The rate of a chemical reaction and the factors that affect it. Chemical reaction rate: conditions, examples

Kinetics- the science of the rates of chemical reactions.

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

True reaction rate:

2. Factors affecting the rate of a chemical reaction

For homogeneous, heterogeneous reactions:

1) concentration of reacting substances;

2) temperature;

3) catalyst;

4) inhibitor.

Only for heterogeneous:

1) the rate of supply of reactants to the interface;

2) surface area.

The main factor - the nature of the reacting substances - the nature of the bond between the atoms in the molecules of the reagents.

NO 2 - nitric oxide (IV) - fox tail, CO - carbon monoxide, carbon monoxide.

If they are oxidized with oxygen, then in the first case the reaction will go instantly, it is worth opening the stopper of the vessel, in the second case the reaction is extended in time.

The concentration of reactants will be discussed below.

Blue opalescence indicates the moment of precipitation of sulfur, the higher the concentration, the higher the rate.

Rice. 10

The greater the concentration of Na 2 S 2 O 3, the less time the reaction takes. The graph (Fig. 10) shows a directly proportional relationship. The quantitative dependence of the reaction rate on the concentration of the reactants is expressed by the MMA (law of mass action), which states: the rate of a chemical reaction is directly proportional to the product of the concentrations of the reactants.

So, basic law of kinetics is an experimentally established law: the reaction rate is proportional to the concentration of the reactants, example: (i.e. for the reaction)

For this reaction H 2 + J 2 = 2HJ - the rate can be expressed in terms of a change in the concentration of any of the substances. If the reaction proceeds from left to right, then the concentration of H 2 and J 2 will decrease, the concentration of HJ will increase in the course of the reaction. For the instantaneous rate of reactions, you can write the expression:

square brackets indicate concentration.

physical meaning k– molecules are in continuous motion, collide, scatter, hit the walls of the vessel. In order for the chemical reaction of HJ formation to occur, the H 2 and J 2 molecules must collide. The number of such collisions will be the greater, the more H 2 and J 2 molecules are contained in the volume, i.e., the greater will be the values of [Н 2 ] and . But the molecules move at different speeds, and the total kinetic energy of the two colliding molecules will be different. If the fastest H 2 and J 2 molecules collide, their energy can be so high that the molecules break into iodine and hydrogen atoms, which fly apart and then interact with other H 2 + J 2 molecules > 2H+2J, then H + J 2 > HJ + J. If the energy of the colliding molecules is less, but high enough to weaken the H - H and J - J bonds, the reaction of formation of hydrogen iodine will occur:

For the majority of colliding molecules, the energy is less than necessary to weaken the bonds in H 2 and J 2 . Such molecules "quietly" collide and also "quietly" disperse, remaining what they were, H 2 and J 2 . Thus, not all, but only a part of the collisions leads to a chemical reaction. The coefficient of proportionality (k) shows the number of effective collisions leading to the reaction at concentrations [H 2 ] = = 1 mol. Value k–const speed. How can the speed be constant? Yes, the speed of uniform rectilinear motion is called a constant vector quantity equal to the ratio of the movement of the body for any period of time to the value of this interval. But the molecules move randomly, so how can the speed be const? But a constant speed can only be at a constant temperature. As the temperature rises, the proportion of fast molecules whose collisions lead to a reaction increases, i.e., the rate constant increases. But the increase in the rate constant is not unlimited. At a certain temperature, the energy of the molecules will become so large that almost all collisions of the reactants will be effective. When two fast molecules collide, a reverse reaction will occur.

A moment will come when the rates of formation of 2HJ from H 2 and J 2 and decomposition will be equal, but this is already a chemical equilibrium. The dependence of the reaction rate on the concentration of the reactants can be traced using the traditional reaction of the interaction of a sodium thiosulfate solution with a sulfuric acid solution.

