Chemical kinetics is the rate of chemical reactions. Subject

Irreversible reactions

1. How will the rate of reaction 2A + B ® A 2 B change if the concentration of substance A is increased by 2 times, and the concentration of substance B is decreased by 2 times?

2. How many times should the concentration of substance B 2 in the system 2A 2 (g) + B 2 (g) ® 2A 2 B (g) be increased so that when the concentration of substance A decreases by 4 times, the rate of the direct reaction does not change?

3. In the system CO + C1 2 ® COC1 2, the concentration of CO was increased from 0.03 to 0.12 mol/l, and the concentration of C1 2 - from 0.02 to 0.06 mol/l. How many times did the rate of the forward reaction increase?

4. How will the rate of the direct reaction N 2 (g) + 3H (g) ® 2 NH 3 change if a) the pressure in the system is increased by 3 times; b) reduce the volume by 2 times; c) increase the concentration of N 2 by 4 times?

5. How many times should the pressure be increased so that the rate of formation of NO 2 by the reaction 2NO + O 2 ® 2 NO 2 increases 1000 times?

6. The reaction between carbon monoxide (II) and chlorine proceeds according to the equation CO + C1 2 ® COC1 2. How will the reaction rate change when a) the CO concentration increases by 2 times; b) concentration of C1 2 2 times; c) the concentrations of both substances are 2 times?

7. The reaction takes place in the gas phase. The reaction involves two substances A and B. It is known that when the concentration of component A is doubled, the rate increases 2 times, and when the concentration of component B doubles, the rate increases 4 times. Write an equation for the reaction that occurs. How will the reaction rate change when the total pressure increases by 3 times?

8. The reaction rate of the interaction of substances A, B and D is studied. At constant concentrations of B and D, an increase in the concentration of substance A by 4 times leads to an increase in the rate by 16 times. If the concentration of substance B increases by 2 times at constant concentrations of substances A and D, then the speed increases only 2 times. At constant concentrations of A and B, doubling the concentration of substance D leads to a 4-fold increase in speed. Write an equation for the reaction.

9. Determine the rate of the chemical reaction A(g) + B(g) ® AB(g), if the reaction rate constant is 2 × 10 -1 l × mol -1 × s, and the concentrations of substances A and B are respectively 0.025 and 0 .01 mol/l. Calculate the reaction rate when the pressure is increased by 3 times.

10. Find the value of the rate constant for the reaction A + 2B ® AB 2, if at the concentrations of substances A and B, respectively equal to 0.1 and 0.05 mol/l, the reaction rate is 7 × 10 -5 mol/(l×s) .

11. In a vessel with a volume of 2 liters, gas A with an amount of substance of 4.5 mol and gas B with an amount of substance of 3 mol were mixed. Gases react in accordance with the equation A + B = C. After 20 seconds, gas C was formed in the system with an amount of substance of 2 mol. Determine the average reaction rate. What quantities of substances A and B did not react?

12. The reaction between substances A and B is expressed by the equation A + B ® C. The initial concentrations are [A] O = 0.03 mol/l, [B] O = 0.05 mol/l. The reaction rate constant is 0.4. Find the initial reaction rate and the reaction rate after some time, when the concentration of the resulting substance C becomes equal to 0.01 mol/l.

13. The reaction between gaseous substances A and B is expressed by the equation A + B ® C. The initial concentrations of the substances are [A] 0 = 0.03 mol/l, [B] 0 = 0.03 mol/l. The reaction rate constant is 0.1. After some time, the concentration of substance A decreased by 0.015 mol/l. How many times must the total pressure be increased so that the rate of a chemical reaction becomes equal to the original rate?

14. How many degrees must the temperature be increased for the reaction rate to increase 27 times? The temperature coefficient of the reaction rate is 3.

15. At 20 o C the reaction occurs in 2 minutes. How long will this reaction take to occur a) at 50 o C, b) at 0 o C? The temperature coefficient of the reaction rate is 2.

16. At a temperature of 30 o C the reaction takes place in 25 minutes, and at 50 o C in 4 minutes. Calculate the temperature coefficient of the reaction rate.

17. The reaction rate at 0 o C is 1 mol/l×s. Calculate the rate of this reaction at 30 o C if the temperature coefficient of rate is 3.

18. With an increase in temperature by 50 o C, the reaction rate increased 32 times. Calculate the temperature coefficient of the rate of a chemical reaction.

19. Two reactions occur at 25 o C at the same rate. The temperature coefficient of the rate of the first reaction is 2.0, and the second is 2.5. Find the ratio of the rates of these reactions at 95 o C.

20. What is the activation energy of the reaction if, with an increase in temperature from 290 to 300 K, the reaction rate increases by 2 times?

21. How many times will the rate of a reaction occurring at 298 K increase if, as a result of using a catalyst, it was possible to reduce the activation energy by 4 kJ/mol?

