Chemical reaction rate
and its dependence on various factors

Lesson using information technology

It is impossible to learn chemistry in any case,
without seeing the practice itself and without taking on chemical operations.
M.V. Lomonosov

The restructuring of higher and secondary specialized education in the country and school reform provide for further improvement of forms, methods and means of teaching, the use of a variety of technologies, including person-centered learning (PLL), problem-search and computer technologies.

We, teachers, are also changing. In my work I try to constantly use new developments and modern educational technologies.

Recently, a lot of materials have appeared on computer disks. They can be used in developing essays, writing term papers, and in students’ independent work. Information technologies allow me to quickly organize training and testing of knowledge, create adaptive programs and apply them in teaching chemistry.

Computer technology and the use of computer technologies today act not only as a means of automating all learning processes, but also as a tool for dramatically increasing the efficiency of students' intellectual activity.

I use computer technology in my lessons for different purposes:

Problem solving, quantitative calculations, data processing (according to the proposed algorithm);

Carrying out self-control and standardized control of knowledge on the content of educational information (tests, control differentiated tasks, maps and other questionnaires);

Automation of a chemical experiment, connection with optical equipment (projection of experiments onto a screen);

Obtaining the necessary reference data, preparing tests, differentiated works, analyzing typical mistakes of students (automated control systems and information banks);

Independent work of students to develop abstracts and term papers, work with the material, perform test work (when receiving the result, exercise self-control).

The proposed lesson from the section “Chemical Kinetics” corresponds to the program of the textbook “Chemistry-10” by the authors L.S. Guzey and R.P. Surovtseva. The study of this topic is preceded by the study of the thermodynamics of reactions. The proposed material does not correspond to the required minimum content, but primarily to the profile level of training.

The lesson uses group work, a differentiated approach, developmental and problem-search technologies, and most importantly, computer technology to conduct a demonstration experiment, which allows you to clearly understand what the rate of a chemical reaction is and how it depends on various factors.

Lesson objectives. Update and deepen knowledge about the rate of chemical reactions; using group work, consider and study various factors: the nature of the reacting substances, the surface area of ​​​​contact of the substances, temperature, catalyst; using a computer measuring unit, clearly demonstrate what the rate of a chemical reaction is and how it depends on the concentration of the reacting substances.

Lesson motto.“There is only what can be measured” (M. Planck).

Class design. The teacher informs in advance the topic of the upcoming lesson, divides the class into four creative groups of 5-6 people, approximately equal in abilities. In the previous lesson, students receive homework - to prepare reports on the practical application of the Arrhenius equation and the types of catalysis.

Equipment and reagents. On the students' desks - textbooks, notebooks, tables, laboratory sheets, racks with test tubes;

group 1: zinc granules, magnesium tape, hydrochloric acid solution;

group 2: glass rod; iron filings, iron nail, copper(II) chloride solution;

group 3: pipette, test tube holder, alcohol lamp, matches; copper(II) oxide, sulfuric acid solution;

group 4(performs a demonstration experiment on a demonstration table): a computer with a measuring unit, an optical density sensor at a wavelength of 525 nm, a cuvette, a magnetic stirrer, a 10 ml syringe, a 100 ml graduated cylinder; solutions of potassium iodide KI 1M, potassium persulfate K 2 S 2 O 8 0.1 M, distilled water.

Students make all notes during the lesson in their notebooks.

PROGRESS OF THE LESSON

Motivation for the importance of the chosen topic

The teacher begins explaining the material with examples of chemical reactions occurring at different rates. Students can give examples of reactions.

Chemical reactions occur at different rates. Some progress slowly, over months, such as the corrosion of iron or the fermentation of grape juice, which results in wine. Others are completed in a few weeks, like the alcoholic fermentation of glucose. Still others end very quickly, such as the precipitation of insoluble salts, and some occur instantly, such as explosions.

Almost instantly, very quickly, many reactions occur in aqueous solutions:

Let's mix aqueous solutions of Na 2 CO 3 and CaCl 2, the reaction product CaCO 3 is insoluble in water and is formed immediately;

If we add excess acid to an alkaline solution of phenolphthalein, the solution becomes discolored instantly. This means that the neutralization reaction, the reaction of converting the colored form of the indicator into a colorless one, proceeds very quickly.

Rust forms slowly on iron objects. Black-brown or greenish-colored corrosion products (patina) slowly form on copper and bronze objects. The speed of all these processes is different.

