Pressure does not affect the state of chemical equilibrium. Chemical equilibrium and creation of conditions for its displacement. Chemical balance. general characteristics
The state of chemical equilibrium is disrupted by various external influences on the system: heating and cooling, pressure changes, addition and removal of individual substances or solvent. As a result, the equality of the rates of forward and reverse reactions is violated and a certain shift in the state of the system occurs.
A shift in chemical equilibrium is a process that occurs in an equilibrium system as a result of an external influence.
A shift in equilibrium leads to the establishment of a new state of equilibrium in the system, characterized by changed concentrations of substances.
Example 10.6. In what direction will the equilibrium of the reaction shift when oxygen is added?
Solution. When oxygen is added, its concentration increases, and hence the speed in the forward direction. The balance will shift to the right. This increases the proportion of conversion of S0 2 to S0 3.
The displacement of equilibrium under any influence obeys Le Chatelier's principle (1884).
An external influence on a system in a state of equilibrium causes a process leading to a decrease in the result of the influence.
When deciding a specific question about the direction of the equilibrium shift, one should clearly understand the essence of the effect produced and its result. For example, a change in concentration cannot be considered as an effect on the system. Substances can be introduced or removed into the system (ego effects), resulting in a change in concentrations. The application of Le Chatelier's principle to the practically important reaction for the production of ammonia is shown in table. 10.1. The first two columns indicate the impact on the system and the result of the impact. Arrows T and >1 indicate an increase or decrease in the corresponding characteristic. The “System Response” column indicates changes that are opposite to the effect of the impact. These changes are associated with the occurrence of a direct or reverse reaction in the system. Some difficulties arise in understanding the influence of pressure on the state of equilibrium. The pressure of a gas mixture, according to the equation of gas state, depends on temperature and volume for a given amount of substance, but a system as such, having a certain volume and temperature, can respond to changes in pressure only by changing the total amount of substance as a result of the reaction. A corollary follows from Le Chatelier’s principle: with increasing pressure, the equilibrium shifts in the direction of decreasing the sum of stoichiometric coefficients for substances in the gaseous state.
Table 10.1
Application of Le Chatelier's principle using the example of the reaction N2 + 3Н2 2NH3, ArH° =-92 kJ/mol
In reversible heterogeneous reactions, a shift in equilibrium is associated with changes in the concentrations of gaseous and dissolved substances. A change in the mass of a solid does not affect the equilibrium position in the system.
Shifting chemical equilibrium is widely used when carrying out reactions in laboratories and in technological processes. In this case, we are not talking about achieving balance, but shifting it one by one. The process is planned from the very beginning so that the established equilibrium is optimal from the point of view of saving the most valuable reagents. Production costs decrease as product yield increases. It depends on temperature and pressure conditions. Using the example of the reaction for producing ammonia, the principle of the approach to choosing process conditions is shown (the signs “+” and “-” symbolize the desired or undesirable nature of the influence on the final result).
From the data presented it follows that in the production of ammonia it is desirable to use high pressure and find the most active catalysts. Temperature has a positive effect from a technological and economic point of view on the reaction rate and a negative effect on the yield of ammonia. Therefore, it is necessary to choose the optimal temperature, which ultimately ensures the minimum cost of producing the product.
Task 2 of 15
2 .
Chemical equilibrium in the system
C 4 H 10 (g) ⇄ C 4 H 6 (g) + 2H 2 (g) − Q
will shift towards the starting substances when
Right
According to Le Chatelier's principle -
Wrong
According to Le Chatelier's principle - If an equilibrium system is influenced from the outside, changing any of the factors that determine the equilibrium position, then the direction of the process in the system that weakens this influence will increase.
When the temperature decreases (external influence - cooling of the system), the system will tend to increase the temperature, which means that the exothermic process (reverse reaction) intensifies, the equilibrium will shift to the left, towards the reagents.
Task 3 of 15
3 .
