities is the choice of enthalpies only for chemical elements as simple substances. It is accepted that the enthalpies of all elements in their standard states are zero under standard conditions – temperature and pressure. Therefore, the chemical elements in thermodynamics are also called standard substances.
All other substances are considered as compounds obtained by chemical reactions between standard substances (chemical elements in the standard state) They are called " individual substances ". The starting point for the enthalpies for chemical compounds (as well as for elements in non-standard states) is the value of the enthalpy of the reaction of their formation from standard substances, as if carried out under standard conditions. In fact, of course, the thermal effect (enthalpy) of the reaction under real conditions is experimentally determined, and then recalculated to standard conditions. This value is taken as standard enthalpy of formation chemical compound as an individual substance.
In practical calculations, it should be remembered that in thermochemistry the following is accepted as a standard sign rule to characterize the enthalpy. If, during the formation of a chemical compound, heat stands out, the sign ” is selected minus” - heat is lost to the system during the isothermal process. If heat is needed to form a chemical compound absorbed, the sign ” is selected plus” - heat is supplied to the system from the environment to maintain isothermality.
Standard States
For such a state, the equilibrium state is chosen, i.e. most stable form of existence (aggregate state, molecular form) chemical element under standard conditions For example, these are elements in the solid state - lead, carbon in the form of graphite, in liquid - mercury and bromine, diatomic molecules of gaseous nitrogen or chlorine, monatomic noble gases, etc.
Standard notation
To denote any thermodynamic property calculated at standard pressure from a standard value and therefore called standard property, the upper right index 0 (zero) of the character is used. That the property is counted down from the selected standard, indicated by the “” sign in front of the algebraic symbol of the thermodynamic function. The temperature corresponding to the value of the function is often given as a right subscript. For example, standard enthalpy substances at 298.15 K is denoted as
The standard enthalpies of individual substances are taken to be the heats of their formation by chemical reactions from standard substances in the standard state. Therefore, thermodynamic functions are sometimes denoted using the index f(from English formation- education): 
Unlike enthalpy, for entropy its absolute value is calculated at any temperature. Therefore, there is no “” sign in the designation of entropy: 
standard entropy substances at 298.15 K, 
standard entropy at temperature T.
Standard properties of substances under standard conditions, i.e. standard thermodynamic functions summarized in tables of thermochemical quantities and published as handbooks of thermochemical quantities of individual substances.
Isobaric processes are most often encountered in reality, since technological processes tend to be carried out in devices that communicate with the atmosphere. Therefore, reference books of thermochemical data for the most part contain, as necessary and sufficient information for calculating any thermodynamic function, quantity 
If the values of the standard absolute entropy and enthalpy of formation are known, as well as dependence of heat capacity on temperature, it is possible to calculate the values or changes in the values of all other thermodynamic functions.
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 series of physical quantities that, all 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.
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 just 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 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, reactants 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 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 parameters (variables) of the state or state functions.
The 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 state parameters. The sequence in which the states of the system continuously pass 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 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 equilibrium.
If we approach the definition of the concept from the point of view of thermodynamics, then 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 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, on the contrary, decreases. In addition, if such a system interacts with a body whose temperature is greater 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. The force propagates in all directions equally precisely because of the unpredictable movement of the 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 the kinetic energy due to the movement of the molecules of a substance, as well as of 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 how 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 entropy 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 such a definition only applies to 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 equal to zero.
Entropy as a thermodynamic parameter of the state of a gas 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 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 wording "energy of the expanded system" is used instead of enthalpy. 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.
The 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
The Helmholtz energy (according to other sources - simply free energy) is the potential amount of energy that will be lost by the system when interacting with bodies that are not part 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 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 changes in other parameters, such as temperature, pressure, and the number of particles, also do not occur.
ELEMENTS OF CHEMICAL THERMODYNAMICS AND KINETICS
Thermodynamic systems: definition, classification of systems (isolated, closed, open) and processes (isothermal, isobaric, isochoric). Standard state.
Thermodynamics - it's science studying the general laws of the course of processes accompanied by the release, absorption and transformation of energy.
Chemical thermodynamics studies the mutual transformations of chemical energy and its other forms - thermal, light, electrical, etc., establishes the quantitative laws of these transitions, and also allows you to predict the stability of substances under given conditions and their ability to enter into certain chemical reactions. Thermochemistry, which is a branch of chemical thermodynamics, studies the thermal effects of chemical reactions.
