The state of equilibrium is characteristic of reversible chemical reactions.
- A reversible reaction is a chemical reaction that, under the same conditions, can occur in the forward and reverse directions.
- A reaction that goes almost to completion in one direction is called irreversible. Conditions for the irreversibility of a reaction are the formation of a precipitate, gas or weak electrolyte. For example: BaCl 2 + H 2 SO 4 = BaSO 4 + 2HClK 2 S + 2HCl = 2KCl + H 2 SHCl + NaOH = NaCl + H 2 O.
- Chemical equilibrium- a state of the system in which the rate of the forward reaction is equal to the rate of the reverse reaction.
The concentrations of all substances in a state of equilibrium (equilibrium concentrations) are constant. Chemical equilibrium is dynamic in nature. This means that both forward and reverse reactions do not stop at equilibrium. Shifting the equilibrium in the desired direction is achieved by changing the reaction conditions.
Le Chatelier's principle— an external influence on a system that is in a state of equilibrium leads to a shift in this equilibrium in a direction in which the effect of the effect is weakened.
>> Chemistry: Reversible and irreversible reactions
CO2+ H2O = H2CO3
Let the resulting acid solution stand on a stand. After some time, we will see that the solution has turned purple again, as the acid has decomposed into its original substances.
This process can be carried out much faster if the solution is a third of carbonic acid. Consequently, the reaction to produce carbonic acid occurs both in the forward and in the reverse direction, that is, it is reversible. The reversibility of a reaction is indicated by two oppositely directed arrows:
Among the reversible reactions underlying the production of the most important chemical products, let us cite as an example the reaction of synthesis (compound) of sulfur (VI) oxide from sulfur (IV) oxide and oxygen.
1. Reversible and irreversible reactions.
2. Berthollet's rule.
Write down the equations for the combustion reactions discussed in the text of the paragraph, noting that as a result of these reactions, oxides of the elements from which the original substances are built are formed.
Give a description of the last three reactions carried out at the end of the paragraph according to plan: a) the nature and number of reagents and products; b) state of aggregation; c) direction: d) presence of a catalyst; e) release or absorption of heat
What inaccuracy was made in the writing of the equation for the reaction of limestone firing proposed in the text of the paragraph?
How true is it to say that compound reactions will generally be exothermic reactions? Justify your point of view using the facts given in the text of the textbook.
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1. Reactions are reversible and irreversible. Signs of irreversibility.
2. Chemical equilibrium. Chemical equilibrium constant.
3. Factors causing a shift in chemical equilibrium. Le Chatelier's principle. Experiment.
4. Application of Le Chatelier's Principle.
5. Solving Unified State Exam tasks.
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During the classes
I. Organizational moment.
II Updating students' knowledge(Slide 4).
1 . Determination of the rate of a chemical reaction.
2 . Formulas for expressing speed and units of speed: a) homogeneous reaction; b) heterogeneous reaction.
3 . List the factors affecting the rate of a chemical reaction.
4. How does the rate of a chemical reaction depend on concentration?
5 . What substances are called catalysts? Inhibitors? What is the difference between their effect on the rate of a chemical reaction? The importance of catalysts and inhibitors in production and in the life of living organisms.
6. What do you need to know about a chemical reaction to determine its rate?
III. Learning new material(Slide 5).
Presentation plan.
1. Reactions are reversible and irreversible. Signs of irreversibility.
2. Chemical balance. Chemical equilibrium constant.
3. Factors causing a shift in chemical equilibrium. Le Chatelier's principle. Experiment.
4. Application of Le Chatelier's Principle.
5. Solving Unified State Examination tasks.
All chemical reactions divided into reversible and irreversible.
(Slide 6).
- Irreversible chemical reactions are reactions that proceed in one direction until the reactants are completely converted into reaction products.
For example:
Na 2 SO 4 + BaCl 2 à BaSO 4 ↓ + 2NaCl
An irreversible reaction ends when at least one of the starting substances is completely consumed. Combustion reactions are irreversible; many thermal decomposition reactions complex substances; most reactions that result in the formation of precipitation or the release of gaseous substances, etc. ( Slide 7).
CuCl 2 + 2KOH= Cu(OH) 2 ↓ +2KOH – precipitate has formed
Na 2 CO 3 + 2HCl=2NaCl + H 2 O + CO 2 – a weak electrolyte is formed, which decomposes into water and carbon dioxide.
H 2 SO 4 + 2KOH = K 2 SO 4 + 2H 2 O – water has formed – a very weak electrolyte.
- Reversible chemical reactions are reactions that simultaneously occur in forward and reverse directions under the same conditions.