Updating Views
about the speed of chemical reactions

Chemical reactions are one of the most important concepts in chemistry. To understand them and competently use them in the educational process, the teacher needs to know and be able to explain the main characteristics of any chemical reaction: thermal effect, equilibrium, speed. Chemical thermodynamics makes it possible to predict in which direction a particular chemical reaction may spontaneously proceed, but chemical thermodynamics alone does not answer the question of how and at what speed the reaction will proceed. The concept of the rate of a chemical reaction is one of the basic ones in chemical kinetics.

To study new material, students use the necessary knowledge about the rate of a chemical reaction; they go through the stage of updating their knowledge. But this concept is deepened by the concepts of the speed of homogeneous and heterogeneous reactions, activation energy, the Arrhenius equation is introduced - this is the zone of proximal development of students (see Appendix No. 1 “Structure of problem-search activity of teachers and students...”).

What do you mean by reaction speed? How can it be measured and changed? The science that studies the patterns of reactions over time - chemical kinetics - will help answer these questions.

Let us recall the basic concepts and patterns used in kinetics (students answer and teacher supplements).

Chemical kinetics is a branch of chemistry whose task is to explain qualitative and quantitative changes in chemical processes that occur over time. Usually this general task is divided into two, more specific ones:

1) identifying the reaction mechanism - establishing the elementary stages of the process and the sequence of their occurrence (qualitative changes);

2) quantitative description of a chemical reaction - the establishment of strict relationships that make it possible to calculate changes in the amounts of initial reagents and products as the reaction proceeds.

The main concept in chemical kinetics is the concept of reaction rate. Chemical reaction rate is determined by the amount of substance reacted per unit time in a unit of reaction space.

If the concentration of one of the reactants decreases from With 1 to With 2 for the period from t 1 to t 2, then in accordance with the definition of the reaction rate is equal to (Fig. 1):

The “–” sign on the right side of the equation means the following. As the reaction progresses ( t 2 – t 1) > 0 the concentration of reagents decreases, therefore, ( c 2 – c 1) < 0, а т.к. скорость реакции всегда положительна, то перед дробью следует поставить знак «–».

Rice. 1. Changing the concentration of the starting substance depending on time. Kinetic curve

Quantitatively, the relationship between the reaction rate and the molar concentrations of the reactants is described by the basic law of chemical kinetics - the law of mass action.

The rate of a chemical reaction at a constant temperature is proportional to the product of the concentrations of the reactants.

For reaction

A A+ b B = With C + d D,

in accordance with the law of mass action, the dependence of speed on the concentrations of reacting substances can be presented as:

Where k– rate constant; n A, n B – reaction orders for reagents A and B, respectively;
n A+ n B is the general reaction order.

In homogeneous reactions, the reacting substances are in the same gas phase or in solution, evenly mixed with each other, the reaction occurs throughout the entire volume of the mixture. The concentration of the reagent is equal to the quotient of the amount of substance divided by the volume of the mixture: With = /V.

Average reaction speed:

The shorter the time period, the more accurate the reaction rate will be.

Heterogeneous reactions occur at the phase interface: gas - solid, gas - liquid, liquid - solid, solid - solid. Reaction speed

measured per unit area of ​​contact of reacting substances S.

When considering the thermal effects of chemical reactions, the transformation of reactant molecules (A + B) into product molecules (C + D) is explained from a thermodynamic point of view as “climbing an energy mountain” in the case of endothermic reactions (Fig. 2, A) or “going downhill” for exothermic reactions (Fig. 2, b).

In order for reactant molecules to react, they must first stock up on additional energy in order to overcome the energy barrier on the way to the reaction products. It is significant that such a barrier also exists in the case of exothermic reactions, so that instead of simply “sliding down the hill,” the molecules have to first “climb the hill.”

Rice. 2. Dependence of energy on time: a – endothermic reaction: A + B C + D – Q; b – exothermic reaction: A + B C + D + Q

The driving force of the reaction is the desire to achieve a minimum of energy.

In order for a reaction to occur, the particles of the reacting substances must collide with each other. As the temperature increases, the number of these collisions increases due to the increase in the kinetic energy of the molecules, therefore the reaction rate increases. But not every collision of molecules of reacting substances leads to their interaction: for molecules to interact, the bonds between the atoms in them must become weaker or break, for which a certain amount of energy must be expended. If the colliding molecules do not have this energy, their collision does not lead to a reaction. The excess energy that molecules must have in order for their collision to lead to the formation of molecules of a new substance is called activation energy this reaction E a, usually measured in J/mol, kJ/mol. Molecules with this energy are called active molecules.

In Fig. 3 shows energy profiles:

a) endothermic reaction, + H = –Q,

N 2 + O 2 2NO – Q;

b) exothermic reaction, – H = +Q,

H 2 + I 2 2HI + Q.

