What are the conditions for thermodynamic equilibrium for isolated systems. Theory of thermodynamic equilibrium. Local thermodynamic equilibrium
THERMODYNAMIC EQUILIBRIUM - thermodynamic state. system that does not change over time and is not accompanied by the transfer of matter or energy through the system. An isolated system that does not exchange matter and energy with the environment always comes to thermodynamic equilibrium over time and cannot spontaneously leave it. The gradual transition of a system from a nonequilibrium state caused by an external influence to a state of thermodynamic equilibrium is called relaxation.
Thermodynamic equilibrium includes: thermal equilibrium - constant temperature in the volume of the system, absence of temperature gradients; mechanical equilibrium, in which no macroscopic movements of parts of the system are possible, i.e. there is equality of pressure in the volume of the system; However, the movements of the system as a whole are permissible - translational movement in the field of action of external forces and rotation. In the case of a heterogeneous system, the coexistence of thermodynamically equilibrium phases is called phase equilibrium. If there are problems between system components chemical reactions, in a state of thermodynamic equilibrium, the rates of direct and reverse processes are equal to each other. At thermodynamic equilibrium in the system, all irreversible transfer processes (thermal conductivity, diffusion, viscous flow, etc.) cease. There is no change in the concentrations of reactants in the system; a closed system is characterized by an equilibrium distribution of components between the phases that make up the system. The state parameters that determine thermodynamic equilibrium, strictly speaking, are not constant, but fluctuate around certain statistical average values; Usually these fluctuations are negligible.
Thermodynamic equilibrium conditions:
Under standard Gibbs energy of formationΔG°, understand the change in the Gibbs energy during the reaction of the formation of 1 mole of a substance in the standard state. This definition implies that the standard Gibbs energy of formation simple substance, stable under standard conditions, is equal to zero.
The change in the Gibbs energy does not depend on the path of the process; therefore, it is possible to obtain different unknown values of the Gibbs energies of formation from equations in which, on the one hand, the sums of the energies of the reaction products are written, and on the other, the sums of the energies of the starting substances.
When using the values of the standard Gibbs energy, the criterion for the fundamental possibility of a process under non-standard conditions is the condition ΔG°<0, а критерием принципиальной невозможности - условие ΔG°>0. At the same time, if the standard Gibbs energy is zero, this does not mean that in real conditions (other than standard) the system will be in equilibrium.
Examples of exergonic and endergonic processes occurring in the body.
Thermal reactions, during which the Gibbs energy decreases (dG<0) и совершается работа называются ЭКЗЕРГОНИЧЕСКИМИ(окисление глюкозы дикислородом- C6H12O6+6O2=6CO2+6H2O, dG=-2880 кДж/моль! Реакции в результате которых энергия Гиббса возрастает (dG>0) and work is done on the system are called ENDERGONIC!
Question 5. Chemical equilibrium.
Chemical equilibrium- state of the system in which the rate of the forward reaction is equal to the rate of the reverse reaction .
Reversible and irreversible reactions.
All chemical reactions can be divided into 2 groups: reversible and irreversible.
Irreversible - These are reactions that go to completion in one direction.
Reversible – are called reactions that can occur under the conditions under consideration in both forward and reverse directions.
A reaction proceeding from left to right is called forward, and from right to left is called reverse.
Chemical equilibrium constant- a value that determines for a given chemical. reactions, the relationship between the thermodynamic activities of the initial substances and products in a chemical state. balance.
For reaction:
The equilibrium constant is expressed by the equality:
Thermodynamic equilibrium conditions:
The dependence of the reaction equilibrium constant on temperature can be described by the isobar equation of a chemical reaction ( isobarsvan't Hoff):
Chemical reaction isotherm equation.
The equation of the isotherm of a chemical reaction makes it possible to calculate the value of the Gibbs energy ΔG (ΔF) during the transition from any state of the system to equilibrium, i.e. answer the question whether a chemical reaction will proceed spontaneously at given concentrations C i (pressures P i) of reagents:
When the equilibrium concentrations of the starting substances and reaction products change by influencing the system, a shift in the chemical equilibrium occurs.
