The main factors affecting the rate of all reactions are the concentration of reactants, temperature, and the presence of a catalyst.

Effect of concentration. Increasing the concentration of interacting substances is one of the most common methods of intensifying processes. The dependence of the rate of chemical reactions on concentration is determined by the law of mass action. According to this law, the speed chemical reaction is directly proportional to the product of the concentrations of the reacting substances in a power equal to the stoichiometric coefficient in front of the formula of the substance in the reaction equation. For example, in the production of molasses for the reaction of neutralizing hydrochloric acid with sodium carbonate, the rate can be calculated using the following equation:

2HCl + Na 2 CO 3 = 2NaCl + H 2 O + CO 2;

The law of mass action is generally written as follows:

where TO- proportionality factor, called the reaction rate constant; C n and Сь - concentration of substances a and B, participating in a chemical reaction; pit - stoichiometric coefficients.

If we accept that, then v = TO, that is, the reaction rate constant is numerically equal to the reaction rate at a concentration of reactants equal to unity. Rate constant depends on the nature of the reacting substances, temperature, the presence of a catalyst and does not depend on the concentration of substances participating in the chemical reaction. The rate constant of this reaction at a given temperature is constant.

To determine the rate constants of the reaction depending on the molecularity and order of the reaction, the corresponding formulas are derived.

Molecularity reaction is determined by the number of molecules participating in the elementary act of chemical interaction. If this requires one molecule, then the reactions are called monomolecular . An example of such a reaction is the decomposition reaction of CaCO3 under the action of high temperature when calcining limestone in kilns at beet sugar factories:

CaCO 3 = CaO + CO 2.

With the participation of two molecules, the reactions are called bimolecular, three - trimolecular . These can be molecules of one or different substances. The above reaction of hydrochloric acid with sodium carbonate is trimolecular.

Reaction order is the sum of the exponents at the concentrations of substances in the equation of the law of mass action. The reaction rate of the first order is proportional to the concentration in the first degree, the reaction rates of the second and third orders are proportional, respectively, to the concentrations in the second and third degrees. However, the order of the reaction can be lower than its molecularity if any substance is in excess and therefore its concentration can be considered practically unchanged. For example, upon inversion of sucrose into aqueous solution Hcl

where a - initial concentration of the substance; NS - the amount of substance that has reacted during a given period of time t; (a - x) - concentration of a substance at a moment in time, i.e.

For a second-order reaction, the reaction rate constant

Hydrolysis time

and the rate constant at a temperature of t + 10 ° Kt + 10, then the ratio of these constants is temperature coefficient of reaction rate :

If we take g = 2 (the maximum value of the coefficient), then with an increase in the reaction temperature by 50 ° C, the reaction rate will increase 32 times.

More precisely, the effect of temperature on the rate of chemical reactions is expressed by a ratio obtained experimentally. This dependence is as follows:

where B and a - constants for a given reaction; T " - temperature, K.

The nature of the influence of temperature and concentration of reactants on the rate of chemical reactions can be explained by the theory of active collisions.

According to this theory chemical interaction between molecules is possible only when they collide, however, effective collisions lead to chemical reactions, that is, not all colliding molecules enter into the reaction, but only molecules that have a certain energy that is excess in comparison with the average. Molecules with this energy are called active . The excess energy of molecules is called activation energy .

For chemical reactions to occur, it is necessary to break the intramolecular bonds in the molecules of the reacting substances. If the colliding molecules have high energy and it is enough to break bonds, then the reaction will proceed; if the energy of the molecules is less than necessary, then the collision will be ineffective and the reaction will not proceed.

As the temperature rises, the number of active molecules increases, the number of collisions between them increases, resulting in an increase in the reaction rate. With an increase in the concentration of reactants, the total number of collisions, including effective ones, also increases, resulting in an increase in the reaction rate.

Effect of the catalyst.Catalyst is a substance that dramatically changes the rate of reaction. In the presence of catalysts, reactions are accelerated thousands of times, and can proceed at lower temperatures, which is economically beneficial. The importance of catalysts in organic synthesis is great - in the processes of oxidation, hydrogenation, dehydrogenation, hydration, etc. The more active the catalyst, the faster the catalytic reactions proceed. Catalysts can accelerate one reaction, a group of reactions, or reactions of different types, that is, they have individual or group specificity, and some of them are suitable for many reactions. For example, hydrogen ions accelerate the reactions of hydrolysis of proteins, starch and other compounds, hydration reactions, etc. There are catalytic reactions in which the catalyst is one of the intermediate or final reaction products. These reactions proceed at a low rate in the initial period and with an increasing rate in the subsequent.

