Equilibrium thermodynamic systems. Thermodynamic equilibrium conditions. Reversibility Criteria as Equilibrium Criteria
Macroscopic systems often have "memory"; they seem to remember their history. For example, if you organize the movement of water in a cup with the help of a spoon, then this movement will continue for some time but inertia. Steel acquires special properties after machining. However, over time, the memory fades. The movement of water in the cup stops, internal stresses in steel weaken due to plastic deformation, concentration inhomogeneities decrease due to diffusion. It can be argued that systems tend to achieve relatively simple states that are independent of the prior history of the system. In some cases, the achievement of this state occurs quickly, in some - slowly. However, all systems tend to states in which their properties are determined by internal factors, and not by previous disturbances. Such simple, limiting states are by definition independent of time. These states are called equilibrium states. Situations are possible when the state of the system is unchanged, but flows of mass or energy take place in it. In this case it comes not about equilibrium, but about a stationary state.
The state of a thermodynamic system, characterized under constant external conditions by the invariability of parameters in time and the absence of flows in the system, is called equilibrium.
Equilibrium state- the limiting state to which the thermodynamic system, isolated from external influences, tends. The isolation condition should be understood in the sense that the rate of equilibrium establishment processes in the system is much higher than the rate of change of conditions at the system boundaries. An example is the combustion of fuel in the combustion chamber of a rocket engine. The residence time of the fuel element in the chamber is very short (10 - 3 - 1 (N s), but the time for establishing equilibrium is about 10 ~ 5 s. Another example is that geochemical processes in the earth's crust proceed very slowly, but the lifetime of thermodynamic systems of this kind is calculated in millions of years, therefore, in this case, the model of thermodynamic equilibrium turns out to be applicable.
Using the introduced concept, we can formulate the following postulate: there are special states of simple systems - those that are completely characterized by macroscopic values of internal energy U, volume V and the number of moles n and n 2> i, chemical components. If the system under consideration has more complex mechanical and electrical properties, then the number of parameters required to characterize the equilibrium state increases (it is necessary to take into account the presence of surface tension forces, gravitational and electromagnetic fields, etc.).
From a practical point of view, the experimenter must always establish whether the system under study is in equilibrium. The absence of visible changes in the system is not enough for this! For example, two bars of steel can have the same chemical composition, but completely different properties due to mechanical processing (forging, pressing), heat treatment, etc. one of them. If the properties of the system under study cannot be described using the mathematical apparatus of thermodynamics, this is can mean that the system is not in equilibrium.
In reality, very few systems reach absolute equilibrium. In particular, in this state, all radioactive materials must be in a stable form.
It can be argued that a system is in equilibrium if its properties are adequately described using the apparatus of thermodynamics.
It is useful to remember that in mechanics the equilibrium mechanical system- the state of a mechanical system under the action of forces, at which all its points are at rest with respect to the considered frame of reference.
Let us consider two examples to clarify the concept of equilibrium in thermodynamics. If you establish contact between the thermodynamic system and environment, then, in the general case, a process will begin, which will be accompanied by a change in some parameters of the system. In this case, some of the parameters will not change. Let the system consist of a cylinder in which the piston is located (Fig. 1.9). At the initial moment of time, the piston is fixed. There is gas to the right and left of it. The pressure to the left of the piston is R A, on the right - R in, and p A> p b If the fastener is removed, the piston is released and begins to move to the right, while the volume of the subsystem BUT will begin to increase, and the right - to decrease (-D V B = D V A). Subsystem BUT loses energy, subsystem IN acquires her, pressure p A falls, pressure p in increases until the pressures to the left and right of the piston become equal. In this case, the gas masses of the subsystems to the left and to the right of the piston do not change. Thus, in the process under consideration, energy is transferred from one subsystem to another due to changes in pressure and volume. The independent variables in the considered process are pressure and volume. These parameters of the state some time after the release of the piston will take constant values and will remain unchanged until the system is influenced from the outside. The achieved state is equilibrium.
Equilibrium state - it is the final state of the process of interaction of one or more systems with their environment.
