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When defining the concept chemical reaction rate it is necessary to distinguish between homogeneous and heterogeneous reactions. If a reaction occurs in a homogeneous system, for example, in a solution or in a mixture of gases, then it occurs throughout the entire volume of the system. Speed ​​of homogeneous reaction is the amount of a substance that reacts or is formed as a result of a reaction per unit time per unit volume of the system. Since the ratio of the number of moles of a substance to the volume in which it is distributed is the molar concentration of the substance, the rate of a homogeneous reaction can also be defined as change in concentration per unit time of any of the substances: the initial reagent or the reaction product. To ensure that the calculation result is always positive, regardless of whether it is based on a reagent or a product, the “±” sign is used in the formula:

Depending on the nature of the reaction, time can be expressed not only in seconds, as required by the SI system, but also in minutes or hours. During the reaction, the magnitude of its speed is not constant, but continuously changes: it decreases, as the concentrations of the starting substances decrease. The above calculation gives the average value of the reaction rate over a certain time interval Δτ = τ 2 – τ 1. True (instantaneous) speed is defined as the limit to which the ratio Δ tends WITH/ Δτ at Δτ → 0, i.e., the true speed is equal to the derivative of the concentration with respect to time.

For a reaction whose equation contains stoichiometric coefficients that differ from unity, the rate values ​​expressed for different substances are not the same. For example, for the reaction A + 3B = D + 2E, the consumption of substance A is one mole, the supply of substance B is three moles, and the input of substance E is two moles. That's why υ (A) = ⅓ υ (B) = υ (D) =½ υ (E) or υ (E) . = ⅔ υ (IN) .

If a reaction occurs between substances located in different phases of a heterogeneous system, then it can only occur at the interface between these phases. For example, the interaction between an acid solution and a piece of metal occurs only on the surface of the metal. Speed ​​of heterogeneous reaction is the amount of a substance that reacts or is formed as a result of a reaction per unit time per unit interface surface:

.

The dependence of the rate of a chemical reaction on the concentration of reactants is expressed by the law of mass action: at a constant temperature, the rate of a chemical reaction is directly proportional to the product of the molar concentrations of the reacting substances raised to powers equal to the coefficients in the formulas of these substances in the reaction equation. Then for the reaction

2A + B → products

the ratio is valid υ ~ · WITH A 2 · WITH B, and to transition to equality a proportionality coefficient is introduced k, called reaction rate constant:

υ = k· WITH A 2 · WITH B = k·[A] 2 ·[B]

(molar concentrations in formulas can be denoted by the letter WITH with the corresponding index and the formula of the substance enclosed in square brackets). The physical meaning of the reaction rate constant is the reaction rate at concentrations of all reactants equal to 1 mol/l. The dimension of the reaction rate constant depends on the number of factors on the right side of the equation and can be c –1 ; s –1 ·(l/mol); s –1 · (l 2 /mol 2), etc., that is, such that in any case, in calculations, the reaction rate is expressed in mol · l –1 · s –1.

For heterogeneous reactions, the equation of the law of mass action includes the concentrations of only those substances that are in the gas phase or in solution. The concentration of a substance in the solid phase is a constant value and is included in the rate constant, for example, for the combustion process of coal C + O 2 = CO 2, the law of mass action is written:

υ = kI·const··= k·,

Where k= kI const.

In systems where one or more substances are gases, the rate of reaction also depends on pressure. For example, when hydrogen interacts with iodine vapor H 2 + I 2 = 2HI, the rate of the chemical reaction will be determined by the expression:

υ = k··.