The catalysts are mainly pure metals (nickel, cobalt, iron, platinum) and in the form of oxides or salts (vanadium oxide, aluminum oxide), compounds of iron, magnesium, calcium, copper, etc. Inorganic catalysts are thermally stable, and reactions with they proceed at relatively high temperatures.

In the environment where the reaction takes place, there are always foreign substances. This circumstance has a different effect on the catalyst: some of them are neutral, others enhance the effect of the catalyst, and still others weaken or suppress it. Substances that poison the catalyst are called catalytic poisons .

There is a concept of catalysis, homogeneous or heterogeneous. In heterogeneous catalysis, the reactants are usually in a liquid or gaseous state, while the catalyst is in a solid state, and the reaction proceeds at the interface between two phases, i.e., on the surface of a solid catalyst.

For example, the catalytic reaction of hydrogenation of fats is three-phase: the catalyst - metallic nickel - forms a solid phase, hydrogen - gaseous, and fat - liquid. Therefore, in this case we are talking about heterogeneous catalysis.

With heterogeneous catalysis great importance have a method of obtaining a catalyst, the conditions of the process, the composition of impurities, etc. Catalysts must have significant selectivity, activity and retain these properties for a long time.

Mechanism homogeneous catalysis explained by the theory of intermediates. When adding a catalyst, the reaction goes through several intermediate stages that require less activation energy than a direct reaction without a catalyst, which leads to an enormous increase in the reaction rate.

A slow process, such as a reaction

A + B = AB,

in the presence of a catalyst TO occurs in two stages: A + K = AK(intermediate); AK + B = AB + K.

Each of these stages proceeds with a low activation energy and, therefore, with a high speed. The catalyst forms an intermediate which, when reacted with another substance, regenerates the catalyst.

Many homogeneous reactions are catalyzed by the action of H + and OH ~ ions. Such reactions include inversion of sucrose, hydrolysis of esters, including fats. Metal ions catalyze oxidation and hydrolysis reactions. For example, copper catalyzes the oxidation of ascorbic acid; therefore, equipment for processing fruits and vegetables cannot be made from copper and its alloys. The oxidation of edible fats is accelerated by the action of ions of copper, iron, manganese; therefore, fats cannot be stored in metal containers.

The main disadvantage of homogeneous catalysis is that it is difficult to separate the catalyst from the final mixture (liquid or gas).

From this, part of it is irretrievably lost, and the product is contaminated with it.

This does not happen with heterogeneous catalysis, and this is the main reason for its widespread use in industry. This type of catalysis is accompanied by the formation of intermediates. They are formed on separate areas of the catalyst surface, in the so-called active sites, which occupy a small part of its surface.

If the active sites are blocked, for example, with catalytic poisons, the catalyst loses its activity. To increase the surface and, consequently, the number of active sites of the catalyst, it is crushed. To prevent the catalyst from being carried away by the gas current, it is applied to an inert carrier with a developed surface (silica gel, asbestos, pumice, etc.).

Most catalytic reactions are positive, i.e., in the presence of a catalyst, their rate increases. However, negative catalysis occurs when the catalyst slows down the rate of the reaction. In this case, the catalyst is called inhibitor. If an inhibitor inhibits the oxidation process, it is called antioxidant or antioxidant.

The study of the rate of a chemical reaction and the conditions affecting its change is engaged in one of the areas of physical chemistry - chemical kinetics. She also examines the mechanisms of these reactions and their thermodynamic validity. These studies are important not only for scientific purposes, but also for monitoring the interaction of components in reactors in the production of all kinds of substances.

The concept of speed in chemistry

The reaction rate is usually called a certain change in the concentrations of the reacting compounds (ΔС) per unit time (Δt). The mathematical formula for the rate of a chemical reaction is as follows:

ᴠ = ± ΔC / Δt.