Na 2 S 2 O 3 + H 2 SO 4 \u003d Na 2 SO 4 + H 2 S 2 O 3, (1)

H 2 S 2 O 3 \u003d Sv + H 2 O + SO 2 ^. (2)

Reaction (1) proceeds almost instantaneously. The rate of reaction (2) depends at a constant temperature on the concentration of the reactant H 2 S 2 O 3 . It is this reaction that we observed - in this case, the rate is measured by the time from the beginning of the pouring of solutions to the appearance of opalescence. In the article L. M. Kuznetsova the reaction of interaction of sodium thiosulfate with hydrochloric acid is described. She writes that when the solutions are drained, opalescence (turbidity) occurs. But this statement by L. M. Kuznetsova is erroneous, since opalescence and clouding are different things. Opalescence (from opal and Latin escentia- suffix meaning weak action) - light scattering by turbid media due to their optical inhomogeneity. light scattering- deviation of light rays propagating in the medium in all directions from the original direction. Colloidal particles are able to scatter light (Tyndall-Faraday effect) - this explains the opalescence, slight turbidity of the colloidal solution. When conducting this experiment, it is necessary to take into account the blue opalescence, and then the coagulation of the colloidal suspension of sulfur. The same density of the suspension is noted by the apparent disappearance of any pattern (for example, the grid at the bottom of the cup), observed from above through the solution layer. Time is counted by a stopwatch from the moment of draining.

Solutions Na 2 S 2 O 3 x 5H 2 O and H 2 SO 4.

The first is prepared by dissolving 7.5 g of salt in 100 ml of H 2 O, which corresponds to a 0.3 M concentration. To prepare a solution of H 2 SO 4 of the same concentration, it is necessary to measure 1.8 ml of H 2 SO 4 (k), ? = = 1.84 g / cm 3 and dissolve it in 120 ml of H 2 O. Pour the prepared solution of Na 2 S 2 O 3 into three glasses: in the first - 60 ml, in the second - 30 ml, in the third - 10 ml. Add 30 ml of distilled H 2 O to the second glass, and 50 ml to the third. Thus, in all three glasses there will be 60 ml of liquid, but in the first the salt concentration is conditionally = 1, in the second - ½, and in the third - 1/6. After the solutions are prepared, pour 60 ml of H 2 SO 4 solution into the first glass with a salt solution and turn on the stopwatch, etc. Considering that the reaction rate decreases with dilution of the Na 2 S 2 O 3 solution, it can be determined as a value inversely proportional to time v= one/? and build a graph by plotting the concentration on the abscissa and the rate of the reaction on the ordinate. From this conclusion - the reaction rate depends on the concentration of substances. The data obtained are listed in Table 3. This experiment can be performed using burettes, but this requires a lot of practice from the performer, because the schedule is sometimes incorrect.

Table 3

Speed and reaction time

The Guldberg-Waage law is confirmed - professor of chemistry Gulderg and the young scientist Waage).

Consider the next factor - temperature.

As the temperature increases, the rate of most chemical reactions increases. This dependence is described by the van't Hoff rule: "When the temperature rises for every 10 ° C, the rate of chemical reactions increases by 2-4 times."

where ? – temperature coefficient, showing how many times the reaction rate increases with an increase in temperature by 10 ° C;

v 1 - reaction rate at temperature t 1 ;

v 2 - reaction rate at temperature t2.

For example, the reaction at 50 °C proceeds in two minutes, how long will the process end at 70 °C if the temperature coefficient ? = 2?

t 1 = 120 s = 2 min; t 1 = 50 °С; t 2 = 70 °C.

Even a slight increase in temperature causes a sharp increase in the reaction rate of active molecular collisions. According to the activation theory, only those molecules participate in the process, the energy of which is greater than the average energy of the molecules by a certain amount. This excess energy is the activation energy. Its physical meaning is the energy that is necessary for the active collision of molecules (rearrangement of orbitals). The number of active particles, and hence the reaction rate, increases with temperature according to an exponential law, according to the Arrhenius equation, which reflects the dependence of the rate constant on temperature

where A - Arrhenius proportionality factor;

k– Boltzmann's constant;

E A - activation energy;

R- gas constant;

T- temperature.

A catalyst is a substance that speeds up the rate of a reaction but is not itself consumed.

Catalysis- the phenomenon of a change in the reaction rate in the presence of a catalyst. Distinguish between homogeneous and heterogeneous catalysis. Homogeneous- if the reactants and the catalyst are in the same state of aggregation. Heterogeneous– if the reactants and the catalyst are in different states of aggregation. About catalysis see separately (further).

Inhibitor A substance that slows down the rate of a reaction.