The object of thermodynamic consideration is called a thermodynamic system or simply a system.
System - any object of nature, consisting of a large number of molecules (structural units) and separated from other objects of nature by a real or imaginary boundary surface (interface).
The state of the system is a set of properties of the system that allow defining the system from the point of view of thermodynamics.
Types of thermodynamic systems:
I. By the nature of the exchange of matter and energy with the environment:
Isolated system - does not exchange matter or energy with the environment (Δm = 0; ΔE = 0) - thermos, Dewar vessel.
Adiabatically isolated - It is impossible to exchange thermal energy with the external environment, exchange of matter is possible.
2. Closed system - does not exchange with the environment as a substance, but can exchange energy (closed flask with reagents).
3. Open system - can exchange both matter and energy with the environment (the human body).
The same system can be in different states. Each state of the system is characterized by a certain set of values of thermodynamic parameters. Thermodynamic parameters include temperature, pressure, density, concentration, etc. A change in at least one thermodynamic parameter leads to a change in the state of the system as a whole. If the thermodynamic parameters are constant at all points of the system (volume), the thermodynamic state of the system is called equilibrium.
II. By state of aggregation:
1. Homogeneous - the absence of sharp changes in physical and chemical properties during the transition from one area of \u200b\u200bthe system to another (they consist of one phase).
2. Heterogeneous - two or more homogeneous systems in one (consists of two or more phases).
A phase is a part of a system that is homogeneous at all points in composition and properties and is separated from other parts of the system by an interface. An example of a homogeneous system is an aqueous solution. But if the solution is saturated and there are salt crystals at the bottom of the vessel, then the system under consideration is heterogeneous (there is a phase boundary). Plain water is another example of a homogeneous system, but water with ice floating in it is a heterogeneous system.
Phase transition - phase transformations (melting of ice, boiling of water).
Thermodynamic process- transition thermodynamic system from one state to another, which is always associated with a violation equilibrium systems.
For example, to reduce the volume of gas contained in a vessel, you need to move the piston. In this case, the gas will be compressed and, first of all, the gas pressure near the piston will increase - the equilibrium will be disturbed. The imbalance will be the greater, the faster the piston moves. If the piston is moved very slowly, then the equilibrium is slightly disturbed and the pressure at different points differs little from the equilibrium value corresponding to a given volume of gas. In the limit with infinitely slow compression, the gas pressure will have a certain value at each moment of time. Consequently, the state of the gas will always be in equilibrium, so that an infinitely slow process will be composed of a sequence of equilibrium states. Such a process is called equilibrium or quasi-static.
The infinitely slow process is an abstraction. In practice, a process can be considered quasi-static if it proceeds so slowly that the deviations of the values of the parameters from the equilibrium values are negligibly small. When changing the direction of the equilibrium process (for example, replacing gas compression with expansion), the system will pass through the same equilibrium states as in the forward course, but in reverse order. Therefore, equilibrium processes are also called reversible. The process by which a system returns to its original state after a series of changes is called circular process or cycle. The concepts of an equilibrium state and a reversible process play an important role in thermodynamics. All quantitative conclusions of thermodynamics are applicable only to equilibrium states and reversible processes.
Classification of thermodynamic processes:
Isothermal - constant temperature - T= const
Isobaric - constant pressure - p= const
Isochoric - constant volume - V= const
Adiabatic - no heat exchange between the system and the environment - d Q=0
standard condition- V chemical thermodynamics conditionally accepted states of individual substances and components of solutions in the evaluation thermodynamic quantities.
The need to introduce "standard states" is due to the fact that thermodynamic laws do not accurately describe the behavior of real substances when the quantitative characteristic is pressure or concentration. Standard states are chosen for reasons of convenience of calculations, and they can change when moving from one problem to another.
In standard states, the values of thermodynamic quantities are called "standard" and denoted by zero in the superscript, for example: G0, H0, m0 are respectively standard Gibbs energy, enthalpy, chemical potential substances. Instead of pressure thermodynamic equations For ideal gases and solutions use volatility, and instead of concentration - activity.
Commission for Thermodynamics international union of theoretical and applied chemistry(IUPAC) has defined that a standard state is the state of a system conditionally chosen as the standard for comparison. The Commission proposed the following standard states of substances:
For the gas phase, this is the (assumed) state chemically pure substance in the gas phase at a standard pressure of 100 kPa (before 1982 - 1 standard atmosphere, 101,325 Pa, 760 mmHg), implying the presence of properties ideal gas.