For example:
H 2 + I 2 ↔ 2HI (1)
CaCO 3 ↔ CaO + CO 2 (2)
Let's consider the reaction equation for the synthesis of hydrogen iodide from hydrogen and iodine (Eq. 1).
Some time after the start of a chemical reaction, not only the final reaction products can be detected in the gas mixture HI , but also the starting materials – H 2 and I 2 . No matter how long the chemical reaction lasts, the reaction mixture at 350°C will always contain approximately 80% HI, 10% H 2 and 10% I 2. If you take HI as the starting substance and heat it to the same temperature, you will find that after some time the ratio between the amounts of all three substances will be the same. Thus, during the formation of hydrogen iodide from hydrogen and iodine, direct and reverse reactions occur simultaneously.
If hydrogen and iodine in concentrations and are taken as starting substances, then the rate of the direct reaction at the initial moment of time was equal to:
V pr =k pr . Feedback speed
V arr =k arr² at the initial moment of time is equal to zero, since there is no hydrogen iodide in the reaction mixture. Gradually, the rate of the direct reaction decreases, because hydrogen and iodine react and their concentrations decrease.In this case, the rate of the reverse reaction increases because the concentration of hydrogen iodide formed gradually increases. When the rates of the forward and reverse reactions become equal, chemical equilibrium occurs. In a state of equilibrium, the same number of HI molecules are formed over a certain period of time, how many of them split into and .
The state of a reversible process, in which the rates of forward and reverse reactions are equal, is called chemical equilibrium.(Slide 8, 9).
dynamic equal - siem. In an equilibrium state, both forward and reverse reactions continue to occur, but since their rates are equal, the concentrations of all substances in the reaction system do not change. These concentrations are called equilibrium concentrations.
The state of chemical equilibrium is characterized by a special value -equilibrium constant. For our example, the equilibrium constant has the form:
Kravn =²/
- The equilibrium constant k is equal to the ratio of the rate constants of the forward and reverse reactions, or the ratiothe product of the equilibrium concentrations of products and reactants raised to powers equal to the coefficients in the reaction equation.The value of the equilibrium constant is determined by the nature of the reacting substances and depends on temperature. (Slide 10).
The value of the equilibrium constant characterizes the completeness of the reversible reaction. If Kravn1, there are practically no initial reactants left in the equilibrium system, the equilibrium is shifted to the right. (Slide 11).
Chemical equilibrium is mobile and can be stored for a long time under constant external conditions:temperature, concentration of starting substances or final products, pressure(if gases are involved in the reaction).
If you change these conditions, you can transfer the system from one equilibrium state to another that meets the new conditions.
This transition is called displacement or shift of equilibrium. (Slide 12).
The displacement control can be predicted using the principle Le Chatelier, 1884
Historical reference.
Henri Louis Le Chatelier (1850-1936), a French chemist, studied the processes of chemical reactions.
The principle of shifting equilibria is the most famous, but far from the only one scientific achievement Le Chatelier.
His Scientific research made him widely known throughout the world. He lived to be 86 years old.(Slide13).
- Henri Louis De Chatelier is known throughout the world. He was not a king or a prince, but he discovered a wonderful principle that is useful to chemists For shifts of all equilibria.
- If an external influence is exerted on a system in a state of chemical equilibrium (changing pressure, concentration of substances or temperature), then the equilibrium will shift towards the preferential occurrence of the process that weakens the effect produced.
Le Chatelier's principle is the principle of “harmfulness”, the principle of “vice versa”. (Slide 14).
The most important external factors that can lead to a shift in chemical equilibrium are: a) the concentration of reacting substances;
b) temperature;
c) pressure.
Effect of concentration of reactants.
If any of the substances participating in the reaction is introduced into the equilibrium system, then the equilibrium shifts towards the reaction during which this substance is consumed. If any substance is removed from an equilibrium system, then the equilibrium shifts towards the reaction during which this substance is formed.
For example , let us consider which substances should be introduced and which should be removed from the equilibrium system to shift the reversible reaction of ammonia synthesis to the right:
N 2(g) + H 2(g) ↔ 2 NH 3(g)
To shift the equilibrium to the right (towards the direct reaction of ammonia formation), it is necessary to introduce nitrogen and hydrogen into the equilibrium mixture (i.e., increase their concentrations) and remove ammonia from the equilibrium mixture (i.e., reduce its concentration).
Conclusions: (Slide 15).
A) if we increase the concentration of the final products, the equilibrium shifts towards the formation of the initial products, i.e. the reverse reaction prevails.