The next factor is surface area. The larger the surface of the reactant, the greater the speed. Consider, for example, the influence of the degree of dispersity on the reaction rate.

CaCO 3 - marble. We lower the tiled marble into hydrochloric acid HCl, wait five minutes, it will dissolve completely.

Powdered marble - we will do the same procedure with it, it dissolved in thirty seconds.

The equation for both processes is the same.

CaCO 3 (tv) + HCl (g) \u003d CaCl 2 (tv) + H 2 O (l) + CO 2 (g) ^.

So, when adding powdered marble, the time is less than when adding tile marble, with the same mass.

With an increase in the interface between phases, the rate of heterogeneous reactions increases.

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

The concept of speed in chemistry

It is customary to call the reaction rate a certain change in the concentrations of the compounds that have entered into the reaction (ΔС) per unit time (Δt). The mathematical formula for the rate of a chemical reaction is as follows:

ᴠ = ±∆C/∆t.

The reaction rate is measured in mol / l s if it occurs in the entire volume (that is, the reaction is homogeneous) and in mol / m 2 s if the interaction takes place 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 reactants, and the "+" sign - to the changing values of the concentrations of the products of the same reaction.

Examples of reactions at different rates

Chemical interactions can occur at different rates. Thus, the growth rate 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. The 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 is cooking, which is accompanied by the decomposition and transformation of the compounds contained in the products. Synthesis of individual polymers requires heating the reaction mixture for a certain time.

An example of chemical reactions, the speed 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. We 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 at 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 quite a long time, however, when the container is shaken or struck, the reaction becomes explosive. Therefore, chemical kinetics has 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;
change in pressure (for gaseous substances);
the area of contact 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 by a molecule during a collision in order for a reaction to occur. It is measured in kJ / mol and the values \u200b\u200bare usually in the range of 50-250.

It is generally accepted that if E a \u003d 150 kJ / mol for any reaction, then at n. y. it practically does not flow. This energy is spent on overcoming the repulsion between the molecules of substances and on weakening the bonds in the initial substances. In other words, the activation energy characterizes the strength of 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 the collisions of particles will lead to a reaction, and its speed will be low.

Influence of concentration

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

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 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 conditionally written as:

αА+ bB = ϲС, then,

in accordance with the above stated 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] - concentrations of the starting compounds,

k is the rate constant of the reaction in question.

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

Si (TV) + Ο 2 (g) \u003d 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's write the 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.

Temperature effect

The dependence of the rate of a chemical reaction on temperature was determined experimentally by the Dutch scientist J. H. Van't Hoff. He found that the rate of many reactions increases by 2-4 times with every 10 degrees rise in temperature. For this rule, there is a mathematical expression that 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 influence 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 molecular collisions, since it depends mainly on the activation energy. Also, a significant role in the efficiency of particle collision 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 reagents, obeys the Arrhenius equation:

k \u003d A 0 e -Ea / RΤ, where

A o is a multiplier;

E a - activation energy.

An example of a task on the van't Hoff law

How should the temperature be changed so that the rate of a chemical reaction, whose temperature coefficient is numerically equal to 3, increases by 27 times?

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 \u003d γ ΔΤ / 10.

We substitute the values: 27=3 ΔΤ/10.

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

Answer: the temperature should be increased by 30 degrees.

Influence of catalysts

In physical chemistry, the rate of chemical reactions is also actively studied by a section called catalysis. He is interested in how and why relatively small amounts of certain substances significantly increase the rate of interaction of others. Substances that can speed up a reaction but are not themselves consumed 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 particle impacts. A catalyst cannot cause a reaction that is energetically impossible.

So hydrogen peroxide is able to decompose with the formation of oxygen and water:

H 2 Ο 2 \u003d H 2 Ο + Ο 2.

But this reaction is very slow and in our medicine cabinets it exists unchanged for quite a long time. When opening only very old vials of peroxide, you can see a small pop caused by oxygen pressure on the walls of the vessel. The addition of just a few grains of magnesium oxide will provoke an active release of gas.

The same peroxide decomposition reaction, but under the action of catalase, occurs during the treatment of wounds. There are many different substances in living organisms 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 bad. Inhibitors are used to protect metal products from corrosion, to extend the shelf life of food, for example, to prevent the oxidation of fats.

