Acceleration and deceleration of chemical processes. Big encyclopedia of oil and gas

Chemical reaction rate- change in the amount of one of the reacting substances per unit of time in a unit of reaction space.

The following factors influence the rate of a chemical reaction:

the nature of the reactants;
concentration of reactants;
contact surface of reactants (in heterogeneous reactions);
temperature;
the action of catalysts.

Active collision theory allows to explain the influence of some factors on the rate of a chemical reaction. The main provisions of this theory:

Reactions occur when particles of reagents collide, which have a certain energy.
The more reagent particles, the closer they are to each other, the more chances they have to collide and react.
Only effective collisions lead to a reaction, i.e. those in which "old ties" are destroyed or weakened and therefore "new" ones can form. For this, the particles must have sufficient energy.
The minimum excess energy required for effective collision of reagent particles is called activation energy Еа.
The activity of chemicals is manifested in the low activation energy of reactions with their participation. The lower the activation energy, the higher the reaction rate. For example, in reactions between cations and anions, the activation energy is very small, so such reactions proceed almost instantly.

Influence of the concentration of reactants on the reaction rate

With an increase in the concentration of reactants, the reaction rate increases. In order to react, two chemical particles must move closer together, so the speed of the reaction depends on the number of collisions between them. An increase in the number of particles in a given volume leads to more frequent collisions and to an increase in the reaction rate.

An increase in the rate of the reaction proceeding in the gas phase will result in an increase in pressure or a decrease in the volume occupied by the mixture.

On the basis of experimental data in 1867, the Norwegian scientists K. Guldberg, and P Vaage, and independently of them in 1865, the Russian scientist N.I. Beketov formulated the basic law of chemical kinetics, establishing dependence of the reaction rate on the concentration of reactants

Mass Action Law (MWL):

The rate of a chemical reaction is proportional to the product of the concentrations of the reactants taken in powers equal to their coefficients in the reaction equation. ("Active mass" is a synonym for the modern concept of "concentration")

aA +bВ =cC +dD, where k- reaction rate constant

ZDM is performed only for elementary chemical reactions proceeding in one stage. If the reaction proceeds sequentially through several stages, then the total rate of the entire process is determined by its slowest part.

Expressions for the rates of various types of reactions

ZDM refers to homogeneous reactions. If the reaction is heterogeneous (the reagents are in different states of aggregation), then only liquid or only gaseous reagents enter into the ZDM equation, and solid reagents are excluded, affecting only the rate constant k.

Molecularity of the reaction Is the minimum number of molecules participating in an elementary chemical process. In terms of molecularity, elementary chemical reactions are divided into molecular (A →) and bimolecular (A + B →); trimolecular reactions are extremely rare.

The rate of heterogeneous reactions

Depends on surface area of contact of substances, i.e. on the degree of grinding of substances, the completeness of mixing of reagents.
An example is wood burning. A whole log burns relatively slowly in air. If you increase the surface of contact of wood with air, splitting the log into chips, the burning rate will increase.
The pyrophoric iron is poured onto a sheet of filter paper. During the fall, the iron particles heat up and set the paper on fire.

Effect of temperature on reaction rate

In the 19th century, the Dutch scientist Van't Hoff experimentally discovered that when the temperature rises by 10 ° C, the rates of many reactions increase by 2-4 times.

Van't Hoff's rule

With an increase in temperature for every 10 ° C, the reaction rate increases by 2-4 times.

Here γ (Greek letter "gamma") - the so-called temperature coefficient or Van't Hoff coefficient, takes values from 2 to 4.

For each specific reaction, the temperature coefficient is determined empirically. It shows how many times the rate of a given chemical reaction (and its rate constant) increases with every 10 degrees increase in temperature.

Van't Hoff's rule is used to approximate the change in the reaction rate constant with increasing or decreasing temperature. A more accurate relationship between the rate constant and temperature was established by the Swedish chemist Svante Arrhenius:

How more E a specific reaction, the smaller(at a given temperature) will be the rate constant k (and rate) of this reaction. An increase in T leads to an increase in the rate constant, which is explained by the fact that an increase in temperature leads to a rapid increase in the number of "energetic" molecules capable of overcoming the activation barrier E a.

Effect of the catalyst on the reaction rate

It is possible to change the reaction rate by using special substances that change the reaction mechanism and direct it along an energetically more favorable path with a lower activation energy.