As is clear from the given example, the parameters of the system in equilibrium depend on the initial state of the system (its subsystems) and the environment. It should be noted that this relationship between the initial and final states is one-sided and does not allow restoring the initial nonequilibrium state based on information about the parameters of the equilibrium state.
Fig. 1.9.
A thermodynamic system is in equilibrium if all state parameters do not change after the system is isolated from other systems and the environment.
The driving force of the considered process of establishing equilibrium was the pressure difference to the left and right of the piston, i.e. the difference in intensive parameters. At the initial moment Ap = p l-r in* 0, at the end Ap = 0, p "A = Pb-
As another example, consider the system shown in Fig. 1.10.
Fig. 1.10.
System shells BUT and IN - non-deformable and heat-resistant (adiabatic). At the initial moment of time, the gas in the system IN is at room temperature, water in the system BUT heated. System pressure IN measured by a pressure gauge. At some point in time, the heat-insulating layer between BUT and IN removed (while the wall remains undeformable, but becomes heat-permeable (diathermic)). System pressure IN begins to grow, it is obvious that energy is transferred from A in B, at the same time, no visible changes in the systems are observed, there are no mechanical movements. Looking ahead, we say that this energy transfer mechanism can be substantiated using the second law of thermodynamics. In the previous example, in the process of establishing equilibrium, two coordinates were changed - pressure and volume. It can be assumed that in the second example, two coordinates must also change, one of which is pressure; the change in the second we could not observe.
Experience shows that after a certain period of time, the state of systems Aw B will cease to change, a state of equilibrium will be established.
Thermodynamics deals with equilibrium states. The term "equilibrium" implies that the action of all forces on the system and within the system is balanced. In this case, the driving forces are equal to zero, and there are no flows. The state of an equilibrium system does not change if the system is isolated from the environment.
Separate types of equilibrium can be considered: thermal (thermal), mechanical, phase and chemical.
In a system in a state thermal equilibrium, the temperature is the same at any point and does not change over time. In a system in a state mechanical equilibrium, the pressure is constant, although the magnitude of the pressure can vary from point to point (column of water, air). Phase equilibrium - equilibrium between two or more phases of a substance (vapor - liquid; ice - water). If the system has reached the state chemical equilibrium, changes in concentration cannot be detected in it chemical substances.
If a thermodynamic system is in equilibrium, it is assumed that equilibrium of all types (thermal, mechanical, phase and chemical) has been achieved in it. Otherwise, the system is nonequilibrium.
Characteristic signs of an equilibrium state:
- 1) does not depend on time (stationarity);
- 2) is characterized by the absence of flows (in particular, heat and mass);
- 3) does not depend on the "history" of the development of the system (the system "does not remember" how it got into a given state);
- 4) resistant to fluctuations;
- 5) in the absence of fields does not depend on the position in the system within the phase.
The state of a thermodynamic system, in which it spontaneously comes after a sufficiently long period of time in conditions of isolation from the environment, after which the parameters of the state of the system do not change over time. The process of transition of a system to an equilibrium state is called relaxation. With thermodynamic equilibrium in the system, all irreversible processes cease - heat conduction, diffusion, chemical reactions, etc. The equilibrium state of the system is determined by the values of its external parameters (volume, strength of the electric or magnetic field, etc.), as well as the value of the temperature. Strictly speaking, the parameters of the state of an equilibrium system are not absolutely fixed - in microvolumes they can experience small fluctuations around their average values (fluctuations). The system is generally insulated using fixed walls that are impermeable to the substance. In the case when the stationary walls insulating the system are practically not thermally conductive, adiabatic insulation takes place, in which the energy of the system remains unchanged. With heat-conducting (diathermic) walls between the system and the external environment, until equilibrium is established, heat exchange is possible. With prolonged thermal contact of such a system with the external environment, which has a very high heat capacity (thermostat), the temperatures of the system and the environment equalize and thermodynamic equilibrium sets in. With walls semi-permeable to the substance, thermodynamic equilibrium occurs if, as a result of the exchange of matter between the system and the external environment, the chemical potentials of the environment and the system are equalized.
