Conditions for shifting chemical equilibrium table. Shift in chemical equilibrium
Chemical equilibrium, corresponding to the equality of the rates of direct and reverse reactions ( = ) and the minimum value of the Gibbs energy (∆ G р,т = 0), is the most stable state of the system under given conditions and remains unchanged as long as the parameters at which the equilibrium is established remain constant .
When conditions change, the equilibrium is disturbed and shifted in the direction of a direct or reverse reaction. The shift in equilibrium is due to the fact that the external influence to a different extent changes the speed of two mutually opposite processes. After some time, the system again becomes equilibrium, i.e. it moves from one equilibrium state to another. The new equilibrium is characterized by a new equality of the rates of forward and reverse reactions and new equilibrium concentrations of all substances in the system.
The direction of equilibrium shift in the general case is determined by the Le Chatelier principle: if an external influence is exerted on a system in a state of stable equilibrium, then the equilibrium shift occurs in the direction of a process that weakens the effect of external influence.
A shift in equilibrium can be caused by a change in temperature, concentration (pressure) of one of the reagents.
Temperature is the parameter on which the value of the equilibrium constant of a chemical reaction depends. The issue of shifting the equilibrium with a change in temperature, depending on the conditions for using the reaction, is solved by using the isobar equation (1.90) - =
1. For an isothermal process ∆ r H 0 (t)< 0, в правой части выражения (1.90) R >0, T > 0, hence the first derivative of the logarithm of the equilibrium constant with respect to temperature is negative< 0, т.е. ln Kp (и сама константа Кр) являются убывающими функциями температуры. При увеличении температуры константа химического равновесия (Кр) уменьшается и что согласно закону действующих масс (2.27), (2.28)соответствует смещению химического равновесия в сторону обратной (эндотермической) реакции. Именно в этом проявляется противодействие системы оказанному воздействию.
2. For an endothermic process ∆ r H 0 (t) > 0, the derivative of the logarithm of the equilibrium constant with respect to temperature is positive (> 0), the theme is ln Kp and Kp are increasing functions of temperature, i.e. in accordance with the law of mass action, with increasing temperature, the equilibrium shifts towards a straight line (endothermic reaction). However, it must be remembered that the rate of both isothermal and endothermic processes increases with increasing temperature, and decreases with decreasing, but the change in rates is not the same with a change in temperature, therefore, by varying the temperature, it is possible to shift the equilibrium in a given direction. A shift in equilibrium can be caused by a change in the concentration of one of the components: the addition of a substance to the equilibrium system or the removal from the system.
According to the Le Chatelier principle, when the concentration of one of the participants in the reaction changes, the equilibrium shifts towards the compensating change, i.e. with an increase in the concentration of one of the starting substances - to the right side, and with an increase in the concentration of one of the reaction products - to the left. If gaseous substances participate in a reversible reaction, then when the pressure changes, all their concentrations change equally and simultaneously. The rates of processes also change, and, consequently, a shift in chemical equilibrium can also occur. So, for example, with an increase in pressure (compared to the equilibrium one) on the CaCO 3 (K) CO (c) + CO 2 (g) system, the rate of the reverse reaction increases = which will lead to a shift in the equilibrium to the left. When the pressure on the same system decreases, the rate of the reverse reaction decreases, and the equilibrium shifts to the right side. With an increase in pressure on the 2HCl H 2 +Cl 2 system, which is in equilibrium, the equilibrium will not shift, because both speeds and will increase equally.
For the 4HCl + O 2 2Cl 2 + 2H 2 O (g) system, an increase in pressure will increase the rate of the direct reaction and shift the equilibrium to the right.
And so, in accordance with the principle of Le Chatelier, with increasing pressure, the equilibrium shifts towards the formation of a smaller number of moles of gaseous substances in the gas mixture and, accordingly, towards a decrease in pressure in the system.
And vice versa, under an external influence that causes a decrease in pressure, the equilibrium shifts towards the formation more moles of gaseous substances, which will cause an increase in pressure in the system and will counteract the effect produced.
Le Chatelier's principle is of great practical importance. On its basis, it is possible to choose such conditions for the implementation of chemical interaction that will ensure the maximum yield of reaction products.
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. Of greatest importance are 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 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 slow down the forward reaction, while the concentration of HI will increase, which will accelerate 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, the partial pressures and concentrations of all gases will double, but the ratio between the rates of the 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 is different in the number of gas molecules: 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 general principle, which 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 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.
Codifier Topics: reversible and irreversible reactions. chemical balance. Displacement of chemical equilibrium under the influence of various factors.