Catalysts- these are substances that take part in a chemical reaction and increase its rate, but after the end of the reaction they remain unchanged qualitatively and quantitatively.

Inhibitors- substances that slow down chemical reactions.

Changing the rate of a chemical reaction or its direction with the help of a catalyst is called catalysis .

Give examples of reactions, an increase or decrease in the rate of which has a positive or negative meaning in production or in everyday life. Please explain.

Answers:

Corrosion of metals: occurs both chemically and electrochemically in various industrial plants. Corrosion of iron pipes should be eliminated by adding alloying elements, varnishes, paints, etc. Make it the best)))

Conditions affecting the rate of chemical reactions

1) The reaction speed depends on nature of reactants... Simply put, different substances react at different rates. For example, zinc reacts violently with hydrochloric acid, and iron rather slowly.

2) The reaction speed is the greater, the higher concentration substances. With a highly diluted acid, zinc will react much longer.

3) The reaction rate increases significantly with increasing temperature... For example, to burn fuel, it is necessary to ignite it, that is, to raise the temperature. For many reactions, an increase in temperature by 10 ° C is accompanied by an increase in the rate by a factor of 2–4.

4) Speed heterogeneous reactions increase with increasing surfaces of reactants... Solids are usually ground for this. For example, in order for powders of iron and sulfur to react when heated, the iron must be in the form of fine sawdust.

Please note that in this case formula (1) is implied! Formula (2) expresses the speed per unit area, therefore it cannot depend on the area.

5) The reaction rate depends on the presence of catalysts or inhibitors.

Catalysts- substances that accelerate chemical reactions, but they themselves are not consumed. An example is the violent decomposition of hydrogen peroxide with the addition of a catalyst - manganese (IV) oxide:

2H 2 O 2 = 2H 2 O + O 2

Manganese (IV) oxide remains at the bottom and can be reused.

Inhibitors- substances that slow down the reaction. For example, corrosion inhibitors are added to the hot water heating system to extend the life of pipes and radiators. In cars, corrosion inhibitors are added to the brake, coolant.

A few more examples.

Chemical reaction. The speed of the reaction and the factors on which it depends. Chemistry lesson. Methodical development is intended for 1st year students.

Lesson type: a lesson familiarization with new material.

Theme: Chemical reaction. The speed of the reaction and the factors on which it depends.

Target: to summarize and deepen knowledge about the rate of a chemical reaction and the factors affecting it.

Tasks:

Educational task:

Developmental tasks

Educational tasks:

Equipment: TV, VCR, film.

Candle, zinc, solutions of hydrochloric and sulfuric acids.

Lesson plan:

Organizing time.

Theme and purpose.

Motivation.

Updating.

Main part.

Output.

Anchoring.

Lesson summary.

Homework.

During the classes:

1. Organizing time.

2. Theme and purpose. Write it down in a notebook.

3. Motivation.

Uch-l: "What do we mean when we say the word SPEED?"

Study:

Uch-l: “How fast can you eat candy? Cutlets?

How fast can you shop? Knit socks? Sawing boards? "

That is, SPEED is the change in a parameter per unit of time. (Write on the board)

Uch-l: Is it possible to talk about the speed of a chemical reaction?

Study: give examples of various chemical processes.

4. Updating.

Uch-l: Let's return to the topic of the lesson. What is a chemical reaction?

Message "Physical and chemical phenomena".

Oral frontal work.

Surovtseva R.P., p. 8, work No. 8. Option 1. (A, B, C, D - work in groups)

5. The main part.

Uch-l: Do all chemical processes go at the same speed?

PROBLEM: What factors determine the rate of a chemical reaction? (WRITE ON BOARD)

Step-by-step solution to the problem:

1. What is called the rate of a chemical reaction? (p. 33, read and write the definition in the notebook).

2. Teacher: So, the rates of chemical reactions are very different.

Some reactions need to be slowed down (rusting, oxidation), some - to be accelerated (getting medicines, other useful products).

REPEATING SAFETY RULES WHEN WORKING WITH REAGENTS !!!

3. Experience No. 1. Film fragment. 6 minutes

(The rate of a chemical reaction depends on the nature of the reactants.)

4. Experience number 2. Burning a candle in air and under a hood.

(For substances in a dissolved state and gases, the rate of a chemical reaction depends on the concentration of the reacting substances.)