B) we increase the concentration of the starting products, the equilibrium shifts towards the formation of the final products, the direct reaction predominates.
C) with a decrease in the concentration of the final products, the equilibrium reaction shifts towards their formation, the direct reaction predominates.
D) when the concentration of the initial reaction products decreases, the reverse reaction predominates.
(Experiment (video experiment) “The influence of the concentration of reactants on the displacement of chemical equilibrium”) ( Slide 16)).
Effect of temperature.
Direct and reverse reactions have opposite thermal effects: if the forward reaction is exothermic, then the reverse reaction is endothermic (and vice versa).
When the system is heated (i.e., its temperature increases), the equilibrium shifts towards the endothermic reaction; upon cooling (lower temperature), the equilibrium shifts towards the exothermic reaction.
For example , the ammonia synthesis reaction is exothermic:
N 2(g) + H 2(g) → 2 NH 3(g) + 92 kJ,
and the decomposition reaction of ammonia is(reverse reaction) is endothermic:
2 NH 3 (g) → N 2 (g) + H 2 (g) - 92 kJ. Therefore, an increase in temperature shifts the equilibrium towards the reverse reaction of ammonia decomposition.
Conclusions: (Slide 17).
A) as the temperature increases, the chemical equilibrium shifts towards an endothermic reaction.
B) as the temperature decreases, the chemical equilibrium shifts towards the exothermic reaction.
(Experiment (video experiment) “The influence of temperature on the displacement of chemical equilibrium”) ( Slide 19)).
Effect of pressure.
Pressure affects the equilibrium of reactions in which gaseous substances take part. If the external pressure increases, then the equilibrium shifts towards the reaction during which the number of gas molecules decreases. Conversely, the equilibrium shifts towards education more gaseous molecules when the external pressure decreases. If the reaction proceeds without changing the number gaseous substances, then pressure does not affect the equilibrium in this system.
For example: for increasing ammonia yield(shift right) it is necessary to increase the pressurein a reversible reaction system
N 2 (g) + H 2 (g) ↔ 2 NH 3 (g), because when a direct reaction occurs number gaseous molecules
decreases (from four molecules of nitrogen and hydrogen gases two molecules of ammonia gas are formed). Conclusions: (Slide 17).
- A) with an increase in pressure, the equilibrium shifts towards the reaction in which the volume of gaseous products formed decreases.
- B) as the pressure decreases, the equilibrium shifts towards the reaction in which the volume of gaseous products formed increases.
Example: 3H 2 + N 2 ↔ 2NH 3
- c) if the volumes of gaseous products are the same in both the forward and reverse reactions, a change in pressure does not shift the equilibrium.
Example: H 2 + Cl 2 = 2HCl
2V=2V
(Experiment (video experiment) “The influence of pressure on the displacement of chemical equilibrium”) ( Slide 18)).
Le Chatelier's principle is applicable not only to chemical reactions, but also to many other processes: evaporation, condensation, melting, crystallization, etc. In the production of the most important chemical products, Le Chatelier's principle and calculations arising from the law of mass action make it possible to find such conditions to carry out a chemical process that provides maximum yield of the desired substance.(Slide 20,21).
IV. Consolidation (Slide 22).
- The chemist pushes the reaction in the back: “Let me move you a little!” She replies: “You know me: I can’t live without fire for an hour or a day! And to improve my mood, I ask, even demand: higher blood pressure! Besides, keep in mind: I am such a reaction that the concentration of reagents is important to me.” And the chemist thought: “Now everything is clear to me. You absorb heat - and it’s wonderful! As soon as the burners are lit under the flask, go ahead, reaction, right along the arrow. These are flowers, but there will also be fruits - The yield of the product will increase the pressure! More concentration...Yes, you’re right: I’ll give you more substances.” The reaction began to work obediently, forming a useful and necessary product. This is the dream the chemist had. What conclusions will he draw?
V. Generalization and conclusions.
Thus, in this lesson we studied chemical equilibrium in more depth - which can arise in reversible chemical reactions, and also gained an understanding of the factors that cause a shift in chemical equilibrium towards a direct or reverse reaction, and we were experimentally convinced of this.
VI . Solving Unified State Exam tasks (Part A).(Slide 23,24).
1. Condition for the irreversibility of a chemical transformation.
A) formation of a weak electrolyte
B) absorption large quantity warmth
B) interaction of weak and strong electrolytes
D) weakening of the color of the solution.