According to the possibility of a reverse reaction, chemical reactions are divided into reversible and irreversible.
Reversible chemical reactions are reactions whose products, under given conditions, can interact with each other.
irreversible reactions These are reactions whose products under given conditions cannot interact with each other.
More details about classification of chemical reactions can be read.
The probability of product interaction depends on the conditions of the process.
So if the system open, i.e. exchanges with environment both matter and energy, then chemical reactions in which, for example, gases are formed, will be irreversible. for instance , when calcining solid sodium bicarbonate:
2NaHCO 3 → Na 2 CO 3 + CO 2 + H 2 O
gas will be released carbon dioxide and escape from the reaction zone. Therefore, such a reaction will irreversible under these conditions. If we consider closed system , which can not exchange matter with the environment (for example, a closed box in which the reaction takes place), then carbon dioxide will not be able to escape from the reaction zone, and will interact with water and sodium carbonate, then the reaction will be reversible under these conditions:
2NaHCO 3 ⇔ Na 2 CO 3 + CO 2 + H 2 O
Consider reversible reactions. Let the reversible reaction proceed according to the scheme:
aA + bB = cC + dD
The rate of the forward reaction according to the law of mass action is determined by the expression: v 1 =k 1 ·C A a ·C B b , the rate of the reverse reaction: v 2 =k 2 ·C C c ·C D d . If at the initial moment of the reaction there are no substances C and D in the system, then particles A and B mainly collide and interact, and a predominantly direct reaction occurs. Gradually, the concentration of particles C and D will also begin to increase, therefore, the rate of the reverse reaction will increase. In some moment the rate of the forward reaction becomes equal to the rate of the reverse reaction. This state is called chemical equilibrium .
In this way, chemical equilibrium is the state of the system in which the rates of the forward and reverse reactions are equal .
Because the rates of the forward and reverse reactions are equal, the rate of formation of substances is equal to the rate of their consumption, and the current concentrations of substances do not change . Such concentrations are called balanced .
Note that in equilibrium both forward and reverse reactions, that is, the reactants interact with each other, but the products also interact at the same rate. At the same time, external factors may influence shift chemical equilibrium in one direction or another. Therefore, chemical equilibrium is called mobile, or dynamic.
Research in the field of moving balance began in the 19th century. In the writings of Henri Le Chatelier, the foundations of the theory were laid, which were later generalized by the scientist Karl Brown. The principle of moving balance, or the principle of Le Chatelier-Brown, states:
If a system in equilibrium is subjected to external factor, which changes any of the equilibrium conditions, then processes in the system are intensified, aimed at compensating for external influences.
In other words: under an external influence on the system, the equilibrium will shift in such a way as to compensate for this external influence.
This principle, which is very important, works for any equilibrium phenomena (not just chemical reactions). However, we will now consider it in relation to chemical interactions. In the case of chemical reactions, external action leads to a change in the equilibrium concentrations of substances.
Three main factors can affect chemical reactions at equilibrium: temperature, pressure, and concentrations of reactants or products.
1. As you know, chemical reactions are accompanied by a thermal effect. If the direct reaction proceeds with the release of heat (exothermic, or + Q), then the reverse reaction proceeds with the absorption of heat (endothermic, or -Q), and vice versa. If you raise temperature in the system, the equilibrium will shift so as to compensate for this increase. It is logical that with an exothermic reaction, the temperature increase cannot be compensated. Thus, as the temperature rises, the equilibrium in the system shifts towards heat absorption, i.e. towards endothermic reactions (-Q); with decreasing temperature - in the direction of an exothermic reaction (+ Q).
2. In the case of equilibrium reactions, when at least one of the substances is in the gas phase, the equilibrium is also significantly affected by the change pressure in system. When the pressure is increased, the chemical system tries to compensate for this effect, and increases the rate of the reaction, in which the amount of gaseous substances decreases. When the pressure is reduced, the system increases the rate of the reaction, in which more molecules of gaseous substances are formed. Thus: with an increase in pressure, the equilibrium shifts towards a decrease in the number of gas molecules, with a decrease in pressure - towards an increase in the number of gas molecules.
Note! Systems where the number of molecules of reactant gases and products is the same are not affected by pressure! Also, a change in pressure practically does not affect the equilibrium in solutions, i.e. in reactions where there are no gases.