5. Experiment No. 3. Place zinc granules in one tube, and powder in another. Pour 2 ml of diluted hydrochloric acid into both tubes.

(For substances in a solid state, the reaction rate is directly proportional to the surface of the reacting substances.)

6. Experiment No. 4. Put two pieces of zinc in two test tubes. Pour 2 ml of diluted sulfuric acid into both tubes. Warm one tube slightly, and keep the other for comparison. Oxygen begins to react with many substances at a noticeable rate even at room temperature (slow oxidation). When the temperature rises, a violent reaction begins, the reaction rate increases sharply.

(As the temperature rises, the rate of most reactions increases.)

6. CONCLUSION: The answer to the problematic question. Read page 34. Conditions affecting the rate of a chemical reaction.

To write down: CATALYSTS AND INHIBITORS.

7. Anchoring.

* Write down the reaction equations.

* Give examples of reactions, an increase or decrease in the rate of which has a positive or negative meaning in production or in everyday life.

8. Lesson summary. Estimates.

Introspection.

Lesson type: lesson-familiarization with new material.

Purpose: introduction of new concepts of inorganic chemistry: the rate of a chemical reaction, factors affecting the rate of a chemical reaction.

This lesson clarifies the features of the rate of a chemical reaction, the factors that affect the rate of a chemical reaction. In subsequent lessons, the processes of the chemical production of sulfuric and nitric acids will be considered, that is, the material of this lesson will be relied on.

The specificity of this lesson lies in the fact that students are first introduced to the concept of the rate of a chemical reaction.

This lesson is 3 in the topic "Fundamentals of Theoretical Chemistry".

The main task of this section is to form the concept of the basic laws of the course of chemical reactions.

The level of development of mental operations in students of this group does not correspond to the socio-psychological standard. Nobody has a high level. In order to develop the ability to draw analogies, to generalize, when planning a lesson, I decided to use the problematic method to study the main material of the lesson.

In this lesson, the following tasks were solved:

Educational task: expand and deepen knowledge of chemical kinetics.

Developmental tasks: to improve the ability of students to analyze, compare, draw conclusions.

Educational tasks: to continue the formation of worldview concepts: about the cognizability of nature, the cause-and-effect relationship between the composition and properties.

Since this lesson is the third in the topic, the following lesson structure was chosen:

Several minutes have been allotted for updating;

Most of the time is devoted to learning new material;

The remaining time was spent on consolidation.

The main emphasis of the lesson was made on the elucidation of the factors affecting the rate of a chemical reaction.

In the lesson were used: explanatory, illustrative, reproductive methods. A problematic method was chosen to reveal the main material. The content of this topic allows you to build it as a system of cognitive problems and conduct a study, constantly involving students in finding answers to certain questions.

Selected forms of training: frontal, group, individual.

The frontal form of work is used when solving basic cognitive problems in order to activate the work of each student, develop the ability to draw analogies, and generalize material.

Individual and group forms of work are used at the stage of actualization, since already familiar material is being repeated.

Control over the assimilation of knowledge, abilities and skills was carried out at various stages of the lesson in various forms and methods:

* at the stage of actualization - an individual survey;

* at the stage of learning new material - visually, individually, frontally.

* individual control was carried out at the stage of consolidating knowledge.

In the lesson, TV, VCR, and a film were used as teaching aids.

The high performance of students in the lesson was maintained due to the problematic presentation of the material (at the main stage of the lesson), the use of technical teaching aids, and work in groups.

I tried to maintain the psychological atmosphere with a benevolent attitude towards the students. I tried to leave my problems outside the classroom.

Chemical processes.

Sulfuric acid production.

Rust formation.

Blackening of silver.

Oxidation of food.

Getting medicines.

Souring milk.

Protein decay.

Sauerkraut.

Laundry.

Cooking food.

Burning a candle.

2. Formation of rust.

3. Blackening of silver.

5. Receiving medicines.

6. Souring milk.

7. Protein decay.

8. Sauerkraut.

9.Laundry washing.

10. Cooking.

11. Candle burning.

12. Combustion of gasoline in the engine

1.Production of sulfuric acid.

2. Formation of rust.

3. Blackening of silver.

4. Oxidation of food.

5. Receiving medicines.

6. Souring milk.

7. Protein decay.

8. Sauerkraut.

9.Laundry washing.

