The rate of chemical reaction in gases. The effect of pressure on the rate of a chemical reaction

The effect of concentration on the rate of a chemical reaction

The dependence of the reaction rate on the concentration of reactants is formulated in law of mass action: “ At a constant temperature, the rate of a chemical reaction is directly proportional to the product of the concentrations of the reacting substances in powers equal to their stoichiometric coefficients.”

For example: for the reaction mA + nB → pAB

mathematical expression of the law of mass action:

υ = k [A] m ∙ [B] n ( otherwise – kinetic equation of the reaction),

where [A] and [B] are the concentrations of reactants A and B; m and n are stoichiometric coefficients; k is a proportionality coefficient called the rate constant.

The physical meaning of the rate constant is that at concentrations of reactants equal to 1.0 mol/l ([A]=[B] = 1mol/l), the rate of a chemical reaction is equal to the rate constant (υ=k). The rate constant depends only on the nature of the reacting substances and on the temperature, but does not depend on the concentration of the substances.

The mathematical representation of the law of mass action for homogeneous and heterogeneous systems has some differences. For heterogeneous reactions, the kinetic equation includes the concentrations of only those substances that are in the system in solution or in the gas phase. The concentration of substances in the solid state on the surface remains constant during the reaction, so its value is taken into account in the reaction rate constant.

For example: for homogeneous reaction 2H 2 (g) + O 2 (g) = 2H 2 O (g)

expression of the law: υ = k ∙ 2 ∙ ;

for a heterogeneous reaction C (tv) + O 2 (g) = CO 2 (g)

expression of the law υ = k eff ∙,

where: k eff – effective rate constant equal to k ∙ [C TV ]

Task

How will the rate of the reaction 2H 2 (g) + O 2 (g) = 2H 2 O (g) change when the concentration of the starting substances is doubled?

Solution

The dependence of the reaction rate on concentration (kinetic equation) will be written: υ = k ∙ 2 ∙

If the concentrations of the starting substances are increased by 2 times, then the kinetic equation has the form: υ" = k ∙ 2 ∙ , then υ"/υ = 8 – the rate of this reaction has increased 8 times.

The dependence of the reaction rate on pressure is described by an expression similar to the law of mass action, where instead of the concentrations of substances, the partial pressures of the reacting gases are used.

For example: for the reaction 2H 2 (g) + O 2 (g) = 2H 2 O (g), the dependence of the reaction rate on pressure will be written: υ = k ∙ P H 2 2 ∙ P O 2

Task

How will the reaction rate change if the total pressure in the system CH 4 (g) + 2O 2 (g) = CO 2 (g) + 2H 2 O (g), if the total pressure in the system is reduced by 5 times?

Solution

The dependence of the reaction rate on pressure will be written:

υ = k ∙ P CH 4 ∙ P 2 O 2 . As the total pressure in the system decreases, the partial pressure of each gas will decrease, that is, υ" = k ∙ P CH 4 /5 ∙ (P O 2 /5) 2. Then υ"/υ = 1/5∙5 2 =1 /125 - reaction speed decreased by 125 times

For substances to react, their molecules must collide. The likelihood of two people colliding on a busy street is much higher than on a deserted one. Same with molecules. Obviously, the probability of molecules colliding in the figure on the left is higher than on the right. It is directly proportional to the number of reagent molecules per unit volume, i.e. molar concentrations of reagents. This can be demonstrated using a model.

In the middle of the 19th century. (1865 - N.N. Beketov, 1867 - K. Guldberg, P. Waage) the basic postulate of chemical kinetics, also called law of mass action :

The numbers n, m in the expression of the law of mass action are called reaction orders for relevant substances. These are experimentally determined quantities. Sum of exponents n, m called general reaction order .

Please note that the degrees at concentrations A and B in general not equal to stoichiometric coefficients in reaction! They become numerically equal only if the reaction proceeds exactly as written (such reactions are called simple or elementary and quite rare). In most cases, the reaction equation reflects only the overall result of a chemical process, and not its mechanism.

The proportionality factor k is called reaction rate constant . The value of the reaction rate constant is constant for a given reaction at a given temperature.

*The law of mass action does not include concentrations of solids, because reactions with solids take place on their surface, where the “concentration” of the substance is constant.

C TV +O 2 =CO 2 , v=k[C] m n =k" n ; k"=k[C] m

The influence of pressure on the rate of a chemical reaction.

Pressure greatly influences the rate of reactions involving gases because it directly determines their concentrations.

