Thermodynamics is a science that studies the general patterns of processes accompanied by the release, absorption and transformation of energy. Chemical thermodynamics studies the mutual transformations of chemical energy and its other forms - heat, light, electricity, etc., establishes the quantitative laws of these transitions, and also makes it possible to predict the stability of substances under given conditions and their ability to enter into certain chemical reactions. The object of thermodynamic consideration is called a thermodynamic system or simply a system.
System- any natural object consisting of large number molecules (structural units) and separated from other natural objects by a real or imaginary boundary surface (interface).
The state of a system is a set of properties of the system that allow us to define the system from the point of view of thermodynamics.
Types of thermodynamic systems:
I. According to the nature of the exchange of matter and energy with environment :
1. Isolated system - does not exchange either matter or energy with the environment (Δm = 0; ΔE = 0) - thermos.
2. Closed system– does not exchange matter with the environment, but can exchange energy (closed flask with reagents).
3. Open system– can exchange with the environment, both matter and energy (human body).
II. By state of aggregation:
1. Homogeneous – absence of sudden changes in physical and chemical properties during the transition from one area of the system to another (consist of one phase).
2. Heterogeneous - two or more homogeneous systems in one (consists of two or more phases).
Phase- this is a part of the system, homogeneous at all points in composition and properties and 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). Another example of a homogeneous system is plain water, but water with ice floating in it is a heterogeneous system.
Phase transition- phase transformations (ice melting, water boiling).
Thermodynamic process- the transition of a thermodynamic system from one state to another, which is always associated with an imbalance of the system.
Classification of thermodynamic processes:
7. Isothermal - constant temperature – T = const
8. Isobaric - constant pressure – p = const
9. Isochoric - constant volume – V = const
Standard condition is the state of the system, conditionally chosen as a standard for comparison.
For gas phase- this is the state of a chemically pure substance in the gas phase under a standard pressure of 100 kPa (until 1982 - 1 standard atmosphere, 101,325 Pa, 760 mm Hg), implying the presence of the properties of an ideal gas.
For pure phase, mixture or solvent in a liquid or solid aggregate state is the state of a chemically pure substance in a liquid or solid phase under standard pressure.
For solution- this is the state of a dissolved substance with a standard molality of 1 mol/kg, under standard pressure or standard concentration, based on the conditions that the solution is diluted indefinitely.
For chemically pure substance- this is a substance in a clearly defined state of aggregation under a clearly defined, but arbitrary, standard pressure.
In defining the standard state standard temperature not included, although they often talk about the standard temperature, which is 25 ° C (298.15 K).
Basic concepts of thermodynamics: internal energy, work, heat
Internal energy U- the total energy reserve, including the movement of molecules, vibrations of bonds, the movement of electrons, nuclei, etc., i.e. all types of energy except kinetic and potential energy systems as a whole.
It is impossible to determine the value of the internal energy of any system, but it is possible to determine the change in internal energy ΔU that occurs in a particular process during the transition of the system from one state (with energy U 1) to another (with energy U 2):
ΔU depends on the type and quantity of the substance in question and the conditions of its existence.
The total internal energy of the reaction products differs from the total internal energy starting materials, because During the reaction, a restructuring of the electronic shells of the atoms of interacting molecules occurs.
Energy can be transferred from one system to another or from one part of a system to another in the form of heat or in the form of work.
Heat (Q)– a form of energy transfer through chaotic, disordered movement of particles.
Work (A)- a form of energy transfer through the ordered movement of particles under the influence of any forces.
The SI unit of measure for work, heat, and internal energy is the joule (J). 1 joule is the work done by a force of 1 newton at a distance of 1 m (1 J = 1 N×m = 1 kg×m 2 /s 2). In the old chemical literature, the calorie (cal) was a widely used unit of heat and energy. 1 Calorie is the amount of heat required to heat 1 g of water by 1°C. 1 Cal = 4.184 J≈4.2 J. It is more convenient to express the heat of chemical reactions in kilojoules or kilocalories: 1 kJ = 1000 J, 1 kcal = 1000 cal.
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 just 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 may 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 state parameters. 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 also 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 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 motion 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 to yourself, 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 will be 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, number of particles, also do not occur.
The main thermodynamic functions used in metallurgical calculations are internal energy U, enthalpy N, entropy S, as well as their most important combinations: isobaric-isothermal G = H - TS and isochoric-isothermal F = U - TS potentials, reduced potential Ф = -G/T.
