The state of equilibrium for a reversible reaction can last for an indefinitely long time (without outside intervention). But if an external influence is applied to such a system (to change the temperature, pressure or concentration of the final or initial substances), then the state of equilibrium will be disturbed. The rate of one of the reactions will become greater than the rate of the other. Over time, the system will again take an equilibrium state, but the new equilibrium concentrations of the initial and final substances will differ from the initial ones. In this case, one speaks of a shift in the chemical equilibrium in one direction or another.

If, as a result of an external influence, the rate of the forward reaction becomes greater than the rate of the reverse reaction, then this means that the chemical equilibrium has shifted to the right. If, on the contrary, the rate of the reverse reaction becomes greater, this means that the chemical equilibrium has shifted to the left.

When the equilibrium is shifted to the right, the equilibrium concentrations of the starting substances decrease and the equilibrium concentrations of the final substances increase in comparison with the initial equilibrium concentrations. Accordingly, the yield of reaction products also increases.

The shift of chemical equilibrium to the left causes an increase in the equilibrium concentrations of the initial substances and a decrease in the equilibrium concentrations of the final products, the yield of which will decrease in this case.

The direction of the chemical equilibrium shift is determined using the Le Chatelier principle: “If an external effect is exerted on a system that is in a state of chemical equilibrium (change the temperature, pressure, concentration of one or more substances participating in the reaction), then this will lead to an increase in the rate of that reaction, the course of which will compensate (reduce) the impact.

For example, with an increase in the concentration of the starting substances, the rate of the direct reaction increases and the equilibrium shifts to the right. With a decrease in the concentration of the starting substances, on the contrary, the rate of the reverse reaction increases, and the chemical equilibrium shifts to the left.

With an increase in temperature (i.e., when the system is heated), the equilibrium shifts towards the occurrence of an endothermic reaction, and when it decreases (i.e., when the system is cooled), it shifts towards the occurrence of an exothermic reaction. (If the forward reaction is exothermic, then the reverse reaction will necessarily be endothermic, and vice versa).

It should be emphasized that an increase in temperature, as a rule, increases the rate of both the forward and reverse reactions, but the rate of the endothermic reaction increases to a greater extent than the rate of the exothermic reaction. Accordingly, when the system is cooled, the rates of forward and reverse reactions decrease, but also not to the same extent: for an exothermic reaction, it is much less than for an endothermic one.

A change in pressure affects the shift in chemical equilibrium only if two conditions are met:

it is necessary that at least one of the substances participating in the reaction be in a gaseous state, for example:

CaCO 3 (t) CaO (t) + CO 2 (g) - a change in pressure affects the displacement of equilibrium.

CH 3 COOH (l.) + C 2 H 5 OH (l.) CH 3 COOS 2 H 5 (l.) + H 2 O (l.) - a change in pressure does not affect the shift in chemical equilibrium, because none of the starting or end substances is in a gaseous state;

if several substances are in the gaseous state, it is necessary that the number of gas molecules on the left side of the equation for such a reaction is not equal to the number of gas molecules on the right side of the equation, for example:

2SO 2 (g) + O 2 (g) 2SO 3 (g) - pressure change affects the equilibrium shift

I 2 (g) + Н 2 (g) 2НI (g) - pressure change does not affect the equilibrium shift

When these two conditions are met, an increase in pressure leads to a shift in the equilibrium towards the reaction, the course of which reduces the number of gas molecules in the system. In our example (catalytic combustion of SO 2), this will be a direct reaction.

A decrease in pressure, on the contrary, shifts the equilibrium in the direction of the reaction proceeding with the formation more gas molecules. In our example, this will be the reverse reaction.

An increase in pressure causes a decrease in the volume of the system, and hence an increase in the molar concentrations of gaseous substances. As a result, the rate of forward and reverse reactions increases, but not to the same extent. Lowering the same pressure in a similar way leads to a decrease in the rates of forward and reverse reactions. But at the same time, the reaction rate, towards which the equilibrium shifts, decreases to a lesser extent.

The catalyst does not affect the equilibrium shift, because it speeds up (or slows down) both the forward and reverse reactions equally. In its presence, the chemical equilibrium is only more quickly (or more slowly) established.