In the Mendeleev-Clapeyron equation:

pV =nRT

we'll move it V to the right side, and RT- to the left and take into account that n/V = c:

p/RT = c

Pressure and molar concentration of a gas are directly proportional. Therefore, we can substitute p/RT into the law of mass action instead of concentration.

The influence of pressure on the rate of a chemical reaction. (Supplementary material).

Chain reactions include in their mechanism many sequentially repeating elementary acts of the same type (chain).

Consider the reaction:

H 2 +Cl 2 = 2HCl

It consists of the following stages, common to all chain reactions:

1) Initiation , or chain initiation

Cl 2 = 2Cl

The decomposition of the chlorine molecule into atoms (radicals) occurs during UV irradiation or heating. The essence of the initiation stage is the formation of active, reactive particles.

2) Chain Development

Cl+H 2 = HCl + HH+Cl 2 = HCl + Cl

As a result of each elementary act of chain development, a new chlorine radical is formed, and this stage is repeated again and again, theoretically, until the reagents are completely consumed.

3) Recombination , or open circuit

2Cl = Cl 2 2H = H 2 H + Cl = HCl

Radicals that happen to be nearby can recombine, forming a stable particle (molecule). They give excess energy to a “third particle” - for example, the walls of a vessel or impurity molecules.

Considered chain reaction is unbranched , since in the elementary act of chain development the number of radicals does not increase . Chain reaction of hydrogen with oxygen is branched , because the number of radicals in the elementary act of chain development increases :

H + O 2 = OH + OO + H 2 = OH + HOH+H 2 = H 2 O+H

Branched chain reactions include many combustion reactions. An uncontrolled increase in the number of free radicals (both as a result of chain branching and for unbranched reactions in the case of too rapid initiation) can lead to a strong acceleration of the reaction and an explosion.

It would seem that the greater the pressure, the higher the concentration of radicals and the more likely an explosion. But in fact, for the reaction of hydrogen with oxygen, an explosion is possible only in certain pressure regions: from 1 to 100 mm Hg. and above 1000 mm Hg. This follows from the reaction mechanism. At low pressure, most of the resulting radicals recombine on the walls of the vessel, and the reaction proceeds slowly. When the pressure rises to 1 mm Hg. radicals reach the walls less often, because react more often with molecules. In these reactions, radicals multiply and an explosion occurs. However, at pressure above 100 mm Hg. the concentrations of substances increase so much that the recombination of radicals begins as a result of triple collisions (for example, with a water molecule), and the reaction proceeds calmly, without explosion (stationary flow). Above 1000 mm Hg. concentrations become very high, and even triple collisions are not enough to prevent the proliferation of radicals.

You know the branched chain reaction of fission of uranium-235, in each elementary act of which 1 neutron is captured (playing the role of a radical) and up to 3 neutrons are emitted. Depending on the conditions (for example, on the concentration of neutron absorbers), it is also possible for it to have a steady flow or an explosion. This is another example of the correlation between the kinetics of chemical and nuclear processes.

Chemical reactions occur at different speeds: at a low speed during the formation of stalactites and stalagmites, at an average speed when cooking food, instantly during an explosion. Reactions occur very quickly in aqueous solutions.

Determining the rate of a chemical reaction, as well as elucidating its dependence on the conditions of the process, is the task of chemical kinetics - the science of the patterns of chemical reactions over time.

If chemical reactions occur in a homogeneous medium, for example in a solution or in the gas phase, then the interaction of the reacting substances occurs throughout the entire volume. Such reactions are called homogeneous.

(v homog) is defined as the change in the amount of substance per unit time per unit volume:

where Δn is the change in the number of moles of one substance (most often the original, but it can also be a reaction product); Δt - time interval (s, min); V is the volume of gas or solution (l).

Since the ratio of the amount of substance to the volume represents the molar concentration C, then

Thus, the rate of a homogeneous reaction is defined as the change in the concentration of one of the substances per unit time:

if the volume of the system does not change.

If a reaction occurs between substances in different states of aggregation (for example, between a solid and a gas or liquid), or between substances that are unable to form a homogeneous medium (for example, between immiscible liquids), then it takes place only on the contact surface of the substances. Such reactions are called heterogeneous.

Defined as the change in the amount of substance per unit time on a unit surface.

where S is the surface area of ​​​​contact of substances (m 2, cm 2).