According to Nernst's theorem for entropy The natural reference point is zero degrees on the Kelvin scale, at which the entropy of crystalline substances is zero. Therefore, from a formal point of view, in principle, it is always possible to 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.
But 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, since 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 of a 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 application problems enough to knowchange quantities thermodynamic functions when changing temperature, pressure, volume, during phase and chemical transformations.
But 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 thermodynamic functions H, F, G, F from internal energy U This enough do for only one of these functions. It really happened the starting point for values has been unifiedenthalpy . Done it by assigning a zero value to the enthalpies of certain substances in certain states under precisely specified physical conditions, which are called standard substances, standard conditions And standard states.
The following is the most common set of agreements discussed, recommended by the International Commission on Thermodynamics of the International Union of Theoretical and Theoretical Sciences. applied chemistry(IUPAC). This set can be called thermodynamic standards, as practically established in the modern literature on chemical thermodynamics.
Standard terms
According to Nernst's theorem, the natural reference point for entropy, or the 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, 0 K is used as a standard temperature. Despite its great logic from a physical and mathematical point of view, this temperature is not widely used as a standard temperature. This is due to the fact that at low temperatures the dependence of the 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 most closely correspond to real physical conditions in chemical laboratories where thermochemical measurements are carried out.
Standard substances
In nature, all isolated, independent substances, called individual in thermodynamics , consist of pure elements of D.I. Mendeleev’s table, or are obtained from chemical reactions between them. That's why sufficient condition to establish a frame of reference 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, 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 a standard state). They are called “ individual substances " The starting point for enthalpies for chemical compounds (as well as for elements in non-standard states) is taken to be 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 is determined experimentally under real conditions, 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 rule of signs to characterize enthalpy. If, during the formation of a chemical compound, heat stands out, the sign is selected " minus” – heat is lost for the system during an 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 one is chosen, i.e. most stable form of existence (state of aggregation, 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 form - mercury and bromine, diatomic molecules of gaseous nitrogen or chlorine, monatomic noble gases, etc.
Standard designations
To denote any thermodynamic property calculated at a standard pressure from a standard value and therefore called standard property, the right upper index 0 (zero) of the symbol is used. That the property is counted from the selected standard, is indicated by the “” symbol in front of the algebraic symbol of the thermodynamic function. The temperature to which the function value corresponds is often given as a right subscript. For example, standard enthalpy substances at 298.15 K is designated as
The standard enthalpies of individual substances are taken to be the heats of their formation by chemical reactions from standard substances in a standard state. Therefore, thermodynamic functions are sometimes denoted using the subscript f(from English formation- education): 
Unlike enthalpy, entropy is calculated by its absolute value 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 compiled into tables of thermochemical quantities and published as reference books of thermochemical quantities of individual substances.
Isobaric processes are most often encountered in reality, since technological processes tend to be carried out in devices communicating with the atmosphere. Therefore, reference books of thermochemical data for the most part contain how 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, then the values or changes in the values of all other thermodynamic functions can be calculated.
Chemical reactions are used not only to obtain final products. Very often it is important to know how much heat can be obtained by burning a particular type of fuel, how much work can be obtained from various chemical reactions. A preliminary solution to the question of the fundamental possibility of a particular reaction occurring is also of enormous importance. All this can be done by carrying out special calculations based on knowledge of the thermodynamic parameters of the substances involved in the chemical process. Since chemical transformations are very diverse, and more than 100 chemical elements can participate in reactions, the problem arises of choosing the starting point for thermodynamic quantities. For this purpose, the concepts are widely used in thermodynamics standard states and standard conditions.
An important feature of chemical reactions is that during the reaction, various chemical elements do not transform into each other. This means that to set the starting point for thermodynamic quantities, we can take all chemical elements in standard states, exactly the same both in relation to the starting substances and in relation to the reaction products.
In the previous section it was shown that the values of the energy parameters of chemical processes generally depend on the reaction path. These are, for example, the heat of a process or the work of a process. But nevertheless, there are conditions when the heat and work of the process are uniquely determined by specifying the final and initial states. At the same time processes must occur at constant volume or pressure. The system temperature at the end of the process must be the same as the temperature at the beginning of the process. In such cases, the scheme for carrying out thermodynamic calculations looks especially simple, as follows from Fig. 11.1.