The reaction rate is measured in mol / l ∙ s, if it occurs throughout the entire volume (that is, the reaction is homogeneous) and in mol / m 2 ∙ s, if the interaction occurs on the surface separating the phases (that is, the reaction is heterogeneous). The “-” sign in the formula refers to the change in the values ​​of the concentrations of the initial reacting substances, and the “+” sign - to the changing values ​​of the concentrations of the products of the same reaction.

Examples of reactions with different rates

Interactions chemical substances can be carried out at different speeds. So, the rate of growth of stalactites, that is, the formation of calcium carbonate, is only 0.5 mm per 100 years. Some biochemical reactions are slow, such as photosynthesis and protein synthesis. Corrosion of metals proceeds at a rather low rate.

The average speed can be characterized by reactions requiring from one to several hours. An example would be the preparation of food, which is accompanied by the decomposition and conversion of compounds contained in foods. The synthesis of individual polymers requires heating the reaction mixture for a certain time.

An example of chemical reactions, the rate of which is quite high, can serve as neutralization reactions, the interaction of sodium bicarbonate with a solution of acetic acid, accompanied by the release of carbon dioxide. You can also mention the interaction of barium nitrate with sodium sulfate, in which the precipitation of insoluble barium sulfate is observed.

A large number of reactions can proceed with lightning speed and are accompanied by an explosion. A classic example is the interaction of potassium with water.

Factors affecting the rate of a chemical reaction

It is worth noting that the same substances can react with each other at different rates. So, for example, a mixture of gaseous oxygen and hydrogen may not show signs of interaction for a rather long time, however, when the container is shaken or hit, the reaction becomes explosive. Therefore, chemical kinetics and identified certain factors that have the ability to influence the rate of a chemical reaction. These include:

• the nature of the interacting substances;
• concentration of reagents;
• temperature change;
• the presence of a catalyst;
• pressure change (for gaseous substances);
• contact area of ​​substances (if we talk about heterogeneous reactions).

Influence of the nature of matter

Such a significant difference in the rates of chemical reactions is explained by different values ​​of the activation energy (E a). It is understood as a certain excess amount of energy in comparison with its average value required for a molecule in a collision in order for a reaction to occur. It is measured in kJ / mol and the values ​​are usually in the range of 50-250.

It is generally accepted that if E a = 150 kJ / mol for any reaction, then at n. at. it practically does not leak. This energy is spent on overcoming the repulsion between the molecules of substances and on weakening the bonds in the original substances. In other words, the activation energy characterizes the strength chemical bonds in substances. By the value of the activation energy, one can preliminarily estimate the rate of a chemical reaction:

• E a< 40, взаимодействие веществ происходят довольно быстро, поскольку почти все столкнове-ния частиц при-водят к их реакции;
• 40-<Е а <120, предполагается средняя реакция, поскольку эффективными будет лишь половина соударений молекул (например, реакция цинка с соляной кислотой);
• E a> 120, only a very small part of particle collisions will lead to a reaction, and its speed will be low.

Effect of concentration

The dependence of the reaction rate on concentration is most accurately characterized by the law of mass action (MLA), which reads:

The rate of a chemical reaction is directly proportional to the product of the concentrations of the reacting substances, the values ​​of which are taken in powers corresponding to their stoichiometric coefficients.

This law is suitable for elementary one-stage reactions, or for any stage of the interaction of substances, characterized by a complex mechanism.

If you want to determine the rate of a chemical reaction, the equation of which can be conventionally written as:

αА + bB = ϲС, then,

in accordance with the above formulation of the law, the speed can be found by the equation:

V = k · [A] a · [B] b, where

a and b are stoichiometric coefficients,

[A] and [B] are the concentrations of the starting compounds,

k is the rate constant of the considered reaction.

The meaning of the rate coefficient of a chemical reaction is that its value will be equal to the rate if the concentrations of the compounds are equal to unity. It should be noted that for a correct calculation using this formula, it is worth taking into account the state of aggregation of the reagents. The concentration of the solid is taken to be unity and is not included in the equation, since it remains constant during the reaction. Thus, only concentrations of liquid and gaseous substances are included in the calculation for ZDM. So, for the reaction of obtaining silicon dioxide from simple substances, described by the equation

Si (tv) + Ο 2 (g) = SiΟ 2 (tv),

speed will be determined by the formula:

Typical task

How would the rate of the chemical reaction of nitrogen monoxide with oxygen change if the concentrations of the starting compounds were doubled?