2. To shift the equilibrium in the system
CaCO 3(t) ↔ CaO (t) + CO 2(t) – Q
In the direction of reaction products it is necessary
A) increase the pressure b) increase the temperature
C) introduce a catalyst d) reduce the temperature
3. As pressure increases, the chemical equilibrium does not shift in the system
A) 2H 2 S (g) + 3O 2 (g) = 2H 2 O (g) + 2SO 2 (g)
B) 2H 2 (g) + O 2 (g) = 2H 2 O (g)
B) H 2 (g) + I 2 (g) = 2HI (g)
D) SO 2(g) + CL 2(g) = SO 2 CL 2(g)
4. Are they true? the following judgments about the shift in chemical equilibrium in the system
2CO (g) + O 2 (g) ↔ 2CO 2 (g) + Q ?
A. When the pressure decreases, the chemical equilibrium in this system will shift towards the reaction product.
B. With increasing concentration carbon dioxide the chemical equilibrium of the system will shift towards the reaction product.
a) only A is true c) both judgments are true
b) only B is true d) both judgments are incorrect
5 . In system
2SO 2 (g) + O 2 (g) ↔ 2SO 3 (g) + Q
A shift in chemical equilibrium towards the starting substances will be facilitated by
a) decrease in pressure
b) decrease in temperature
c) increase in SO concentration 2
d) decrease in SO concentration 3
6. Chemical equilibrium in the system
C 4 H 10 (g) ↔ C 4 H 6 (g) + 2H 2 (g) -Q
side of the reverse reaction, if
A) increase the temperature
B) reduce the concentration of H 2
B) add a catalyst
D) increase blood pressure
Now check the correctness of your answers. (Slide 25).
1 – a
2 – b
3 – in
4 – a
5 – a
6 – g
VII. § 14, ex. 1-8. (Slide 26).
Reversibility of chemical reactions.Chemical balance.
Grade 11
(profile level)
Chemistry teacher, MBOU Secondary School, Kadgaron village Khetagurova F.A.
2012-2013 academic year year.
Used Books.
1. O.S. Gabrielyan, G.G. Lysova “Chemistry” - M.: “Drofa”, 2009.
2. O. S. Gabrielyan, I. G. Ostroumov " general chemistry" - Olma textbook, 2008.
3. O.S. Gabrielyan, G.G. Lysova, A.G. Vvedenskaya " Desk book Chemistry teacher", Part I, 11th grade. - M.: “Drofa”, 2009.
4.T.P. Troegubov “Lesson-based developments in chemistry” - M.: “Vako”, 2009.
5. A.S. Egorov “Chemistry Tutor” - “Phoenix”, 2008.
6. S. A. Litvinova, N. V. Mankevich " Inorganic chemistry. All school course in tables" - Minsk: " Modern school: Kuzma", 2009.
7. A.N. Levkin, A.A. Kartsova, S.E. Dombrovskaya, E.D. Krutetskaya “Chemistry: Unified State Exam: Educational and reference materials. (Series “Final control: Unified State Exam”) - M.; St. Petersburg: Education, 2011.
8. G.P. Khomchenko “A manual on chemistry for those entering universities” - M.: “New Wave”., 2004.
9.V.N.Doronkin, A.G.Berezhnaya, T.V.Sazhneva, V.A.Fevraleva “Chemistry. Thematic tests. Preparation for the Unified State Exam” - Rostov-on-Don “Legion”, 2010.
10. D. M. Dobrotin, A. A. Kaverina, M. G. Snastina “Unified State Examination-2011. Chemistry: typical exam options: 30 options." - FIPI, M.; " National education" 2011.
Reversibility of chemical reactions. Chemical balance.
Grade 11
Basic concepts: Reversible and irreversible chemical reactions, chemical equilibrium, equilibrium concentrations, equilibrium constant, reaction rate, Le Chatelier's principle. Equipment: F eCl 3 solution; KNCS; KCl; starch paste; test tubes, water, alcohol lamp, holder.
During the classes. Frontal survey 1. Determination of the rate of a chemical reaction. 2. Formulas for expressing speed and units of speed: a) homogeneous reaction; b) heterogeneous reaction. 3. List the factors that affect the rate of a chemical reaction. 4. How does the rate of a chemical reaction depend on concentration? 5. What substances are called catalysts? Inhibitors? What is the difference between their effect on the rate of a chemical reaction? The importance of catalysts and inhibitors in production and in the life of living organisms. 6. What do you need to know about a chemical reaction to determine its rate?
Learning new material. Presentation plan. 1. Reactions are reversible and irreversible. Signs of irreversibility 2. Chemical equilibrium. Chemical equilibrium constant. 3. Factors causing a shift in chemical equilibrium. Le Chatelier's principle. Experiment. 4. Application of Le Chatelier's Principle. 5. Solving Unified State Exam tasks.