3. Also, the equilibrium in chemical systems is affected by the change concentration reactants and products. As the concentration of the reactants increases, the system tries to use them up and increases the rate of the forward reaction. With a decrease in the concentration of reagents, the system tries to accumulate them, and the rate of the reverse reaction increases. With an increase in the concentration of products, the system also tries to use them up, and increases the rate of the reverse reaction. With a decrease in the concentration of products, the chemical system increases the rate of their formation, i.e. the rate of the forward reaction.
If in chemical system the rate of the forward reaction increases right , towards the formation of products and reagent consumption . If the rate of the reverse reaction increases, we say that the balance has shifted to the left , towards food consumption and increasing the concentration of reagents .
for instance, in the ammonia synthesis reaction:
N 2 + 3H 2 \u003d 2NH 3 + Q
an increase in pressure leads to an increase in the reaction rate, in which a smaller number of gas molecules are formed, i.e. direct reaction (the number of reactant gas molecules is 4, the number of gas molecules in the products is 2). As the pressure increases, the equilibrium shifts to the right, towards the products. At rise in temperature balance will shift towards an endothermic reaction, i.e. to the left, towards the reagents. An increase in the concentration of nitrogen or hydrogen will shift the equilibrium towards their consumption, i.e. to the right, towards the products.
Catalyst does not affect the balance, because speeds up both the forward and reverse reactions.
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 a chemical reaction obeys 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, to 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 the hydrolysis of a salt of a weak acid, the second for a salt of a weak base, and the third for a 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). V
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 solids are 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 periodic process, is the concentration at the outlet of the reactor or the current moment of the periodic 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 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.
The state in which the rates of the forward and reverse reactions are equal is called chemical equilibrium. Reversible reaction equation in general form:
Forward reaction rate v 1 =k 1 [A] m [B] n , rate of reverse reaction v 2 =k 2 [С] p [D] q , where in square brackets are the equilibrium concentrations. By definition, at chemical equilibrium v 1 =v 2, from where
K c \u003d k 1 / k 2 \u003d [C] p [D] q / [A] m [B] n,
where K c is the chemical equilibrium constant expressed in terms of molar concentrations. Reduced mathematical expression often called the law of mass action for a reversible chemical reaction: the ratio of the product of the equilibrium concentrations of the reaction products to the product of the equilibrium concentrations of the starting substances.
The position of chemical equilibrium depends on the following reaction parameters: temperature, pressure and concentration. The influence that these factors have on a chemical reaction is subject to a pattern that was expressed in general terms in 1884 by the French scientist Le Chatelier. The modern formulation of Le Chatelier's principle is as follows:
If an external influence is exerted on a system that is in a state of equilibrium, then the system will move to another state in such a way as to reduce the effect of external influence.
Factors affecting chemical equilibrium.
1. Effect of temperature. In each reversible reaction, one of the directions corresponds to an exothermic process, and the other to an endothermic one.
As the temperature rises, the chemical equilibrium shifts in the direction of the endothermic reaction, and as the temperature decreases, in the direction of the exothermic reaction.
2. Influence of pressure.
In all reactions involving gaseous substances, accompanied by a change in volume due to a change in the amount of substance in the transition from the starting substances to the products, the equilibrium position is affected by the pressure in the system.
The influence of pressure on the equilibrium position obeys the following rules:
With increasing pressure, the equilibrium shifts in the direction of the formation of substances (initial or products) with a smaller volume.
3. Influence of concentration. 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 equilibrium shifts in the direction of the formation of 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.
Questions for self-control:
1. What is the rate of a chemical reaction and what factors does it depend on? On what factors does the rate constant depend?
2. Write an equation for the reaction rate of the formation of water from hydrogen and oxygen and show how the rate changes if the hydrogen concentration is tripled.
3. How does the reaction rate change over time? What reactions are called reversible? What is the state of chemical equilibrium? What is called the equilibrium constant, on what factors does it depend?
4. What external influences Can the chemical balance be disturbed? In which direction will the equilibrium shift as the temperature changes? Pressure?
5. How can I shift reversible reaction in a certain direction and bring it to an end?
Lecture No. 12 (problem)
Solutions
Target: Give qualitative conclusions about the solubility of substances and a quantitative assessment of solubility.
Keywords: Solutions - homogeneous and heterogeneous; true and colloidal; solubility of substances; concentration of solutions; solutions of nonelectroyls; laws of Raoult and van't Hoff.
Plan.
1. Classification of solutions.
2. Concentration of solutions.
3. Solutions of non-electrolytes. Raoult's laws.
Classification of solutions
Solutions are homogeneous (single-phase) systems of variable composition, consisting of two or more substances (components).