22. What is the value of the activation energy of the reaction, the rate of which at 300 K is 10 times greater than at 280 K.

23. The activation energy of the reaction O 3 (g) +NO(g) ® O 2 (g) +NO 2 (g) is 40 kJ/mol. How many times will the reaction rate change when the temperature increases from 27 to 37 o C?

24. One catalyst reduces the activation energy at 300 K by 20 kJ/mol, and the other by 40 kJ/mol. Which catalyst is more effective? Justify the answer by calculating the ratio of reaction rates when using a particular catalyst.

25. At 150 o C, some reaction ends in 16 minutes. Taking the temperature coefficient of the reaction rate equal to 2.5, calculate the time after which this reaction will end if it is carried out a) at 200 o C, b) at 80 o C.

26. When the temperature increases by 10 o C, the rate of a chemical reaction doubles. At 20 o C it is equal to 0.04 mol/(l×s). What will be the rate of this reaction at a) 40 o C, b) 0 o C?

27. At 20 o C, the rate of the chemical reaction is 0.04 mol/(l×s). Calculate the rate of this reaction at 70 o C, if it is known that the activation energy is 70 kJ/mol.

28. Calculate the temperature coefficient of the reaction g, if the rate constant of this reaction at 120 o C is equal to 5.88 × 10 -4, and at 170 o C - 6.7 × 10 -2.

29. How many times will the rate of a chemical reaction change when the temperature increases from 300 K to 400 K, if the temperature coefficient g = 2? What is the activation energy for this reaction?

30. How many times will the rate of the chemical reaction A + 2B ® C increase when the pressure in the system increases by 4 times and the temperature simultaneously increases by 40 o C. The reactants are gases. The temperature coefficient of the reaction is 2.

31. How many times will the rate of the chemical reaction 2A(g) + B(g) ® 2C(g) decrease when the pressure of all substances in the system decreases by 3 times and the temperature of the system simultaneously decreases by 30 o C? The temperature coefficient of the reaction rate g is 2.

32. The reaction between gaseous substances A and B is expressed by the equation A + B ® C. The initial concentrations of the substances are [A] 0 = 0.05 mol/l and [B] 0 = 0.05 mol/l. After some time, the concentration of substances decreased by half. Determine how it is necessary to change the temperature so that the reaction rate becomes equal to the initial rate, if a) the temperature coefficient of the reaction is 2, b) the activation energy is 70 kJ, the reaction temperature is 27 o C?

33. It is known that when the temperature increases from 290 to 300 K, the rate of a chemical reaction doubles. Calculate the activation energy. How will the rate of this reaction change at 310 K if a catalyst is introduced into the system that lowers the activation energy of this reaction by 10 kJ/mol?

Chemical equilibrium

1. At a certain temperature, equilibrium in the 2NO 2 «2NO+O 2 system was established at concentrations = 0.4 mol/l, = 0.2 mol/l, = 0.1 mol/l. Find the equilibrium constant and the initial concentration of NO 2 if the initial oxygen concentration is zero. What conditions will promote a shift in equilibrium towards the formation of NO if the direct reaction is endothermic?

2. The equilibrium constant of the system A+B«C+D is equal to unity. What percentage of substance A will be converted if you mix 3 moles of substance A and 5 moles of substance B? What conditions will contribute to a shift in equilibrium towards the formation of B if the direct reaction is exothermic?

3. For the system

CO (G) + H 2 O (G) “CO 2 (G) + H 2 (G)

0 = 0 =0.03 mol/l, 0 = 0 =0. Calculate the equilibrium constant if the equilibrium concentration carbon dioxide equal to 0.01 mol/l. What conditions will contribute to a shift in equilibrium towards the formation of CO if the direct reaction is endothermic?

4. For the system

2NO (G) +Cl 2 (G) “2NOCl (G)

0 =0.5 mol/l, 0 =0.2 mol/l, 0 =0 mol/l. Find the equilibrium constant if by the time of its onset 20% of nitric oxide has reacted. What conditions will contribute to a shift in equilibrium towards the formation of NOCl if the direct reaction is exothermic?

H 2(G) + I 2(G) «2HI (G) ,

if 1 mole of iodine and 2 moles of hydrogen are placed in a vessel with a capacity of 10 liters (KC = 50). What conditions will contribute to a shift in equilibrium towards the formation of iodine if the direct reaction is exothermic?

6. For the system CO (G) + H 2 O (G) “CO 2 (G) + H 2 (G), 0 = 0 =1 mol/l, 0 = 0 =0. Calculate the composition of the equilibrium mixture (% vol.), if the equilibrium constant K C = 1. What conditions will contribute to a shift in equilibrium towards the formation of hydrogen if the reverse reaction is exothermic?

7. In a closed vessel the reaction AB (G) “A (G) + B (G) takes place. Equilibrium constant K C =0.04. Find the initial concentration of AB if the equilibrium concentration of AB is 0.02 mol/l. What conditions will contribute to a shift in equilibrium towards the formation of A if the reverse reaction is exothermic?