Equilibrium in reaction
CaCO 3 (tv) = CaO (tv) + CO 2 (g) - Q
will shift towards products when
Right
According to Le Chatelier's principle - e If an equilibrium system is influenced from the outside, changing any of the factors that determine the equilibrium position, then the direction of the process in the system that weakens this influence will intensify -
Wrong
According to Le Chatelier's principle - e If an equilibrium system is influenced from the outside, changing any of the factors that determine the equilibrium position, then the direction of the process in the system that weakens this influence will intensify - when the temperature increases (heating), the system will tend to lower the temperature, which means that the process absorbing heat intensifies, the equilibrium will shift towards the endothermic reaction, i.e. towards the products.
Task 4 of 15
4 .
Equilibrium in reaction
C 2 H 4 (g) + H 2 O (g) = C 2 H 5 OH (g) + Q
will shift towards the product when
Right
According to Le Chatelier's principle - e
Wrong
According to Le Chatelier's principle - e If an equilibrium system is influenced from the outside, changing any of the factors that determine the equilibrium position, then the direction of the process in the system that weakens this influence will increase - as the total pressure increases, the system will tend to lower it, the equilibrium will shift towards a smaller amount of gaseous substances, i.e. towards products.
Task 5 of 15
5 .
O 2 (g) + 2CO (g) ⇄ 2CO 2 (g) + Q
A. As the temperature decreases, the chemical equilibrium in this system will shift towards the reaction products.
B. When the concentration of carbon monoxide decreases, the equilibrium of the system will shift towards the reaction products.
Right
Wrong
Only A is true, according to Le Chatelier’s principle, as the temperature decreases, the chemical equilibrium shifts towards the exothermic reaction, i.e. the reaction products. Statement B is incorrect, because when the concentration of carbon monoxide decreases, the system will tend to increase it, that is, the direction in which it is formed will increase, the equilibrium of the system shifts to the left, towards the reagents.
Task 6 of 15
6 .
As pressure increases, the yield of product(s) in a reversible reaction increases
Right
Wrong
According to Le Chatelier's principle - e If an equilibrium system is influenced from the outside, changing any of the factors that determine the equilibrium position, then the direction of the process in the system that weakens this influence will increase - as pressure increases, the system will tend to lower it, and the equilibrium will shift towards a smaller amount of gaseous substances. That is, in reactions in which the amount of gaseous substances on the right side of the equation (in the products) is less than on the left side (in the reactants), an increase in pressure will lead to an increase in the yield of product(s), in other words, the equilibrium will shift towards the products. This condition is met only in the second option - on the left side - 2 moles of gas, on the right side - 1 mole of gas.
In this case, solid and liquid substances do not contribute to the equilibrium shift. If the quantities of gaseous substances on the right and left sides of the equation are equal, a change in pressure will not lead to a shift in equilibrium.
Task 7 of 15
7 .
To shift the chemical equilibrium in the system
H 2 (g) + Br 2 (g) ⇄ 2HBr (g) + Q
towards the product is necessary
Right
Wrong
According to Le Chatelier's principle, the system reacts to external influences. Therefore, the equilibrium can be shifted to the right, towards the product, if the temperature is reduced, the concentration of the starting substances is increased, or the amount of reaction products is reduced. Since the quantities of gaseous substances on the right and left sides of the equation are equal, a change in pressure will not shift the equilibrium. The addition of bromine will lead to an intensification of processes that consume it, i.e. the balance will shift towards products.
Task 8 of 15
8 .
In system
2SO 2 (g) + O 2 (g) ⇄ 2SO 3 (g) + Qa shift in chemical equilibrium to the right will occur when
Right
Wrong
Reduce the temperature (i.e. the direct reaction is exothermic), increase the concentration of the starting substances or reduce the amount of reaction products, or increase the pressure (since the direct reaction occurs with a decrease in the total volume of gaseous substances).
Task 9 of 15
9 .
Are the following judgments about the shift in chemical equilibrium in the system correct?
CO (g) + Cl 2 (g) ⇄ COCl 2 (g) + Q
A. When pressure increases, the chemical equilibrium shifts towards the reaction product.
B. As the temperature decreases, the chemical equilibrium in this system will shift towards the reaction product.
Right
According to Le Chatelier's principle, the system reacts to external influences. Therefore, to shift the equilibrium to the right, towards the product, you can reduce temperature increase blood pressure
Wrong
According to Le Chatelier's principle, the system reacts to external influences. Therefore, to shift the equilibrium to the right, towards the product, you can reduce temperature(i.e. the direct reaction is exothermic), increase the concentration of starting substances or reduce the amount of reaction products or increase blood pressure(because the direct reaction occurs with a decrease in the total volume of gaseous substances). Thus, both judgments are correct.