Solution: This process corresponds to the reaction equation:

2ΝΟ + Ο 2 = 2ΝΟ 2.

Let us write expressions for the initial (ᴠ 1) and final (ᴠ 2) reaction rates:

ᴠ 1 = k · [ΝΟ] 2 · [Ο 2] and

ᴠ 2 = k · (2 ​​· [ΝΟ]) 2 · 2 · [Ο 2] = k · 4 [ΝΟ] 2 · 2 [Ο 2].

ᴠ 1 / ᴠ 2 = (k · 4 [ΝΟ] 2 · 2 [Ο 2]) / (k · [ΝΟ] 2 · [Ο 2]).

ᴠ 2 / ᴠ 1 = 4 2/1 = 8.

Answer: increased by 8 times.

Influence of temperature

The dependence of the rate of a chemical reaction on temperature was determined empirically by the Dutch scientist J. H. Van't Hoff. He found that the rate of many reactions increases by a factor of 2-4 with an increase in temperature for every 10 degrees. There is a mathematical expression for this rule, which looks like:

ᴠ 2 = ᴠ 1 γ (Τ2-Τ1) / 10, where

ᴠ 1 and ᴠ 2 - corresponding speeds at temperatures Τ 1 and Τ 2;

γ - temperature coefficient, equal to 2-4.

At the same time, this rule does not explain the mechanism of the effect of temperature on the value of the rate of a particular reaction and does not describe the entire set of regularities. It is logical to conclude that with an increase in temperature, the chaotic movement of particles increases and this provokes a greater number of their collisions. However, this does not particularly affect the efficiency of collision of molecules, since it depends mainly on the activation energy. Also, a significant role in the efficiency of particle collisions is played by their spatial correspondence to each other.

The dependence of the rate of a chemical reaction on temperature, taking into account the nature of the reactants, obeys the Arrhenius equation:

k = A 0 e -Ea / RΤ, where

And about is a multiplier;

E a is the activation energy.

An example of a problem for Van't Hoff's law

How should the temperature be changed so that the rate of a chemical reaction, for which the temperature coefficient is numerically equal to 3, grows by a factor of 27?

Solution. Let's use the formula

ᴠ 2 = ᴠ 1 γ (Τ2-Τ1) / 10.

From the condition ᴠ 2 / ᴠ 1 = 27, and γ = 3. You need to find ΔΤ = Τ 2 -Τ 1.

Transforming the original formula, we get:

V 2 / V 1 = γ ΔΤ / 10.

Substitute the values: 27 = 3 ΔΤ / 10.

Hence it is clear that ΔΤ / 10 = 3 and ΔΤ = 30.

Answer: the temperature should be increased by 30 degrees.

Effect of catalysts

In physical chemistry, the rate of chemical reactions is also actively studied by the section called catalysis. He is interested in how and why relatively small amounts of certain substances significantly increase the rate of interaction of others. Such substances that can accelerate the reaction, but are not consumed in it themselves, are called catalysts.

It has been proven that catalysts change the mechanism of the chemical interaction itself, contribute to the appearance of new transition states, which are characterized by lower energy barrier heights. That is, they contribute to a decrease in the activation energy, and hence to an increase in the number of effective collisions of particles. The catalyst cannot cause a reaction that is energetically impossible.

So hydrogen peroxide is able to decompose to form oxygen and water:

H 2 Ο 2 = H 2 Ο + Ο 2.

But this reaction is very slow and in our first-aid kits it exists unchanged for quite a long time. Opening only very old vials of peroxide, you will notice a slight popping caused by the pressure of oxygen on the walls of the vessel. The addition of just a few grains of magnesium oxide will provoke active gas evolution.

The same reaction of the decomposition of peroxide, but under the action of catalase, occurs when treating wounds. Living organisms contain many different substances that increase the rate of biochemical reactions. They are called enzymes.

Inhibitors have the opposite effect on the course of reactions. However, this is not always a bad thing. Inhibitors are used to protect metal products from corrosion, to extend the shelf life of food, for example, to prevent fat oxidation.

Contact area of ​​substances

In the event that the interaction takes place between compounds that have different states of aggregation, or between substances that are not able to form a homogeneous medium (immiscible liquids), then this factor also significantly affects the rate of the chemical reaction. This is due to the fact that heterogeneous reactions are carried out directly at the interface between the phases of the interacting substances. Obviously, the wider this boundary, the more particles have the opportunity to collide, and the faster the reaction proceeds.