8. In a closed vessel with a volume of 10 liters at a temperature of 800˚C, the equilibrium CaCO 3 (T) “CaO (T) + CO 2 (G) has been established. Equilibrium constant K P =300 kPa. What mass of CaCO 3 decomposed? What conditions will contribute to a shift in equilibrium towards the formation of carbon dioxide if the direct reaction is endothermic?

9. In a closed vessel at a certain temperature, the equilibrium Fe (T) + H 2 O (G) “FeO (T) + H 2 (G) has been established. Determine the fraction of reacted water if K P = 1 and the initial partial pressure of hydrogen is zero. What conditions will contribute to a shift in equilibrium towards the formation of hydrogen if the reverse reaction is exothermic?

10. Determine the equilibrium concentration of hydrogen in the system 2HI (G) “H 2 (G) + I 2 (G) if the initial concentration of HI was 0.05 mol/l and the equilibrium constant K C = 0.02. What conditions will contribute to a shift in equilibrium towards the formation of HI if the direct reaction is endothermic?

1. Basic concepts and postulates of chemical kinetics

Chemical kinetics - a branch of physical chemistry that studies rates chemical reactions. The main tasks of chemical kinetics: 1) calculation of reaction rates and determination of kinetic curves, i.e. dependence of the concentrations of reactants on time ( direct task); 2) determination of reaction mechanisms from kinetic curves ( inverse problem).

The rate of a chemical reaction describes the change in concentrations of reactants per unit time. For reaction

a A+ b B+... d D+ e E+...

the reaction rate is determined as follows:

where square brackets indicate the concentration of the substance (usually measured in mol/l), t- time; a, b, d, e- stoichiometric coefficients in the reaction equation.

The reaction rate depends on the nature of the reactants, their concentration, temperature and the presence of a catalyst. The dependence of the reaction rate on concentration is described by the basic postulate of chemical kinetics - law of mass action:

The rate of a chemical reaction at each moment in time is proportional to the current concentrations of the reactants, raised to certain powers:

,

Where k- rate constant (independent of concentration); x, y- some numbers that are called order of reaction by substance A and B, respectively. In general, these numbers have nothing to do with the coefficients a And b in the reaction equation. Sum of exponents x+ y called in general reactions. The order of the reaction can be positive or negative, integer or fractional.

Most chemical reactions consist of several steps called elementary reactions. An elementary reaction is usually understood as a single act of formation or cleavage of a chemical bond, proceeding through the formation of a transition complex. The number of particles participating in an elementary reaction is called molecularity reactions. There are only three types of elementary reactions: monomolecular (A B + ...), bimolecular (A + B D + ...) and trimolecular (2A + B D + ...). For elementary reactions, the overall order is equal to the molecularity, and the orders by substance are equal to the coefficients in the reaction equation.

EXAMPLES

Example 1-1. The rate of NO formation in the reaction 2NOBr (g) 2NO (g) + Br 2 (g) is 1.6. 10 -4 mol/(l.s). What is the rate of reaction and the rate of NOBr consumption?

Solution. By definition, the reaction rate is:

Mol/(l.s).

From the same definition it follows that the rate of NOBr consumption is equal to the rate of NO formation with the opposite sign:

mol/(l.s).

Example 1-2. In the 2nd order reaction A + B D, the initial concentrations of substances A and B are equal to 2.0 mol/L and 3.0 mol/L, respectively. The reaction rate is 1.2. 10 -3 mol/(l.s) at [A] = 1.5 mol/l. Calculate the rate constant and reaction rate at [B] = 1.5 mol/L.

Solution. According to the law of mass action, at any moment of time the reaction rate is equal to:

.

By the time when [A] = 1.5 mol/l, 0.5 mol/l of substances A and B have reacted, so [B] = 3 – 0.5 = 2.5 mol/l. The rate constant is:

L/(mol. s).

By the time when [B] = 1.5 mol/l, 1.5 mol/l of substances A and B have reacted, therefore [A] = 2 – 1.5 = 0.5 mol/l. The reaction rate is:

Mol/(l.s).