Task 10 of 15
10 .
In system
SO 2 (g) + Cl 2 (g) ⇄ SO 2 Cl 2 (g) + Q
the shift of chemical equilibrium to the right contributes to
Right
Wrong
Task 11 of 15
11 .
In which system does an increase in hydrogen concentration shift the chemical equilibrium to the left?
Right
Wrong
According to Le Chatelier's principle, when the concentration of a component increases, the system will tend to reduce its concentration, i.e., consume it. In a reaction where hydrogen is the product, an increase in its concentration shifts the chemical equilibrium to the left, towards its consumption.
Task 12 of 15
12 .
As the total pressure increases, the equilibrium will shift towards the products in the reaction
Right
According to Le Chatelier's principle - e If an equilibrium system is influenced from the outside, changing any of the factors that determine the equilibrium position, then the direction of the process in the system that weakens this influence will increase -
Wrong
According to Le Chatelier's principle - e If an equilibrium system is influenced from the outside, changing any of the factors that determine the equilibrium position, then the direction of the process in the system that weakens this influence will increase - as the total pressure increases, the system will tend to lower it, and the equilibrium will shift towards a smaller amount of gaseous substances. Only in the fourth option do the products contain less gaseous substances, i.e. a direct reaction proceeds with a decrease in volume, so an increase in total pressure will shift the equilibrium towards the products in this reaction.
Task 1 of 15
1 .
As the total pressure decreases, the equilibrium will shift towards the products in the reaction
Right
e
Wrong
According to Le Chatelier's principle - a decrease in pressure will lead to an intensification of processes that increase pressure, which means that the equilibrium will shift towards a larger number of gaseous particles (which create pressure). Only in the second case are there more gaseous substances in the products (on the right side of the equation) than in the reactants (on the left side of the equation).
The state of chemical equilibrium depends on a number of factors: temperature, pressure, concentration of reacting substances. Let's take a closer look at the influence of these factors.
A change in the concentration of the components of an equilibrium system at a constant temperature shifts the equilibrium, however, the value of the equilibrium constant does not change. If the concentration of substance A (or B) is increased for a reaction, then the rate of the forward reaction will increase, but the rate of the reverse reaction at the initial time will not change. The balance will be disrupted. Then the concentration of the starting substances will begin to decrease, and the concentration of the reaction products will increase, and this will happen until a new equilibrium is established. In such cases, the equilibrium is said to shift towards the formation of reaction products or shift to the right.
Reasoning in the same way, determine for yourself where the equilibrium will shift if the concentration of substance C is increased; reduce the concentration of substance D.
By changing the concentrations of the components, you can shift the equilibrium in the desired direction, increasing or decreasing the yield of reaction products; achieving more complete use of starting substances or, conversely,
To complete the second task, remember that the direct reaction will proceed until one of the components A or B is completed. From the reaction equation it is clear that the reagents react in equimolar* quantities, and their concentrations are equal according to the conditions of the problem. Consequently, substances A and B, entering into a reaction, will end at the same time. From the reaction equation it is also clear that when one mole of substance A is converted, two moles of substance C and one mole of substance D are formed. Therefore, to the amount of substances C and D already present in the system, some more will be added. After a simple calculation, we get the desired result:
[A] = [B] = 0 mol/l; [C] = 2 +2 = 4 mol/l; [D] = 2 +1 = 3 mol/l.
Carry out a similar reasoning for the third task, remembering that substances C and D react in a 2:1 ratio, and the calculation must be carried out according to the amount of the substance that is in short supply (identify this substance). Do the calculations and get the result:
[A] = [B] = 1+2/2 = 2 mol/l; [C] = 0 mol/l; [D] = 2-2/2 = 1 mol/l.
The equilibrium constant of the reaction A + B C + D is equal to unity. Initial concentration [A]o = 0.02 mol/l. What percentage of substance A will undergo transformation if the initial concentrations of [B]o are 0.02; 0.1; 0.2?