If you increase the pressure, for example, by 3 times, then the volume occupied by the system will decrease by the same amount, and, consequently, the concentrations of each of the reacting substances will increase by the same amount. The reaction rate in this case will increase 9 times

Dependence of reaction rate on temperature described by van't Hoff's rule: with every 10 degree increase in temperature, the reaction rate increases by 2-4 times. This means that as the temperature increases in an arithmetic progression, the rate of a chemical reaction increases exponentially. The base in the progression formula is temperature coefficient of reaction rateγ, showing how many times the rate of a given reaction increases (or, which is the same thing, the rate constant) with an increase in temperature by 10 degrees. Mathematically, Van't Hoff's rule is expressed by the formulas:

or

where and are the reaction rates, respectively, at the initial t 1 and final t 2 temperatures. Van't Hoff's rule can also be expressed by the following relations:

; ; ; ,

where and are, respectively, the rate and rate constant of the reaction at temperature t; and – the same values ​​at temperature t +10n; n– number of “ten-degree” intervals ( n =(t 2 –t 1)/10), by which the temperature has changed (can be an integer or fractional number, positive or negative).

Examples of problem solving

Example 1. How will the rate of the reaction 2CO + O 2 = 2CO 2, occurring in a closed vessel, change if the pressure is doubled?

Solution:

The rate of this chemical reaction is determined by the expression:

υ start = k· [CO] 2 · [O 2 ].

An increase in pressure leads to a 2-fold increase in the concentration of both reagents. Taking this into account, we rewrite the expression of the law of mass action:

υ 1 = k· 2 · = k·2 2 [CO] 2 ·2[O 2 ] = 8 k·[CO] 2 ·[O 2 ] = 8 υ beginning

Answer: The reaction speed will increase 8 times.

Example 2. Calculate how many times the reaction rate will increase if the temperature of the system is increased from 20 °C to 100 °C, taking the value of the temperature coefficient of the reaction rate equal to 3.

Solution:

The ratio of reaction rates at two different temperatures is related to the temperature coefficient and temperature change by the formula:

Calculation:

Answer: The reaction speed will increase by 6561 times.

Example 3. When studying the homogeneous reaction A + 2B = 3D, it was found that during 8 minutes of the reaction, the amount of substance A in the reactor decreased from 5.6 mol to 4.4 mol. The volume of the reaction mass was 56 l. Calculate the average rate of a chemical reaction for the studied period of time for substances A, B and D.

Solution:

We use the formula in accordance with the definition of the concept of “average rate of a chemical reaction” and substitute numerical values, obtaining the average rate for reagent A:

From the reaction equation it follows that, compared with the rate of loss of substance A, the rate of loss of substance B is twice as large, and the rate of increase in the amount of product D is three times as large. Hence:

υ (A) = ½ υ (B) =⅓ υ (D)

and then υ (B) = 2 υ (A) = 2 2.68 10 –3 = 6.36 10 –3 mol l –1 min –1 ;

υ (D) = 3 υ (A) = 3 2.68 10 –3 = 8.04 10 –3 mol l –1 min –1

Answer: υ(A) =2.68·10 –3 mol·l–1 ·min–1; υ (B) = 6.36·10–3 mol·l–1 min–1; υ (D) = 8.04·10–3 mol·l–1 min–1.

Example 4. To determine the rate constant of the homogeneous reaction A + 2B → products, two experiments were carried out at different concentrations of substance B and the reaction rate was measured.

The rate of a chemical reaction depends on the following factors:

1) The nature of the reacting substances.

2) The contact surface of the reagents.

3) Concentration of reactants.

4) Temperature.

5) Presence of catalysts.

The rate of heterogeneous reactions also depends on:

a) the size of the phase interface (with an increase in the phase interface, the rate of heterogeneous reactions increases);

b) the rate of supply of reacting substances to the phase interface and the rate of removal of reaction products from it.

Factors affecting the rate of a chemical reaction:

1. Nature of the reagents. The nature of the chemical bonds in compounds and the structure of their molecules play an important role. For example, the release of hydrogen by zinc from a solution of hydrochloric acid occurs much faster than from a solution of acetic acid, since the polarity of the H-C1 bond is greater than the O-H bond in the CH 3 COOH molecule, in other words, due to the fact that HCl - is a strong electrolyte, and CH 3 COOH is a weak electrolyte in aqueous solution.

2. Contact surface of reagents. The larger the contact surface of the reacting substances, the faster the reaction proceeds. The surface area of ​​solids can be increased by grinding them, and for soluble substances by dissolving them. Reactions in solutions occur almost instantly.