The nature of the equilibrium shift can be predicted using Le Chatelier’s principle:
When the concentration of one of the starting substances increases, the equilibrium shifts towards the formation of reaction products;
When the concentration of one of the reaction products decreases, the equilibrium shifts towards the formation of the starting substances.
As the temperature rises chemical equilibrium shifts in the direction of the endothermic reaction, and with decreasing temperature - in the direction of the exothermic reaction.
As pressure increases, the equilibrium shifts in the direction in which the total number of moles of gases decreases and vice versa.
General principle for predicting equilibrium shifts:
The impact of any factor on an equilibrium system stimulates a shift in equilibrium in a direction that helps restore the original characteristics of the displacement.
Homeostasis - a set of complex adaptive reactions of the animal and human body, aimed at eliminating or maximizing the effect of various factors of the external or internal environment of the body . The ability of the body to maintain the constancy of its internal state.
Stationary state- this is a typical state of living objects. It is characterized by a constant energy level, and the change in entropy tends to zero. All basic physiological processes are realized in a stationary state of the system.
1. Extreme properties of thermodynamic potentials.
2. Conditions for equilibrium and stability of a spatially homogeneous system.
3. General conditions for phase equilibrium in thermodynamic systems.
4. Phase transitions 1st kind.
5. Phase transitions of the second order.
6. Generalization of semi-phenomenological theory.
Issues of stability of thermodynamic systems were considered in the previous topic in relation to the problem of chemical equilibrium. Let us pose the problem of theoretical substantiation of the previously formulated conditions (3.53) on the basis of the II law of thermodynamics, using the properties of thermodynamic potentials.
Let us consider a macroscopic infinitesimal change in the state of the system: 1 -2, in which all its parameters are related to an infinitesimal value:
Respectively:
Then, in the case of a quasi-static transition, from the generalized formulation of the I and II laws of thermodynamics (2.16) it follows:
If 1-2 is non-quasi-static, then the following inequalities hold:
In expression (4.3), quantities with a prime correspond to a non-quasi-static process, and quantities without a prime correspond to a quasi-static process. The first inequality of system (4.3) characterizes the principle of maximum heat absorption obtained on the basis of generalization of numerous experimental data, and the second - the principle of maximum work.
Writing the work for a non-quasi-static process in the form and introducing the parameters and in a similar way, we obtain:
Expression (4.4) is absolutely equivalent to the Clausius inequality.
Let us consider the main consequences (4.4) for in various ways descriptions of thermodynamic systems:
1. Adiabatically isolated system: (). Respectively. Then:
This means that if we fix the state variables of the system, then, due to (4.5), its entropy will arise until the system, according to the zero law of thermodynamics, reaches a state of equilibrium. That is, the equilibrium state corresponds to the maximum entropy:
Variations in (4.6) are made according to those parameters that, given the specified fixed parameters of the system, can take on non-equilibrium values. It could be concentration n, pressure r, temperature, etc.
2. System in the thermostat (). Accordingly, which allows us to rewrite (4.4) in the form:
Taking into account the form of the expression for free energy: and equality, we obtain:
Thus, the course of nonequilibrium processes for a system placed in a thermostat is accompanied by a decrease in its free energy. And the equilibrium value corresponds to its minimum:
3. System under the piston (), i.e. .In this case, relation (4.4) takes the form:
Thus, equilibrium in the system under the piston occurs when the minimum value of the Gibbs potential is reached:
4. System with imaginary walls (). Then. Then
which allows you to write
Accordingly, in a system with imaginary walls, nonequilibrium processes are directed towards decreasing the potential, and equilibrium is achieved under the condition:
The condition determines the very state of equilibrium of the system and is widely used in the study of multicomponent or multiphase systems. The conditions of minimum or maximum determine the criteria for the stability of these equilibrium states in relation to spontaneous or artificially created disturbances of the system.