Let us denote by x the equilibrium concentration of substance A and write down the expression for the equilibrium constant. The equilibrium concentration of substance B will also be equal to x. The concentrations of the reaction products (C and D) will be equal to each other and equal to 0.02. (Show this using the reaction equation.)
Let us write down the expression for the equilibrium constant.
Kravn. = (0.02 - x)(0.02 - x)/x2 = 1
Having solved the equation for x, we get the result: x = 0.01. Consequently, in the first case, half of substance A (or 50%) underwent transformation.
For the second case, the equilibrium constant will be equal to
Kravn. = (0.02 - x)(0.02 - x)/(0.1- (0.02 - x)) = 1
Get this expression yourself and, having solved the equation, check the result obtained (x = 0.003). Consequently, (0.02 - 0.003) moles of substance A entered the reaction, which is 83.5%.
Solve the problem for the third case yourself, and also solve the same problem, denoting by x the amount of the substance that reacted.
An important conclusion can be drawn from the results obtained. To increase the proportion of a substance that reacts at a constant equilibrium constant, it is necessary to increase the amount of the second reagent in the system. A similar problem arises, for example, when recycling waste chemically.
As the temperature increases, the rate of both the forward and reverse reactions will increase, but if the forward reaction is endothermic (?H > 0), then the rate of the forward reaction will increase more than the rate of the reverse reaction, and the equilibrium will shift towards the formation of products, or to the right. With a negative thermal effect of the forward reaction (exothermic reaction), the rate of the reverse reaction will increase more strongly, and the equilibrium will shift to the left.
Consider on your own all possible cases of shifting the equilibrium as the temperature decreases.
Figure 5 shows that the difference E"a - E"a is equal to?H of the reaction, which means the value of the equilibrium constant depends on the magnitude of the thermal effect of the reaction, i.e. depends on whether the reaction is endo- or exothermic.
The equilibrium constant of a certain reaction at 293°K is 5 10-3, and at 1000°K it is 2 10-6. What is the sign of the thermal effect of this reaction?
From the conditions of the problem it follows that as the temperature increases, the equilibrium constant decreases. Let's use expression (22) and see what the sign of the DH reaction must be for the constant to decrease.
Kravn. is represented by an exponential function, the value of which decreases with decreasing argument, in our case - the value of the expression DH/RT. In order for the value of the argument to decrease, the value of DH must be negative. Therefore, the reaction in question is exothermic.
Changes in pressure significantly affect the state of systems containing gaseous components. In this case, in accordance with gas laws, the volume of the system changes, and this leads to a change in the concentration of gaseous substances (or their partial pressures). So, with increasing pressure, the volume will decrease, and the concentration of gaseous substances will increase. An increase in concentration leads, as we already know, to a shift in equilibrium towards the consumption of the reagent, which has increased its concentration. In this case, this can be formulated somewhat differently. ?As pressure increases, the equilibrium shifts towards a smaller amount of gaseous substances or, more simply, towards a decrease in the number of molecules of gaseous substances. The concentration of solid and liquid substances does not change with pressure.
Consider the classic example of the synthesis of ammonia from nitrogen and hydrogen
3H2 + N2 - 2NH3 , (DN< 0).
Since the system consists only of gaseous substances, and with the formation of ammonia the number of molecules decreases, then with increasing pressure the equilibrium will shift to the right, towards a greater yield of ammonia. Therefore, industrial synthesis of ammonia is carried out at elevated pressure.
Propose your own temperature conditions for the synthesis of ammonia, knowing the thermal effect of the reaction and subject to the maximum yield of the product. How do these conditions relate to the kinetic factors of the process?
How will the equilibrium of the following reactions be affected by an increase in pressure?
chemical kinetics catalyst inhibitor
CaCO3(c.) - CaO(c.) + CO2(g.);
4Fe(c.) + 3O2(g.) - 2Fe2O3(c.).
In the first reaction, only carbon dioxide CO2 is gaseous, therefore, with increasing pressure, the equilibrium will shift to the left, towards a decrease in the amount of gaseous substance.
Consider the second case yourself.
How should the pressure in these reactions be changed to achieve a higher yield of products?