3. Concentration of reagents. For interaction to occur, particles of reacting substances in a homogeneous system must collide. When increasing concentrations of reactants the speed of reactions increases. This is explained by the fact that as the amount of substance per unit volume increases, the number of collisions between particles of reacting substances increases. The number of collisions is proportional to the number of particles of reacting substances in the volume of the reactor, i.e., their molar concentrations.

The quantitative dependence of the reaction rate on the concentration of reactants is expressed law of mass action (Guldberg and Waage, Norway, 1867): the rate of a chemical reaction is proportional to the product of the concentrations of the reacting substances.

For reaction:

aA + bB ↔ cC + dD

the reaction rate in accordance with the law of mass action is equal to:

υ = k[A]υ a ·[B]υ b,(9)

where [A] and [B] are the concentrations of the starting substances;

k-reaction rate constant, which is equal to the reaction rate at the concentrations of the reactants [A] = [B] = 1 mol/l.

The reaction rate constant depends on the nature of the reactants, temperature, but does not depend on the concentration of substances.

Expression (9) is called kinetic equation of the reaction. The kinetic equations include the concentrations of gaseous and dissolved substances, but do not include the concentrations of solid substances:

2SO 2 (g) + O 2 (g) = 2SO 3 (g); υ = k 2 · [O 2 ];

CuO (tv.) + H 2 (g) = Cu (tv.) + H 2 O (g); υ = k.

Using kinetic equations, you can calculate how the reaction rate changes when the concentration of reactants changes.

The influence of the catalyst.

5. Reaction temperature. Active collision theory

In order for an elementary act of chemical interaction to take place, the reacting particles must collide with each other. However, not every collision results in a chemical reaction. Chemical interaction occurs when particles approach distances at which redistribution of electron density and the formation of new chemical bonds are possible. The interacting particles must have sufficient energy to overcome the repulsive forces that arise between their electron shells.

Transition state- a state of the system in which the destruction and creation of connections are balanced. The system remains in a transition state for a short time (10–15 s). The energy that must be expended to bring the system into a transition state is called activation energy. In multistep reactions that include several transition states, the activation energy corresponds to the highest energy value. After overcoming the transition state, the molecules scatter again with the destruction of old bonds and the formation of new ones or with the transformation of the original bonds. Both options are possible, as they occur with the release of energy. There are substances that can reduce the activation energy for a given reaction.

Active molecules A 2 and B 2 upon collision combine into an intermediate active complex A 2 ... B 2 with weakening and then breaking of the A-A and B-B bonds and strengthening of the A-B bonds.

The “activation energy” of the reaction for the formation of НI (168 kJ/mol) is significantly less than the energy required to completely break the bond in the initial molecules of Н2 and I2 (571 kJ/mol). Therefore, the reaction path through the formation active (activated) complex energetically more favorable than the path through the complete rupture of bonds in the original molecules. The vast majority of reactions occur through the formation of intermediate active complexes. The principles of the theory of the active complex were developed by G. Eyring and M. Polyani in the 30s of the 20th century.

Activation energy represents the excess kinetic energy of particles relative to the average energy required for the chemical transformation of colliding particles. Reactions are characterized by different activation energies (E a). In most cases, the activation energy of chemical reactions between neutral molecules ranges from 80 to 240 kJ/mol. For biochemical processes values E a often lower - up to 20 kJ/mol. This is explained by the fact that the vast majority of biochemical processes proceed through the stage of enzyme-substrate complexes. Energy barriers limit the reaction. Due to this, in principle, possible reactions (with Q< 0) практически всегда не протекают или замедляются. Реакции с энергией активации выше 120 кДж/моль настолько медленны, что их протекание трудно заметить.

For a reaction to occur, the molecules must be oriented in a certain way and have sufficient energy when they collide. The probability of proper collision orientation is characterized by activation entropy ∆S a. The redistribution of electron density in the active complex is favored by the condition when, upon collision, molecules A 2 and B 2 are oriented, as shown in Fig. 3a, whereas with the orientation shown in Fig. 3b, the probability of reaction is even much less - in Fig. 3c.