During the reaction, chemical bonds in active molecules weaken and new bonds arise between particles of reacting substances, a transition state is formed - an activated complex, when the old bonds are not completely destroyed, but new ones have already begun to be built. Activation energy is the energy required for the formation of an activated complex. The energy barrier varies; the lower it is, the easier and faster the reaction occurs.

The point located at the top of the energy barrier is called transition state. From this point, the system can freely pass into the reaction product or return to its original state (Fig. 4).

Activation energy is the factor by which the nature of the reactants influences the rate of a reaction. For some reactions it is small, for others it is large. If the activation energy is small (< 40 кДж/моль), то большая часть столкновений между молекулами реагирующих веществ приводит к реакции. Скорость таких реакций велика. Если энергия активации велика (>40 kJ/mol), then in this case only a small part of the collisions of molecules or other particles leads to a reaction. The speed of such a reaction is low.

The reaction rate at a given time can be calculated if you know the number of active collisions of reacting particles per unit time. Therefore, the dependence of the reaction rate on temperature can be written as:

0 exp(– E a/ RT),

where 0 is the reaction rate, provided that each collision leads to interaction ( E a = 0). This expression for the reaction rate is - Arrhenius equation- an important equation in chemical kinetics (for its practical application, see Appendix No. 2, students make reports).

Why do chemical reactions occur at different rates? This is the main question that faces the teacher and the children in the lesson. Students answer it theoretically by conducting laboratory experiments in groups and solving problems.

Group work

The work of the groups includes the following activities:

Experimental study of factors affecting the rate of chemical reactions;

Observation and analysis of the experimental results obtained;

Completing laboratory sheets reflecting progress and conclusions.

A prerequisite for successful work in groups and the implementation of assigned tasks is to provide each student’s workplace with the necessary equipment and visual aids. During work, the teacher approaches all groups and, if necessary, provides advisory assistance. Below is the content of the tasks for each group.

Laboratory experiment No. 1.
Dependence of the rate of a chemical reaction
from the nature of the reactants

Target. Reinforce the concept of “rate of a chemical reaction” and identify its dependence on the nature of the reacting substances.

Equipment and reagents. Rack with test tubes; zinc granules, magnesium tape, hydrochloric acid solution.

Demonstration experience.
Reaction rate and its dependence
on the concentration of starting substances

Target. Visually demonstrate what the rate of a chemical reaction is and how it depends on the concentration of the starting substances.

Equipment and reagents. Computer with measuring unit, optical density sensor at wavelength = 525 nm, cuvette, magnetic stirrer, 5 ml syringe, 100 ml graduated cylinder; solutions – 1M KI, 0.1M K 2 S 2 O 8, distilled water.

Chemical essence of the process. The reaction of oxidation of iodide ion with persulfate is studied:

2I – + S 2 O 8 2– = I 2 + 2SO 4 2– .

The reaction is carried out in excess of potassium iodide. The released iodine turns the solution brown. The iodine concentration is determined by the color intensity of the solution using an optical density sensor at 525 nm.

Preparing for work. An optical density sensor tuned to a wavelength of 525 nm is connected to the first channel of the measuring unit. Turn on the sensor in time-dependent mode, pour 10 ml of 1M KI solution and 90 ml of distilled water into the cuvette. Set up the sensor.

Execution. Start the mixing process. Take 5 ml of K 2 S 2 O 8 solution into a syringe, quickly pour it into the cuvette, simultaneously starting the measurement process by pressing the “Start” screen button. The measurement is stopped when the optical density reaches 0.5.

Repeat the experiment using 20 ml of KI solution and 80 ml of water.

Comments. The rate of a reaction is the change in the concentration of reactants or reaction products per unit time. The speed of the reaction depends on the concentration of the starting reagents at a given time.

Inferred concepts. Reaction rate, its dependence on concentration.

Conclusions. As reactants are consumed during the reaction, the rate slows down.

As the concentration of the starting reagent increases, the reaction rate increases. Moreover, in this case, when the concentration was doubled, the reaction rate also doubled.

Laboratory experiment No. 2.
Effect of temperature on speed

Target. Reinforce the concept of “rate of a chemical reaction” and explore the effect of temperature on the rate of a chemical reaction.

Equipment and reagents. Rack with test tubes, pipette, alcohol lamp, test tube holder; copper(II) oxide, sulfuric acid solution (1:3).

Laboratory experiment No. 3.
Dependence of the rate of a chemical reaction
from the contact surface area
reactants

Target. Reinforce the concept of “rate of a chemical reaction” and identify its dependence on the size of the contact surface of the reacting substances.

Equipment and reagents. Rack with test tubes, glass rod; iron filings, iron nail, copper(II) chloride solution.