All cases of changes in the state of an equilibrium system under external influences can be generalized by formulating Le Chatelier’s principle:
If an external influence is exerted on a system in a state of equilibrium, then the equilibrium shifts in the direction that weakens the effect of the external influence.
Check whether Le Chatelier's principle holds in all the cases discussed above.
Give yourself examples of shifting equilibria when external conditions change and explain them based on Le Chatelier’s principle.
So, we have considered the main issues related to the patterns of chemical reactions. Knowledge of these patterns will allow you to meaningfully influence the conditions for carrying out certain processes in order to obtain an optimal result.
Questions for self-control
- 1. What reactions are called reversible?
- 2. How and why do the rates of forward and reverse reactions change over time?
- 3. What is called chemical equilibrium?
- 4. What quantity quantitatively characterizes chemical equilibrium?
- 5. What does the value of the equilibrium constant depend on: the concentration of reactants; the nature of the reacting substances; general pressure; temperature; presence of a catalyst?
- 6. What signs are characteristic of true chemical equilibrium?
- 7. What is the difference between false chemical equilibrium and true equilibrium?
- 8. Give the formulation of Le Chatelier’s principle.
- 9. Formulate consequences from Le Chatelier’s principle.
Task
Indicate how it will affect:
a) increase in pressure;
b) increase in temperature;
c) an increase in oxygen concentration to balance the system:
2CO (G) + O 2 (G) ↔ 2CO 2 (G) + Q
Solution:
a) A change in pressure shifts the equilibrium of reactions involving gaseous substances (d). Let us determine the volumes of gaseous substances before and after the reaction using stoichiometric coefficients:
According to Le Chatelier's principle, with increasing pressure , the balance shifts towards educationI substances occupying less o b b we eat, therefore the equilibrium will shift to the right, i.e. towards the formation of CO 2, towards the direct reaction (→) .
b) According to Le Chatelier's principle, as the temperature rises, the balance shifts towards the endothermic reaction (- Q ), i.e. towards the reverse reaction - the decomposition reaction of CO 2 (←) , because according to the law of conservation of energy:
Q - 2 CO (g) + O 2 (g) ↔ 2 CO 2 (g) + Q
V) With increasing oxygen concentration the equilibrium of the system shifts towards the production of CO 2 (→) becausean increase in the concentration of reactants (liquid or gaseous) shifts towards products, i.e. towards direct reaction.
Additionally:
Example 1. How many times will the rate of forward and reverse reaction in the system change:
2 SO 2 (d) +O 2 (g) = 2SO 3 (G)
if the volume of the gas mixture is reduced by three times? In which direction will the equilibrium of the system shift?
Solution. Let us denote the concentrations of reactants: [SO 2 ]= a , [ABOUT 2 ] = b , [ SO 3 ] = With. According to the law of mass velocityv direct and reverse reactions before volume changes:
v etc = Ka 2 b
v arr. = TO 1 With 2 .
After reducing the volume of a homogeneous system by three times, the concentration of each of the reactants will increase three times: [SO 2 ] = 3 A , [ABOUT 2 ] = 3 b ; [ SO 3 ] = 3 With . At new speed concentrationsv ’ forward and reverse reaction:
v ’ etc = TO (3 A ) 2 (3 b ) = 27 Ka 2 b
v ’ arr. = TO 1 (3 With ) 2 = 9 TO 1 With 2
From here:
Consequently, the rate of the forward reaction increased by 27 times, and the rate of the reverse reaction by only nine times. The balance of the system has shifted towards educationSO 3 .
Example 2. Calculate how many times the rate of a reaction occurring in the gas phase will increase when the temperature increases from 30 to 70 O C, if the temperature coefficient of the reaction is 2.
Solution. The dependence of the rate of a chemical reaction on temperature is determined by the empirical Van't Hoff rule according to the formula:
Therefore, the reaction rateν T 2 at a temperature of 70 O With more reaction speedν T 1 at a temperature of 30 O C 16 times.
Example 3. Equilibrium constant of a homogeneous system:
CO(g) + H 2 O(g) = CO 2 (g) + N 2 (G)
at 850 O C is equal to 1. Calculate the concentrations of all substances at equilibrium if the initial concentrations are: [CO] ref =3 mol/l, [H 2 ABOUT] ref = 2 mol/l.