Rice. 3. Favorable (a) and unfavorable (b, c) orientations of molecules A 2 and B 2 upon collision

The equation characterizing the dependence of the rate and reaction on temperature, activation energy and activation entropy has the form:

(10)

Where k- reaction rate constant;

A- to a first approximation, the total number of collisions between molecules per unit of time (second) per unit volume;

e- the base of natural logarithms;

R- universal gas constant;

T- absolute temperature;

E a- activation energy;

∆S a- change in activation entropy.

Equation (11) was derived by Arrhenius in 1889. Pre-exponential factor A proportional to the total number of collisions between molecules per unit time. Its dimension coincides with the dimension of the rate constant and depends on the total order of the reaction.

Exhibitor equal to the proportion of active collisions from their total number, i.e. colliding molecules must have sufficient interaction energy. The probability of their desired orientation at the moment of impact is proportional to .

When discussing the law of mass action for speed (9), it was specifically stated that the rate constant is a constant value that does not depend on the concentrations of the reagents. It was assumed that all chemical transformations occur at a constant temperature. At the same time, the rate of chemical transformation can change significantly with decreasing or increasing temperature. From the point of view of the law of mass action, this change in speed is due to the temperature dependence of the rate constant, since the concentrations of the reacting substances change only slightly due to thermal expansion or compression of the liquid.

The most well-known fact is that the rate of reactions increases with increasing temperature. This type of temperature dependence of speed is called normal (Fig. 3 a). This type of dependence is characteristic of all simple reactions.

Rice. 3. Types of temperature dependence of the rate of chemical reactions: a - normal;

b - abnormal; c - enzymatic

However, chemical transformations are now well known, the rate of which decreases with increasing temperature; this type of temperature dependence of the rate is called abnormal . An example is the gas-phase reaction of nitrogen (II) oxide with bromine (Fig. 3 b).

Of particular interest to physicians is the temperature dependence of the rate of enzymatic reactions, i.e. reactions involving enzymes. Almost all reactions occurring in the body belong to this class. For example, when hydrogen peroxide decomposes in the presence of the enzyme catalase, the rate of decomposition depends on temperature. In the range 273-320 TO The temperature dependence is normal. As the temperature increases, the speed increases, and as the temperature decreases, it decreases. When the temperature rises above 320 TO There is a sharp anomalous drop in the rate of peroxide decomposition. A similar picture occurs for other enzymatic reactions (Fig. 3c).

From the Arrhenius equation for k it is clear that, since T included in the exponent, the rate of a chemical reaction is very sensitive to temperature changes. The dependence of the rate of a homogeneous reaction on temperature can be expressed by the van’t Hoff rule, according to which with every 10° increase in temperature, the reaction rate increases by 2-4 times; a number showing how many times the rate of a given reaction increases with an increase in temperature by 10° is called temperature coefficient of reaction rate -γ.

This rule is expressed mathematically by the following formula:

(12)

where γ is the temperature coefficient, which shows how many times the reaction rate increases when the temperature increases by 10 0; υ 1 –t 1 ; υ 2 – reaction rate at temperature t2.

As the temperature increases in an arithmetic progression, the speed increases in a geometric progression.

For example, if γ = 2.9, then with an increase in temperature by 100° the reaction rate increases by 2.9 10 times, i.e. 40 thousand times. Deviations from this rule are biochemical reactions, the speed of which increases tens of times with a slight increase in temperature. This rule is only valid to a rough approximation. Reactions involving large molecules (proteins) are characterized by a large temperature coefficient. The rate of denaturation of protein (egg albumin) increases 50 times with an increase in temperature by 10 °C. After reaching a certain maximum (50-60 °C), the reaction rate sharply decreases as a result of thermal denaturation of the protein.