Presentation of the results of group work, their discussion

The order in which the results are presented is determined by the group numbers (in turn). Students speak at the board using tables filled in based on the results of their laboratory experiments. A brief discussion of the results of the groups’ work is organized and conclusions are formulated. The teacher points out another factor that affects the rate of a chemical reaction - the presence of a catalyst.

Catalysts- these are substances that speed up a chemical reaction, inhibitors- These are substances that slow down a chemical reaction. The catalysts and inhibitors themselves are not consumed in the reaction and are not included in the reaction products.

Catalysis is the process of changing the rate of a reaction under the influence of a catalyst. The action of the catalyst is selective. Reactions that occur with the participation of a catalyst are called catalytic reactions.

Mechanism of homogeneous catalysis

Often reactions are slow, because... their activation energy E a is large (Fig. 5):

A + B A B AB.

Catalyst (K) speeds up the reaction:

Activation energies E"a and E"" and are small, so the reactions proceed quickly.

With the participation of a catalyst, a decrease occurs E and, a gain in energy is formed and the reaction proceeds faster.

V i d y c a t a l i s a

1. Homogeneous catalysis– starting materials and catalyst – single-phase system.

For example, natural fluctuations in the thickness of the Earth's ozone layer are associated with changes in solar activity. In the upper layers of the atmosphere, the destruction of the ozone layer occurs, catalyzed by nitrogen oxides:

2. Heterogeneous catalysis– the starting materials and the catalyst form a multiphase system.

The mechanism of heterogeneous catalysis includes five stages:

Diffusion - reacting molecules diffuse to the surface of the catalyst;

Adsorption - reactants accumulate on the surface of the catalyst;

Chemical reaction - the surface of the catalyst is heterogeneous, there are active centers on it, they weaken the bonds between atoms in the adsorbed molecules, the reacting molecules are deformed, sometimes break up into atoms, which facilitates the occurrence of a chemical reaction;

Desorption - product molecules are first retained by the surface of the catalyst and then released;

Diffusion - product molecules diffuse from the surface of the catalyst.

Figuratively speaking, the mechanism of action of the catalyst can be compared to tourists crossing a mountain pass. Tourists unfamiliar with the area will choose the most obvious but most difficult route, requiring a long climb and descent over the top of the mountain. An experienced guide (catalyst) will lead his group along the path, past the top. Although this path is winding, it is less difficult; it is easier to reach the final point along it, after which the guide returns to the starting point.

A special group consists of catalysts that act in living organisms. Such catalysts are called enzymes or enzymes.

Enzymes (enzymes)- these are protein molecules that accelerate chemical processes in biological systems (there are about 30 thousand different enzymes in the body, each of them accelerates the corresponding reaction).

Demonstration experience.
Catalytic decomposition of hydrogen peroxide
(conducted by teacher)

2H 2 O 2 2H 2 O + O 2 .

5 ml of a pharmaceutical solution of hydrogen peroxide is poured into three test tubes. The first test tube is a control one; for comparison, a piece of raw meat is dropped into the second test tube with tweezers, and a piece of raw carrot is placed into the third test tube. “Boiling” is observed in two test tubes, except the first. Smoldering splinters are introduced into the second and third test tubes, which flare up because oxygen is released. The teacher explains that the decomposition of hydrogen peroxide occurs without a catalyst, but much more slowly. The reaction may take several months. Rapid reactions in other test tubes demonstrate the work of the enzyme catalase, which is found in both plant and animal cells.

The effectiveness of the catalase enzyme can be illustrated by data on the decomposition of H 2 O 2 in an aqueous solution.

Enzymes are learned in more detail when studying the 11th grade chemistry course.

The development of sustained attention, the ability to observe experience, conduct analysis, and draw conclusions begins with a demonstration experiment. The group form of work allows you to effectively obtain knowledge, fostering a sense of teamwork.

The use of a set of equipment with a computer measuring unit and sensors (temperature, optical density, electrical conductivity, pH level) significantly expands the capabilities of the demonstration experiment, because allows us to look inside the process, which we could not do previously by studying this topic only theoretically. The study of quantitative laws is one of the key and most complex topics in chemistry (see Appendix No. 3 “Parameters used in quantitative chemical calculations”).

In this lesson, we are interested in reaction parameters. In previous lessons, students became familiar with thermodynamic parameters, and the parameters of matter and environment will be studied in subsequent lessons.

Lesson summary, reflective analysis

The teacher sums up the lesson. Students fill out student work control sheets, on which they indicate their class, last name, first name, evaluate their work in the lesson, group work, understanding of the topic (“bad”, “good”, “excellent”).

Students answer questions.