Solution. At equilibrium, the rates of the forward and reverse reactions are equal, and the ratio of the constants of these rates is constant and is called the equilibrium constant of the given system:
v pr = TO 1 [DREAM 2 ABOUT]
v arr. = K 2 [CO 2 ][N 2 ]
In the problem statement the initial concentrations are given, while in the expressionTO R includes only the equilibrium concentrations of all substances in the system. Let us assume that by the moment of equilibrium concentration [CO 2 ] R = X mol/l. According to the equation of the system, the number of moles of hydrogen formed will also beX mol/l. The same number of moles (X mol/l) CO and H 2 O is spent for educationX moles CO 2 and N 2 . Therefore, the equilibrium concentrations of all four substances are:
[CO 2
]
R = [H 2
]
R =
X
mol/l;
[CO] R = (3 – X ) mol/l;
[H 2 ABOUT] R = (2 – X ) mol/l.
Knowing the equilibrium constant, we find the valueX , and then the initial concentrations of all substances:
Thus, the desired equilibrium concentrations are:
[CO 2 ] R = 1.2 mol/l;
[H 2 ] R = 1.2 mol/l;
[CO] R = 3 – 1.2 = 1.8 mol/l;
[H 2 ABOUT] R = 2 – 1.2 = 0.8 mol/l.
Example 4. At a certain temperature, the equilibrium concentrations in the system
2CO (g) + O 2 (g) ↔ 2CO 2 (g) were: = 0.2 mol/l, = 0.32 mol/l, = 0.16 mol/l. Determine the equilibrium constant at this temperature and the initial concentrations of CO and O 2 if the initial mixture did not contain CO 2.
Solution:
1). Since equilibrium concentrations are given in the problem statement, the equilibrium constant is equal to 2:
2). If the initial mixture did not contain CO 2, then at the moment of chemical equilibrium 0.16 mol of CO 2 was formed in the system.
According to UHR:
2CO (g) + O 2 (g) ↔ 2CO 2 (g)
The formation of 0.16 mol CO 2 required:
υ reacted (CO) = υ (CO 2) = 0.16 mol
υ reacted (O 2) = 1/2υ (CO 2) = 0.08 mol
Hence,
υ initial = υ reacted + υ equilibrium
υ initial (CO) = 0.16 +0.2 = 0.36 mol
υ initial (O 2) = 0.08 +0.32 = 0.4 mol
|
Substance |
CO2 |
||
|
From original |
0,36 |
||
|
C reacted |
0,16 |
0,08 |
0,16 |
|
C equilibrium |
0,32 |
0,16 |
Example 5.Determine the equilibrium concentration of HI in the system
H 2 (g) + I 2 (g) ↔ 2HI (g) ,
if at a certain temperature the equilibrium constant is 4, and the initial concentrations of H 2, I 2 and HI are, respectively, 1, 2 and 0 mol/l.
Solution. Let x mol/l be formed at some point in time HI
|
Substance |
H 2 |
I 2 |
|
|
from source , mol/l |
|||
|
with pro-react. , mol/l |
x/2 |
x/2 |
|
|
c equal , mol/l |
1-x/2 |
PCl 5 (g) = RS l 3 (d) + WITH l 2(G); Δ N= + 92.59 kJ. How to change: a) temperature; b) pressure; c) concentration to shift the equilibrium towards a direct reaction - decompositionPCl 5 ? Solution. A displacement or shift in chemical equilibrium is a change in the equilibrium concentrations of reacting substances as a result of a change in one of the reaction conditions. The direction in which the equilibrium has shifted is determined by Le Chatelier’s principle: a) since the decomposition reactionPCl 5 endothermic (Δ N > 0) then to shift the equilibrium towards the direct reaction it is necessary to increase the temperature; b) since in this system the decomposition of PCl 5 leads to an increase in volume (two gaseous molecules are formed from one gas molecule), then to shift the equilibrium towards a direct reaction it is necessary to reduce the pressure; c) a shift in equilibrium in the indicated direction can be achieved by increasing the concentration of RSl 5 , and a decrease in the concentration of PCl 3 or Cl 2 . |
The state in which the rates of forward and reverse reactions are equal is called chemical equilibrium. The equation for a reversible reaction in general form:
Forward reaction rate v 1 =k 1 [A] m [B] n, reverse reaction speed v 2 =k 2 [C] p [D] q, where in square brackets are equilibrium concentrations. By definition, at chemical equilibrium v 1 =v 2, where from
K c =k 1 /k 2 = [C] p [D] q / [A] m [B] n,
where Kc is the chemical equilibrium constant, expressed in terms of molar concentrations. The given mathematical expression is 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 in a state of equilibrium, the system will move to another state in such a way as to reduce the effect of the external influence.