One of the conditions for thermodynamic equilibrium is mechanical equilibrium, in which no macroscopic movements of the parts of the system are possible, but the translational movement and rotation of the system as a whole are admissible. In the absence of external fields and rotation of the system, the condition for its mechanical equilibrium is the constancy of pressure in the entire volume of the system. Others necessary condition thermodynamic equilibrium is the constancy of temperature and chemical potential in the volume of the system. Sufficient conditions for thermodynamic equilibrium can be obtained from the second law of thermodynamics (the principle of maximum entropy); these, for example, include: an increase in pressure with a decrease in volume (at constant temperature) and a positive value of heat capacity at constant pressure. In the general case, the system is in a state of thermodynamic equilibrium when the thermodynamic potential of the system, corresponding to the variables that are independent under experimental conditions, is minimal. For example:
An isolated (absolutely not interacting with the environment) system is the maximum entropy.
A closed system (exchanges only heat with the thermostat) - a minimum of free energy.
System with fixed temperature and pressure - minimum Gibbs potential.
A system with fixed entropy and volume is a minimum of internal energy.
System with fixed entropy and pressure - minimum enthalpy.
13. Le Chatelier - Brown principle
If the system, which is in stable equilibrium, is influenced from the outside, changing any of the equilibrium conditions (temperature, pressure, concentration), then the processes in the system aimed at compensating for the external influence are intensified.
Influence of temperature depends on the sign of the thermal effect of the reaction. With increasing temperature, chemical equilibrium shifts in the direction of the endothermic reaction, with decreasing temperature, in the direction of the exothermic reaction. In the general case, when the temperature changes, the chemical equilibrium shifts towards the process, the sign of the change in entropy in which coincides with the sign of the change in temperature. For example, in the ammonia synthesis reaction:
N2 + 3H2 ⇄ 2NH3 + Q - the thermal effect under standard conditions is +92 kJ / mol, the reaction is exothermic, therefore, an increase in temperature leads to a shift in equilibrium towards the starting materials and a decrease in the product yield.
Pressure significantly affects on the position of equilibrium in reactions involving gaseous substances, accompanied by a change in volume due to a change in the amount of substance during the transition from initial substances to products: with increasing pressure, the equilibrium shifts in the direction in which the total number of moles of gases decreases and vice versa.
In the ammonia synthesis reaction, the amount of gases is halved: N2 + 3H2 ↔ 2NH3, which means that with increasing pressure, the equilibrium shifts towards the formation of NH3.
The introduction of inert gases into the reaction mixture or the formation of inert gases during the reaction also acts, as well as a decrease in pressure, since the partial pressure of the reacting substances decreases. It should be noted that in this case, a gas that does not participate in the reaction is considered as an inert gas. In systems with a decrease in the number of moles of gases, inert gases shift the equilibrium towards the initial substances, therefore, in production processes, in which inert gases can form or accumulate, periodic purging of the gas ducts is required.
Effect of concentration on a state of equilibrium obeys following rules:
With an increase in the concentration of one of the initial substances, the equilibrium shifts in the direction of the formation of the reaction products;
With an increase in the concentration of one of the reaction products, the equilibrium shifts in the direction of the formation of the starting substances.
Thermodynamic equilibrium Is a completely stable state in which the system can be for an unlimited period of time. When removing an isolated system from equilibrium, it tends to return to this state spontaneously (thermos with hot water and a piece of ice).
In a state of thermodynamic equilibrium in the system, not only all parameters are constant in time, but there are no stationary flows due to the action of any external sources.
For open and closed systems, a characteristic stationary state (the parameters of the system do not change over time).
Equilibrium system- the parameters in different parts of the system are the same. There are no driving forces. If such a system is isolated, then it can be in a state of equilibrium for an unlimited time.
Non-equilibrium system- their parameters are different at different points of the volume, which leads to the presence of constant gradients and forces, and the flows of matter and energy created by them due to the flow of energy from the external environment. If such a system is isolated, then it irreversibly evolves to the state of TD of equilibrium.