1. In what mood do you leave the lesson?

2. Why is the lesson interesting for each group and each student?

3. What is the benefit of this lesson for you?

4. What difficulties did you encounter in the lesson?

Different classes offer different questions. From experience we can say that at the reflective stage, students give a high rating to the lesson (“5”, less often “4”), note the unusualness, clarity, richness of the lesson, high emotional level, logic, and interesting information material. The most important technology in a lesson is collaboration between teacher and students. Together, common goals are achieved, students better assimilate the material and apply the acquired knowledge.

Homework

Along with the textbook paragraphs, each group receives an individual task to study the influence of a particular factor on the rate of a chemical reaction.

Task 1. At t= 30 °C the reaction proceeds in 25 minutes, and at t= 50 °C – in 4 minutes. Calculate the temperature coefficient of the reaction.

Task 2. The interaction of aluminum with chlorine proceeds according to the equation:

2Al (solid) + 3Cl 2 (g) = 2AlCl 3 (solid).

The initial concentration of chlorine is 0.05 mol/l. The reaction rate constant is 0.2 l/(mol s).

Write a mathematical expression for the reaction rate. How does the reaction rate change compared to the initial one if the pressure in the system is increased 6 times?

Task 3. In two identical vessels, decomposition reactions were carried out with the formation of oxygen and hydrogen. In 10 s, 22.4 liters of O 2 were obtained in the first vessel, and 4 g of H 2 in the second vessel. Which chemical reaction has the fastest rate? How many times?

Task 4. Suggest ways to increase the reaction rate 16 times by changing the concentrations of the starting substances:

a) 2Cu (solid) + O 2 (g) = 2CuO (solid);

b) 2H 2 (g.) + O 2 (g.) = 2H 2 O (g.).

A special feature of the lesson is that it offers material that goes beyond the scope of the textbook. This is necessary both to increase general erudition and for future applicants. Additional material in the specialized class is based mainly on materials from entrance exams to various universities.

The goal of educational technologies is to increase the efficiency of the educational process. The main thing in any technology is to focus on the student’s personality. Pedagogical technology is a set of interrelated means, methods, and processes necessary for a targeted impact on the formation of a personality with given qualities. I use a student-centered approach in my lessons. As a result, students are able to approach the study of the material more consciously and creatively. It is the technology of cooperation between teacher and student that is important in achieving high results. The active use of elements of pedagogical technologies in the classroom contributes to the development of the student's motivational sphere, intelligence, independence, and the ability to control and manage his educational and cognitive activities.

My subject is chemistry, but I also teach human studies. Using new approaches in education allows you to look at your subject differently. The main thing is to see a person in every student.

Chemistry is the science of substances. I approach the study of substances not only from the point of view of their practical significance for society, but also from the position of a philosophical understanding of the world. In the lessons of chemistry and human studies, I show the integrity of the world and man, I try to reveal to children the infinity and harmony of life, to cultivate the desire to understand and know themselves, the desire to improve themselves, to work on themselves in order to improve their lives. I am pleased with the guys' interest in these problems. And I think it’s useful for us teachers to reflect on this as well. Only by improving and developing ourselves can we teach children.

APPENDIX No. 1

The structure of problem-search activities of teachers and students
on the study of the properties of substances and the essence of chemical reactions
(possible use of information technology)

APPENDIX No. 2

Practical use of the Arrhenius equation

Example 1. The speed (frequency) of crickets' beeping obeys, although not quite strictly, the Arrhenius equation, gradually increasing in the temperature range from 14.2 °C to 27 °C, with an effective activation energy E a = 51 kJ/mol. Based on the frequency of chirps, you can determine the temperature quite accurately: you need to count their number in 15 seconds and add 40, you get the temperature in degrees Fahrenheit (F) (Americans still use this temperature scale).

So, at 55 F (12.8 °C) the chirping frequency is 1 chirp/s, and at 100 F (37.8 °C) - 4 chirps/s.

Example 2. In the temperature range from 18 °C to 34 °C, the sea turtle's heart rate is consistent with the Arrhenius equation, which gives the activation energy
E a = 76.6 kJ/mol, but at lower temperatures the activation energy increases sharply. This may be due to the fact that at low temperatures the turtle does not feel very well and its heart rate begins to be controlled by other biochemical reactions.

Example 3. Particularly interesting are attempts to “put Arrhenius dependence” on human psychological processes. Thus, people with different body temperatures (from 36.4 °C to 39 °C) were asked to count the seconds. It turned out that the higher the temperature, the faster the counting.
(E a = 100.4 kJ/mol). Thus, our subjective sense of time obeys the Arrhenius equation. The author of the sociological study, G. Hogland, suggested that this is due to certain biochemical processes in the human brain.