For example, it goes much faster in the form of small chips than in the form of a log. For the same purpose, many solids are ground into a fine powder before being added to the solution. So, powdered chalk (calcium carbonate) acts faster with hydrochloric acid than a piece of the same mass. However, in addition to increasing the area, this technique also leads to a chaotic rupture of the crystal lattice of the substance, which means it increases the reactivity of the particles.

Mathematically, the rate of a heterogeneous chemical reaction is found as the change in the amount of substance (Δν) that occurs per unit of time (Δt) per unit surface

(S): V = Δν / (S Δt).

Influence of pressure

The change in pressure in the system has an effect only when gases take part in the reaction. An increase in pressure is accompanied by an increase in the molecules of the substance per unit volume, that is, its concentration increases proportionally. Conversely, lowering the pressure leads to an equivalent decrease in the concentration of the reagent. In this case, the formula corresponding to the ZDM is suitable for calculating the rate of a chemical reaction.

Task. How will the rate of the reaction described by the equation

2ΝΟ + Ο 2 = 2ΝΟ 2,

if the volume of a closed system is reduced by three times (T = const)?

Solution. As the volume decreases, the pressure increases proportionally. Let's write expressions for the initial (V 1) and final (V 2) reaction rates:

V 1 = k · 2 · [Ο 2] and

V 2 = k · (3 ·) 2 · 3 · [Ο 2] = k · 9 [ΝΟ] 2 · 3 [Ο 2].

To find how many times the new speed is greater than the initial one, you should separate the left and right parts of the expressions:

V 1 / V 2 = (k · 9 [ΝΟ] 2 · 3 [Ο 2]) / (k · [ΝΟ] 2 · [Ο 2]).

The concentration values ​​and rate constants are reduced, and it remains:

V 2 / V 1 = 9 3/1 = 27.

Answer: the speed has increased 27 times.

Summing up, it should be noted that the speed of interaction of substances, or rather, the quantity and quality of collisions of their particles, is influenced by many factors. First of all, it is the activation energy and the geometry of molecules, which are almost impossible to correct. As for the rest of the conditions, for an increase in the reaction rate, it follows:

• increase the temperature of the reaction medium;
• increase the concentration of the starting compounds;
• increase the pressure in the system or reduce its volume when it comes to gases;
• to bring dissimilar substances to the same state of aggregation (for example, by dissolving in water) or to increase the area of ​​their contact.

Chemical reaction rate- change in the amount of one of the reactants 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

As the temperature rises 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 .

The main factors affecting the rate of chemical reactions are the concentration of reactants, temperature, and the presence of a catalyst.

Concentration. Increasing the concentration of interacting substances is one of the most common methods of intensifying processes. The dependence of the rate of chemical reactions on concentration is determined by the law of mass action. According to this law, the rate of a chemical reaction is directly proportional to the product of the concentrations of the reacting substances to a power equal to the stoichiometric coefficient in front of the formula of the substance in the reaction equation. For example, in the production of molasses for the reaction of neutralization of hydrochloric acid with sodium carbonate, the rate can be calculated by the equation:

2HCI + Na 2 CO 3 = 2NaCI + H 2 O + CO 2

General law of mass action:

where K is the coefficient of proportionality, which is otherwise called the reaction rate constant; C a and C b - the concentration of the substance a and b participating in the chemical reaction; n and m are stoichiometric coefficients.

If we assume that C a and C b = 1, then v = K, i.e. the reaction rate constant is numerically equal to the reaction rate when the concentration of the reactants is equal to unity. The rate constant depends on the nature of the reacting substances, temperature, the presence of a catalyst and does not depend on the concentration of substances participating in the chemical reaction. The reaction rate constant at a given rate and temperature is constant.

To determine the rate constants of the reaction depending on the molecularity and the order of the reaction, the corresponding formulas are derived.

Molecularity of the reaction is determined by the number of molecules participating in the elementary act of chemical interaction. If this requires one molecule, then the reactions are called monomolecular. An example of such a reaction is the reaction of decomposition of CaCO 3 under the influence of high temperature during the burning of limestone in kilns at beet sugar factories:

CaCO 3 = CaO + CO 2

Reactions involving two molecules are called bimolecular, three - trimolecular. These can be molecules of one or different substances. The reaction of hydrochloric acid with sodium carbonate, above, is three-molecular.