If the system is affected by several factors at the same time, then each of them acts independently of the others. For example, in the synthesis of ammonia

N 2 (gas) + 3H 2 (gas) 2NH 3 (gas)

the reaction is carried out with heating and in the presence of a catalyst to increase its rate. But at the same time, the effect of temperature leads to the fact that the reaction equilibrium is shifted to the left, towards the reverse endothermic reaction. This causes a decrease in the output of NH 3 . In order to compensate for this undesirable effect of temperature and increase the ammonia yield, at the same time the pressure in the system is increased, which shifts the reaction equilibrium to the right, i.e. towards the formation of a smaller number of gas molecules.

At the same time, the most optimal conditions for the reaction (temperature, pressure) are selected empirically, under which it would proceed at a sufficiently high rate and give an economically viable yield of the final product.

The Le Chatelier principle is similarly used in the chemical industry in the production of a large number various substances of great importance for the national economy.

Le Chatelier's principle is applicable not only to reversible chemical reactions, but also to various other equilibrium processes: physical, physicochemical, biological.

The body of an adult is characterized by the relative constancy of many parameters, including various biochemical indicators, including the concentration of biologically active substances. However, such a state cannot be called equilibrium, because it does not apply to open systems.

The human body, like any living system, constantly exchanges various substances with the environment: it consumes food and releases the products of their oxidation and decay. Therefore, the body is characterized steady state, defined as the constancy of its parameters at a constant rate of exchange of matter and energy with the environment. In the first approximation, the stationary state can be considered as a series of equilibrium states interconnected by relaxation processes. In a state of equilibrium, the concentrations of substances participating in the reaction are maintained by replenishing the initial products from the outside and removing the final products to the outside. Changing their content in the body does not lead, in contrast to closed systems, to a new thermodynamic equilibrium. The system returns to its original state. Thus, the relative dynamic constancy of the composition and properties of the internal environment of the body is maintained, which determines the stability of its physiological functions. This property of a living system is called differently homeostasis.

In the course of the life of an organism in a stationary state, in contrast to a closed equilibrium system, there is an increase in entropy. However, along with this, the reverse process simultaneously proceeds - a decrease in entropy due to the consumption of nutrients with a low entropy value from the environment (for example, high-molecular compounds - proteins, polysaccharides, carbohydrates, etc.) and the release of decay products into the environment. According to the position of I.R. Prigozhin, the total production of entropy for an organism in a stationary state tends to a minimum.

A great contribution to the development of nonequilibrium thermodynamics was made by I. R. Prigozhy, Laureate Nobel Prize 1977, who stated that “in any non-equilibrium system, there are local areas that are in an equilibrium state. In classical thermodynamics, equilibrium refers to the whole system, and in non-equilibrium - only to its individual parts.

It has been established that entropy in such systems increases during the period of embryogenesis, during the processes of regeneration and the growth of malignant neoplasms.

If a external conditions chemical process do not change, then the state of chemical equilibrium can be maintained for an arbitrarily long time. By changing the reaction conditions (temperature, pressure, concentration), one can achieve displacement or shift of chemical equilibrium in the required direction.

The shift of equilibrium to the right leads to an increase in the concentration of substances whose formulas are on the right side of the equation. The shift of equilibrium to the left will lead to an increase in the concentration of substances whose formulas are on the left. In this case, the system will move to a new state of equilibrium, characterized by other values ​​of the equilibrium concentrations of the participants in the reaction.

The shift in chemical equilibrium caused by changing conditions obeys the rule formulated in 1884 by the French physicist A. Le Chatelier (Le Chatelier's principle).

Le Chatelier's principle:if a system in a state of chemical equilibrium is affected in any way, for example, by changing the temperature, pressure, or concentrations of reagents, then the equilibrium will shift in the direction of the reaction that weakens the effect .

Influence of concentration change on the shift of chemical equilibrium.

According to Le Chatelier's principle an increase in the concentration of any of the participants in the reaction causes a shift in the equilibrium towards the reaction that leads to a decrease in the concentration of this substance.

The influence of concentration on the state of equilibrium obeys the following rules:

With an increase in the concentration of one of the starting substances, the rate of the direct reaction increases and the equilibrium shifts in the direction of the formation of reaction products and vice versa;

With an increase in the concentration of one of the reaction products, the rate of the reverse reaction increases, which leads to a shift in the equilibrium in the direction of the formation of the starting substances and vice versa.