TASKS

1-1. How is the rate of the ammonia synthesis reaction 1/2 N 2 + 3/2 H 2 = NH 3 expressed in terms of the concentrations of nitrogen and hydrogen? (answer)

1-2. How will the rate of the ammonia synthesis reaction 1/2 N 2 + 3/2 H 2 = NH 3 change if the reaction equation is written as N 2 + 3H 2 = 2NH 3? (answer)

1-3. What is the order of elementary reactions: a) Cl + H 2 = HCl + H; b) 2NO + Cl 2 = 2NOCl? (answer)

1-4. Which of the following quantities can take a) negative; b) fractional values: reaction rate, reaction order, reaction molecularity, rate constant, stoichiometric coefficient? (answer)

1-5. Does the rate of a reaction depend on the concentration of reaction products? (answer)

1-6. How many times will the rate of the gas-phase elementary reaction A = 2D increase when the pressure increases by 3 times? (answer)

1-7. Determine the order of the reaction if the rate constant has the dimension l 2 / (mol 2 . s). (answer)

1-8. The rate constant of a 2nd order gas reaction at 25 o C is equal to 10 3 l/(mol. s). What is this constant equal to if the kinetic equation is expressed in terms of pressure in atmospheres? (answer)

1-9. For gas phase reaction n th order nA B, express the rate of formation of B in terms of the total pressure. (answer)

1-10. The rate constants for the forward and reverse reactions are 2.2 and 3.8 l/(mol. s). By which of the following mechanisms can these reactions occur: a) A + B = D; b) A + B = 2D; c) A = B + D; d) 2A = B.(answer)

1-11. The decomposition reaction 2HI H 2 + I 2 has a 2nd order with a rate constant k= 5.95. 10 -6 l/(mol. s). Calculate the reaction rate at a pressure of 1 atm and a temperature of 600 K. (answer)

1-12. The rate of the 2nd order reaction A + B D is 2.7. 10 -7 mol/(l.s) at concentrations of substances A and B, respectively, 3.0. 10 -3 mol/l and 2.0 mol/l. Calculate the rate constant.(answer)

1-13. In the 2nd order reaction A + B 2D, the initial concentrations of substances A and B are equal to 1.5 mol/l. The reaction rate is 2.0. 10 -4 mol/(l.s) at [A] = 1.0 mol/l. Calculate the rate constant and reaction rate at [B] = 0.2 mol/L. (answer)

1-14. In the 2nd order reaction A + B 2D, the initial concentrations of substances A and B are equal to 0.5 and 2.5 mol/l, respectively. How many times is the reaction rate at [A] = 0.1 mol/l less than the initial rate? (answer)

1-15. The rate of the gas-phase reaction is described by the equation w = k. [A] 2 . [B]. At what ratio between the concentrations of A and B will the initial reaction rate be maximum at a fixed total pressure? (answer)

2. Kinetics of simple reactions

In this section, based on the law of mass action, we will compose and solve kinetic equations for irreversible reactions of a whole order.

0th order reactions. The rate of these reactions does not depend on concentration:

,

where [A] is the concentration of the starting substance. Zero order occurs in heterogeneous and photochemical reactions.

1st order reactions. In type A–B reactions, the rate is directly proportional to the concentration:

.

When solving kinetic equations, the following notation is often used: initial concentration [A] 0 = a, current concentration [A] = a - x(t), Where x(t) is the concentration of the reacted substance A. In this notation, the kinetic equation for the 1st order reaction and its solution have the form:

Solution kinetic equation written in another form, convenient for analyzing the reaction order:

.

The time during which half of substance A decays is called the half-life t 1/2. It is defined by the equation x(t 1/2) = a/2 and equal

2nd order reactions. In reactions of type A + B D + ..., the rate is directly proportional to the product of concentrations:

.

Initial concentrations of substances: [A] 0 = a, [B] 0 = b; current concentrations: [A] = a- x(t), [B] = b - x(t).

When solving this equation, two cases are distinguished.

1) identical initial concentrations of substances A and B: a = b. The kinetic equation has the form:

.

The solution to this equation is written in various forms:

The half-lives of substances A and B are the same and equal to:

2) The initial concentrations of substances A and B are different: a b. The kinetic equation has the form:
.

The solution to this equation can be written as follows:

The half-lives of substances A and B are different: .

Nth order reactions n A D + ... The kinetic equation has the form:

.

Solution of the kinetic equation:

. (2.1)

The half-life of substance A is inversely proportional to ( n-1)th degree of initial concentration:

. (2.2)

Example 2-1. The half-life of the radioactive isotope 14 C is 5730 years. During archaeological excavations, a tree was found whose 14 C content was 72% of normal. How old is the tree?
Solution. Radioactive decay is a 1st order reaction. The rate constant is:

The life time of a tree can be found from solving the kinetic equation, taking into account the fact that [A] = 0.72. [A] 0:

Example 2-2. It has been established that a 2nd order reaction (one reagent) is 75% complete in 92 minutes at an initial reagent concentration of 0.24 M. How long will it take for the reagent concentration to reach 0.16 M under the same conditions?
Solution. Let us write the solution of the kinetic equation for a 2nd order reaction with one reagent twice:

,

where, by condition, a= 0.24 M, t 1 = 92 min, x 1 = 0.75. 0.24 = 0.18 M, x 2 = 0.24 - 0.16 = 0.08 M. Let's divide one equation by another:

Example 2-3. For an elementary reaction n A B we denote the half-life of A by t 1/2, and the decay time of A by 75% by t 3/4. Prove that the ratio t 3/4 / t 1/2 does not depend on the initial concentration, but is determined only by the order of the reaction n.Solution. Let us write the solution of the kinetic equation for the reaction twice n-th order with one reagent:

and divide one expression by another. Constants k And a both expressions will cancel and we get:

.