The German researcher H. von Foerstler measured the rate of forgetting in people with different temperatures. He gave people a sequence of different signs and measured the time during which people remembered this sequence. The result was the same as Hoagland's: Arrhenius dependence with E a = 100.4 kJ/mol.

These examples show that many processes in nature, including psychological ones, obey the Arrhenius equation with fairly high activation energies E A. This last point is especially important because E and physical processes (for example, viscous fluid flow) usually do not exceed 20 kJ/mol. A high activation energy usually means that chemical bonds are broken. So in all the examples discussed, there are undoubtedly real chemical reactions taking place (obviously enzymatic).

APPENDIX No. 3

Reaction rate constant k in equation (72) there is a function of temperature; An increase in temperature generally increases the rate constant. The first attempt to take into account the influence of temperature was made by Van't Hoff, who formulated the following empirical (i.e., based on experimental data) rule: With an increase in temperature for every 10 degrees, the rate constant of an elementary chemical reaction increases by 2–4 times.

The value showing how many times the rate constant increases when the temperature increases by 10 degrees is van't Hoff temperature coefficient(γ). Mathematically, van't Hoff's rule can be written as follows:

Van't Hoff's rule is applicable only in a narrow temperature range, since the temperature coefficient of the reaction rate γ is itself a function of temperature; at very high and very low temperatures, γ becomes equal to unity (i.e., the rate of a chemical reaction ceases to depend on temperature).

The interaction of particles occurs during their collisions; however, not every collision results in a chemical interaction between the particles. Arrhenius postulated that collisions of molecules will be effective (i.e., will lead to a reaction) only if the colliding molecules have a certain amount of energy - activation energy. Activation energy E A – the necessary excess of energy (compared to the average energy of the reacting substances) that molecules must have in order for their collision to lead to a chemical interaction.

Consider the path of some elementary reaction

A ––> B

Since the chemical interaction of particles is associated with the breaking of old chemical bonds and the formation of new ones, it is believed that every elementary reaction passes through the formation of some unstable intermediate compound called activated complex:

A ––> K # ––> B

The formation of an activated complex always requires the expenditure of a certain amount of energy, which is caused, firstly, by the repulsion of electron shells and atomic nuclei when particles approach each other and, secondly, by the need to construct a certain spatial configuration of atoms in the activated complex and redistribute the electron density. Thus, on the way from the initial state to the final state, the system must overcome a kind of energy barrier (Fig. 26). The activation energy of a reaction is equal to the excess of the average energy of the activated complex over the average energy level of the reactants. Obviously, if the forward reaction is exothermic, then the activation energy of the reverse reaction E" A higher than the activation energy of the direct reaction E A. For an endothermic reaction, an inverse relationship is observed between E" A And E" A. The activation energies of the forward and reverse reactions are related to each other through the change in internal energy during the reaction - the thermal effect of the reaction ( D.U. in Fig. 26.).

Rice. 26. Energy profile of a chemical reaction. E ref– average energy of particles of starting substances, E cont– average energy of reaction product particles.

Since temperature is a measure of the average kinetic energy of particles, an increase in temperature leads to an increase in the proportion of particles whose energy is equal to or greater than the activation energy, which leads to an increase in the reaction rate constant (Fig. 27):

Fig.27. Energy distribution of particles. Here n E /N– fraction of particles with energy E; E 1 T 1, E 2- average particle energy at temperature T 2, E 3- average particle energy at temperature T 3 ;(T 1

The dependence of the rate constant on temperature is described by the Arrhenius equation:

Here A– pre-exponential factor. From equation (58) it is easy to show its physical meaning: the quantity A equal to the reaction rate constant at a temperature tending to infinity.

Let us take the logarithm of relation (88):

As can be seen from the last expression, the logarithm of the rate constant depends linearly on the inverse temperature (Fig. 28); activation energy value E A and the logarithm of the pre-exponential factor A can be determined graphically (respectively, the tangent of the angle of inclination of the straight line to the abscissa axis and the segment cut off by the straight line on the ordinate axis).

Fig.28. Dependence of the logarithm of the rate constant of a chemical reaction on the inverse temperature.

Knowing the activation energy of a reaction and the rate constant at a certain temperature T 1, using the Arrhenius equation, you can calculate the value of the rate constant at any temperature T 2.

Problem 347.
Schematically depict the energy diagram of the exothermic reaction A + B ↔ AB. Which reaction - forward or reverse - is characterized by a higher rate constant?
Solution:
The reaction equation has the form: A + B ↔ AB. Since the reaction is exothermic, the final state of the system (substance AB) must have a lower energy level than the initial substances (substances A and B).