For pure phase, mixture or solvent in liquid or solid state of aggregation- This is the state of a chemically pure substance in a liquid or solid phase under standard pressure.
For a solution, this is the (assumed) state of the solute with the standard molality 1 mol/kg, under standard pressure or standard concentration, based on the conditions that the solution is diluted indefinitely.
For a chemically pure substance, it is a substance in a well-defined state of aggregation under a well-defined, but arbitrary, standard pressure.
The IUPAC definition of a standard state does not include a standard temperature, although one often speaks of a standard temperature, which is 25 °C (298.15 K).
7. Reaction speed: average and true. The law of active masses.
Thermodynamic systems: definition, classification of systems (isolated, closed, open) and processes (isothermal, isobaric, isochoric). Standard state.
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Author Chemical Encyclopedia b.b. N.S.ZefirovSTANDARD CONDITION in chemical thermodynamics, the state of a system chosen as the state of reference when evaluating thermodynamic quantities. Need to select STANDARD STATE p. due to the fact that within the framework of chemical thermodynamics, abs. can not be calculated. values of Gibbs energies, chemical potentials, enthalpies and other thermodynamic quantities for a given substance; settlement is possible only in relation. the values of these values in this state in comparison with their value in the STANDARD STATE c.
STANDARD CONDITION p. choose for reasons of convenience of calculations; it can change when moving from one task to another. Values of thermodynamic quantities in STANDARD CONDITION p. are called standard and are usually denoted by zero upwards. index, for example G 0 , H 0 , m 0 - respectively, the standard Gibbs energy, enthalpy, chemical potential of the substance. For a chemical reaction, D G 0 , D H 0 , D S 0 are equal to the changes, respectively, G 0 , H 0 and S 0 of the reacting system in the process of transition from the initial substances to the STANDARD STATE c. to reaction products in STANDARD STATE c.
STANDARD CONDITION p. characterized by standard conditions: pressure p 0, temperature T 0, composition (molar fraction x 0). The IUPAC Commission on Thermodynamics identified (1975) as the main STANDARD STATE p. for all gaseous substances, a pure substance (x 0 \u003d 1) in the state of an ideal gas with a pressure p 0 \u003d 1 atm (1.01 10 5 Pa) for any fixed. temperature. For solid and liquid substances, the basic STANDARD STATE c. is the state of a pure (x 0 \u003d 1) substance under external pressure p 0 \u003d 1 atm. To the definition STANDARD CONDITION p. IUPAC T 0 is not included, although one often speaks of a standard temperature of 298.15 K.
Mn. gases at a pressure of 1 atm cannot be considered as an ideal gas. STANDARD CONDITION p. in these cases, not real, but some hypothetical. state. Similar to the arts. select STANDARD CONDITION p. due to the simplicity of calculations of thermodynamic functions for an ideal gas.
For the process of formation of a chemical compound from simple substances, standard Gibbs energies, enthalpies, and entropies are given in thermodynamic reference books.
To determine these quantities, some simple substances are chosen, for which, by definition, the following conditions are met: = 0, = 0, = 0. As STANDARD STATE c. for simple substances, a stable phase and chemical state of the element is assumed at a given temperature. This state does not always coincide with the natural; so, STANDARD CONDITION p. the simple substance fluorine at all temperatures is a pure ideal gas at 1 atm, consisting of F 2 molecules; in this case, dissociation of F 2 into atoms is not taken into account. STANDARD CONDITION p. may be different in different temperature ranges. For Na, for example, in the range from 0 to T pl (370.86 K) STANDARD STATE s. simple substance - pure metal. Na at 1 atm; in the range from T pl to T kip (1156.15 K) - pure liquid Na at 1 atm; above 1156.15 K is an ideal gas at 1 atm, consisting solely of Na atoms. So arr., the standard enthalpy of formation of solid NaF below 370.86 K corresponds to a change in the enthalpy in the reaction Na (tv) + 1/2 F 2 = = NaF (tv), and in the range of 370.86-1156.15 K corresponds to a change enthalpies in the reaction Na (liquid) + 1/2 F 2 = NaF (TB).