Change in thermodynamic parameter in a reaction
initial substances - final substances
is equal to the difference between the corresponding formation parameters of the final and initial substances. For example, the enthalpy change in a reaction is
The change in the reaction of other quantities is calculated in a similar way. For entropy, absolute values of substances are used
Designation of thermodynamic quantities in Fig. 11.1 is provided with additional indices. The index “o” indicates that the value under consideration characterizes the standard state of the substance.
The index "g" comes from English word reaction and will be widely used in the future to characterize quantities that change in reactions. Index "f" (formation) indicates a change in the quantity under consideration in the reaction of formation of a compound from simple substances. However, the use of the index “g” (or “f”) also has another important function: a change in any thermodynamic quantity, written in the form A g M, characterizes
Rice. 11.1. Scheme for calculating the thermodynamic parameters of chemical reactions rate of change M in a reaction with a change of one for a very large system, when the changes do not affect the properties of the system. In other words, quantities with the index “g” (or “f”) characterize the differential properties of the system:
and, for example,
while keeping the system parameters unchanged (except for the value?). So, AM is the change in the value of M, a A x M is the rate of change of the value of M with the depth of reaction. The value A x M characterizes the slope of the curve of dependence of M on ?,.
The values required for calculations are taken directly from thermodynamic tables, which are created on the basis of experimental and theoretical data.
Currently, the totality of all chemical elements in the form of simple substances that are in the most stable forms at 25 ° C is used as a single zero reference. For example, carbon is taken in the form of graphite, bromine - in the form of a liquid. Exceptions are made for phosphorus and tin. For phosphorus, white phosphorus (compound P4) is taken as the basic substance, and for tin, white tin ((3-tin), since these substances are more accessible. Selected a collection of simple substances forms the basis for thermodynamic calculations, and each simple substance included in the basis is basic substance.
To perform thermodynamic calculations, use the parameters of the substance in standard conditions, which, in accordance with the IUPAC recommendation (for use since 1982), are selected as follows:
1. The temperature of the substance in the standard state is equal to the temperature of the system:
2. Pressure above a substance in a standard state or pressure of a gaseous substance in a standard state (р°) equals 1 bar:
Until 1982, the pressure in the standard state was one atmosphere (1 atm = 101325 Pa). Although the possible differences in the reference data are small, it is nevertheless recommended to pay attention to the system of units used for pressure in the standard state.
- 3. For gaseous substances, hypothetical states in the form of ideal gases are chosen as standard states.
- 4. For liquid and solid substances, take real states at р°= 1 bar and temperature T.
- 5. Sometimes hypothetical states of matter are introduced into consideration, for example, water in the form of a gas at a pressure of 1 bar and a temperature below 100 °C or in the form of ice at 25 °C.
- 6. Thermodynamic quantities characterizing substances in standard states are called standard.
Substances are said to be in standard states at temperature T° = 298.15 K are under standard conditions. Please note that there is no need to confuse standard states and standard conditions: standard states are possible at any temperature, standard conditions refer only to temperature 25 °C.
It should be noted that in practice, other standard states are sometimes used if this seems more convenient. For solid and liquid substances, the concept of a standard state is often used at any pressure, and not just at р°= 1 bar. To denote standard quantities related to such standard conditions, we will use the superscript “*” (for example, AN*).
For mixtures and solutions, the state of an ideal mixture or solution with a concentration of a substance equal to one (molarity or molality) is used as a standard.
Sometimes states with T= Dsystems) and V= I' = 1 l.
Conventionally accepted states of individual substances and components of solutions when assessing 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 are designated by zero in the superscript, for example: G 0, H 0, m 0 are, respectively, the standard Gibbs energy, enthalpy, and chemical potential of the substance. Instead of pressure, fugacity (volatility) is used in thermodynamic equations for ideal gases and solutions, and activity is used instead of concentration.
IUPAC standard states
The Commission on Thermodynamics of the International Union of Pure and Applied Chemistry (IUPAC) determined that the standard state is the state of the system, arbitrarily 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 under a standard pressure of 100 kPa (before 1982 - 1 standard atmosphere, 101,325 Pa, 760 mmHg), implying the presence of ideal gas properties.
- For a pure phase, mixture or solvent in a liquid or solid aggregate state, 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 a standard molality of 1 mol/kg, under standard pressure or standard concentration, assuming that the solution is infinitely dilute.
- For a chemically pure substance, this is a substance in a clearly defined state of aggregation under a clearly defined, but arbitrary, standard pressure.
The IUPAC definition of a standard state does not include a standard temperature, although the standard temperature is often referred to as 25 °C (298.15 K).