This result can be generalized by proving that the ratio of the times for which the degree of conversion is a and b depends only on the order of the reaction:

.

TASKS

2-2. The first order reaction proceeds 30% in 7 minutes. How long will it take for the reaction to be 99% complete? (answer)

2-3. The half-life of the radioactive isotope 137 Cs, which entered the atmosphere as a result of the Chernobyl accident, is 29.7 years. After what time will the amount of this isotope be less than 1% of the original? (answer)

2-4. The half-life of the radioactive isotope 90 Sr, which enters the atmosphere when nuclear tests, - 28.1 years. Let's assume that the body of a newborn child absorbed 1.00 mg of this isotope. How much strontium will remain in the body after a) 18 years, b) 70 years, if we assume that it is not excreted from the body? (answer)

2-5. The rate constant for the first order reaction SO 2 Cl 2 = SO 2 + Cl 2 is 2.2. 10 -5 s -1 at 320 o C. What percentage of SO 2 Cl 2 will decompose when kept for 2 hours at this temperature? (answer)

2-6. 1st order reaction rate constant

2N 2 O 5 (g) 4NO 2 (g) + O 2 (g)

at 25 o C is equal to 3.38. 10 -5 s -1 . What is the half-life of N 2 O 5? What will be the pressure in the system after a) 10 s, b) 10 min, if the initial pressure was 500 mm Hg? Art. (answer)

2-7. The first order reaction is carried out with varying amounts of the starting material. Will the tangents to the initial sections of the kinetic curves intersect at one point on the x-axis? Explain your answer. (answer)

2-8. The first order reaction A 2B occurs in the gas phase. The initial pressure is p 0 (B missing). Find the dependence of total pressure on time. After what time will the pressure increase by 1.5 times compared to the original? What is the progress of the reaction by this time? (answer)

2-9. The second order reaction 2A B occurs in the gas phase. The initial pressure is p 0 (B missing). Find the dependence of total pressure on time. After what time will the pressure decrease by 1.5 times compared to the original? What is the progress of the reaction by this time? (answer)

2-10. Substance A was mixed with substances B and C in equal concentrations of 1 mol/l. After 1000 s, 50% of substance A remains. How much substance A will remain after 2000 s if the reaction has: a) zero, b) first, c) second, c) third general order? (answer)

2-11. Which of the reactions - first, second or third order - will end faster if the initial concentrations of substances are 1 mol/l and all rate constants expressed in terms of mol/l and s are equal to 1? (answer)

2-12. Reaction

CH 3 CH 2 NO 2 + OH - H 2 O + CH 3 CHNO 2 -

has second order and rate constant k= 39.1 l/(mol. min) at 0 o C. A solution was prepared containing 0.004 M nitroethane and 0.005 M NaOH. How long will it take for 90% of nitroethane to react?

2-13. The rate constant for the recombination of H + and FG - (phenylglyoxynate) ions into an UFG molecule at 298 K is equal to k= 10 11.59 l/(mol. s). Calculate the time it takes for the reaction to complete 99.999% if the initial concentrations of both ions are 0.001 mol/L. (answer)

2-14. The rate of oxidation of 1-butanol by hypochlorous acid does not depend on the alcohol concentration and is proportional to 2. How long will it take for the oxidation reaction at 298 K to complete 90% if the initial solution contained 0.1 mol/L HClO and 1 mol/L alcohol? The reaction rate constant is k= 24 l/(mol min). (answer)

2-15. At certain temperature A 0.01 M solution of ethyl acetate is saponified by a 0.002 M solution of NaOH by 10% in 23 minutes. After how many minutes will it be saponified to the same degree with a 0.005 M KOH solution? Consider that this reaction is of second order, and the alkalis are completely dissociated. (answer)

2-16. The second order reaction A + B P is carried out in a solution with initial concentrations [A] 0 = 0.050 mol/L and [B] 0 = 0.080 mol/L. After 1 hour, the concentration of substance A decreased to 0.020 mol/l. Calculate the rate constant and half-lives of both substances.

LESSON 10 10th grade(first year of study)

Fundamentals of chemical kinetics. State of chemical equilibrium Plan

1. Chemical kinetics and the field of its study.

2. Rate of homogeneous and heterogeneous reactions.

3. Dependence of the reaction rate on various factors: the nature of the reactants, the concentration of the reagents (law of mass action), temperature (van't Hoff rule), catalyst.