STANDARD CONDITION p. an ion in an aqueous solution is introduced to enable the experimentally determined enthalpies of dissolution D aq H 0 (H 2 O) to be converted into the enthalpy of formation of a chemical compound. So, if the standard enthalpy of dissolution in water KCl is known, and D H 0 arr [K + , solution] and [Cl - , solution] - respectively, the enthalpy of formation of K + and Cl ions in the STANDARD STATE s. in an aqueous solution, then the standard enthalpy of formation of KCl can be calculated by the equation: [KCl, TV] = = - D aq H 0 (H 2 0) + [K +, solution] + [Cl -, solution].
As STANDARD CONDITION p. ion in an aqueous solution, according to the recommendations of IUPAC, take the state of this ion in the hypothetical. a one molar aqueous solution in which the enthalpy for the ion in question is equal to its enthalpy in an infinitely dilute solution. In addition, it is assumed that the enthalpy of formation of the H + ion in the STANDARD STATE c., i.e. [H + , solution, H 2 O] is zero. As a result, it becomes possible to obtain the relative standard enthalpies of formation of other ions in solution based on the most reliable (key) values of the enthalpies of formation of chemical compounds. In turn, the obtained values of the enthalpies of formation of ions in solution serve to determine the unknown enthalpies of formation of a chemical compound in cases where the standard enthalpies of dissolution are measured.
STANDARD CONDITION p. components of two- and multi-component systems is introduced as a reference state when calculating thermodynamic activities, Gibbs energies, enthalpies, entropy of mixing (the last three values in the STANDARD STATE s. are equal to zero). The so-called symmetrical choice STANDARD STATE s. is possible, in which as STANDARD STATE s. component uses its basic STANDARD CONDITION s., as determined according to IUPAC. If the multi-component system is liquid, then as STANDARD STATE c. components, their liquid state is taken. An alternative is the antisymmetric choice of STANDARD STATE s., where the solvent is kept STANDARD STATE s., chosen according to the IUPAC recommendations, and for the solute A as STANDARD STATE s. its state in a solution of unit concentration is chosen, which has the properties of an infinitely dilute solution. Select STANDARD STATUS p. in this case is associated with a certain concentration. scale (molar fraction, molarity, molality). Antisymmetric selection STANDARD STATE p. useful in cases where the solute does not exist in the phase in its pure form (for example, HCl does not exist as a liquid at room temperature).
The concept STANDARD CONDITION p. introduced by G. Lewis in the beginning. 20th century
Literature: Lewis J., Randall M., Chemical thermodynamics, trans. from English, M., 1936; Belousov V.P., Panov M.Yu., Thermodynamics of aqueous solutions of non-electrolytes, L., 1983: Voronin G.F., Fundamentals of thermodynamics, M., 1987, p. 91, 98, 100. M.V. Korobov.
Chemical encyclopedia. Volume 4 >>
Conventionally Accepted States of Individual Substances and Solution Components in Estimating Thermodynamic Quantities.
The need to introduce "standard states" is due to the fact that thermodynamic laws do not accurately describe the behavior of real substances when pressure or concentration serves as a quantitative characteristic. Standard states are chosen for reasons of convenience of calculations, and they can change when moving from one problem to another.
In standard states, the values of thermodynamic quantities are called "standard" and denoted by zero in the superscript, for example: G 0, H 0, m 0 are, respectively, the standard Gibbs energy, enthalpy, chemical potential of the substance. Instead of pressure in thermodynamic equations for ideal gases and solutions, fugacity (volatility) is used, and instead of concentration, activity is used.
IUPAC standard states
The Commission on Thermodynamics of the International Union of Pure and Applied Chemistry (IUPAC) has defined that the standard state is the state of a system conventionally chosen as a standard for comparison. The Commission proposed the following standard states of substances:
- For the gas phase, it is the (assumed) state of a chemically pure substance in the gas phase at a standard pressure of 100 kPa (before 1982, 1 standard atmosphere, 101,325 Pa, 760 mmHg), implying the presence of the properties of an ideal gas.
- For a pure phase, mixture or solvent in a liquid or solid state of aggregation, this is the state of a chemically pure substance in a liquid or solid phase under standard pressure.
- For a solution, it is the (assumed) state of the solute at a standard molality of 1 mol/kg, under standard pressure or standard concentration, assuming that the solution is unrestrictedly diluted.
- For a chemically pure substance, it is a substance in a well-defined state of aggregation under a well-defined, but arbitrary, standard pressure.
The IUPAC definition of a standard state does not include a standard temperature, although one often speaks of a standard temperature, which is 25 °C (298.15 K).