In addition, the presence of extreme properties of thermodynamic potentials makes it possible to use variational methods to study them by analogy with the variational principles of mechanics. However, for these purposes the use of a statistical approach is required.
Let us consider the conditions of equilibrium and stability of thermodynamic systems using the example of a gas placed in a cylinder above a piston. In addition, to simplify the analysis, we will neglect external fields, assuming. Then the state variables are ().
It was previously noted that a thermodynamic system can be influenced either by doing work on it or by imparting a certain amount of heat to it. Therefore, balance and stability in relation to each of the noted influences should be analyzed.
The mechanical impact is associated with the displacement of a loose piston. In this case, the work on the system is equal to
As an internal parameter that can change and according to which variation should be carried out, we choose volume.
Representing the Gibbs potential through free energy
and varying, we write:
From the last equality it follows:
Expression (4.13) should be considered as an equation regarding the equilibrium value of the volume at given parameters systems ().
The stability conditions for the equilibrium state have the form:
Taking into account (4.13), the last condition can be rewritten as:
Condition (4.14) imposes certain requirements on the equation of state. Thus, the isotherms of an ideal gas
satisfy the stability condition everywhere. At the same time, the van der Waals equation
or Dieterighi equations
have areas where the stability conditions are not met, and which do not correspond to real equilibrium states, i.e. is being implemented experimentally.
If at some point there are isotherms, then to check stability they use special methods mathematical analysis, i.e. check the fulfillment of the conditions:
In a similar way, the stability requirements for the equation of state can be formulated for other parameters of the system. Consider as an example the dependence chemical potential. Let us introduce the density of the number of particles. Then the chemical potential can be represented as:
Let's calculate the differential depending on the state variables:
When writing the last expression, it was taken into account that the thermodynamic identity (3.8) was used. Then
That is, the stability condition for the chemical potential takes the form
IN critical point in the presence of deflection we have:
Let's move on to analyzing the system's resistance to thermal effects associated with the transfer of a certain amount of heat. Then, as a variational parameter, we consider the entropy of the system S. To take into account the thermal effect, we fix the mechanical parameters. Then it is convenient to choose a set as the variables of the thermodynamic state, and free energy as the thermodynamic potential.
Carrying out the variation, we find:
From the equilibrium condition we obtain
Equations (4.21) should be considered as an equation for the equilibrium value of entropy. From the positivity of the second free energy variation:
Since the temperature always takes positive values, from (4.22) it follows:
Expression (4.23) is the desired stability condition thermodynamic system in relation to heating. Some authors consider the positivity of heat capacity as one of the manifestations of the Le Chatelier-Brown principle. When communicating the amount of heat to a thermodynamic system:
Its temperature arises, which, in accordance with the second law of thermodynamics in the formulation of Clausius (1850), leads to a decrease in the amount of heat entering the system. In other words, in response to external influences - the message of the amount of heat - the thermodynamic parameters of the system (temperature) change in such a way that the external influences are weakened.
Let us first consider a one-component system in a two-phase state. Hereinafter, by phase we will mean a homogeneous substance in chemical and physical terms.
Thus, we will consider each phase as a homogeneous and thermodynamically stable subsystem, characterized general meaning pressure (in accordance with the requirement of absence of heat flows). Let us study the equilibrium condition of a two-phase system with respect to a change in the number of particles and located in each of the phases.
Taking into account the assumptions made, the most convenient is to use a description of the system under the piston with fixation of parameters (). Here - total number particles in both phases. Also, for simplicity, let’s “turn off” external fields ( A=0).
In accordance with the chosen method of description, the equilibrium condition is condition (4.10) for the minimum of the Gibbs potential:
which is supplemented by the condition of constancy of the number of particles N:
Performing variation in (4.24a) taking into account (4.24b) we find:
Thus, the general criterion for the equilibrium of a two-phase system is the equality of their chemical potentials.