According to the nature of the state of aggregation, solutions can be gaseous, liquid and solid. Usually, a component that under given conditions is in the same state of aggregation as the resulting solution is considered a solvent, the remaining components of the solution are solutes. In the case of the same aggregate state of the components, the solvent is the component that prevails in the solution.
Depending on the size of the particles, solutions are divided into true and colloidal. In true solutions (often referred to simply as solutions), the solute is dispersed to the atomic or molecular level, the particles of the solute are not visible either visually or under a microscope, they move freely in the solvent medium. True solutions are thermodynamically stable systems, infinitely stable over time.
The driving forces for the formation of solutions are the entropy and enthalpy factors. When dissolving gases in a liquid, the entropy always decreases ΔS< 0, а при растворении кристаллов возрастает (ΔS >0). The stronger the interaction between the solute and the solvent, the greater the role of the enthalpy factor in the formation of solutions. The sign of the change in the enthalpy of dissolution is determined by the sign of the sum of all thermal effects of the processes accompanying dissolution, of which the main contribution is made by destruction crystal lattice into free ions (ΔH > 0) and interaction of formed ions with solvent molecules (solvation, ΔH< 0). При этом независимо от знака энтальпии при растворении (абсолютно нерастворимых веществ нет) всегда ΔG = ΔH – T·ΔS < 0, т. к. переход вещества в раствор сопровождается значительным возрастанием энтропии вследствие стремления системы к разупорядочиванию. Для жидких растворов (расплавов) процесс растворения идет самопроизвольно (ΔG < 0) до установления динамического равновесия между раствором и твердой фазой.
The concentration of a saturated solution is determined by the solubility of the substance at a given temperature. Solutions with a lower concentration are called unsaturated.
Solubility for various substances varies considerably and depends on their nature, the interaction of the particles of the solute with each other and with solvent molecules, as well as on external conditions (pressure, temperature, etc.)
In chemical practice, solutions prepared on the basis of a liquid solvent are most important. It is liquid mixtures in chemistry that are simply called solutions. The most widely used inorganic solvent is water. Solutions with other solvents are called non-aqueous.
Solutions are of extremely great practical importance; many chemical reactions take place in them, including those underlying the metabolism in living organisms.
Solution concentration
An important characteristic of solutions is their concentration, which expresses the relative amount of components in the solution. There are mass and volume concentrations, dimensional and dimensionless.
TO dimensionless concentrations (shares) include the following concentrations:
Mass fraction of solute W(B) expressed as a fraction of a unit or as a percentage:
where m(B) and m(A) are the mass of the solute B and the mass of the solvent A.
The volume fraction of a dissolved substance σ(B) is expressed in fractions of a unit or volume percent:
where V i is the volume of the component of the solution, V(B) is the volume of the dissolved substance B. Volume percentages are called degrees *) .
*) Sometimes the volume concentration is expressed in thousandths (ppm, ‰) or in parts per million (ppm), ppm.
The mole fraction of a solute χ(B) is expressed by the relation
The sum of the mole fractions of the k components of the solution χ i is equal to one
TO dimensional concentrations include the following concentrations:
The solute molality C m (B) is determined by the amount of substance n(B) in 1 kg (1000 g) of the solvent, the unit is mol/kg.
Molar concentration of substance B in solution C(B) - the content of the amount of dissolved substance B per unit volume of the solution, mol/m 3, or more often mol/liter:
where μ(B) is the molar mass of B, V is the volume of the solution.
Molar concentration equivalents of substance B C E (B) (normality - obsolete.) is determined by the number of equivalents of a solute per unit volume of the solution, mol / liter:
where n E (B) is the amount of substance equivalents, μ E is the molar mass of the equivalent.
The titer of a solution of substance B( T B) is determined by the mass of the solute in g contained in 1 ml of the solution:
g/ml or 
g/ml.
Mass concentrations ( mass fraction, percentage, molal) do not depend on temperature; volumetric concentrations refer to a specific temperature.
All substances are capable of solubility to some extent and are characterized by solubility. Some substances are infinitely soluble in each other (water-acetone, benzene-toluene, liquid sodium-potassium). Most compounds are sparingly soluble (water-benzene, water-butyl alcohol, water-table salt), and many are slightly soluble or practically insoluble (water-BaSO 4 , water-gasoline).
The solubility of a substance under given conditions is its concentration in a saturated solution. In such a solution, equilibrium is reached between the solute and the solution. In the absence of equilibrium, the solution remains stable if the concentration of the solute is less than its solubility (unsaturated solution), or unstable if the solution contains substances greater than its solubility (supersaturated solution).