4. Reversible and irreversible chemical reactions.

5. Chemical equilibrium and conditions for its displacement. Le Chatelier's principle.

The branch of chemistry that studies the rates and mechanisms of chemical reactions is called chemical kinetics. One of the main concepts in this section is the concept of the rate of a chemical reaction. Some chemical reactions occur almost instantly (for example, a neutralization reaction in solution), others take thousands of years (for example, the transformation of graphite into clay during the weathering of rocks).

The rate of a homogeneous reaction is the amount of a substance that reacts or is formed as a result of a reaction per unit time per unit volume of the system:

In other words, the rate of a homogeneous reaction is equal to the change in the molar concentration of any of the reactants per unit time. The reaction rate is a positive quantity, therefore, when expressing it through a change in the concentration of the reaction product, a “+” sign is given, and when the reagent concentration changes, a “–” sign is given.

The rate of a heterogeneous reaction is the amount of substance that reacts or is formed as a result of a reaction per unit time per unit surface area of ​​the phase:

The most important factors influencing the rate of a chemical reaction are the nature and concentration of the reagents, temperature, and the presence of a catalyst.

Influence nature of the reagents manifests itself in the fact that, under the same conditions, different substances interact with each other at different rates, for example:

When increasing reagent concentrations the number of collisions between particles increases, which leads to an increase in the reaction rate. The quantitative dependence of the reaction rate on the concentration of the reagents is expressed by the law of effective mass (K.M. Guldberg and P. Waage, 1867; N.I. Beketov , 1865). The rate of a homogeneous chemical reaction at a constant temperature is directly proportional to the product of the concentration of the reacting substances in powers equal to their stoichiometric coefficients (concentration solids are not taken into account), for example:

where A and B are gases or liquids, k – reaction rate constant equal to the reaction rate at a reactant concentration of 1 mol/l. Constant k depends on the properties of the reacting substances and temperature, but does not depend on the concentration of the substances.

Dependence of reaction speed on temperature is described by the experimental rule of Van t-Goff (1884). When the temperature increases by 10°, the rate of most chemical reactions increases by 2–4 times:

where is the temperature coefficient.

Catalyst is a substance that changes the rate of a chemical reaction, but is not consumed as a result of this reaction. There are positive catalysts (specific and universal), negative (inhibitors) and biological (enzymes, or enzymes). The change in reaction rate in the presence of catalysts is called catalysis. There are homogeneous and heterogeneous catalysis. If the reactants and the catalyst are in the same state of aggregation, catalysis is homogeneous; in different – ​​heterogeneous.

Homogeneous catalysis:

heterogeneous catalysis:

The mechanism of action of catalysts is very complex and not fully understood. There is a hypothesis about the formation of intermediate compounds between the reagent and the catalyst:

A + cat. ,

B AB + cat.

Promoters are used to enhance the action of catalysts; There are also catalytic poisons that weaken the effect of catalysts.

The rate of a heterogeneous reaction is affected by interfacial area(the degree of grinding of the substance) and the rate of supply of reagents and removal of reaction products from the phase interface.

All chemical reactions are divided into two types: reversible and irreversible.

Chemical reactions that proceed in only one direction are called irreversible., i.e. the products of these reactions do not interact with each other to form the starting materials. 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 + 2HCl,

K 2 S + 2HCl = 2KCl + H 2 S,

HCl + NaOH = NaCl + H 2 O.

Reversible reactions are those that occur simultaneously in the forward and reverse directions., For example:

When a reversible chemical reaction occurs, the rate of the direct reaction initially has a maximum value, and then decreases due to a decrease in the concentration of the starting substances. The reverse reaction, on the contrary, at the initial moment of time has a minimum speed, which gradually increases. Thus, at a certain point in time there comes state of chemical equilibrium, at which the rate of the forward reaction is equal to the rate of the reverse reaction. The state of chemical equilibrium is dynamic - 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.

The ratio of the rate constants of forward and reverse reactions is a constant value and is called the equilibrium constant ( TO R ) . Solid concentrations are not included in the equilibrium constant expression. The equilibrium constant of the reaction depends on temperature and pressure, but does not depend on the concentration of the reactants and on the presence of a catalyst, which accelerates the progress of both forward and reverse reactions. The more TO p, the higher the practical yield of reaction products. If TO p > 1, then the reaction products predominate in the system; If TO R< 1, в системе преобладают реагенты.

Chemical equilibrium is mobile, i.e. when external conditions change, the speed of the forward or reverse reaction may increase. The direction of the equilibrium shift is determined by the principle formulated by the French scientist Le Chatelier in 1884. If an external influence is exerted on an equilibrium system, then the equilibrium shifts towards the reaction that counteracts this influence. Equilibrium shifts are affected by changes in reactant concentrations, temperature, and pressure.

An increase in the concentration of reagents and the removal of products lead to a shift in equilibrium towards the direct reaction.

When the system is heated, the equilibrium shifts towards the endothermic reaction, and when cooled, towards the exothermic reaction.