Reversible and irreversible reactions. Reversible chemical reactions are reactions that simultaneously occur in forward and reverse directions under the same conditions. For example: H 2 + I 2 ↔ 2HI CaCO 3 ↔ CaO + CO 2 Irreversible chemical reactions are reactions that proceed in one direction until the reactants are completely converted into reaction products. For example: Na 2 SO 4 + BaCl 2 BaSO 4 ↓ + 2NaCl
Signs of irreversibility. CuCl 2 + 2KOH= Cu(OH) 2 ↓ +2KOH – a precipitate formed Na 2 CO 3 + 2HCl=2NaCl + H 2 O + CO 2 – a weak electrolyte was formed, which decomposes into water and carbon dioxide. H 2 SO 4 + 2KOH = K 2 SO 4 + 2H 2 O - water was formed - a very weak electrolyte.
Chemical balance. Let's return to the reversible reaction of hydrogen with iodine vapor. In accordance with the law of mass action kinetic equation direct reaction has the form: V pr = k pr Over time, the rate of the direct reaction decreases, because the starting materials are consumed. At the same time, with the accumulation of hydrogen iodide in the system, the rate of its decomposition reaction increases: V arr = k arr [HI] ² In any reversible reaction, sooner or later a moment will come when the rates of the direct and reverse processes become equal. The state of a reversible process, in which the rates of forward and reverse reactions are equal, is called chemical equilibrium.
Chemical equilibrium constant. The state of chemical equilibrium is characterized by a special value - the equilibrium constant. For our example, the equilibrium constant has the form: K equal = ² / The equilibrium constant k is equal to the ratio of the rate constants of the forward and reverse reactions, or the ratio of the product of the equilibrium concentrations of products and reagents raised to powers equal to the coefficients in the reaction equation. The value of the equilibrium constant is determined by the nature of the reacting substances and depends on temperature.
The value of the equilibrium constant characterizes the completeness of the reversible reaction. If K is equal to 1, there are practically no initial reactants left in the equilibrium system, and the equilibrium is shifted to the right.
Factors causing a shift in chemical equilibrium. The state of chemical equilibrium can be maintained for a long time under constant external conditions: temperature, concentration of starting substances or final products, pressure (if gases are involved in the reaction). If you change these conditions, you can transfer the system from one equilibrium state to another that meets the new conditions. This transition is called a displacement or equilibrium shift. The displacement control can be predicted using Le Chatelier's principle, 1884.
Historical reference. Henri Louis Le Chatelier (1850-1936), a French chemist, studied the processes of chemical reactions. The principle of displacement of equilibrium is the most famous, but far from the only scientific achievement of Le Chatelier. His scientific research has made him widely known throughout the world. He lived to be 86 years old.
Le Chatelier's principle. Henri Louis De Chatelier is known throughout the world. He was not a king or a prince, But he discovered a wonderful principle, Which is useful to chemists For shifting all kinds of equilibria. If an external influence is exerted on a system in a state of chemical equilibrium (changing pressure, concentration of substances or temperature), then the equilibrium will shift towards the preferential occurrence of the process that weakens the effect produced. Le Chatelier's principle is the principle of “harmfulness”, the principle of “vice versa”.
Change in concentration: A) if we increase the concentration of the final products, the equilibrium shifts towards the formation of the initial products, i.e. the reverse reaction prevails. B) we increase the concentration of the starting products, the equilibrium shifts towards the formation of the final products, the direct reaction predominates. C) with a decrease in the concentration of the final products, the equilibrium reaction shifts towards their formation, the direct reaction predominates. D) when the concentration of the initial reaction products decreases, the reverse reaction predominates.
Effect of pressure changes. A) with an increase in pressure, the equilibrium shifts towards the reaction in which the volume of gaseous products formed decreases. B) as the pressure decreases, the equilibrium shifts towards the reaction in which the volume of gaseous products formed increases. Example: 3H 2 + N 2 ↔ 2NH 3 c) if the volumes of gaseous products are the same in both direct and reverse reactions - change pressure does not shift the equilibrium. Example: H 2 + Cl 2 =2HCl 2V=2V
Effect of temperature change. A) as the temperature increases, the chemical equilibrium shifts towards an endothermic reaction. B) as the temperature decreases, the chemical equilibrium shifts towards the exothermic reaction. Example: N 2 (g) + H 2 (g) → 2 NH 3 (g) +92 kJ, 2 NH 3 (g) → N 2 (g) + H 2 (g) - 92 kJ.
The meaning of Le Chatelier's principle.
Ammonia and methanol production.