For many chemical reactions the law of mass action for speed is unknown. In such cases, the expression can be used to describe the temperature dependence of the conversion rate:

Pre-exponent And with does not depend on temperature, but depends on concentration. The unit of measurement is mol/l∙s.

The theoretical dependence allows the speed to be calculated in advance at any temperature if the activation energy and pre-exponential are known. Thus, the influence of temperature on the rate of chemical transformation is predicted.

Complex reactions

The principle of independence. Everything discussed above related to relatively simple reactions, but in chemistry so-called complex reactions are often encountered. Such reactions include those discussed below. When deriving kinetic equations for these reactions, the principle of independence is used: If several reactions occur in a system, then each of them is independent of the others and its rate is proportional to the product of the concentrations of its reactants.

Parallel reactions- These are reactions that occur simultaneously in several directions.

The thermal decomposition of potassium chlorate occurs simultaneously in two reactions:

Sequential reactions- These are reactions that occur in several stages. These are the majority of reactions in chemistry.

.

Conjugate reactions. If several reactions occur in a system and the occurrence of one of them is impossible without the other, then these reactions are called conjugated , and the phenomenon itself - by induction .

2HI + H 2 CrO 4 → I 2 + Cr 2 O 3 + H 2 O.

This reaction is practically not observed under normal conditions, but if FeO is added to the system, the following reaction occurs:

FeO + H 2 CrO 4 → Fe 2 O 3 + Cr 2 O 3 + H 2 O

and at the same time the first reaction occurs. The reason for this is the formation in the second reaction of intermediate products involved in the first reaction:

FeO 2 + H 2 CrO 4 → Cr 2 O 3 + Fe 5+;

HI + Fe 5+ → Fe 2 O 3 + I 2 + H 2 O.

Chemical induction- a phenomenon in which one chemical reaction (secondary) depends on another (primary).

A+ IN- primary reaction,

A+C- secondary reaction,

then A is an activator, IN- inductor, C - acceptor.

During chemical induction, unlike catalysis, the concentrations of all reaction participants decrease.

Induction factor determined from the following equation:

.

Depending on the magnitude of the induction factor, the following cases are possible.

I > 0 - damping process. The reaction rate decreases over time.

I < 0 - ускоряющийся процесс. Скорость реакции увеличи­вается со временем.

The phenomenon of induction is important because in some cases the energy of the primary reaction can compensate for the energy consumed in the secondary reaction. For this reason, for example, it turns out to be thermodynamically possible to synthesize proteins by polycondensation of amino acids.

Chain reactions. If a chemical reaction proceeds with the formation of active particles (ions, radicals), which, entering subsequent reactions, cause the appearance of new active particles, then this sequence of reactions is called chain reaction.

The formation of free radicals is associated with the expenditure of energy to break bonds in the molecule. This energy can be imparted to molecules by illumination, electrical discharge, heating, irradiation with neutrons, α- and β-particles. To carry out chain reactions at low temperatures, initiators are introduced into the reaction mixture - substances that easily form radicals: sodium vapor, organic peroxides, iodine, etc.

The reaction of the formation of hydrogen chloride from simple compounds, activated by light.

Total reaction:

H 2 + C1 2 2HC1.

Individual stages:

Сl 2 2Сl∙ photoactivation of chlorine (initiation)

Cl∙ + H 2 = HCl + H∙ chain development

H∙ + Cl 2 = HCl + Cl∙, etc.

H∙ + Cl∙ = HCl open circuit

Here H∙ and Cl∙ are active particles (radicals).

In this reaction mechanism, three groups of elementary stages can be distinguished. The first is a photochemical reaction chain nucleation. Chlorine molecules, having absorbed a light quantum, dissociate into free atoms that are highly reactive. Thus, during the nucleation of a chain, the formation of free atoms or radicals from valence-saturated molecules occurs. The process of chain nucleation is also called initiation. Chlorine atoms, having unpaired electrons, are able to react with molecular hydrogen, forming molecules of hydrogen chloride and atomic hydrogen. Atomic hydrogen, in turn, interacts with a chlorine molecule, as a result of which a hydrogen chloride molecule and atomic chlorine are again formed, etc.