7. The first law of thermodynamics. Discovery history. Formulation, physical and biological meaning.
The discovery of the first law of thermodynamics is historically associated with the establishment of the equivalence of heat and mechanical work. This discovery is associated with the names of R. Mayer and D. Joule. The main work of Mayer, in which he developed his ideas in detail and systematically, was published in 1845 and was called "The Organic Movement in its Connection with Metabolism." Mayer immediately formulated the first law of thermodynamics as a principle that obeys all forms of motion in nature. He pointed out that the source of mechanical and thermal effects in a living organism is not vitality, as the vitalists argued, but the chemical processes that take place in it as a result of the absorption of oxygen and food.
Joule came to establish the equivalence of heat and mechanical work inductively, i.e. directly by experimental measurement of the transformation mechanical movement into the warmth.
The first law of thermodynamics is formulated as follows: “The total energy in an isolated system is a constant value and does not change in time, but only passes from one form to another.
The heat σQ absorbed by the system from the external environment is used to increase the internal energy dU of the system and perform work σA against external forces.
If heat is transferred into the system, then ΔQ> 0.
If heat is transferred system, then ΔQ< 0.
Work perfect system considered positive.
Work perfect over the system - negative.
The first law of thermodynamics explains the impossibility of the existence of a perpetual motion machine of the first kind, i.e. such a motor that would do work without the expenditure of energy.
In the 19th century, it was proven that the first law of thermodynamics is applicable to living systems. This proof is reflected in the work "On Heat", 1873. Lavoisier, Laplace - ice calorimeter to determine the amount of heat released. The meaning of the experiment was that breathing is similar to slow combustion (multi-stage process). The breathing process serves as a source of heat for living organisms. Also in the experiments, a pneumatic installation was used, which made it possible to calculate the amount of carbon dioxide released.
When burning carbohydrates in a calorimeter
C 6 H 12 O 6 + 6O 2 = 6CO 2 + 6H 2 O - carbohydrates are oxidized to carbon dioxide and water.
The amount of energy released from each gram of glucose in this reaction is 4.1 kcal.
The pathways of transformation of food products in metabolic processes in living organisms and in chemical reactions outside a living cell are equivalent in terms of total thermal effects.
(Hence a consequence of the first law of TD - Hess's law: the thermal effect does not depend on its intermediate stages, it is determined only by the initial and final states of the system.)
Thermodynamic functions of state (thermodynamic potential). Gibbs free energy. Examples of using thermodynamic representations.
The purpose of introducing thermodynamic potentials is to use such a set of natural independent variables describing the state of a thermodynamic system, which is most convenient in a particular situation, while maintaining the advantages that the use of characteristic functions with the dimension of energy gives. In particular, the decrease in thermodynamic potentials in equilibrium processes occurring at constant values of the corresponding natural variables is equal to useful external work.
Thermodynamic potentials were introduced by W. Gibbs.
The following thermodynamic potentials are distinguished:
internal energy
enthalpy
free energy of Helmholtz
Gibbs potential
high thermodynamic potential
Free energy (Gibbs G) of a biological system is determined by the presence and magnitude of the gradient:
G = RT ln Ф1 / Ф2
R - universal gas constant,
T - thermodynamic temperature in Kelvin
F1 and F2 are the values of the parameter that determines the gradients.
Examples: First Law of Thermodynamics- law of energy conservation: Energy is not created or disappeared. For any chemical process, the total energy in a closed system always remains constant. Ecology studies the connection between sunlight and ecological systems within which transformations of light energy take place. Energy is not created anew and does not disappear anywhere. Light as a form of energy can be converted into work, heat, or the potential energy of food chemicals. It follows from this that if any system (both inanimate and living) receives or spends energy, then the same amount of energy must be withdrawn from its environment. Energy can only be redistributed or pass into another form depending on the situation, but at the same time it cannot arise from anywhere or disappear without a trace.