Consolidation. The chemist pushes the reaction in the back: “Let me move you a little!” “She replies: “You know me: I can’t live without fire for an hour or a day!” And to improve my mood, I ask, even demand: higher blood pressure! Besides, keep in mind: I am such a reaction that the concentration of reagents is important to me.” And the chemist thought: “Now everything is clear to me. You absorb heat - and it’s wonderful! As soon as the burners are lit under the flask, go ahead, reaction, right along the arrow. These are flowers, but there will also be fruits - The yield of the product will increase the pressure! More concentration... Yes, you’re right: I’ll give you more substances.” The reaction began to work obediently, forming a useful and necessary product. This is the dream the chemist had. What conclusions will he draw?
Unified State Examination tasks. 1. Condition for the irreversibility of a chemical transformation. a) formation of a weak electrolyte b) absorption of a large amount of heat c) interaction of weak and strong electrolytes d) weakening of the color of the solution. 2. To shift the equilibrium in the system CaCO 3(s) ↔ CaO (s) + CO 2(s) – Q towards the reaction products, it is necessary to a) increase the pressure b) increase the temperature c) introduce a catalyst d) reduce the temperature 3. With an increase pressure, the chemical equilibrium does not shift in the system a) 2H 2 S (g) + 3O 2 (g) = 2H 2 O (g) + 2SO 2 (g) b) 2H 2 (g) + O 2 (g) = 2H 2 O (g) c) H 2 (g) + I 2 (g) = 2HI (g) g) SO 2 (g) + CL 2 (g) = SO 2 CL 2 (g)
4. Are the following judgments about the shift in chemical equilibrium in the system 2CO (g) + O 2 (g) ↔ 2CO 2 (g) + Q correct? A. When the pressure decreases, the chemical equilibrium in this system will shift towards the reaction product. B. As the concentration of carbon dioxide increases, the chemical equilibrium of the system will shift towards the reaction product. a) only A is true c) both judgments are correct b) only B is true d) both judgments are incorrect 5. In the system 2 SO 2 (g) + O 2 (g) ↔ 2SO 3 (g) + Q the chemical equilibrium shifts towards the original substances will contribute to a) a decrease in pressure c) an increase in the concentration of SO 2 b) a decrease in temperature d) a decrease in the concentration of SO 3 6. Chemical equilibrium in the system C 4 H 10 (g) ↔ C 4 H 6 (g) + 2H 2 ( d) -Q side of the reverse reaction, if a) increase the temperature c) add a catalyst b) reduce the concentration of H 2 d) increase the pressure
Check yourself! 1 – a 2 – b 3 – c 4 – a 5 – a 6 – d
Homework. § 14, ex. 1-8.
Chemical reactions proceeding in one direction are called irreversible.
Most chemical processes are reversible. This means that under the same conditions both forward and reverse reactions occur (especially if we're talking about about closed systems). 
For example:
a) reaction
V open system irreversible;
b) the same reaction
in a closed system reversible.
Chemical equilibrium
Let us consider in more detail the processes occurring during reversible reactions, for example, for a conditional reaction:
Based on the law of mass action rate of forward reaction:
Since the concentrations of substances A and B decrease over time, the rate of the direct reaction also decreases.
The appearance of reaction products means the possibility of a reverse reaction, and over time the concentrations of substances C and D increase, which means that the reverse reaction speed.
Sooner or later a state will be reached in which the rates of forward and reverse reactions become equal = .
The state of the system in which the rate of the forward reaction is equal to the rate of the reverse reaction is called chemical equilibrium.
In this case, the concentrations of reactants and reaction products remain unchanged. They are called equilibrium concentrations. At the macro level, it seems that overall nothing is changing. But in fact, both the direct and reverse processes continue to occur, but with equal speed. Therefore, such equilibrium in the system is called mobile and dynamic.
Let us denote the equilibrium concentrations of substances [A], [B], [C], [D]. Then since = , k 1 [A] α [B] β = k 2 [C] γ [D] δ , where
where α, β, γ, δ are exponents, equal to the coefficients in the reversible reaction; K equal - chemical equilibrium constant.
The resulting expression quantitatively describes state of equilibrium and represents mathematical expression law of mass action for equilibrium systems.
At a constant temperature, the equilibrium constant is constant value for a given reversible reaction. It shows the relationship between the concentrations of reaction products (numerator) and starting substances (denominator), which is established at equilibrium.
Equilibrium constants are calculated from experimental data, determining the equilibrium concentrations of starting substances and reaction products at a certain temperature.