The difference between the activation energies of the forward and reverse reactions is equal to the thermal effect: H = E a(Inv.) - E a(Inv.) . This reaction occurs with the release of heat, i.e. is exothermic,< 0. Исходя из этого, энергия активации прямой реакции имеет меньшее значение, чем энергия активации обратной реакции:
E a(Ex.)< Еа (Обр.) .

The graph shows that the activation energy of the forward reaction is less than the activation energy of the reverse reaction.

Energy diagram of the exothermic reaction A + B ↔ AB:

As follows from the Arrhenius equation, the lower the activation energy, the greater the reaction rate constant. Therefore, the forward reaction, as a reaction with a lower activation energy, is characterized by a higher rate constant than the reverse reaction, a reaction with a lower activation energy.

Answer: k (Right) > k (Rev.) .

Problem 348.
Schematically depict the energy diagram of the following transformations: ,
if k 1 > k 2 > k 3 , and for the system as a whole H > 0.
Solution:
According to the conditions of the problem, if k 1 > k 2 > k 3, H > 0.

The energy diagram of the reaction A↔B→C looks like:

Since the rate constant of the forward reaction k1 is greater than the rate constant of the reverse reaction k2, the activation energy of the forward reaction must be less than the activation energy of the reverse reaction
(E a(Ex.)< E а(Обр.) . Это означает, что в результате превращения вещества сдается второй стадии реакции – (В→С), где k 2 >k 3 , then the energy barrier for this process will increase (E a ​​3 > E a 2). In accordance with these data, the maximum energy in the BC section should be higher than in the VA section. Considering that, according to the conditions of the problem for the reaction as a whole, H > 0, the maximum energy should be even greater than at the beginning of the reaction, i.e. The energy barrier for the BC process must be greater than for the AB process. This is what is shown in the energy diagram. Overall the process is endothermic
H > 0 (H 1< H 2).

Chain reactions

Problem 349.
Why in the chain reaction H 2 + C1 2 ↔ 2HC1 does the nucleation of the chain begin with the Cl* radical, and not with the H* radical?
Solution:
Chain reactions occur with the participation of active centers - atoms, ions or radicals - particles that have unpaired electrons and are therefore highly reactive (active).

In the reaction H 2 + C1 2 ↔ 2HC1 the following processes occur:

a) absorption of a quantum of radiant energy (hv) by a chlorine molecule leads to its excitation - the appearance of energetic vibrations of atoms in it, which leads to the disintegration of the chlorine molecule into atoms, i.e. a photochemical reaction occurs:

Cl2+ hv↔ Cl* .

b) The resulting chlorine atoms (radicals) Cl* attack hydrogen molecules, and in this case an HCl molecule and a hydrogen atom H* are formed:

Cl* + H2 ↔ HCl + H*

c) A hydrogen atom attacks a chlorine molecule, and in this case an HCl molecule and a chlorine atom Cl* are formed:

H* + Cl 2 ↔ HCl + Cl*

Thus, this reaction is a chain photochemical reaction, and the process of generation of radicals in the first reaction chain begins with the formation of the Cl* radical, which is formed when a chlorine molecule is irradiated with radiant energy. Absorption of a quantum of light or radiant energy (hv) by a hydrogen molecule does not occur because the energy of the quantum is insufficient to break the bond between hydrogen atoms, since the H-H bond is stronger than the
Cl-Cl.

The theory was formulated by S. Arrhenius in 1889. This theory is based on the idea that for a chemical reaction to occur, collisions between the molecules of the starting substances are necessary, and the number of collisions is determined by the intensity of the thermal motion of the molecules, i.e. depends on temperature. But not every collision of molecules leads to a chemical transformation - only an active collision leads to it.

Activation energy is a characteristic of each reaction and determines the influence of the nature of the reactants on the rate of a chemical reaction.

Arrhenius equation:

Energy profile of exo- and endothermic reaction

Exothermic reaction - with the release of heat.
CH 4 (g) + 2O 2 (g) = CO 2 (g) + 2H 2 O (g) + Q

Endothermic - with heat absorption. CaCO 3 (cr) = CaO (cr) + CO 2 (g) - Q,

Catalysis. The concept of homogeneous and heterogeneous catalysis. Catalysts positive and negative

The phenomenon of catalysis is a change in the rate of a reaction under the influence of certain substances, which at the end of the reaction remain chemically unchanged.

The homogeneous catalyst and reactant form one phase.