A change in the amount of a substance by which the reaction rate is determined is an external factor observed by the researcher. In fact, all processes are carried out at the micro level. Obviously, in order for some particles to react, they must first collide, and collide effectively: not scatter like balls in different directions, but in such a way that “old bonds” are destroyed or weakened in the particles and “new ones” can form. ", and for this the particles must have sufficient energy.

Calculated data show that, for example, in gases, collisions of molecules at atmospheric pressure amount to billions per second, that is, all reactions should occur instantly. But that's not true. It turns out that only a very small fraction of molecules have the necessary energy to lead to effective collisions.

The minimum excess energy that a particle (or pair of particles) must have for an effective collision to occur is called activation energy Ea.

Thus, on the path of all particles entering the reaction there is an energy barrier equal to the activation energy E a. When it is small, there are many particles that can overcome it, and the reaction rate is high. Otherwise, a “push” is required. When you bring a match to light an alcohol lamp, you impart the additional energy E a necessary for the effective collision of alcohol molecules with oxygen molecules (overcoming the barrier).

The speed of a chemical reaction depends on many factors. The main ones are: the nature and concentration of the reacting substances, pressure (in reactions involving gases), temperature, the action of catalysts and the surface of the reacting substances in the case of heterogeneous reactions.

Temperature

As the temperature increases, in most cases the rate of a chemical reaction increases significantly. In the 19th century Dutch chemist J. X. van't Hoff formulated the rule:

Every 10 °C increase in temperature leads to an increase inreaction speed 2-4 times(this value is called the temperature coefficient of the reaction).

As the temperature increases, the average speed of molecules, their energy, and the number of collisions increase slightly, but the proportion of “active” molecules participating in effective collisions that overcome the energy barrier of the reaction increases sharply. Mathematically, this dependence is expressed by the relation:

where v t 1 and v t 2 are the reaction rates, respectively, at the final t 2 and initial t 1 temperatures, and γ is the temperature coefficient of the reaction rate, which shows how many times the reaction rate increases with every 10 °C increase in temperature.

However, to increase the reaction rate, increasing the temperature is not always applicable, since the starting substances may begin to decompose, solvents or the substances themselves may evaporate, etc.

Endothermic and exothermic reactions

The reaction of methane with atmospheric oxygen is known to be accompanied by the release of a large amount of heat. Therefore, it is used in everyday life for cooking, heating water and heating. Natural gas supplied to homes through pipes consists of 98% methane. The reaction of calcium oxide (CaO) with water is also accompanied by the release of a large amount of heat.

What can these facts indicate? When new chemical bonds are formed in the reaction products, more energy than is required to break chemical bonds in reagents. Excess energy is released as heat and sometimes light.

CH 4 + 2O 2 = CO 2 + 2H 2 O + Q (energy (light, heat));

CaO + H 2 O = Ca (OH) 2 + Q (energy (heat)).

Such reactions should occur easily (as a stone rolls easily downhill).

Reactions in which energy is released are called EXOTHERMAL(from the Latin “exo” - out).

For example, many redox reactions are exothermic. One of these beautiful reactions is intramolecular oxidation-reduction occurring inside the same salt - ammonium dichromate (NH 4) 2 Cr 2 O 7:

(NH 4) 2 Cr 2 O 7 = N 2 + Cr 2 O 3 + 4 H 2 O + Q (energy).

Another thing is the backlash. They are analogous to rolling a stone up a mountain. It has still not been possible to obtain methane from CO 2 and water, and strong heating is required to obtain quicklime CaO from calcium hydroxide Ca(OH) 2. This reaction occurs only with a constant flow of energy from outside:

Ca(OH) 2 = CaO + H 2 O - Q (energy (heat))

This suggests that breaking chemical bonds in Ca(OH) 2 requires more energy than can be released during the formation of new chemical bonds in CaO and H 2 O molecules.

Reactions in which energy is absorbed are called ENDOTHERMIC(from “endo” - inward).

Concentration of reactants

A change in pressure when gaseous substances participate in the reaction also leads to a change in the concentration of these substances.

For chemical interactions between particles to occur, they must effectively collide. The higher the concentration of reactants, the more collisions and, accordingly, the higher the reaction rate. For example, acetylene burns very quickly in pure oxygen. In this case, a temperature sufficient to melt the metal develops. Based on a large amount of experimental material, in 1867 the Norwegians K. Guldenberg and P. Waage and independently of them in 1865, the Russian scientist N.I. Beketov formulated the basic law of chemical kinetics, establishing the dependence of the reaction rate on the concentration of the reacting substances.