Substance contact area

In the event that the interaction takes place between compounds that have different aggregate states, or between substances that are not able to form a homogeneous medium (immiscible liquids), then this factor also significantly affects the rate of a 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.

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 a 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 a change in the amount of substance (Δν) occurring per unit time (Δt) per unit surface

(S): V = Δν/(S Δt).

Pressure influence

A 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 a substance per unit volume, that is, its concentration increases proportionally. Conversely, a decrease in pressure leads to an equivalent decrease in the concentration of the reagent. In this case, the formula corresponding to ZDM is suitable for calculating the rate of a chemical reaction.

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

2ΝΟ + Ο 2 = 2ΝΟ 2 ,

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

Solution. As the volume decreases, the pressure increases proportionally. Let's write down the 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 divide 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 the rate constants are reduced, and remains:

V 2 /V 1 \u003d 9 3/1 \u003d 27.

Answer: the speed has increased by 27 times.

Summing up, it should be noted that the rate of interaction of substances, or rather, the number 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 remaining conditions, for an increase in the reaction rate it follows:

increase the temperature of the reaction medium;
increase the concentration of the original compounds;
increase the pressure in the system or reduce its volume, if we are talking about gases;
bring dissimilar substances to one state of aggregation (for example, by dissolving in water) or increase the area of their contact.

In life, we are faced with different chemical reactions. Some of them, like the rusting of iron, can go on for several years. Others, such as the fermentation of sugar into alcohol, take several weeks. Firewood in the stove burns out in a couple of hours, and gasoline in the engine burns out in a split second.

To reduce equipment costs, chemical plants increase the rate of reactions. And some processes, such as food spoilage, metal corrosion, need to be slowed down.

The rate of a chemical reaction can be expressed as change in the amount of matter (n, modulo) per unit time (t) - compare the speed of a moving body in physics as a change in coordinates per unit time: υ = Δx/Δt . So that the rate does not depend on the volume of the vessel in which the reaction takes place, we divide the expression by the volume of reacting substances (v), i.e., we obtain change in the amount of a substance per unit time per unit volume, or change in the concentration of one of the substances per unit time:

n 2 − n 1
υ = –––––––––– = –––––––– = Δс/Δt (1)
(t 2 − t 1) v Δt v

where c = n / v is the concentration of the substance,

Δ (pronounced "delta") is the generally accepted designation for a change in magnitude.

If substances have different coefficients in the equation, the reaction rate for each of them, calculated by this formula, will be different. For example, 2 moles of sulfur dioxide reacted completely with 1 mole of oxygen in 10 seconds in 1 liter:

2SO 2 + O 2 \u003d 2SO 3

The oxygen velocity will be: υ \u003d 1: (10 1) \u003d 0.1 mol / l s

Sour gas speed: υ \u003d 2: (10 1) \u003d 0.2 mol / l s- this does not need to be memorized and spoken in the exam, an example is given in order not to get confused if this question arises.

The rate of heterogeneous reactions (involving solids) is often expressed per unit area of contacting surfaces:

Δn
υ = –––––– (2)
Δt S

Reactions are called heterogeneous when the reactants are in different phases:

a solid with another solid, liquid or gas,
two immiscible liquids
gas liquid.

Homogeneous reactions occur between substances in the same phase:

between well-miscible liquids,
gases,
substances in solutions.

Conditions affecting the rate of chemical reactions

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

2) The reaction rate is greater, the higher concentration substances. With a highly dilute acid, the zinc will take significantly longer to react.

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

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

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

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

Catalysts Substances that speed up chemical reactions but are not themselves consumed. An example is the rapid decomposition of hydrogen peroxide with the addition of a catalyst - manganese (IV) oxide:

2H 2 O 2 \u003d 2H 2 O + O 2

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

Inhibitors- substances that slow down the reaction. For example, to extend the life of pipes and batteries, corrosion inhibitors are added to the water heating system. In automobiles, corrosion inhibitors are added to the brake fluid.

A few more examples.