The order of the reaction is the sum of the exponents at the concentrations of substances in the equation of the law of mass action. The reaction rate of the first order is proportional to the concentration in the first degree, the reaction rates of the second and third orders are proportional, respectively, to the concentrations in the second and third degrees. However, the order of the reaction can be lower than its molecularity if any substance is in excess and therefore its concentration can be considered practically unchanged.

Temperature is an important factor in determining the speed of the reaction. With increasing temperature, the reaction rate increases, which is associated with an increase in the reaction rate constant. According to the Van't Hoff rule, an increase in temperature by 10 0 С increases the reaction rate by 2-4 times (on average, 3 times). This rule is approximate and applies to reactions occurring in the temperature range from 0 to 300 0 С and in a small temperature range.

The nature of the influence of temperature and concentration of reactants on the rate of chemical reactions can be explained by the theory of active collisions. According to this theory, chemical interaction between molecules is possible only when they collide; however, effective collisions lead to chemical reactions, i.e. not all colliding molecules enter into the reaction, but only those molecules that have a certain energy, excess in comparison with the average. Molecules with this energy are called active. The excess energy of molecules is called activation energy and depends on the nature of the reacting substances.

For chemical reactions to occur, it is necessary to break the intramolecular bonds in the molecules of the reacting substances. If the colliding molecules have high energy and it is enough to break bonds, then the reaction will proceed; if the energy of the molecules is less than necessary, then the collision will be ineffective and the reaction will not proceed.

As the temperature rises, the number of active molecules increases, the number of collisions between them increases, resulting in an increase in the reaction rate. With an increase in the concentration of reactants, the total number of collisions, including effective ones, also increases, resulting in an increase in the reaction rate.

A catalyst is a substance that dramatically alters the rate of a reaction. In the presence of catalysts, reactions are accelerated thousands of times, and can proceed at lower temperatures, which is economically beneficial. The importance of catalysts in organic synthesis is great - in the processes of oxidation, hydrogenation, dehydrogenation, hydration, etc. The more active the catalyst, the faster the catalytic reactions proceed. Catalysts can accelerate one reaction, a group of reactions, or reactions of different types, i.e. they have individual or group specificity, and some of them are suitable for many reactions. For example, hydrogen ions accelerate the hydrolysis reactions of proteins, starch and other compounds.

There are catalytic reactions in which the catalyst is one of the intermediate or end products of the reaction. These reactions proceed at a low rate in the initial period and with an increasing rate in the subsequent.

Metals are mainly used as catalysts in pure form (nickel, cobalt, iron, platinum) and in the form of oxides or salts (compounds of iron, magnesium, calcium, copper, etc.). Inorganic catalysts are thermally stable and reactions with them proceed at relatively high temperatures.

The presence of foreign substances in the medium where the reaction takes place affects the catalyst in different ways: some are neutral, others enhance the effect of the catalyst, and others weaken or suppress it. The substances that poison the catalyst are called catalytic poisons.

Depending on whether the catalyst is in the same phase as the reacting substances, being uniformly distributed in the reaction medium, or forms an independent phase, one speaks of homogeneous or heterogeneous catalysis. In heterogeneous catalysis, the reactants are, as a rule, in a liquid or gaseous state, while the catalyst is in a solid state, and the reaction proceeds at the boundary of two phases, i.e. on the surface of a solid catalyst. For example, the catalytic reaction of hydrogenation of fats is three-phase: the catalyst (metallic nickel) forms a solid phase, hydrogen forms a gaseous phase, and fat forms a liquid phase. Therefore, in this case we are talking about heterogeneous catalysis.

In heterogeneous catalysis, the method of obtaining the catalyst, the conditions of the process, the composition of impurities, etc., are of great importance. Catalysts should have significant selectivity, activity, and retain these properties for a long time.

In order to explain the mechanism of homogeneous catalysis, the theory of intermediates is used. When adding a catalyst, the reaction goes through several intermediate stages that require less activation energy than a direct reaction without a catalyst, which leads to a tremendous increase in the reaction rate.