The value of the equilibrium constant characterizes the yield of reaction products and the completeness of its progress. If we get K » 1, this means that at equilibrium [C] γ [D] δ "[A] α [B] β , i.e., the concentrations of reaction products prevail over the concentrations of the starting substances, and the yield of reaction products is high.
At K equal to « 1, the yield of reaction products is correspondingly low. For example, for the hydrolysis reaction of acetic acid ethyl ester
equilibrium constant:
at 20 °C it has a value of 0.28 (that is, less than 1).
This means that a significant portion of the ester was not hydrolyzed.
In the case of heterogeneous reactions, the expression of the equilibrium constant includes the concentrations of only those substances that are in the gas or liquid phase. For example, for the reaction
The equilibrium constant is expressed as follows:
The value of the equilibrium constant depends on the nature of the reactants and temperature.
The constant does not depend on the presence of a catalyst, since it changes the activation energy of both the forward and reverse reactions by the same amount. The catalyst can only accelerate the onset of equilibrium without affecting the value of the equilibrium constant.
The state of equilibrium is maintained indefinitely under constant external conditions: temperature, concentration of starting substances, pressure (if gases participate in the reaction or are formed).
By changing these conditions, it is possible to transfer the system from one equilibrium state to another that meets the new conditions. This transition is called displacement or shift in equilibrium.
Let's consider different ways to shift the equilibrium using the example of the reaction between nitrogen and hydrogen to form ammonia:
Effect of changing the concentration of substances
When nitrogen N2 and hydrogen H2 are added to the reaction mixture, the concentration of these gases increases, which means the rate of forward reaction increases. The equilibrium shifts to the right, towards the reaction product, that is, towards ammonia NH 3.
N 2 +3H 2 → 2NH 3
The same conclusion can be drawn by analyzing the expression for the equilibrium constant. As the concentration of nitrogen and hydrogen increases, the denominator increases, and since K is equal. - the value is constant, the numerator must increase. Thus, the amount of the reaction product NH 3 in the reaction mixture will increase.
An increase in the concentration of the ammonia reaction product NH 3 will lead to a shift of equilibrium to the left, towards the formation of the starting substances. This conclusion can be drawn based on similar reasoning.
Effect of Pressure Change
A change in pressure affects only those systems where at least one of the substances is in a gaseous state. As pressure increases, the volume of gases decreases, which means their concentration increases.
Let's assume that the pressure in a closed system is increased, for example, by 2 times. This means that the concentrations of all gaseous substances (N 2, H 2, NH 3) in the reaction under consideration will increase by 2 times. In this case, the numerator in the expression for K equal will increase by 4 times, and the denominator by 16 times, i.e., the equilibrium will be disrupted. To restore it, the concentration of ammonia must increase and the concentrations of nitrogen and hydrogen must decrease. The balance will shift to the right. Changes in pressure have virtually no effect on the volume of liquid and solids, i.e. does not change their concentration. Hence, the state of chemical equilibrium of reactions that do not involve gases does not depend on pressure.
Effect of temperature change
As the temperature increases, the rates of all reactions (exo- and endothermic) increase. Moreover, an increase in temperature has a greater effect on the rate of those reactions that have a higher activation energy, which means endothermic.
Thus, the rate of the reverse reaction (endothermic) increases more than the rate of the forward reaction. The equilibrium will shift towards the process accompanied by the absorption of energy.
The direction of the equilibrium shift can be predicted using Le Chatelier's principle:
If an external influence is exerted on a system that is in equilibrium (concentration, pressure, temperature changes), then the equilibrium shifts to the side that weakens this influence.
Thus:
As the concentration of reactants increases, the chemical equilibrium of the system shifts towards the formation of reaction products;
As the concentration of reaction products increases, the chemical equilibrium of the system shifts towards the formation of the starting substances;
As pressure increases, the chemical equilibrium of the system shifts towards the reaction in which the volume of gaseous substances formed is smaller;
As the temperature increases, the chemical equilibrium of the system shifts towards the endothermic reaction;
As the temperature decreases, it moves towards an exothermic process.
Le Chatelier's principle is applicable not only to chemical reactions, but also to many other processes: evaporation, condensation, melting, crystallization, etc. In the production of the most important chemical products, Le Chatelier's principle and calculations arising from the law of mass action make it possible to find such conditions to carry out chemical processes that provide maximum yield of the desired substance.
Reference material for taking the test:
Mendeleev table
Solubility table
All chemical reactions can be divided into two groups: irreversible and reversible reactions. Irreversible reactions proceed to completion - until one of the reactants is completely consumed. Reversible reactions do not proceed to completion: in a reversible reaction, none of the reactants are completely consumed. This difference is due to the fact that an irreversible reaction can only proceed in one direction. A reversible reaction can occur in both forward and reverse directions.