If the expressions for chemical potentials are known, then the solution to equation (4.25) will be a certain curve
called a phase equilibrium curve or discrete phase equilibrium curve.
Knowing the expressions for chemical potentials, from equality (2.1):
we can find the specific volumes for each of the phases:
That is, (4.26) can be rewritten in the form of equations of state for each of the phases:
Let us generalize the results obtained to the case n phases and k chemically non-reacting components. For free i th components, equation (4.25) will take the form:
It is easy to see that expression (4.28) represents the system ( n- 1) independent equations. Accordingly, from the equilibrium conditions for k component we get k(n-1) independent equations ( k(n-1) connections).
The state of the thermodynamic system in this case is determined by temperature, pressure p And k-1 values of the relative concentrations of the components in each phase. Thus, the state of the system as a whole is specified by the parameter.
Taking into account the imposed connections, we find the number of independent parameters of the system (degree of freedom).
Equality (4.29) is called the Gibbs phase rule.
For a one-component system () in the case of two phases () there is one degree of freedom, i.e. we can arbitrarily change only one parameter. In the case of three phases (), there are no degrees of freedom (), that is, the coexistence of three phases in a one-component system is possible only at one point, called the triple point. For water, the triple point corresponds to the following values: .
If the system is not single-component, more complex cases are possible. Thus, a two-phase () two-component system () has two degrees of freedom. In this case, instead of a phase equilibrium curve, we obtain a region in the form of a strip, the boundaries of which correspond to the phase diagrams for each of the pure components, and the internal regions correspond to different values of the relative concentrations of the components. One degree of freedom in in this case corresponds to the curve of coexistence of three phases, and corresponds to the fourth point of coexistence of four phases.
As discussed above, the chemical potential can be represented as:
Accordingly, the first derivatives of the chemical potential are equal to the specific values of entropy, taken with the opposite sign, and volume:
If at points satisfying phase equilibrium:
the first derivatives of the chemical potential for different phases experience a discontinuity:
a thermodynamic system is said to undergo a first-order phase transition.
Phase transitions of the first order are characterized by the presence of latent heat of the phase transition, which is different from zero, and a jump in the specific volumes of the system. The latent specific heat of phase transition is determined from the relation:
and the jump in specific volume is equal to:
Examples of first-order phase transitions are the processes of boiling and evaporation of liquids. Melting of solids, transformation of crystal structure, etc.
Let us consider two nearby points on the phase equilibrium curve () and (), the parameters of which differ by infinitesimal values. Then equation (4.25) is also valid for differentials of chemical potentials:
it follows from here:
Performing transformations in (4.34), we obtain:
Expression (4.35) is called the Clapeyron-Clausius equation. This equation makes it possible to obtain the form of the phase equilibrium curve using the experimentally known values of the heat of phase transition and phase volumes and without invoking the concept of chemical potential, which is quite difficult to determine both theoretically and experimentally.
The so-called metastable states are of great practical interest. In these states, one phase continues to exist in the stability region of the other phase:
Examples of fairly stable metastable states are diamonds, amorphous glass (along with crystalline rock crystal), etc. In nature and industrial installations, the metastable states of water are widely known: superheated liquid and supercooled steam, as well as supercooled liquid.
An important circumstance is that the condition for the experimental implementation of these states is the absence of a new phase, impurities, impurities, etc. in the system, i.e. absence of a center of condensation, vaporization and crystallization. In all these cases, the new phase appears initially in small quantities (drops, bubbles or crystals). Therefore, surface effects commensurate with volumetric ones become significant.