For example, if in an equilibrium system:

SO 2 (g) + NO 2 (g) SO 3 (g) + NO (g)

increase the concentration of SO 2 or NO 2, then, in accordance with the law of mass action, the rate of the direct reaction will increase. This will shift the equilibrium to the right, which will cause the consumption of the starting materials and an increase in the concentration of the reaction products. A new state of equilibrium will be established with new equilibrium concentrations of the initial substances and reaction products. When the concentration of, for example, one of the reaction products decreases, the system will react in such a way as to increase the concentration of the product. The advantage will be given to the direct reaction, leading to an increase in the concentration of the reaction products.

Influence of pressure change on the shift of chemical equilibrium.

According to Le Chatelier's principle an increase in pressure leads to a shift in equilibrium towards the formation of a smaller amount of gaseous particles, i.e. towards smaller volume.

For example, in reversible reaction:

2NO 2 (g) 2NO (g) + O 2 (g)

from 2 mol NO 2 2 mol NO and 1 mol O 2 are formed. Stoichiometric coefficients before formulas gaseous substances indicate that the forward reaction leads to an increase in the number of moles of gases, and the reverse reaction, on the contrary, reduces the number of moles of a gaseous substance. If an external influence is exerted on such a system, for example, by increasing pressure, then the system will react in such a way as to weaken this impact. The pressure can decrease if the equilibrium of this reaction shifts towards a smaller number of moles of a gaseous substance, and hence a smaller volume.

On the contrary, an increase in pressure in this system is associated with a shift in equilibrium to the right - towards the decomposition of NO 2, which increases the amount of gaseous matter.

If the number of moles of gaseous substances remains constant before and after the reaction, i.e. the volume of the system does not change during the reaction, then a change in pressure equally changes the rates of the forward and reverse reactions and does not affect the state of chemical equilibrium.

For example, in react:

H 2 (g) + Cl 2 (g) 2HCl (g),

total the mole of gaseous substances before and after the reaction remains constant and the pressure in the system does not change. The equilibrium in this system does not change with pressure.

Influence of temperature change on the shift of chemical equilibrium.

In each reversible reaction, one of the directions corresponds to an exothermic process, and the other to an endothermic one. So in the ammonia synthesis reaction, the forward reaction is exothermic, and the reverse reaction is endothermic.

N 2 (g) + 3H 2 (g) 2NH 3 (g) + Q (-ΔH).

When the temperature changes, the rates of both the forward and reverse reactions change, however, the change in rates does not occur to the same extent. In accordance with the Arrhenius equation, an endothermic reaction, characterized by a large value of activation energy, reacts to a change in temperature to a greater extent.

Therefore, in order to estimate the effect of temperature on the direction of the shift in chemical equilibrium, it is necessary to know the thermal effect of the process. It can be determined experimentally, for example, using a calorimeter, or calculated based on G. Hess's law. It should be noted that a change in temperature leads to a change in the value of the constant of chemical equilibrium (K p).

According to Le Chatelier's principle An increase in temperature shifts the equilibrium towards an endothermic reaction. As the temperature decreases, the equilibrium shifts in the direction of the exothermic reaction.

In this way, temperature rise in the ammonia synthesis reaction will lead to a shift in equilibrium towards the endothermic reactions, i.e. to the left. The advantage is obtained by the reverse reaction proceeding with the absorption of heat.

If the system is in a state of equilibrium, then it will remain in it as long as the external conditions remain constant. If the conditions change, then the system will go out of balance - the rates of the direct and reverse processes will change differently - the reaction will proceed. Highest value have cases of imbalance due to a change in the concentration of any of the substances involved in the equilibrium, pressure or temperature.

Let's consider each of these cases.

An imbalance due to a change in the concentration of any of the substances involved in the reaction. Let hydrogen, hydrogen iodide and iodine vapor be in equilibrium with each other at a certain temperature and pressure. Let us introduce an additional amount of hydrogen into the system. According to the law of mass action, an increase in hydrogen concentration will entail an increase in the rate of the forward reaction - the reaction of synthesis of HI, while the rate of the reverse reaction will not change. In the forward direction, the reaction will now proceed faster than in the reverse. As a result, the concentrations of hydrogen and iodine vapor will decrease, which will entail a slowdown in the forward reaction, and the concentration of HI will increase, which will cause an acceleration of the reverse reaction. After some time, the rates of the forward and reverse reactions will again become equal - a new equilibrium will be established. But at the same time, the HI concentration will now be higher than it was before the addition, and the concentration will be lower.