For reactions involving gaseous substances, an increase in pressure shifts the equilibrium towards a reaction that occurs with a decrease in the number of gas molecules. If the reaction proceeds without changing the number of molecules of gaseous substances, then the change in pressure does not in any way affect the shift in equilibrium.

Problem 325.
Find the value of the rate constant for the reaction A + B ⇒ AB, if at concentrations of substances A and B equal to 0.05 and 0.01 mol/l, respectively, the reaction rate is 5 . 10 -5 mol/(l. min).
Solution:
Speed chemical reaction is expressed by the equation:

v- ,k- reaction rate constant

Answer: 0.1/mol. min.

Problem 326.
How many times will the rate of the reaction 2A + B ⇒ A 2 B change if the concentration of substance A is increased by 2 times, and the concentration of substance B is decreased by 2 times?
Solution:

v- ,k- reaction rate constant, [A] and [B] – concentrations of starting substances.

Due to an increase in the concentration of substance A by 2 times and a decrease in the concentration of substance B by 2 times, the reaction rate will be expressed by the equation:

Comparing the expressions for v and v" we find that the reaction rate has increased by 2 times.

Answer: increased by 2 times.

Problem 327.
How many times should the concentration of substance B 2 in the system be increased?
2A 2(g) + B 2(g) = 2A 2 B, so that when the concentration of substance A decreases by 4 times, the rate of the direct reaction does not change?
Solution:
The concentration of substance A was reduced by 4 times. We denote the change in the concentration of substance B by x. Then, before the concentration of substance A changes, the reaction rate can be expressed by the equation:

v- ,k- reaction rate constant, [A] and [B] – concentrations of starting substances.
After changing the concentration of substance A 2, the reaction rate will be expressed by the equation:

According to the conditions of the problem, v = v" or

Thus, it is necessary to increase the concentration of substance B 2 in the system 2A 2 (g) + B 2 (g) = 2A 2 B by 16 times, so that when the concentration of substance A 2 decreases by 4 times, the rate of the direct reaction does not change.

Answer: 16 times.

Problem 328.
Two vessels of the same capacity are introduced: into the first - 1 mole of gas A and 2 moles of gas B, into the second - 2 moles of gas A and 1 mole of gas B. The temperature in both vessels is the same. Will the reaction rate between gases A and B in these vessels differ if the reaction rate is expressed by: a) equation b) equation
Solution:
a) If the reaction rate is expressed by the equation, then, taking into account the concentrations of substances A and B in the vessels, we write down the expressions for the reaction rates for the vessels:

Thus,

b) If the reaction rate is expressed by the equation, then, taking into account the concentrations of substances A and B in the vessels, we write down the expressions for the reaction rates for the vessels:

Thus,

Answer: a) no, b) yes.

Problem 329.
Some time after the start of the reaction 3A+B ⇒ 2C+D concentrations of substances were: [A] = 0.03 mol/l; [B] =0.01 mol/l; [C] = 0.008 mol/l. What are the initial concentrations of substances A and B?

Solution:
To find the concentrations of substances A and B, we take into account that, according to the reaction equation, from 3 moles of substance A and 1 mole of substance B, 1 mole of substance C is formed. Since, according to the conditions of the problem, 0.008 moles of substance C were formed in each liter of the system, then it was consumed 0.012 mol of substance A (3/2 . 0.008 = 0.012) and 0.004 mol of substance B (1/2 . 0.008 = 0.004). Thus, the initial concentrations of substances A and B will be equal:

[A] 0 = 0.03 + 0.012 = 0.042 mol/l;
[B] 0 = 0.01 + 0.004 = 0.014 mol/l.

Answer:[A] 0 = 0.042 mol/l; [B] 0 = 0.014 mol/l.

Problem 330.
In the system CO + C1 2 = COC1 2, the concentration was increased from 0.03 to 0.12 mol/l, and the chlorine concentration from 0.02 to 0.06 mol/l. How many times did the rate of the forward reaction increase?
Solution:
Before the concentration changes, the reaction rate can be expressed by the equation:

v is the reaction rate, k is the reaction rate constant, [CO] and are the concentrations of the starting substances.

After increasing the concentration of reactants, the reaction rate is:

Let's calculate how many times the reaction rate has increased:

Answer: 12 times.