The radiant energy of the Sun, hitting the Earth, tends to turn into diffused heat. The fraction of light energy converted by green plants into the potential energy of their biomass is much less than the input (qconc< Qсол). Незначительная часть энергии отражается, основная же ее часть превращается в теплоту, покидающую затем и растения, и экосистему и биосферу.
The second law of thermodynamics states: the processes associated with the transformation of energy can occur spontaneously only under the condition that the energy passes from a concentrated form to a dispersed form (degrades). This law is called entropy law. Heat is not transferred spontaneously from a colder body to a hotter one (although the first law does not prohibit such a transition). There are many examples of unidirectional processes in nature. For example, gases are mixed in a vessel, but do not separate themselves; a lump of sugar dissolves in water, but does not come back out in the form of a lump. A measure of the amount of bound energy that becomes unavailable for use is entropy(from the Greek. inward and transformation). Those. entropy is a measure of disorder, a measure of the amount of bound energy that becomes unavailable for use. In closed systems, entropy (S) cannot decrease; its change (ΔS) is zero for reversible processes or greater than zero for irreversible processes. The system and its environment, left to themselves, tend to a state of maximum entropy (disorder). In this way, spontaneous processes go in the direction of increasing disorder.
The second law of thermodynamics can also be formulated as follows: since some of the energy is always dissipated in the form of unavailable heat energy losses, the efficiency of converting light energy into potential energy of chemical compounds is always less than 100%. There is another formulation of the law: any kind of energy ultimately changes to the form that is least usable and most easily dissipated.
The relationship between producer plants and animal consumers is governed by the flow of energy accumulated by plants, which is then used by animals. The entire living world receives the necessary energy from organic substances created by plants and, to a lesser extent, chemosynthetic organisms. Food created by the photosynthetic activity of green plants contains potential energy chemical bonds, which, when consumed by animal organisms, turns into other forms. Animals, absorbing the energy of food, also convert most of it into heat, and less into the chemical potential energy of the protoplasm synthesized by them.
Enthalpy. Hess's law. Examples of use in biological systems.
Enthalpy is a property of a substance that indicates the amount of energy that can be converted into heat. It is a function of state. Denoted as ΔH, measured in J / kg. The non-systemic unit of measurement is kcal / kg.
Hess's law: The thermal effect of a multistage process does not depend on its intermediate stages, but is determined only by the initial and final state of the system. Consequently, the thermal effect of a chemical reaction depends only on the type and state of the initial substances and does not depend on the path of its course.
Calorie- off-system unit of the amount of heat. The average value of physiologically available energy in 1 gram (in kcal): proteins - 4.1; carbohydrates - 4.1; fat - 9.3.
The amount of energy absorbed by living organisms along with nutrients is equal to the heat released during the same time. Consequently, organisms themselves are not a source of any new form of energy.
Types of heat, heat production. Specific heat production. Examples.
Quantity of heat- the energy that the body receives or loses during heat transfer. The amount of heat is one of the main thermodynamic quantities. The amount of heat is a function of the process, not a function of state (i.e., the amount of heat received by a system depends on the way it was brought into its current state.)
Heat production, heat generation, heat production in the body as a result of energy transformations in living cells; associated with the continuously occurring biochemical synthesis of proteins and other organic compounds, with osmotic work (transfer of ions against the concentration gradient), with the mechanical work of muscles (heart muscle, smooth muscles of various organs, skeletal muscles). Even with complete muscular rest, such work in total is quite large, and a person of average weight and age at an optimal temperature of the environment releases about 1 kcal (4.19 kJ) per kg of body weight in 1 hour.
In homeothermic animals at rest:
50% of all heat is generated in the abdominal organs,
20% - in skeletal muscles,
10% - during the work of the respiratory and circulatory organs.
(At rest, about 50% of all heat is formed in the abdominal organs (mainly in the liver), 20% each in skeletal muscles and the central nervous system, and about 10% during the work of the respiratory and circulatory organs. T. is also called chemical thermoregulation.)