A slow process, such as a reaction

in the presence of catalyst K proceeds in two stages:

A + K = AK (intermediate);

AK + B = AB + K.

Each of these stages proceeds with a low activation energy and, therefore, with a high speed. The catalyst forms an intermediate which, when reacted with another substance, regenerates the catalyst.

Many homogeneous reactions are catalyzed by the action of H + and OH - ions. Such reactions include inversion of sucrose, hydrolysis of esters, including fats. Metal ions catalyze oxidation and hydrolysis reactions. For example, copper catalyzes the oxidation of ascorbic acid; therefore, equipment for processing fruits and vegetables cannot be made from copper and its alloys. The oxidation of edible fats is accelerated by the action of ions of copper, iron, manganese; therefore, fats cannot be stored in metal containers.

The main disadvantage of homogeneous catalysis is the difficulty in separating the catalyst from the final mixture (liquid or gas), as a result of which part of it is irretrievably lost, and the product is contaminated with it. Heterogeneous catalysis does not suffer from this drawback, which is one of the most important reasons for its widespread use in industry. This type of catalysis is accompanied by the formation of intermediates. They are formed on separate areas of the catalyst surface in the so-called active sites, which occupy a small part of its surface. If the active sites are blocked, for example, with catalytic poisons, the catalyst loses its activity. To increase the surface and, consequently, the number of active sites of the catalyst, it is crushed. To prevent the catalyst from being carried away by the gas current, it is applied to an inert carrier with a developed surface (silica gel, asbestos, pumice, etc.).

Most catalytic reactions are positive, i.e. in the presence of a catalyst, their rate increases. However, negative catalysis occurs when the catalyst slows down the rate of the reaction. In this case, the catalyst is called an inhibitor. If an inhibitor inhibits the oxidation process, it is called an antioxidant or antioxidant.

The reaction rate depends on the nature and concentration of the reactants, temperature, pressure, the presence of a catalyst and its properties, the degree of grinding of the solid phase, irradiation with light quanta and other factors.

1. The nature of the reacting substances... The nature of the reacting substances is understood as the nature of the chemical bond in the molecules of the reactants and its strength. The breaking of bonds and the formation of new bonds determine the value of the rate constant, and, thereby, affect the process of the reaction.

The value of the activation energy is the factor through which the effect of the nature of the reacting substances on the reaction rate affects: if the activation energy is low, then the rate of such a reaction is high, for example, all ion exchange reactions proceed almost instantly, the rates of reactions involving radicals are very high; if the activation energy is high, then the rate of such a reaction is low, for example, these are many reactions between substances with covalent chemical bonds, between gaseous substances.

2. Concentration of reactants... The quantitative characteristic of the dependence of the reaction rate on concentration is determined by law of mass action (Guldberg and Waage, 1867): the rate of a chemical reaction is directly proportional to the concentration of reactants raised to powers equal to the stoichiometric coefficients in the reaction equation.

For the reaction aA + bB = cC + dD, the mathematical expression for the law of mass action is:

υ = k · [A] a · [B] b or υ = k C A a C B b,

where v - the rate of the chemical reaction; [A], [V] or C A, C B- molar concentrations of reactants; a, b - stoichiometric coefficients of reactants; k- coefficient of proportionality.

Similar expressions are called kinetic equations of reactions ... Aspect ratio k in the kinetic equation is called rate constant ... The rate constant is numerically equal to the reaction rate at a reactant concentration of 1 mol / l; k depends on the nature of the reacting substances, temperature, method of expressing the concentration, but does not depend on the magnitude of the concentration of the reacting substances.

For heterogeneous reactions of solids concentration are not included in the rate equation, since the reaction takes place only at the interface. For example, the kinetic equation for the reaction of coal combustion C (tv) + O 2 (g) = CO 2 (g) will have the form: υ = k · [О 2].

The sum of the indicators of the degrees of concentration of reagents in the kinetic equation of the reaction is called order of chemical reaction ... The order for a given substance ( private order ) is defined as an exponent at the concentration of this substance. For example, the general order of the reaction: H 2 + I 2 = 2HI is equal to two, the particular orders of hydrogen and iodine are equal to one, since υ = k

3. Temperature. The dependence of the reaction rate on temperature is expressed van't Hoff rule (1884): when the temperature rises for every ten degrees, the reaction rate increases by about 2 - 4 times... Mathematical expression van't Hoff rule:

υ 2 = υ 1 γ ∆ t / 10

where υ 1 and υ 2 - the reaction rate at t 1 and t 2; ∆t= t 2 - t 1; γ - temperature coefficient showing how many times the reaction rate increases when the temperature rises by 10 ºС.