For simplicity, we will limit ourselves to considering the simplest case of the coexistence of two spatially disordered phase states - liquid and vapor. Consider a liquid containing a small bubble of saturated vapor. In this case, the surface tension force acts along the interface. To take it into account, we enter the parameters:
Here is the surface area of the film,
Surface tension coefficient. The “-” sign in the second equality (4.36) corresponds to the fact that the film is contracted and the work of the external force is aimed at increasing the surface:
Then, taking into account surface tension, the Gibbs potential will change by:
Introducing the model of the system under the piston and, taking into account the equality, we write the expression for the Gibbs potential in the form
Here and are the specific values of free energy, and are the specific volumes of each phase. For fixed values of (), quantity (4.39) reaches a minimum. In this case, the Gibbs potential can be varied according to. These quantities are related using the relation:
Where R can be expressed through: . Let us choose quantities as independent parameters, then the Gibbs potential (4.39) can be rewritten as:
(taken into account here)
Carrying out variation (4.40), we write:
Taking into account the independence of quantities, we reduce (4.41) to the system
Let us analyze the resulting equality. From (4.42a) it follows:
Its meaning is that the pressure in phase 1 is equal to the external pressure.
By introducing expressions for the chemical potentials of each phase and taking into account
Let's write (4.42b) in the form:
Here is the pressure in phase II. The difference between equation (4.44) and the phase equilibrium condition (4.25) is that the pressure in (4.44) in each of the phases can be different.
From equality (4.42c) it follows:
Comparing the resulting equality with (4.44) and the expression for the chemical potential, we obtain a formula for the gas pressure inside a spherical bubble:
Equation (4.45) represents the known course general physics Laplace's formula. Generalizing (4.44) and (4.45), we write the equilibrium conditions between a liquid and a vapor bubble in the form:
In the case of studying the problem of phase transition, liquid - solid the situation is significantly complicated due to the need to take into account the geometric features of the crystals and the anisotropy of the direction of preferential crystal growth.
Phase transitions are also observed in more complex cases, in which only the second derivatives of the chemical potential with respect to temperature and pressure undergo discontinuity. In this case, the phase equilibrium curve is determined not by one, but by three conditions:
Phase transitions that satisfy equations (4.47) are called phase transitions of the second order. Obviously, the latent heat of phase transition and the change in specific volume in this case are equal to zero:
To receive differential equation phase equilibrium curve, the Clapeyron-Clausius equation (4.35) cannot be used, because by directly substituting the values (4.48) into expression (4.35), uncertainty is obtained. Let us take into account that when moving along the phase equilibrium curve, the condition and is preserved. Then:
Let's calculate the derivatives in (4.49)
Substituting the resulting expressions into (4.49), we find:
System linear equations(4.51), written relatively and is homogeneous. Therefore, its nontrivial solution exists only if the determinant made up of coefficients is equal to zero. Therefore, let's write down
Taking into account the obtained condition and choosing any equation from system (4.51), we obtain:
Equations (4.52) for the phase equilibrium curve in the case of a second-order phase transition are called Ehrenfest equations. In this case, the phase equilibrium curve can be determined by known characteristics jumps in heat capacity, coefficient of thermal expansion, coefficient of elasticity.
Phase transitions of the second order occur much earlier than phase transitions of the first order. This is obvious even from condition (4.47), which is much more stringent than the phase equilibrium curve equation (4.10) with conditions (4.31). Examples of such phase transitions include the transition of a conductor from a superconducting state to a normal state in the absence of magnetic field.
In addition, there are phase transitions with equal to zero latent heat, for which, during the transition, the presence of a singularity in the caloric equation is observed (the heat capacity suffers a discontinuity of the second kind). This type of phase transition is called phase transition type. Examples of such transitions are the transition of liquid helium from a superfluid state to a normal state, a transition at the Curie point for ferromagnets, transitions from an inelastic state to an elastic state for alloys, etc.
To visually display 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 equilibrium states:1. Stable balance.
2. Labile (unstable) equilibrium.
3. Metastable equilibrium.
Using the matchbox model, it becomes clear that the center of gravity of a box standing on an edge (metastable equilibrium) must 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 lowest potential energy (Fig. 9.1.1).
Thermal equilibrium is characterized by the absence of temperature gradients in the system. Chemical equilibrium occurs when there is no net reaction between two substances that causes a change, i.e. all reactions occur in the forward and reverse directions equally quickly.