All real processes are accompanied by the dissipation of some part of the energy into heat.Heat- a degraded form of energy. Heat- this special type of energy (low quality) cannot be transferred without loss to other types of energy. Thermal energy is associated with the chaotic movement of molecules, other types of energy are based on the ordered movement of molecules.
There is a classification of types of energy according to the ability of the type of energy to transform into other types of energy.
A. - max efficient, converts into all other types of energy. Gravitational, nuclear, light, electric,
B. - chemical,
C. - thermal.
Allocate primary and secondary heat, as well as specific heat production.
Primary heat- this is the result of the inevitable dissipation of energy in the course of dissimilation reactions due to irreversible biochemical reactions. Primary heat is released immediately after the body absorbs oxygen and food, regardless of whether it is doing work or not. It goes to heating the body and dissipates in the surrounding space.
Highlighting secondary heat observed only when the energy of high-energy compounds (ATP, GTP) is realized. Goes to do useful work.
Specific heat production is the amount of heat released by a unit of mass of an animal per unit of time:
q = QT / μT,,Where:
QT- the amount of heat released per unit of time,
μT- unit of mass,
q- specific heat production.
Heat production is proportional to the weight of the animal:
q = a + b / M 2/3,Where:
a - the number of cells,
b - surface area,
M is the body weight of the animal.
(Specific heat production decreases with increasing weight of the animal).
For a visual display of the equilibrium conditions, one should proceed from a simple mechanical model, which, depending on the change in potential energy depending on the position of the body, reveals three states of equilibrium:1. Stable balance.
2. Stable (unstable) balance.
3. Metastable equilibrium.
On the model of a matchbox, it becomes clear that the center of gravity of a box standing on an edge (metastable equilibrium) should only be raised in order for the box to fall on the wide side through the labile state, i.e. into a mechanically stable state of equilibrium, which reflects the state of the lowest potential energy (Figure 9.1.1).
Thermal equilibrium is characterized by the absence of temperature gradients in the system. Chemical equilibrium occurs when there is no resulting reaction between two substances that causes a change, i.e. all reactions go in the forward and backward directions equally quickly.
Thermodynamic equilibrium exists if mechanical, thermal and chemical equilibrium conditions are met in the system. This happens when free energy has a minimum. At constant pressure, as is generally accepted in metallurgy, Gibbs free energy C, called free enthalpy, should be taken as free energy:
In this case, H is the enthalpy, or heat content, or the sum of internal energy E and displacement energy pV with pressure p and volume V in accordance with
Assuming a constant volume V, the Helmholtz free energy F can be applied:
From these relations it turns out that the equilibrium state is characterized by extreme values. This means that the Gibbs free energy is minimal. Equation (9.1.1) implies that the Gibbs free energy is determined by two components, namely the enthalpy, or heat content H and entropy S. This fact is essential for understanding the temperature dependence of the existence of various phases.
The behavior of the Gibbs free energy with a change in temperature is different for substances in a gaseous, liquid or solid phase. This means that, depending on the temperature for a certain phase (which is equivalent to the state of aggregation), the Gibbs free energy is minimal. Thus, depending on the temperature, in stable equilibrium there will always be that phase, the Gibbs free energy of which at the considered temperature is correspondingly the lowest (Fig. 9.1.2).
The fact that the Gibbs free energy is composed of enthalpy and entropy becomes clear by the example of the temperature dependence of the zones of existence of various modifications of tin. Thus, tetragonal (white) β-tin is stable at temperatures> 13 ° C, cubic, diamond-like (gray) α-tin exists in stable equilibrium below 13 ° C (allotropy).
If, under normal conditions of 25 ° C and 1 bar, the heat content of the stable β-phase is taken to be 0, then a heat content of 2 kJ / mol is obtained for gray tin. In terms of the heat content at a temperature of 25 ° C, β-tin would have to turn into α-tin upon liberation of 2 kJ / mol, provided that the system with a lower heat content should be stable. In fact, such a transformation does not occur, since here the phase stability is ensured by an increase in the amplitude of the entropy.