Heterogeneous catalyst and reactant are in different phases

Distinguish positive catalysis(acceleration of reactions) and negative catalysis(slowing down reactions)

Energy profile of the catalytic reaction

The concept of enzymatic catalysis. Features of the catalytic activity of enzymes

Enzyme catalysis– catalytic reactions occurring with the participation of enzymes – biological catalysts of protein nature. Enzyme catalysis has two characteristic features: 1) high specificity 2) High activity

The catalytic activity of an enzyme can most accurately be expressed using an indicator called molar activity, which is measured in cathals per 1 mole of enzyme (cat×mol -1 f.). This indicator shows how many substrate molecules are converted in 1 second by one enzyme molecule.

Reversible and irreversible in the direction of the reaction

Reversible reactions- chemical reactions occurring simultaneously in two opposite directions (forward and reverse), for example:

3H 2 + N 2 ⇆ 2NH 3

Irreversible These are chemical processes whose products are not able to react with each other to form the starting substances. Examples of irreversible reactions include the decomposition of Berthollet salt when heated

2КlО3 > 2Кl + ЗО2,

Chemical equilibrium constant. Thermodynamic equilibrium conditions in thermodynamic systems

The equilibrium constant is the ratio of the products of the concentrations of reaction products to the product of the concentrations of the starting substances

Thermodynamic equilibrium- a state of a system in which the macroscopic quantities of this system (temperature, pressure, volume, entropy) remain unchanged over time under conditions of isolation from the environment.

Diffusion

Diffusion is the mixing of molecules of a substance during their random thermal movement.
the process of mutual penetration of molecules or atoms of one substance between molecules or atoms of another, leading to spontaneous equalization of their concentrations throughout the occupied volume

examples: 1) dissolving milk in coffee;
2) making tea;
3) distribution of odors;

Osmosis. Endo-exosmosis

Osmosis is the result of inequality in the chemical potentials of water on opposite sides of the membrane. An ideal semi-permeable membrane allows water molecules to pass through but does not allow solute molecules to pass through.

One-way diffusion of solvent through a semi-permeable membrane separating the solution from the pure solvent.

observed when liquids come into contact through membranes.

ENDOOSMOS biol. the process of leakage (diffusion) of liquids and certain solutes from the external environment into the cell

EXOOSMOS biol. the process of leakage (diffusion) of liquids and certain solutes from a cell into the surrounding external environment

Osmosis directed into a limited volume of liquid is called endosmosome, out - exosmosome.

22. Osmotic pressure (van't Hoff's law)

Osmotic pressure is equal to the pressure that the solute would have if it were in a gaseous state in the volume of solution

Collision theory allows us to establish a mathematical relationship between the rate of reaction and the frequency of collisions, as well as the probability that the collision energy E exceeds the minimum energy Em required for the reaction to occur. This relation has the form

Reaction Rate = (Collision Frequency) (Probability that E > Em) From this relationship the following equation can be derived:

where k is the reaction rate constant; P-steric factor, having a value from 0 to 1 and corresponding to that part of the colliding molecules that have the necessary mutual orientation upon collision; Z-number of collisions, which is related to the frequency of collisions; Ea is the activation energy corresponding to the minimum collision energy that reacting molecules must have; L-gas constant; T-absolute temperature.

The two factors, P and Z, can be combined into one constant A, which is called the pre-exponential factor or Arrhenius constant. The result is the famous Arrhenius equation, which we already met in the previous section:

TRANSITION STATE THEORY

Transition state theory considers reacting molecules as a single system. It examines in detail the changes in the geometric arrangement of atoms in this system as it transforms reactants into products. The geometric position of the atoms in such a molecular system is called configuration. As the configuration of reactants turns into a configuration of products, there is a gradual increase in the potential energy of the system until it reaches a maximum. At the moment of reaching maximum energy, the molecules have a critical configuration, which is called a transition state or activated complex. Only those molecules that have sufficient total energy are able to achieve this critical configuration. As the configuration of this transition state changes to the product configuration, a decrease in potential energy occurs (Figure 9.12). The reaction coordinate in these two diagrams represents changes in the geometric arrangement of the atoms of the reacting molecules, considered as a single system, as that system undergoes a transformation starting with the reactant configuration, moving to the critical configuration, and ending with the product configuration. If intermediates are formed in a reaction, then the appearance of each intermediate corresponds to a minimum on the graph of potential energy versus reaction coordinate (Fig. 9.13).

Rice. 9.12. Energy profile of a reaction - a graph of the dependence of potential energy on the reaction coordinate, a for an exothermic reaction; b-for an endothermic reaction.

Transition state theory can be used to predict the constants A and El in the Arrhenius equation. The use of this theory and modern computer technology makes it possible to establish an accurate picture of the occurrence of chemical reactions at the molecular level.