The influence of the nature of reacting particles is determined by their atomic composition, spatial structure, and molecular properties. The rate of a chemical reaction is determined by the rate of breaking one and the formation of other chemical bonds. These transformations occur in the elementary act of the reaction. It is known that a change in the length of a chemical bond, bond angles, and other geometric parameters of a molecule is accompanied by a change in its potential energy. Therefore, the interaction of particles in an elementary act of reaction must also be characterized by a change in the potential energy of the entire system. Since reacting molecules usually contain many atoms, the elementary act of a chemical reaction is characterized by multidimensional potential energy surface. This potential energy surface reflects the influence of a change in each geometric parameter of one molecule on the energy of its interaction with another molecule and vice versa.

However, the interaction usually occurs in one specific place of the molecule - its reaction center. Therefore, it is possible to trace the change in the potential energy of the reacting system by considering a limited number of parameters associated only with the reaction center. These can be, for example, the lengths of two bonds: formed and broken, the bond angle between them. Then, instead of the potential energy surface, we can consider the change in the potential energy of the reacting system with respect to this limited set of its coordinates, called reaction coordinate.

Effect of temperature on the rate of reactions.

As the temperature rises, the rate of a chemical reaction increases. In the equation of chemical kinetics  = С А С В, the influence of temperature practically affects the change in the reaction rate constant . As the temperature increases, the value of the constant  increases, therefore, the rate of the reaction itself increases.

If through  T we denote the rate constant of a given reaction at a temperature T, and through  T + 10K - the rate constant of the same reaction at a temperature (T + 10K), the ratio of the second value to the first will give the so-called reaction rate temperature coefficient ():

 \u003d  T + 10K /  T (22.)

According to the approximate (empirical) van't Hoff rule, the value of the temperature coefficient  ranges from 2–4, i.e. with an increase in temperature by 10 K, the rate of a chemical reaction increases two to four times.

Fig.1. dependence of the temperature coefficient of the reaction rate on the temperature in the reactions

formation (1) and decomposition of HI (2).

According to the van't Hoff rule, the temperature coefficient of velocity  for each chemical reaction must be a constant value. However, in reality, it greatly decreases with increasing temperature, which is clearly seen from Fig. 1, which shows the curves  = f (T) for the reactions of formation and decomposition of hydrogen iodide. An increase in temperature by 30 K (from 743 to 773 K) entails a decrease in the temperature coefficient of the first reaction by 1.64 times, the second - by 1.71 times. For these reactions, the van't Hoff rule is valid only in a relatively narrow temperature range.

A more accurate dependence of the rate constant of a chemical reaction on temperature was found by Arrhenius (1889). The Arrhenius equation has the form

ln  = B  A / T, (23.)

where  - reaction rate constant; A and B are constants characteristic of this reaction; T is the thermodynamic temperature.

Equation (6.) shows that the logarithm of the rate constant is linearly dependent on the inverse temperature.

The rate of any chemical reaction depends on the number of collisions of the reacting molecules, since the number of collisions is proportional to the concentrations of the reactants. However, not all collisions of molecules are accompanied by an interaction. Obviously, the reaction rate depends not only on the number of collisions, but also on some properties of the colliding molecules. This phenomenon finds an explanation in the Arrhenius activation theory.

According to this theory, only those molecules are reactive that have the energy reserve necessary for the implementation of a particular reaction, i.e. excess energy compared to the average energy of the molecule. Such molecules are called active molecules. This excess energy of an active molecule, due to which a chemical reaction becomes possible, is called activation energy. This energy is usually expressed in kJ/kmol. The activation energy can be less than the bond breaking energy in the molecule, since in order for the molecule to react, it is not at all necessary to completely break the bonds, it is enough to weaken them.

The value of the activation energy depends on the structure of the molecule and on what reaction this molecule enters into, i.e. Each chemical reaction is characterized by its own value of activation energy. It can be reduced under the influence of external factors: an increase in temperature, radiant energy, catalysts, etc. The activation energy manifests itself in active molecules in different ways: active molecules can have a higher speed of movement, increased vibration energy of atoms in a molecule, etc.

The rate of a chemical reaction depends on the value of the activation energy: the higher it is, the slower the reaction will proceed. On the other hand, the lower the energy barrier of the reaction, the greater the number of molecules will have the necessary excess energy and the faster this reaction will proceed. So, the rate of a chemical reaction ultimately depends on the ratio between the number of active and inactive molecules.