Let's look at two examples.
Example 1. The interaction between zinc and concentrated nitric acid proceeds according to the equation:
With a sufficient amount of nitric acid, the reaction will only end when all the zinc has dissolved. In addition, if you try to carry out this reaction in the opposite direction - passing nitrogen dioxide through a solution of zinc nitrate, then metallic zinc and nitric acid will not work - this reaction cannot proceed in the opposite direction. Thus, the interaction of zinc with nitric acid is an irreversible reaction.
Example 2. Ammonia synthesis proceeds according to the equation:
If you mix one mole of nitrogen with three moles of hydrogen, create conditions in the system that are favorable for the reaction to occur, and after a sufficient time, analyze the gas mixture, the results of the analysis will show that not only the reaction product (ammonia) will be present in the system, but also the initial substances (nitrogen and hydrogen). If now, under the same conditions, not a nitrogen-hydrogen mixture, but ammonia is placed as the starting substance, then it will be possible to find that part of the ammonia will decompose into nitrogen and hydrogen, and the final ratio between the quantities of all three substances will be the same as in that case , when starting from a mixture of nitrogen and hydrogen. Thus, ammonia synthesis is a reversible reaction.
In equations of reversible reactions, arrows can be used instead of the equal sign; they symbolize the reaction occurring in both forward and reverse directions.
In Fig. Figure 68 shows the change in the rates of forward and reverse reactions over time. At first, when mixing the starting substances, the rate of the forward reaction is high, and the rate of the reverse reaction is zero. As the reaction proceeds, the starting substances are consumed and their concentrations fall.
Rice. 63. Change in the speed of forward and reverse reactions over time.
As a result, the rate of the forward reaction decreases. At the same time, reaction products appear and their concentration increases. As a result, a reverse reaction begins to occur, and its speed gradually increases. When the rates of forward and reverse reactions become equal, chemical equilibrium occurs. Thus, in the last example, an equilibrium is established between nitrogen, hydrogen and ammonia.
Chemical equilibrium is called dynamic equilibrium. This emphasizes that at equilibrium both forward and reverse reactions occur, but their rates are the same, as a result of which changes in the system are not noticeable.
A quantitative characteristic of chemical equilibrium is a value called the chemical equilibrium constant. Let's consider it using the example of the iodide-hydrogen synthesis reaction:
According to the law of mass action, the rates of forward and reverse reactions are expressed by the equations:
At equilibrium, the rates of forward and reverse reactions are equal to each other, hence
The ratio of the rate constants of the forward and reverse reactions is also a constant. It is called the equilibrium constant of this reaction (K):
From here finally
On the left side of this equation are those concentrations of interacting substances that are established at equilibrium - equilibrium concentrations. The right side of the equation is a constant (at constant temperature) quantity.
It can be shown that in general case reversible reaction
the equilibrium constant will be expressed by the equation:
Here capital letters denote the formulas of substances, and small ones denote coefficients in the reaction equation.
Thus, at constant temperature, the equilibrium constant of a reversible reaction is constant value, showing the ratio between the concentrations of reaction products (numerator) and starting substances (denominator), which is established at equilibrium.
The equilibrium constant equation shows that under equilibrium conditions, the concentrations of all substances participating in the reaction are related to each other. A change in the concentration of any of these substances entails changes in the concentrations of all other substances; as a result, new concentrations are established, but the ratio between them again corresponds to the equilibrium constant.
The numerical value of the equilibrium constant, to a first approximation, characterizes the yield of a given reaction. For example, when the reaction yield is high, because in this case
i.e., at equilibrium, the concentrations of the reaction products are much greater than the concentrations of the starting substances, and this means that the yield of the reaction is high. When (for a similar reason) the yield of the reaction is low.
In the case of heterogeneous reactions, the expression of the equilibrium constant, as well as the expression of the law of mass action (see § 58), includes the concentrations of only those substances that are in the gas or liquid phase. For example, for the reaction
the equilibrium constant has the form:
The value of the equilibrium constant depends on the nature of the reacting substances and on the temperature. It does not depend on the presence of catalysts. As already mentioned, the equilibrium constant is equal to the ratio of the rate constants of the forward and reverse reactions. Since the catalyst changes the activation energy of both forward and reverse reactions by the same amount (see § 60), it does not affect the ratio of their rate constants.
Therefore, the catalyst does not affect the value of the equilibrium constant and, therefore, can neither increase nor decrease the yield of the reaction. It can only speed up or slow down the onset of equilibrium.