The process of changing concentrations caused by imbalance is called displacement or equilibrium shift. If in this case there is an increase in the concentrations of substances on the right side of the equation (and, of course, at the same time a decrease in the concentrations of substances on the left), then they say that the equilibrium shifts to the right, i.e., in the direction of the flow of the direct reaction; with a reverse change in concentrations, they speak of a shift of equilibrium to the left - in the direction of the reverse reaction. In this example, the equilibrium has shifted to the right. At the same time, the substance, the increase in the concentration of which caused an imbalance, entered into a reaction - its concentration decreased.

Thus, with an increase in the concentration of any of the substances participating in the equilibrium, the equilibrium shifts towards the consumption of this substance; when the concentration of any of the substances decreases, the equilibrium shifts towards the formation of this substance.

An imbalance due to a change in pressure (by reducing or increasing the volume of the system). When gases are involved in the reaction, the equilibrium can be disturbed by a change in the volume of the system.

Consider the effect of pressure on the reaction between nitrogen monoxide and oxygen:

Let the mixture of gases , and be in chemical equilibrium at a certain temperature and pressure. Without changing the temperature, we increase the pressure so that the volume of the system decreases by 2 times. At the first moment partial pressures and the concentrations of all gases will double, but the ratio between the rates of forward and reverse reactions will change - the equilibrium will be disturbed.

Indeed, before the pressure was increased, the gas concentrations had equilibrium values ​​, and , and the rates of the forward and reverse reactions were the same and were determined by the equations:

At the first moment after compression, the concentrations of gases will double in comparison with their initial values ​​and will be equal to , and , respectively. In this case, the rates of forward and reverse reactions will be determined by the equations:

Thus, as a result of an increase in pressure, the rate of the forward reaction increased by 8 times, and the reverse - only by 4 times. The equilibrium in the system will be disturbed - the direct reaction will prevail over the reverse. After the speeds become equal, the equilibrium will be established again, but the quantity in the system will increase, the equilibrium will shift to the right.

It is easy to see that the unequal change in the rates of forward and reverse reactions is due to the fact that in the left and in right parts the equation of the reaction under consideration, the number of gas molecules is different: one molecule of oxygen and two molecules of nitrogen monoxide (only three molecules of gases) are converted into two molecules of gas - nitrogen dioxide. The pressure of a gas is the result of the impact of its molecules on the walls of the vessel; ceteris paribus, the pressure of a gas is the higher, the more molecules are contained in a given volume of gas. Therefore, a reaction proceeding with an increase in the number of gas molecules leads to an increase in pressure, and a reaction proceeding with a decrease in the number of gas molecules leads to its decrease.

With this in mind, the conclusion about the effect of pressure on chemical equilibrium can be formulated as follows:

With an increase in pressure by compressing the system, the equilibrium shifts towards a decrease in the number of gas molecules, i.e., towards a decrease in pressure; with a decrease in pressure, the equilibrium shifts towards an increase in the number of gas molecules, i.e., towards an increase in pressure.

In the case when the reaction proceeds without changing the number of gas molecules, the equilibrium is not disturbed by compression or expansion of the system. For example, in the system

the balance is not disturbed by a change in volume; HI output is independent of pressure.

Disequilibrium due to temperature change. The equilibrium of the vast majority of chemical reactions shifts with temperature. The factor that determines the direction of the equilibrium shift is the sign of the thermal effect of the reaction. It can be shown that when the temperature rises, the equilibrium shifts in the direction of the endothermic reaction, and when it decreases, it shifts in the direction of the exothermic reaction.

Thus, the synthesis of ammonia is an exothermic reaction

Therefore, with an increase in temperature, the equilibrium in the system shifts to the left - towards the decomposition of ammonia, since this process proceeds with the absorption of heat.

Conversely, the synthesis of nitric oxide (II) is an endothermic reaction:

Therefore, when the temperature rises, the equilibrium in the system shifts to the right - in the direction of formation.