The dependence of the reaction rate constant on temperature is expressed by the Arrhenius equation (1889):

k = A e - E / RT

where E is the activation energy, cal / mol; J / mol; e is the base of the natural logarithm; A - constant, independent of temperature; R is the gas constant.

The effect of temperature on the reaction rate is explained by the fact that as the temperature rises sharply (exponentially) the number of active molecules increases.

4. Surface of reactants and pressure. V heterogeneous reactions the interaction of substances occurs at the interface, and the larger the area of ​​this surface, the higher the reaction rate... In this case, an increase in the contact surface corresponds to an increase in the concentration of reactants.

On the speed of reactions involving gaseous substances, is affected by the change pressure... A decrease or increase in pressure leads to corresponding changes in volume, and since the quantities of substances do not change, the concentrations of the reacting substances will change.

5. Catalysis. One of the methods of accelerating a chemical reaction is catalysis, which is carried out by introducing catalysts that increase the rate of the reaction, but are not consumed as a result of its occurrence. The mechanism of action of the catalyst is reduced to a decrease in the activation energy of the reaction, i.e. to a decrease in the difference between the average energy of active molecules and the average energy of molecules of the starting substances. In this case, the rate of the chemical reaction increases. As a rule, the term “ catalyst »Is applied to those substances that increase the rate of a chemical reaction. Substances that reduce the rate of reaction are called inhibitors .

The catalysts are directly involved in the process, but at the end of it they can be isolated from the reaction mixture in the initial amount. The catalysts are characterized by selectivity , i.e. the ability to influence the progress of the reaction in a certain direction, therefore, different products can be obtained from the same starting materials, depending on the catalyst used.

Biocatalysts take a special place –enzymes which are proteins. Enzymes affect the rates of strictly defined reactions, that is, they have a very high selectivity. Enzymes speed up reactions billions and trillions of times at room temperature. At elevated temperatures, they lose their activity, as protein denaturation occurs.

There are two types of catalysis: homogeneous catalysis when the catalyst and starting materials are in the same phase, and heterogeneous , when the catalyst and the starting materials are in different phases, i.e. the reactions take place on the surface of the catalyst. The catalyst does not affect the state of equilibrium in the system, but only changes the rate at which this state is reached. This follows from the fact that equilibrium corresponds to a minimum of the isobaric-isothermal potential (Gibbs energy), and the equilibrium constant has the same value, both in the presence of a catalyst and without it.

Homogeneous catalyst action consists in the fact that it reacts with one of the starting materials with the formation of an intermediate compound, which, in turn, enters into chemical reactions with another starting material, giving the desired reaction product and "freeing" the catalyst. Thus, in the case of homogeneous catalysis, the process proceeds in several stages, but with lower values ​​of the activation energy for each stage than for the direct non-catalytic process.

Let substance A react with substance B to form compound AB:

The reaction proceeds at an insignificant rate. When adding a catalyst K reactions proceed: A + K = AK and AK + B = AB + K.

Adding these two equations, we get: A + B = AB.

An example of a reaction proceeding with the participation of a homogeneous catalyst is the oxidation reaction of sulfur (IV) oxide to sulfur (VI) oxide: without catalyst: SO 2 + 0.5O 2 = SO 3;

with catalyst NO 2: SO 2 + NO 2 = SO 3 + NO, NO + 0.5O 2 = NO 2.

Heterogeneous catalyst action consists in the fact that gas (or liquid) molecules are adsorbed on the surface of the catalyst crystal, which leads to a redistribution of the electron density in the adsorbed molecules and weakening of the chemical bond in them up to the complete dissociation of the molecule into atoms. This greatly facilitates the interaction of the adsorbed molecules (atoms) of the reacting substances with each other. The larger the surface, the more effective the catalyst. Metals (nickel, platinum, palladium, copper), crystalline aluminosilicates, zeolites, Al 2 O 3, Al 2 (SO 4) 3, etc. are widely used as heterogeneous catalysts.