Thermodynamic equilibrium exists if the system performs mechanical, thermal and chemical conditions balance. This occurs when free energy is at a minimum. At constant pressure, as is generally accepted in metallurgy, the free energy should be taken as the Gibbs free energy C, called free enthalpy:
In this case, H is the enthalpy, or heat content, or the sum of the internal energy E and the displacement energy pV with pressure p and volume V in accordance with
Assuming 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. From equation (9.1.1) it follows that the Gibbs free energy is determined by two components, namely enthalpy, or heat content H and entropy S. This fact is essential for understanding the temperature dependence of the existence of different phases.
The behavior of the Gibbs free energy with temperature changes is different for substances in the 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 whose Gibbs free energy at the temperature in question is correspondingly the lowest (Fig. 9.1.2).
The fact that the Gibbs free energy is composed of enthalpy and entropy becomes clear from 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 as 0, then for gray tin a heat content of 2 kJ/mol is obtained. According to the heat content at a temperature of 25 °C, β-tin should be converted to α-tin upon release 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 phase stability is ensured by an increase in the entropy amplitude.
Due to the increase in entropy during the transformation of α-tin to β-tin under normal conditions, the increase in enthalpy is more than compensated for, so that the Gibbs free energy C=H-TS for the modification of white β-tin actually fulfills the minimum condition.
Just like energy, the entropy of a system behaves additively, i.e. the entire entropy of a system is formed from the sum of individual entropies. Entropy is a state parameter 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 visualized by the fact that when mixing particles that do not uniformly fill space (as, for example, when mixing gases), the state of homogeneous distribution is most likely, i.e. as random a distribution as possible. This expresses the entropy S as a measure of arbitrary distribution in the system and is defined as the logarithm of probability:
where k is Boltzmann's constant; w is the probability of distribution, for example, of two types of gas molecules.
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Thermodynamic condition of chemical equilibrium
The thermodynamic equilibrium condition for a process occurring under isobaric-isothermal conditions is that the change in Gibbs energy (D rG(T)=0). When the reaction occurs n and A+ n b B= n with C+ n d D
the change in standard Gibbs energy is˸
D rG 0 T=(n c×D f G 0 C+ n d×D f G 0 D)–(n a×D f G 0 A+ n b×D f G 0 B).
This expression corresponds to an ideal process in which the concentrations of reactants are equal to unity and remain unchanged during the reaction. During real processes, the concentrations of reagents change; the concentration of starting substances decreases, and the concentration of reaction products increases. Taking into account the concentration dependence of the Gibbs energy (see section 1 . 3. 4) its change during the reaction is equal to
D r G T=–
– =
=(n c×D f G 0 C+ n d×D f G 0 D)–(n a×D f G 0 A+ n b×D f G 0 B) +
+ R× T×(n c×ln C C + n d×ln C D–n a×ln C A–n b×ln C B)
D r G T=D rG 0 T+R× T× ,
where is the dimensionless concentration i-th substance; X i– mole fraction i-th substance; p i– partial pressure i-th substance; r 0 = =1.013×10 5 Pa – standard pressure; with i– molar concentration i-th substance; With 0 =1 mol/l – standard concentration.
In a state of balance
D rG 0 T+R×T× = 0,
Magnitude TO 0 is called standard (thermodynamic) equilibrium constant of the reaction. Thus, at a certain temperature T as a result of the forward and reverse reactions occurring in the system, equilibrium is established at certain concentrations of the reactants - equilibrium concentrations (With i) p . The values of equilibrium concentrations are determined by the value of the equilibrium constant, which is a function of temperature and depends on enthalpy (D r N 0) and entropy (D r S 0) reactions˸
D rG 0 T+R× T×ln K 0 = 0,
since D rG 0 T=D r N 0 T – T×D r S 0 T,
If the enthalpy values (D r N 0 T) and entropy (D r S 0 T) or D rG 0 T reaction, then the value of the standard equilibrium constant can be calculated.