These processes, characterized by the repetition of the same elementary stages (links) and proceeding with the preservation of free radicals, lead to the consumption of starting substances and the formation of reaction products. Such groups of reactions are called reactions of development (or continuation) of the chain.

The stage of the chain reaction in which the death of free radicals occurs is called open circuit. Chain termination can occur as a result of the recombination of free radicals, if the energy released during this process can be given to some third body: the wall of the vessel or molecules of inert impurities (stages 4, 5). That is why the rate of chain reactions is very sensitive to the presence of impurities, to the shape and size of the vessel, especially at low pressures.

The number of elementary links from the moment the chain begins to break is called the chain length. In the example under consideration, up to 10 5 HCl molecules are formed for each quantum of light.

Chain reactions during which there is no “multiplying” of the number of free radicals are called unbranched or simple chain reactions . In each elementary stage of an unbranched chain process, one radical “gives birth” to one molecule of the reaction product and only one new radical (Fig. 41).

Other examples of simple chain reactions: a) chlorination of paraffin hydrocarbons Cl∙ + CH 4 → CH 3 ∙ + HC1; CH 3 ∙ + Cl - → CH 3 Cl + Cl ∙ etc.; b) radical polymerization reactions, for example, polymerization of vinyl acetate in the presence of benzoyl peroxide, which easily decomposes into radicals; c) the interaction of hydrogen with bromine, which occurs according to a mechanism similar to the reaction of chlorine with hydrogen, only with a shorter chain length due to its endothermicity.

If, as a result of the act of growth, two or more active particles appear, then this chain reaction is branched.

In 1925, N. N. Semenov and his collaborators discovered reactions containing elementary stages, as a result of which not one, but several chemically active particles - atoms or radicals - appear. The appearance of several new free radicals leads to the appearance of several new chains, i.e. one chain branches. Such processes are called branched chain reactions (Fig. 42).

An example of a highly branched chain process is the oxidation of hydrogen at low pressures and temperatures of about 900°C. The reaction mechanism can be written as follows.

1. H 2 + O 2 OH∙ + OH∙ chain initiation

2. OH∙ + H2 → H2O + H∙ chain development

3. H∙ + O 2 → OH∙ + O: chain branching

4. O: + H 2 → OH∙ +H∙

5. OH∙ +H 2 → H 2 O + H∙ continuation of the chain

6. Н∙ + Н∙ + wall → Н 2 open circuit on the wall of the vessel

7. H∙ + O 2 + M → HO 2 ∙ + M open circuit in the volume.

M is an inert molecule. The radical HO 2 ∙, formed during a triple collision, is inactive and cannot continue the chain.

At the first stage of the process, hydroxyl radicals are formed, which ensure the development of a simple chain. In the third stage, as a result of interaction with the original molecule of one radical, two radicals are formed, and the oxygen atom has two free valences. This ensures branching of the chain.

As a result of chain branching, the reaction rate rapidly increases in the initial period of time, and the process ends with a chain ignition-explosion. However, branched chain reactions end in explosion only when the rate of branching is greater than the rate of chain termination. Otherwise, the process is slow.

When the reaction conditions change (changes in pressure, temperature, mixture composition, size and condition of the walls of the reaction vessel, etc.), a transition from a slow reaction to an explosion may occur and vice versa. Thus, in chain reactions there are limiting (critical) states at which chain ignition occurs, from which thermal ignition that occurs in exothermic reactions as a result of ever-increasing heating of the reacting mixture with weak heat removal should be distinguished.

Oxidation of vapors of sulfur, phosphorus, carbon monoxide (II), carbon disulfide, etc. occurs through a branched chain mechanism.

The modern theory of chain processes was developed by Nobel Prize laureates (1956) Soviet academician N. N. Semenov and English scientist Hinshelwood.

Chain reactions should be distinguished from catalytic reactions, although the latter are also cyclic in nature. The most significant difference between chain reactions and catalytic ones is that with a chain mechanism, the reaction can flow in the direction of increasing the energy of the system due to spontaneous reactions. A catalyst does not cause a thermodynamically impossible reaction. In addition, in catalytic reactions there are no process stages such as chain nucleation and chain termination.