The rate of a chemical reaction is proportional to the product of the concentrations of the reacting substances, taken in powers equal to their coefficients in the reaction equation.

This law is also called law of mass action.

For the reaction A + B = D, this law will be expressed as follows:

For the reaction 2A + B = D, this law will be expressed as follows:

Here C A, C B are the concentrations of substances A and B (mol/l); k 1 and k 2 are proportionality coefficients, called reaction rate constants.

The physical meaning of the reaction rate constant is not difficult to establish - it is numerically equal to the reaction rate in which the concentrations of the reactants are 1 mol/l or their product is equal to unity. In this case, it is clear that the reaction rate constant depends only on temperature and does not depend on the concentration of substances.

Law of mass action does not take into account the concentration of reactants in the solid state, because they react on surfaces and their concentrations are usually constant.

For example, for a coal combustion reaction, the reaction rate expression should be written as follows:

i.e., the reaction rate is proportional only to the oxygen concentration.

If the reaction equation describes only a total chemical reaction taking place in several stages, then the rate of such a reaction can depend in a complex way on the concentrations of the starting substances. This dependence is determined experimentally or theoretically based on the proposed reaction mechanism.

Action of catalysts

It is possible to increase the rate of a reaction by using special substances that change the reaction mechanism and direct it along an energetically more favorable path with a lower activation energy. They are called catalysts (from the Latin katalysis - destruction).

The catalyst acts as an experienced guide, guiding a group of tourists not through a high pass in the mountains (overcoming it requires a lot of effort and time and is not accessible to everyone), but along detour paths known to him, along which one can overcome the mountain much easier and faster.

True, using the roundabout route you can get not exactly where the main pass leads. But sometimes this is exactly what is required! This is exactly how catalysts that are called selective act. It is clear that there is no need to burn ammonia and nitrogen, but nitrogen oxide (II) is used in the production of nitric acid.

Catalysts- these are substances that participate in a chemical reaction and change its speed or direction, but at the end of the reaction they remain unchanged quantitatively and qualitatively.

Changing the rate of a chemical reaction or its direction using a catalyst is called catalysis. Catalysts are widely used in various industries and transport (catalytic converters that convert nitrogen oxides from car exhaust gases into harmless nitrogen).

There are two types of catalysis.

Homogeneous catalysis, in which both the catalyst and the reactants are in the same state of aggregation (phase).

Heterogeneous catalysis, in which the catalyst and reactants are in different phases. For example, the decomposition of hydrogen peroxide in the presence of a solid manganese (IV) oxide catalyst:

The catalyst itself is not consumed as a result of the reaction, but if other substances are adsorbed on its surface (they are called catalytic poisons), then the surface becomes inoperable and regeneration of the catalyst is required. Therefore, before carrying out the catalytic reaction, the starting materials are thoroughly purified.

For example, in the production of sulfuric acid by the contact method, a solid catalyst is used - vanadium (V) oxide V 2 O 5:

In the production of methanol, a solid “zinc-chrome” catalyst (8ZnO Cr 2 O 3 x CrO 3) is used:

Biological catalysts - enzymes - work very effectively. By chemical nature they are proteins. Thanks to them, complex chemical reactions occur at high speed in living organisms at low temperatures.

Other interesting substances are known - inhibitors (from the Latin inhibere - to delay). They react with active particles at high speed to form low-active compounds. As a result, the reaction slows down sharply and then stops. Inhibitors are often specifically added to various substances to prevent unwanted processes.

For example, hydrogen peroxide solutions are stabilized using inhibitors.

The nature of the reacting substances (their composition, structure)

Meaning activation energies is the factor through which the influence of the nature of the reacting substances on the reaction rate is affected.

If the activation energy is low (< 40 кДж/моль), то это означает, что значительная часть столкнове­ний между частицами реагирующих веществ при­водит к их взаимодействию, и скорость такой ре­акции очень большая. Все реакции ионного обмена протекают практически мгновенно, ибо в этих ре­акциях участвуют разноименно заряженные ионы, и энергия активации в данных случаях ничтожно мала.

If the activation energy is high(> 120 kJ/mol), this means that only a tiny fraction of collisions between interacting particles lead to a reaction. The rate of such a reaction is therefore very low. For example, the progress of the ammonia synthesis reaction at ordinary temperatures is almost impossible to notice.