The reaction equilibrium constant characterizes ideal gas mixtures and solutions. Intermolecular interactions in real gases and solutions lead to a deviation of the calculated equilibrium constants from the real ones. To take this into account, instead of the partial pressures of the components of gas mixtures, their fugacity is used, and instead of the concentration of substances in solutions, their activity is used. Fugativity i th component is related to the partial pressure by the relation f i=g i× p i, where g i– fugacity coefficient. The activity and concentration of the component are related by the relation and i=g i× With i, where g i– activity coefficient.
It should be noted that in a fairly wide range of pressures and temperatures, gas mixtures can be considered ideal and the equilibrium composition of the gas mixture can be calculated by calculating the fugacity coefficient g i@ 1. In the case of liquid solutions, especially electrolyte solutions, the activity coefficients of their components can differ significantly from unity (g i¹ 1) and to calculate the equilibrium composition it is necessary to use activities.
Thermodynamic condition of chemical equilibrium - concept and types. Classification and features of the category "Thermodynamic condition of chemical equilibrium" 2015, 2017-2018.
The state of a thermodynamic system to which it spontaneously comes after a sufficiently long period of time under conditions of isolation from the environment, after which the parameters of the system’s state no longer change over time. The process of a system transitioning to an equilibrium state called relaxation. At thermodynamic equilibrium, all irreversible processes in the system cease - thermal conductivity, diffusion, chemical reactions, etc. The equilibrium state of the system is determined by the values of its external parameters (volume, electric or magnetic field strength, etc.), as well as 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). Insulation of the system is generally carried out using fixed walls that are impenetrable to substances. In the case when the fixed walls insulating the system are practically not thermally conductive, adiabatic insulation occurs, 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 are equalized and thermodynamic equilibrium occurs. With semi-permeable walls for matter, 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 parts of the system are possible, but translational motion and rotation of the system as a whole are permissible. In the absence of external fields and rotation of the system, the condition for its mechanical equilibrium is the constancy of pressure throughout the entire volume of the system. To others a 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 include, for example, an increase in pressure with a decrease in volume (at constant temperature) and a positive value of heat capacity at constant pressure. In general, a system is in a state of thermodynamic equilibrium when thermodynamic potential system corresponding to the variables independent in the experimental conditions is minimal. For example:
Isolated (absolutely not interacting with environment) system - maximum entropy.
A closed system (exchanges only heat with the thermostat) is a minimum of free energy.
A system with fixed temperature and pressure is the minimum Gibbs potential.
A system with fixed entropy and volume is a minimum of internal energy.
A system with fixed entropy and pressure - minimum enthalpy.
13. Le Chatelier-Brown principle
If a system that is in stable equilibrium is influenced from the outside by changing any of the equilibrium conditions (temperature, pressure, concentration), then processes in the system aimed at compensating for the external influence are intensified.
Effect of temperature depends on the sign of the thermal effect of the reaction. As the temperature increases, the chemical equilibrium shifts in the direction of the endothermic reaction, and as the temperature decreases, in the direction of the exothermic reaction. In the general case, when the temperature changes, the chemical equilibrium shifts towards a process in which the sign of the entropy change coincides with the sign of the temperature change. 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 the equilibrium towards the starting substances and a decrease in the yield of the product.
Pressure significantly affects on the equilibrium position in reactions involving gaseous substances, accompanied by a change in volume due to a change in the amount of substance during the transition from starting 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 where the number of moles of gases decreases, inert gases shift the equilibrium towards the parent substances, therefore, in production processes in which inert gases can form or accumulate, periodic purging of gas lines is required.
Effect of concentration the state of equilibrium is subject to the following rules:
When the concentration of one of the starting substances increases, the equilibrium shifts towards the formation of reaction products;
When the concentration of one of the reaction products increases, the equilibrium shifts towards the formation of the starting substances.
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