The regularities that are manifested in the considered examples of violation of chemical equilibrium are special cases of the general principle that determines the influence various factors to balanced systems. This principle, known as Le Chatelier's principle, can be formulated as follows when applied to chemical equilibria:

If any impact is exerted on a system that is in equilibrium, then as a result of the processes occurring in it, the equilibrium will shift in such a direction that the impact will decrease.

Indeed, when one of the substances participating in the reaction is introduced into the system, the equilibrium shifts towards the consumption of this substance. "When the pressure rises, it shifts so that the pressure in the system decreases; when the temperature rises, the equilibrium shifts towards an endothermic reaction - the temperature in the system drops.

Le Chatelier's principle applies not only to chemical, but also to various physico-chemical equilibria. Equilibrium shift when changing the conditions of such processes as boiling, crystallization, dissolution occurs in accordance with the Le Chatelier principle.

Chemical equilibrium is a state of the system where both reactions - direct and reverse - have the same speed. What characterizes this phenomenon, and what factors affect the chemical equilibrium?

chemical balance. general characteristics

Chemical equilibrium refers to the state chemical system, at which the initial amount of substances in the reaction does not change over time.

Chemical equilibrium can be divided into three types:

• true balance- this is an equilibrium for which constancy is characteristic in time, provided there is no external influence. If external conditions change, the state of the system also changes, but after the conditions are restored, the state also becomes the same. The state of true equilibrium can be considered from two sides: from the side of the reaction products and from the side of the starting substances.
• metastable (apparent) equilibrium- this state occurs when any of the conditions of true equilibrium is not met.
• retarded (false) balance is a state of the system that changes irreversibly when external conditions change.

Equilibrium shift in chemical reactions

Chemical equilibrium depends on three parameters: temperature, pressure, concentration of a substance. The French chemist Henri Louis Le Chatelier in 1884 formulated the principle of dynamic equilibrium, according to which an equilibrium system tends to return to a state of equilibrium under external influence. That is, with an external influence, the equilibrium will shift in such a way that this influence is neutralized.

Rice. 1. Henri Louis Le Chatelier.

The principles formulated by Le Chatelier are also called the principles of "shifting the equilibrium in chemical reactions."

The following factors influence the chemical balance:

• temperature. As the temperature rises, the chemical equilibrium shifts towards absorption of the reaction. If the temperature is lowered, then the equilibrium shifts in the direction of evolution of the reaction.

Rice. 2. Effect of temperature change on chemical equilibrium.

The absorption reaction is called an endothermic reaction, and the release reaction is called exothermic.

• pressure. If the pressure in a chemical reaction increases, then the chemical equilibrium shifts towards the smallest volume of the substance. If the pressure decreases, then the equilibrium shifts in the direction of the largest volume of the substance. This principle applies only to gases, and it does not apply to solids.
• concentration. If, during a chemical reaction, the concentration of one of the substances is increased, then the equilibrium will shift towards the products of the reaction, and if the concentration is reduced, then the equilibrium will shift towards the starting substances.

Rice. 3. Effect of concentration change on chemical equilibrium.

The catalyst does not belong to the factors that affect the shift of the chemical equilibrium.

What have we learned?

At chemical equilibrium, the rates in each pair of reactions are equal to each other. Chemical equilibrium, studied in grade 9, can be divided into three types: true, metastable (apparent), inhibited (false). For the first time, the thermodynamic theory of chemical equilibrium was formulated by the scientist Le Chatelier. Only three factors influence the equilibrium of the system: pressure, temperature, concentration of the initial substance.

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Main article: Le Chatelier-Brown principle

The position of chemical equilibrium depends on the following reaction parameters: temperature, pressure and concentration. The influence that these factors have on chemical reaction, obey the pattern that was expressed in general view in 1885 by the French scientist Le Chatelier.

Factors affecting chemical equilibrium:

1) temperature

As the temperature increases, the chemical equilibrium shifts towards an endothermic (absorption) reaction, and as it decreases, towards an exothermic (isolation) reaction.

CaCO 3 =CaO+CO 2 -Q t →, t↓ ←

N 2 +3H 2 ↔2NH 3 +Q t ←, t↓ →

2) pressure

When the pressure increases, the chemical equilibrium shifts towards a smaller volume of substances, and when it decreases, towards a larger volume. This principle only applies to gases, i.e. if solids are involved in the reaction, they are not taken into account.