If the activation energies of chemical reactions have intermediate values ​​(40120 kJ/mol), then the rates of such reactions will be average. Such reactions include the interaction of sodium with water or ethyl alcohol, decolorization of bromine water with ethylene, the interaction of zinc with hydrochloric acid, etc.

Contact surface of reacting substances

The rate of reactions occurring on the surface of substances, i.e. heterogeneous ones, depends, other things being equal, on the properties of this surface. It is known that powdered chalk dissolves much faster in hydrochloric acid than a piece of chalk of equal weight.

The increase in reaction rate is primarily due to increasing the contact surface of the starting substances, as well as a number of other reasons, for example, a violation of the structure of the “correct” crystal lattice. This leads to the fact that particles on the surface of the resulting microcrystals are much more reactive than the same particles on a “smooth” surface.

In industry, to carry out heterogeneous reactions, a “fluidized bed” is used to increase the contact surface of the reacting substances, the supply of starting substances and the removal of products. For example, in the production of sulfuric acid, pyrites are fired using a “fluidized bed”.

Reference material for taking the test:

Periodic table

Solubility table

The effect of pressure on the reaction rate depends on order reactions. If the temperature remains unchanged and the composition of the initial gas mixture is given, then using the equation of state for each concentration we can write: p a=aR m T, p b=bR m T. Here A, b,…, are molar concentrations, and p a, p b, ..., are the partial pressures of the corresponding gases. If the total number of moles per unit volume is z, then in exactly the same way you can write p=zR m T, Where r- general pressure. Hence , , ...etc. Quantities...etc. there are relative volumetric concentrations. Denoting them by A, IN...etc., we get: p a=Ap,

Where ; p b =Bp, . Let's consider monomolecular process described by the equation:

in this case, the rate of transformation of the substance is directly proportional to the pressure: ~ p.

For bimolecular reactions:

i.e. ~ p 2. Accordingly for trimolecular reactions we get:

Where k- reaction rate constant.

2.2. Activation energy. Arrhenius's law

The number of mutual collisions of reacting molecules increases ~, which contributes to an increase in the reaction rate. For example, for many reactions, an increase in temperature of just 10°C leads to an increase in the rate constant by a factor of 2-4.

Example. Half-life of hydrogen iodide according to the equation 2HJ→H 2 +J 2. At T = 373K half-life is 314,000 years, with T=666K it decreases to 1.3 hours, and at T=973K t 1/2 = 0.12sec.

Arrhenius: for a chemical reaction to occur, a preliminary weakening or breaking of the internal bonds of a stable molecule is necessary, for which a certain amount of energy must be expended E . The greater the thermal energy of colliding molecules, the greater the likelihood of rearrangement of internal bonds and the creation of new molecules. At E= const the frequency of collisions ending in a reaction will increase significantly faster than .

The energy required to overcome the energy barrier that prevents the approach of reacting molecules and the formation of reaction products is called activation energy E a. Thus, the elementary act of a chemical reaction occurs only during the collision of those molecules whose kinetic energy is greater E a.

Activation energy E a usually higher than the average energy of thermal motion of molecules. The lower the activation energy, the more often collisions of molecules will occur, leading to the formation of reaction products, and the higher the rate of the chemical reaction will be. Increase T leads to an increase in the number of molecules with excess energy exceeding E a. This explains the increase in the rate of chemical reaction with increasing temperature (Fig. 2.1).

Rice. 2.1. Heat of combustion Q and activation energy E=u max - u 1

In the simplest cases, the rate constants of chemical reactions can be determined on the basis of the general relations of molecular kinetic theory (see, for example,).

Let us denote by p A And p in number of molecules A and B in 1 cm 3 . The reaction rate will be equal to the number Z such collisions of molecules A and B per unit time, the energy of which is greater than the activation energy E . For an ideal gas Z is determined based on the Maxwell–Boltzmann energy distribution law:

Here is the average effective diameter of colliding molecules, is the reduced molecular weight, R m = 8.315∙10 7 erg/deg - universal gas constant, m A, m B - molecular weights.

In most cases, the experimental values ​​are significantly less than the theoretical ones. Therefore, the so-called probability or steric coefficient is introduced into the calculation formula R. As a result, the formula for calculating the rate of a bimolecular reaction, called Arrhenius formula, takes the following form:

Comparing the resulting formula with equation (2.8) for second-order reactions, we can obtain an expression for the rate constant of this reaction:

The strong effect of temperature on reaction rates is attributed mainly to the Arrhenius factor. Therefore, in approximate calculations, the pre-exponential factor is often taken to be independent of T.