Polymerization reactions. A special case of a chain reaction is a polymerization reaction.

Polymerization is a process in which the reaction of active particles (radicals, ions) with low-molecular compounds (monomers) is accompanied by the sequential addition of the latter with an increase in the length of the material chain (molecule length), i.e., with the formation of a polymer.

Monomers are organic compounds, usually containing unsaturated (double, triple) bonds in the molecule.

Main stages of the polymerization process:

1. Initiation(under the influence of light, heat, etc.):

A: A→A" + A"- homolytic decomposition with the formation of radicals (active valence-unsaturated particles).

A: B→ A - + B +- heterolytic decomposition with the formation of ions.

2. Chain height: A" + M→ AM"

(or A - + M→ AM", or IN + + M→ VM +).

3. Open circuit: AM" + AM"→ polymer

(or AM" + B +→ polymer, VM + + A"→ polymer).

The speed of a chain process is always greater than that of a non-chain process.

The rate of a chemical reaction depends on many factors, including the nature of the reactants, the concentration of the reactants, temperature, and the presence of catalysts. Let's consider these factors.

1). Nature of reactants. If there is an interaction between substances with an ionic bond, then the reaction proceeds faster than between substances with a covalent bond.

2.) Concentration of reactants. For a chemical reaction to take place, the molecules of the reacting substances must collide. That is, the molecules must come so close to each other that the atoms of one particle experience the action of the electric fields of the other. Only in this case will electron transitions and corresponding rearrangements of atoms be possible, as a result of which molecules of new substances are formed. Thus, the rate of chemical reactions is proportional to the number of collisions that occur between molecules, and the number of collisions, in turn, is proportional to the concentration of the reactants. Based on experimental material, the Norwegian scientists Guldberg and Waage and, independently of them, the Russian scientist Beketov in 1867 formulated the basic law of chemical kinetics - law of mass action(ZDM): at a constant temperature, the rate of a chemical reaction is directly proportional to the product of the concentrations of the reacting substances to the power of their stoichiometric coefficients. For the general case:

the law of mass action has the form:

The recording of the law of mass action for a given reaction is called basic kinetic equation of the reaction. In the basic kinetic equation, k is the reaction rate constant, which depends on the nature of the reactants and temperature.

Most chemical reactions are reversible. During such reactions, their products, as they accumulate, react with each other to form the starting substances:

Forward reaction rate:

Feedback speed:

At the moment of equilibrium:

Hence the law of mass action in a state of equilibrium takes the form:

,

where K is the reaction equilibrium constant.

3) Effect of temperature on reaction rate. The rate of chemical reactions, as a rule, increases when the temperature is exceeded. Let's consider this using the example of the interaction of hydrogen with oxygen.

2H 2 + O 2 = 2H 2 O

At 20 0 C, the reaction rate is practically zero and it would take 54 billion years for the interaction to progress by 15%. At 500 0 C, it will take 50 minutes to form water, and at 700 0 C the reaction occurs instantly.

The dependence of the reaction rate on temperature is expressed van't Hoff's rule: with an increase in temperature by 10 o, the reaction rate increases by 2–4 times. Van't Hoff's rule is written:

4) Effect of catalysts. The rate of chemical reactions can be controlled using catalysts– substances that change the rate of a reaction and remain after the reaction in unchanged quantities. Changing the rate of a reaction in the presence of a catalyst is called catalysis. Distinguish positive(reaction speed increases) and negative(reaction rate decreases) catalysis. Sometimes a catalyst is formed during a reaction; such processes are called autocatalytic. There are homogeneous and heterogeneous catalysis.

At homogeneous In catalysis, the catalyst and reactants are in the same phase. For example:

At heterogeneous In catalysis, the catalyst and reactants are in different phases. For example:

Heterogeneous catalysis is associated with enzymatic processes. All chemical processes occurring in living organisms are catalyzed by enzymes, which are proteins with certain specialized functions. In solutions in which enzymatic processes take place, there is no typical heterogeneous environment, due to the absence of a clearly defined phase interface. Such processes are referred to as microheterogeneous catalysis.