CaCO 3 =CaO+CO 2 P ←, P↓ →

1mol=1mol+1mol

3) concentration of starting substances and reaction products

With an increase in the concentration of one of the starting substances, the chemical equilibrium shifts towards the reaction products, and with an increase in the concentration of the reaction products, towards the starting substances.

S 2 +2O 2 =2SO 2 [S],[O] →, ←

Catalysts do not affect the shift of chemical equilibrium!

Basic quantitative characteristics of chemical equilibrium: chemical equilibrium constant, degree of conversion, degree of dissociation, equilibrium yield. Explain the meaning of these quantities on the example of specific chemical reactions.

In chemical thermodynamics, the law of mass action relates the equilibrium activities of the initial substances and reaction products, according to the relation:

Substance activity. Instead of activity, concentration (for a reaction in an ideal solution), partial pressures (reaction in a mixture of ideal gases), fugacity (reaction in a mixture of real gases) can be used;

Stoichiometric coefficient (for initial substances it is assumed to be negative, for products - positive);

Chemical equilibrium constant. The index "a" here means the use of the activity value in the formula.

The efficiency of the reaction is usually evaluated by calculating the yield of the reaction product (Section 5.11). However, the efficiency of the reaction can also be assessed by determining what part of the most important (usually the most expensive) substance turned into the target product of the reaction, for example, what part of SO 2 turned into SO 3 during the production of sulfuric acid, that is, find degree of conversion original substance.

Let a brief scheme of the ongoing reaction

Then the degree of transformation of substance A into substance B (A) is determined by the following equation

where n proreag (A) is the amount of the substance of reagent A that reacted to form product B, and n initial (A) - the initial amount of the substance of the reagent A.

Naturally, the degree of conversion can be expressed not only in terms of the amount of substance, but also in terms of any quantities proportional to it: the number of molecules (formula units), mass, volume.

If reactant A is taken in short supply and the loss of product B can be neglected, then the degree of conversion of reactant A is usually equal to the yield of product B

An exception is reactions in which the starting material is obviously consumed to form several products. So, for example, in the reaction

Cl 2 + 2KOH \u003d KCl + KClO + H 2 O

chlorine (reagent) is equally converted into potassium chloride and potassium hypochlorite. In this reaction, even with a 100% yield of KClO, the degree of conversion of chlorine into it is 50%.

The quantity known to you - the degree of protolysis (paragraph 12.4) - is a special case of the degree of conversion:

Within the framework of TED, similar quantities are called degree of dissociation acids or bases (also referred to as the degree of protolysis). The degree of dissociation is related to the dissociation constant according to the Ostwald dilution law.

Within the framework of the same theory, the equilibrium of hydrolysis is characterized by degree of hydrolysis (h), while using the following expressions relating it to the initial concentration of the substance ( With) and dissociation constants of weak acids (K HA) and weak bases formed during hydrolysis ( K MOH):

The first expression is valid for salt hydrolysis weak acid, the second is the salt of a weak base, and the third is the salt of a weak acid and a weak base. All these expressions can only be used for dilute solutions with a degree of hydrolysis of not more than 0.05 (5%).

Usually, the equilibrium yield is determined by the known equilibrium constant, with which it is associated in each particular case by a certain ratio.

The yield of the product can be changed by shifting the equilibrium of the reaction in reversible processes, by the influence of factors such as temperature, pressure, concentration.

In accordance with the Le Chatelier principle, the equilibrium degree of conversion increases with increasing pressure in the course of simple reactions, while in other cases the volume of the reaction mixture does not change and the yield of the product does not depend on pressure.

The influence of temperature on the equilibrium yield, as well as on the equilibrium constant, is determined by the sign of the thermal effect of the reaction.

For a more complete assessment of reversible processes, the so-called yield from the theoretical (the yield from equilibrium) is used, which is equal to the ratio of the actually obtained product w to the amount that would have been obtained in the equilibrium state.

THERMAL DISSOCIATION chemical

a reaction of reversible decomposition of a substance caused by an increase in temperature.