Analysis of formula (2.12) shows that as T increases, the growth rate of W first increases, reaches a certain maximum value, and then decreases; in other words, the W versus T curve has an inflection point. Equating the second derivative of W with respect to T to zero, we find the temperature corresponding to the inflection point:

It is easy to see that this temperature is quite high. For example, at E = 20000 cal/(g-mol) T p = 5000 K. When using formula (2.12) for numerical calculations, the dimensions of the quantities included in it should be taken into account.

Formula (2.12) can be written as follows:

where is the pre-exponential factor, i.e. total number of collisions at n A =n B =1 molecule/cm 3. Sometimes R also included in the pre-exponential factor.

For estimated calculations of the order of the reaction rate, the value k 0 can be taken for temperature T=300K equal to 10 -10 cm 3 /(molecule∙sec) (for d avg "4∙10 -8 and m A =m B "30).

The rate of a chemical reaction at a given temperature is proportional to the product of the concentrations of the reacting substances to a degree equal to the stoichiometric coefficient appearing before the formula of the given substance in the reaction equation.

The law of mass action is valid only for the simplest interaction reactions in their mechanism, occurring in gases or in dilute solutions .

1. aA(W) + bB (W) ↔ cC (W) + dD (W) ; (T=const)

2. 3H 2(G) + N 2(G) ↔ 2NH 3(G) ;

For heterogeneous reactions:

1. aA (t) + bB (G) = cC (G) + dD (G); 2. C (t) + O 2 (G) = CO 2 (G);

The law of mass action does not take into account the concentrations of substances in the solid phase. The greater the surface area of ​​the solid phase, the higher the rate of the chemical reaction.

k - chemical reaction rate constant is determined by the nature of the reacting substances and depends on temperature, on the presence of a catalyst in the system, but does not depend on the concentration of the reacting substances. The rate constant represents the rate of a chemical reaction (), if the concentrations of the reactants are .

3. Dependence of the rate of a chemical reaction on pressure. For gaseous systems, an increase in pressure or a decrease in volume is equivalent to an increase in concentration and vice versa.

Task: How will the rate of the chemical reaction 2SO 2 (g) + O 2 (g) 2SO 3 (g) change if the pressure in the system is increased by 4 times?

In accordance with the law of mass action for a direct reaction, we write the expression:

Let = a mol/l, = b mol/l, then according to the law of mass action

A decrease in volume by 4 times corresponds to an increase in concentration in the system by 4 times, then:

The effect of temperature on the rate of a chemical reaction is approximately determined van't Hoff's rule. When the temperature increases by 10 0 C, the rate of the chemical reaction increases 2-4 times.

The mathematical notation of Van't Hoff's rule: γ is the temperature coefficient of the reaction rate or the Van't Hoff coefficient for most reactions lies in the range of 2-4.

Task. How many times will the rate of a chemical reaction occurring in the gas phase change if the temperature changes from 80 0 C to 120 0 C ( γ = 3)?

In accordance with Van't Hoff's rule, we write:

An increase in the rate of a chemical reaction with increasing temperature is explained not only by an increase in the kinetic energy of interacting molecules. For example, the number of molecular collisions increases in proportion to the square root of the absolute temperature. When substances are heated from zero to one hundred degrees Celsius, the speed of movement of molecules increases by 1.2 times, and the speed of a chemical reaction increases by approximately 59 thousand times. Such a sharp increase in the reaction rate with increasing temperature is explained by the proportion of active molecules whose collisions lead to chemical interaction. According to the theory of active collisions, only active molecules, whose energy exceeds the average energy of the molecules of a given substance, i.e. molecules with activation energy.

Activation energy (E A)- this is the excess energy compared to the average reserve that molecules must have to carry out a chemical reaction. If E A< 40 кДж/моль - реакции протекают быстро, если Е А >120 kJ/mol - reactions do not occur, if E A = 40-120 kJ/mol - reactions proceed under normal conditions. An increase in temperature reduces the activation energy, makes substances more reactive, and the rate of interaction increases.

Established a more accurate dependence of the rate of a chemical reaction on temperature C. Arrhenius: the reaction rate constant is proportional to the base of the natural logarithm raised to a power (-EA /RT). ,

A - pre-exponential factor, determines the number of active collisions;

e - exponent (base of natural logarithm).

Taking the logarithm of the expression, we obtain the equation:

. The Arrhenius equation shows that the lower the activation energy, the higher the reaction rate. Catalysts are used to reduce the activation energy.