Due to the increase in entropy during the transformation of α-tin into β-tin under normal conditions, the increase in enthalpy is compensated with an excess, so that the Gibbs free energy C = H-TS for the modification of white β-tin actually satisfies the minimum condition.
In the same way as energy, the entropy of the system behaves additively, i.e. the entire entropy of the system is formed from the sum of individual entropies. Entropy is a parameter of state and, thus, can characterize the state of a system.
Always fair
where Q is the heat supplied to the system.
For reversible processes, the equal sign matters. For an adiabatically isolated system, dQ = 0, thus dS> 0. Statistically, entropy can be graphically depicted by the fact that when mixing particles that do not exhibit uniform filling of space (as, for example, when mixing gases), the state of homogeneous distribution is most likely, i.e. maximum random distribution. This expresses the entropy S as a measure of an arbitrary distribution in the system and is defined as the logarithm of the probability:
where k is the Boltzmann constant; w is the probability of distribution, for example, of two types of gas molecules.
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Any thermodynamic system (TS) can be either in equilibrium or in nonequilibrium states. The general condition for equilibrium in mechanics is the equality to zero of the sum of work at small displacements corresponding to the system constraints. An extremum of potential energy corresponds to this condition. If this is a minimum, then when shifting from the equilibrium state, positive work is consumed ( dℒ> 0) and the equilibrium state is stable. In thermodynamics, the role of potential energy is played by characteristic functions.
The conditions of thermodynamic equilibrium for various conjugations of the TS with the environment with the fixation of two parameters are determined by the behavior of the characteristic functions, which make it possible to judge the direction of the occurrence of chemical reactions and phase transitions.
For simple ( dℒ= 0), closed TS when fixing two parameters, we have:
For irreversible processes:
those. irreversible, nonequilibrium processes in a simple, closed TS proceed in the direction of a decrease in the corresponding thermodynamic potential. In the equilibrium state, the value of the corresponding potential reaches a minimum, and the equilibrium conditions of the TS have the form:
With a deviation from the equilibrium state in any direction, the corresponding thermodynamic potential increases.
Consider the equilibrium of closed vehicles, which, in addition to the conditions of conjugation with the environment, are affected by only one force of a non-mechanical nature. Then the combined expressions of the 1st and 2nd equations of thermodynamics will take the form:
ℒ, (35)
Non-mechanical work ℒ in (35) we will represent in the form:
ℒ, J, (36)
Where BUT- thermodynamic affinity, J / mol, x- the path of the thermodynamic process, mol.
Thermodynamic affinity is introduced by the ratio:
J / mol, (37)
where is the uncompensated heat, i.e. the amount of work that has dissipated (dissipated) into the energy of the thermal motion of particles along the length of the process path. Thus, thermodynamic affinity is the amount of energy of the ordered motion of particles (work) that has been dissipated (dissipated) along the path of the process inside the vehicle. When BUT= 0 - the process is reversible, at BUT> 0 - the process is irreversible. After substitution of expression (36) for ℒ into equations (35) we get:
In this way, U = U(S, V, x), H = H(S, p, x), F = F(T, V, x), G = G(T, p, x) and when fixing the first two parameters in equations (38) we will have:
Thus, the Gibbs potential when fixing the values T and R from (38) is equal to:
Consequently, the thermodynamic affinity is determined through the partial derivatives of the characteristic functions along the path of the process.
An example of thermodynamic affinity is chemical affinity. In this case, the size of the path of the process is called the path of the chemical reaction.
When the TS tends to the equilibrium state, the Gibbs potential tends to the minimum of its value ( G T, p ®G T, p min) at fixed values T and R, which is reached at an equilibrium value (in this case, the value of thermodynamic affinity BUT= 0), as can be seen from the figure below:
The equilibrium state of the TS can also be characterized by the change in entropy. In irreversible, nonequilibrium processes inside an adiabatic, closed TS, the change in entropy dS = dS in> 0, i.e. entropy grows and reaches a maximum in a state of equilibrium: dS=0, S = S max... When the vehicle is swinging relative to the equilibrium state, the entropy will decrease, and the thermodynamic potentials will increase.