Factors influencing 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 process.
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.
2. Effect of pressure.
In all 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, the equilibrium position is influenced by the pressure in the system.
The influence of pressure on the equilibrium position obeys the following rules:
As pressure increases, the equilibrium shifts towards the formation of substances (initial or products) with a smaller volume.
3. Effect of concentration. The influence of concentration on 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.
Questions for self-control:
1. What is the rate of a chemical reaction and what factors does it depend on? What factors does the rate constant depend on?
2. Create an equation for the reaction rate of the formation of water from hydrogen and oxygen and show how the rate changes if the concentration of hydrogen is increased threefold.
3. How does the reaction rate change over time? What reactions are called reversible? What characterizes the state of chemical equilibrium? What is called the equilibrium constant, on what factors does it depend?
4. What external influences can disrupt the chemical balance? In which direction will the equilibrium mix when the temperature changes? Pressure?
5. How can a reversible reaction be shifted in a certain direction and completed?
Lecture No. 12 (problematic)
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 non-electroyls; Raoult's and van't Hoff's laws.
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 their state of aggregation, solutions can be gaseous, liquid and solid. Typically, a component that, under given conditions, is in the same state of aggregation as the resulting solution is considered a solvent, while the remaining components of the solution are considered solutes. In the case of the same state of aggregation of the components, the solvent is considered to be the component that predominates in the solution.
Depending on the particle size, solutions are divided into true and colloidal. In true solutions (often called simply 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, and move freely in the solvent environment. True solutions are thermodynamically stable systems that are indefinitely stable in time.
The driving forces for the formation of solutions are entropy and enthalpy factors. When gases are dissolved in a liquid, 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 the destruction of the crystal lattice into free ions (ΔH > 0) and the interaction of the resulting ions with solvent molecules (soltivation, 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 lower concentrations are called unsaturated.
Solubility for various substances varies widely and depends on their nature, the interaction of solute particles with each other and with solvent molecules, as well as on external conditions (pressure, temperature, etc.)
In chemical practice, the most important solutions are those prepared on the basis of a liquid solvent. Liquid mixtures in chemistry 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 metabolism in living organisms.
Concentration of solutions
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 solute B and the mass of solvent A.
The volume fraction of the solute σ(B) is expressed in fractions of a unit or volume percent:
where Vi is the volume of the solution component, V(B) is the volume of the dissolved substance B. Volume percentages are called degrees *).
*) Sometimes the volume concentration is expressed in parts per thousand (ppm, ‰) or in parts per million (ppm), ppm.
The mole fraction of the dissolved substance 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 unity
TO dimensional concentrations include the following concentrations:
The molality of the solute C m (B) is determined by the amount of substance n(B) in 1 kg (1000 g) of solvent, the dimension is mol/kg.
Molar concentration of substance B in solution C(B) – content of the amount of dissolved substance B per unit volume of solution, mol/m3, or more often mol/liter:
where μ(B) is the molar mass of B, V is the volume of the solution.
Molar concentration of equivalents of substance B C E (B) (normality - outdated) is determined by the number of equivalents of a dissolved substance per unit volume of solution, mol/liter:
where n E (B) is the amount of substance equivalents, μ E is the molar mass of the equivalent.
Titer of solution of substance B( T B) is determined by the mass of the solute in g contained in 1 ml of 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 dissolving to one degree or another and are characterized by solubility. Some substances are unlimitedly 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 achieved between the solute and the solution. In the absence of equilibrium, a solution remains stable if the concentration of the solute is less than its solubility (unsaturated solution), or unstable if the solution contains a solute more than its solubility (supersaturated solution).