Rate of chemical reactions The branch of chemistry that studies the rate and mechanism of chemical reactions is called chemical kinetics. The rate of a chemical reaction is the number of elementary acts of interaction per unit of time in a unit of reaction space. This definition is valid for both homogeneous and heterogeneous processes. In the first case, the reaction space is the volume of the reaction vessel, and in the second, the surface on which the reaction occurs. Since the interaction changes the concentrations of reagents or reaction products per unit time. In this case, there is no need to monitor changes in the concentration of all substances participating in the reaction, since its stoichiometric equation establishes the relationship between the concentrations of the reactants. The concentration of reactants is most often expressed as the number of moles in 1 liter (mol/L). The rate of a chemical reaction depends on the nature of the reacting substances, concentration, temperature, size of the contact surface of the substances, the presence of catalysts and others. , and talk about a monomolecular reaction; when a collision of two different molecules occurs in an elementary act, the dependence has the following form: u - k[A][B], and they speak of a bimolecular reaction; when a collision of three molecules occurs in an elementary act, the dependence of speed on concentration is true: v - k [A] [B] [C], and they speak of a trimolecular reaction. In all analyzed dependencies: v - reaction rate; [A], [B], [C] - concentrations of reacting substances; k - proportionality coefficient; called the reaction rate constant. v = k, when the concentrations of reactants or their product are equal to unity. The rate constant depends on the nature of the reactants and on the temperature. The dependence of the rate of simple reactions (i.e., reactions occurring through one elementary act) on concentration is described by the law of mass action established by K. Guldberg and P. Waage in 1867: the rate of a chemical reaction is directly proportional to the product of the concentration of the reacting substances raised to the power their stoichiometric coefficients. For example, for the reaction 2NO + 02 = 2N02; v - k2 and will increase three times Find: Solution: 1) Write the reaction equation: 2СО + 02 = 2С02. According to the law of mass action v - k[C0]2. 2) Let us denote [CO] = a; = b, then: v = k a2 b. 3) When the concentration of the starting substances increases by 3 times, we obtain: [CO] = 3a, a = 3b. 4) Calculate the speed of reaction u1: - k9a23b - k27a% a if k27 D2b 27 v k a2b Answer: 27 times. Example 3 How many times will the rate of a chemical reaction increase when the temperature increases by 40 °C if the temperature coefficient of the reaction rate is 3? Given: At = 40 °C Y - 3 Find: 2 Solution: 1) According to Van't Hoff's rule: h-U vt2 = vh y 10, 40 and, - vt > 3 10 - vt -81. 2 1 1 Answer: 81 times. a Example 4 The reaction between substances A and B proceeds according to the scheme 2A + B * "C. The concentration of substance A is 10 mol/l, and substance B is 6 mol/l. The reaction rate constant is 0.8 l2 4 mol"2 sec"1. Calculate the rate of the chemical reaction at the initial moment, as well as at the moment when 60% of substance B remains in the reaction mixture. Given: k - 0.8 l2 mol"2 sec"1 [A] = 10 mol/l [B] = 6 mol/l Find: "start! ^ Solution: 1) Find the reaction rate at the initial moment: v - k[A]2 [B], r> = 0.8 102 b - 480 mol - l sec"1. beginning 2) After some time, 60% of substance B will remain in the reaction mixture. Then: Therefore, [B] decreased by: 6 - 3.6 = 2.4 mol/l. 3) From the reaction equation it follows that substances A and B interact with each other in a ratio of 2:1, therefore [A] decreased by 4.8 mol/l and became equal to: [A] = 10 - 4.8 = 5.2 mol/l. 4) Calculate if: d) = 0.8 * 5.22 3.6 = 77.9 mol l "1 * sec"1. Answer: g>begin ~ 480 mol l sec"1, g/ = 77.9 mol l-1 sec"1. Example 5 The reaction at a temperature of 30 °C takes 2 minutes. How long will it take for this reaction to complete at a temperature of 60 °C, if in this temperature range temperature coefficient of reaction rate is 2? Given: t1 = 30 °C t2 = 60 °C 7 = 2 t = 2 min = 120 sec Find: h Solution: 1) In accordance with the van’t Hoff rule: vt - = y 1 vt - = 23 = 8. Vt 2) The reaction speed is inversely proportional to the reaction time, therefore: Answer: t = 15 sec. Questions and tasks for independent solution 1. Define reaction rate. Give examples of reactions occurring at different rates. 2. The expression for the true rate of a chemical reaction occurring at a constant volume of the system is written as follows: dC v = ±--. d t Indicate in which cases a positive one is necessary, and in which - negative signs on the right side of the expression. 3. On what factors does the rate of a chemical reaction depend? 4. What is called activation energy? What factor influences the rate of a chemical reaction does it characterize? 5. What explains the strong increase in reaction rate with increasing temperature? 6. Define the basic law of chemical kinetics - the law of mass action. Who and when was it formulated? 7. What is the rate constant of a chemical reaction called and what factors does it depend on? 8. What is a catalyst and how does it affect the rate of a chemical reaction? 9. Give examples of processes in which inhibitors are used. 10. What are promoters and where are they used? 11. What substances are called “catalytic poisons”? Give examples of such substances. 12. What is homogeneous and heterogeneous catalysis? 24. At a temperature of 40 °C, the reaction proceeds in 36 minutes, and at 60 °C - in 4 minutes. Calculate the temperature coefficient of the reaction rate. 25. The reaction rate at 10 °C is 2 mol/l. Calculate the rate of this reaction at 50 °C if the temperature coefficient of the reaction rate is 2.