With T. d., several (2H2H + OSaO + CO) or one simpler substance is formed from one substance

Equilibrium etc. is established according to the acting mass law. It

can be characterized either by the equilibrium constant or by the degree of dissociation

(the ratio of the number of decayed molecules to the total number of molecules). AT

in most cases, T. d. is accompanied by the absorption of heat (increment

enthalpy

DN>0); therefore, in accordance with the Le Chatelier-Brown principle

heating intensifies it, the degree of displacement of T. d. with temperature is determined

the absolute value of DN. The pressure prevents T. d. the stronger, the larger

change (increase) in the number of moles (Di) of gaseous substances

the degree of dissociation does not depend on pressure. If a solids not

form solid solutions and are not in a highly dispersed state,

then the pressure T. d. is uniquely determined by the temperature. To implement T.

e. solid substances (oxides, crystalline hydrates, etc.)

it's important to know

temperature, at which the dissociation pressure becomes equal to the external one (in particular,

atmospheric) pressure. Since the escaping gas can overcome

ambient pressure, then upon reaching this temperature, the decomposition process

immediately intensifies.

Dependence of the degree of dissociation on temperature: the degree of dissociation increases with increasing temperature (an increase in temperature leads to an increase in the kinetic energy of dissolved particles, which contributes to the decay of molecules into ions)

The degree of conversion of the starting materials and the equilibrium yield of the product. Methods for their calculation at a given temperature. What data is needed for this? Give a scheme for calculating any of these quantitative characteristics of chemical equilibrium using an arbitrary example.

The degree of conversion is the amount of the reacted reagent related to its initial amount. For the simplest reaction, where is the concentration at the inlet to the reactor or at the beginning of the batch process, is the concentration at the outlet of the reactor or the current moment of the batch process. For an arbitrary reaction, for example, , in accordance with the definition, the calculation formula is the same: . If there are several reagents in the reaction, then the degree of conversion can be calculated for each of them, for example, for the reaction The dependence of the degree of conversion on the reaction time is determined by the change in the concentration of the reagent with time. At the initial moment of time, when nothing has changed, the degree of transformation is equal to zero. Then, as the reagent is converted, the degree of conversion increases. For an irreversible reaction, when nothing prevents the reagent from being completely consumed, its value tends (Fig. 1) to unity (100%). Fig.1 The higher the reagent consumption rate, determined by the value of the rate constant, the faster the degree of conversion increases, which is shown in the figure. If the reaction is reversible, then when the reaction tends to equilibrium, the degree of conversion tends to an equilibrium value, the value of which depends on the ratio of the rate constants of the forward and reverse reactions (on the equilibrium constant) (Fig. 2). Fig.2 Yield of the target product Yield of the product is the amount of the target product actually obtained, related to the amount of this product that would have been obtained if the entire reagent had passed into this product (to the maximum possible amount of the resulting product). Or (via the reagent): the amount of the reagent actually converted into the target product, divided by the initial amount of the reagent. For the simplest reaction, the yield is , and keeping in mind that for this reaction, , i.e. for the simplest reaction, the yield and degree of conversion are one and the same quantity. If the transformation takes place with a change in the amount of substances, for example, then, in accordance with the definition, the stoichiometric coefficient must be included in the calculated expression. In accordance with the first definition, the imaginary amount of the product obtained from the entire initial amount of the reagent will be half as much for this reaction as the initial amount of the reagent, i.e. , and the calculation formula . In accordance with the second definition, the amount of the reagent actually converted into the target product will be twice as much as the amount of this product formed, i.e. , then the calculation formula . Naturally, both expressions are the same. For a more complex reaction, the calculation formulas are written in exactly the same way in accordance with the definition, but in this case the yield is no longer equal to the degree of conversion. For example, for the reaction . If there are several reagents in the reaction, the yield can be calculated for each of them; if, in addition, there are several target products, then the yield can be calculated for any target product for any reagent. As can be seen from the structure of the calculation formula (the denominator contains a constant value), the dependence of the yield on the reaction time is determined by the time dependence of the concentration of the target product. So, for example, for the reaction this dependence looks like in Fig.3. Fig.3

The degree of conversion as a quantitative characteristic of chemical equilibrium. How will the increase in total pressure and temperature affect the degree of conversion of the reagent ... in a gas-phase reaction: ( given the equation)? Give the rationale for the answer and the corresponding mathematical expressions.