10. Cooking.

11. Candle burning.

12. Combustion of gasoline in the engine

1.Production of sulfuric acid.

2. Formation of rust.

3. Blackening of silver.

4. Oxidation of food.

5. Receiving medicines.

6. Souring milk.

7. Protein decay.

8. Sauerkraut.

9.Laundry washing.

10. Cooking.

11. Candle burning.

12. Combustion of gasoline in the engine

1.Production of sulfuric acid.

2. Formation of rust.

3. Blackening of silver.

4. Oxidation of food.

5. Receiving medicines.

6. Souring milk.

7. Protein decay.

8. Sauerkraut.

9.Laundry washing.

10. Cooking.

11. Candle burning.

12. Combustion of gasoline in the engine

1.Production of sulfuric acid.

2. Formation of rust.

3. Blackening of silver.

4. Oxidation of food.

5. Receiving medicines.

6. Souring milk.

7. Protein decay.

8. Sauerkraut.

9.Laundry washing.

10. Cooking.

11. Candle burning.

12. Combustion of gasoline in the engine

The presence of foreign substances in it has a great influence on the course of the chemical process. These are the walls of the vessel in which the process takes place, and the air environment of the process, and special substances introduced into the process and designed to influence its course.

Those foreign substances that contribute to the chemical process are called catalysts; which inhibit the process - inhibitors.

Consider the mechanisms of action of both.

These mechanisms are based on such phenomena as the resonance of thermal vibrations, deformation of atoms and molecules, and a change in the electrical state.

Let's start with resonance. The stuck together sections of the grooves of different atoms vibrate. If these vibrations are swayed, that is, they are brought into resonance, then this will accelerate the breaking of bonds of the stuck together sections and the separation of atoms.

Thermal vibrations can be swayed by non-contact and contact methods.

The non-contact method is carried out using light, visible and invisible.

So silver chloride Cl 2 (Ag) and silver bromide Br (Ag) decompose under the influence of light with the release of pure silver Ag. Chemical photography is based on this.

The substances that gave rise to light in this case can be considered accelerators (catalysts).

In the contact method, the vibrating atoms of the accelerator come into contact with the stuck together sections of the separated atoms and molecules and swing them in resonance until they are completely ruptured. This is equivalent to an increase in the temperature of the particles being separated.

Example. Berthollet's salt (K) Cl 2 (K) (O) 6 decomposes into potassium chloride (K) Cl 2 (K) and pure oxygen O 2 at a temperature of 400 degrees. But if the molecules of Berthollet's salt come into contact with the molecules of manganese oxide O 2 (Mg), then this decomposition occurs already at a temperature of 200 degrees. Manganese oxide in this example plays the role of an accelerator (catalyst).

When not the disintegration of molecules is required, but the combination of atoms into molecules (that is, the reverse process), the phenomenon of deformation of adhesion elements is used.

Suppose, in its usual form, the suction groove of an atom (or molecule) is not convenient for sticking together. It can be made more convenient if the atom is preliminarily attached with its reverse side to the atom (molecule) of the catalyst.

Example. In free form and under normal conditions, the suction trough of sulfur dioxide S (O 2 (is sharply bent and inconvenient for sticking with an additional oxygen atom (O. If a sulfur dioxide molecule collides with a vanadium oxide molecule O 5 (V 2) and sticks to it) sulfur side at least for a moment, its concave groove will open and an additional oxygen atom will adhere to it; sulfur trioxide S (O 2 (O (.

This chemical process takes place at a temperature of 500 degrees. With such heating, the molecules of atmospheric oxygen O 2 decompose into atoms, and the newly formed molecules of sulfur trioxide are easily detached from vanadium oxide.

Vanadium oxide acts in this case as an accelerator.

The accelerating (catalytic) effect on the course of the chemical process of electricity is especially expressive when substances are dissolved. Solvents act as accelerators in such cases.

First, the solvent molecules adhere to the surface molecules of the soluble substance and squeeze out electrons from them. At the second stage, the squeezed out electrons are introduced between the molecules of the soluble substance and, like a wedge, break their mutual bonds.

As a result, the soluble substance is fragmented into molecules.

The mechanisms of retarders (inhibitors) that inhibit the chemical process are the same, only their goals are opposite. So light, thermal vibrations and electrons, acting in their own style, can slow down those chemical processes in which molecules must combine.