Let us define the basic concept of chemical kinetics - the rate of a chemical reaction:

The rate of a chemical reaction is the number of elementary acts of a chemical reaction occurring per unit time per unit volume (for homogeneous reactions) or per unit surface (for heterogeneous reactions).

The rate of a chemical reaction is the change in the concentration of reactants per unit time.

The first definition is the most restrictive; It follows from it that the rate of a chemical reaction can also be expressed as a change in time of any parameter of the state of the system, depending on the number of particles of any reacting substance, per unit volume or surface - electrical conductivity, optical density, dielectric constant, etc. and so on. However, most often in chemistry the dependence of the concentration of reagents on time is considered. In the case of one-way (irreversible) chemical reactions (hereinafter only one-way reactions are considered), it is obvious that the concentrations of the starting substances are constantly decreasing over time (ΔC in< 0), а концентрации продуктов реакции увеличиваются (ΔС прод >0). The reaction rate is considered positive, so the mathematical definition average reaction speed in the time interval Δt is written as follows:

(II.1)

At different time intervals, the average rate of a chemical reaction has different values; true (instantaneous) reaction rate is defined as the derivative of concentration with respect to time:

(II.2)

There is a graphical representation of the dependence of the concentration of reagents on time kinetic curve (Figure 2.1).

Rice. 2.1 Kinetic curves for starting substances (A) and reaction products (B).

The true reaction rate can be determined graphically by drawing a tangent to the kinetic curve (Fig. 2.2); the true reaction rate at a given time is equal in absolute value to the tangent of the tangent angle:

Rice. 2.2 Graphic definition of V source.

(II.3)

It should be noted that if the stoichiometric coefficients in the equation of a chemical reaction are not the same, the magnitude of the reaction rate will depend on the change in the concentration of which reagent was determined. Obviously, in the reaction

2H 2 + O 2 → 2H 2 O

the concentrations of hydrogen, oxygen and water change to varying degrees:

ΔC(H 2) = ΔC(H 2 O) = 2 ΔC(O 2).

The rate of a chemical reaction depends on many factors: the nature of the reactants, their concentration, temperature, the nature of the solvent, etc.

One of the tasks facing chemical kinetics is determining the composition of the reaction mixture (i.e., the concentrations of all reagents) at any time, for which it is necessary to know the dependence of the reaction rate on concentrations. In general, the greater the concentration of reactants, the greater the rate of the chemical reaction. Chemical kinetics is based on the so-called. basic postulate of chemical kinetics:

The rate of a chemical reaction is directly proportional to the product of the concentrations of the reacting substances, taken to certain powers.

That is, for the reaction

AA + bB + dD + ... → eE + ...

You can write down

(II.4)

The proportionality coefficient k is chemical reaction rate constant. The rate constant is numerically equal to the reaction rate at concentrations of all reactants equal to 1 mol/l.

The dependence of the reaction rate on the concentrations of the reactants is determined experimentally and is called kinetic equation chemical reaction. Obviously, in order to write the kinetic equation, it is necessary to experimentally determine the value of the rate constant and exponents at the concentrations of the reacting substances. The exponent for the concentration of each of the reactants in the kinetic equation of a chemical reaction (in equation (II.4) x, y and z, respectively) is private reaction order for this component. The sum of the exponents in the kinetic equation of a chemical reaction (x + y + z) is general reaction order . It should be emphasized that the reaction order is determined only from experimental data and is not related to the stoichiometric coefficients of the reactants in the reaction equation. The stoichiometric equation of a reaction is a material balance equation and in no way can determine the nature of the course of this reaction over time.

In chemical kinetics, it is customary to classify reactions according to the magnitude of the overall reaction order. Let us consider the dependence of the concentration of reactants on time for irreversible (one-sided) reactions of zero, first and second orders.