In the theory of active collisions, Arrhenius showed that the number of active molecules can be calculated according to the Maxwell-Boltzmann law:

N a = N total e E/RT, (24.)

where N a is the number of active molecules; Ntot is the total number of molecules; e is the base of natural logarithms; E is the activation energy; T - thermodynamic temperature; R is the universal gas constant.

Thus, the increase in the reaction rate with increasing temperature is explained by the fact that with increasing temperature, not only the average kinetic energy of molecules increases, but also the fraction of molecules with energies above a certain level increases sharply, i.e. the proportion of active molecules capable of reaction.

Speed reaction is determined by the change in the molar concentration of one of the reactants:

V \u003d ± ((C 2 - C 1) / (t 2 - t 1)) \u003d ± (DC / Dt)

Where C 1 and C 2 are the molar concentrations of substances at times t 1 and t 2, respectively (sign (+) - if the rate is determined by the reaction product, sign (-) - by the original substance).

Reactions occur when molecules of reactants collide. Its speed is determined by the number of collisions and the likelihood that they will lead to a transformation. The number of collisions is determined by the concentrations of the reacting substances, and the probability of a reaction is determined by the energy of the colliding molecules.
Factors affecting the rate of chemical reactions.
1. The nature of the reactants. An important role is played by the nature of chemical bonds and the structure of the molecules of the reagents. Reactions proceed in the direction of the destruction of less strong bonds and the formation of substances with stronger bonds. Thus, high energies are required to break bonds in H 2 and N 2 molecules; such molecules are not very reactive. To break bonds in highly polar molecules (HCl, H 2 O), less energy is required, and the reaction rate is much higher. Reactions between ions in electrolyte solutions proceed almost instantaneously.
Examples
Fluorine reacts explosively with hydrogen at room temperature; bromine reacts with hydrogen slowly even when heated.
Calcium oxide reacts vigorously with water, releasing heat; copper oxide - does not react.

2. Concentration. With an increase in concentration (the number of particles per unit volume), collisions of reactant molecules occur more often - the reaction rate increases.
The law of active masses (K. Guldberg, P. Waage, 1867)
The rate of a chemical reaction is directly proportional to the product of the concentrations of the reactants.

AA + bB + . . . ® . . .

[A] a [B] b . . .

The reaction rate constant k depends on the nature of the reactants, temperature, and catalyst, but does not depend on the concentrations of the reactants.
The physical meaning of the rate constant is that it is equal to the reaction rate at unit concentrations of the reactants.
For heterogeneous reactions, the concentration of the solid phase is not included in the reaction rate expression.

3. Temperature. For every 10°C increase in temperature, the reaction rate increases by a factor of 2-4 (Van't Hoff's rule). With an increase in temperature from t 1 to t 2, the change in the reaction rate can be calculated by the formula:

		(t 2 - t 1) / 10
Vt 2 / Vt 1	= g

(where Vt 2 and Vt 1 are the reaction rates at temperatures t 2 and t 1, respectively; g is the temperature coefficient of this reaction).
Van't Hoff's rule is applicable only in a narrow temperature range. More accurate is the Arrhenius equation:

e-Ea/RT

where
A is a constant depending on the nature of the reactants;
R is the universal gas constant;

Ea is the activation energy, i.e. the energy that colliding molecules must have in order for the collision to result in a chemical transformation.
Energy diagram of a chemical reaction.


exothermic reaction	Endothermic reaction

A - reagents, B - activated complex (transition state), C - products.
The higher the activation energy Ea, the more the reaction rate increases with increasing temperature.

4. The contact surface of the reactants. For heterogeneous systems (when substances are in different states of aggregation), the larger the contact surface, the faster the reaction proceeds. The surface of solids can be increased by grinding them, and for soluble substances by dissolving them.

5. Catalysis. Substances that participate in reactions and increase its rate, remaining unchanged by the end of the reaction, are called catalysts. The mechanism of action of catalysts is associated with a decrease in the activation energy of the reaction due to the formation of intermediate compounds. At homogeneous catalysis the reagents and the catalyst constitute one phase (they are in the same state of aggregation), with heterogeneous catalysis- different phases (they are in different states of aggregation). In some cases, the course of undesirable chemical processes can be drastically slowed down by adding inhibitors to the reaction medium (the phenomenon negative catalysis").