We are constantly confronted with various chemical interactions. Combustion natural gas, rusting of iron, souring of milk - far from all the processes that are studied in detail in a school chemistry course.

Some reactions take fractions of seconds, while some interactions take days or weeks.

Let's try to identify the dependence of the reaction rate on temperature, concentration, and other factors. In the new educational standard, a minimum amount of study time is allocated for this issue. In the tests of the unified state exam, there are tasks on the dependence of the reaction rate on temperature, concentration, and even calculation tasks are offered. Many high school students experience certain difficulties in finding answers to these questions, so we will analyze this topic in detail.

The relevance of the issue under consideration

Information about the reaction rate is of great practical and scientific importance. For example, in a specific production of substances and products, the productivity of equipment and the cost of goods directly depend on this value.

Classification of ongoing reactions

There is a direct relationship between the state of aggregation of the initial components and products formed in the course of heterogeneous interactions.

In chemistry, a system is usually understood as a substance or a combination of them.

Homogeneous is such a system that consists of one phase (the same state of aggregation). As an example, we can mention a mixture of gases, several different liquids.

A heterogeneous system is a system in which the reactants are in the form of gases and liquids, solids and gases.

There is not only a dependence of the reaction rate on temperature, but also on the phase in which the components involved in the analyzed interaction are used.

For a homogeneous composition, the process is characteristic throughout the entire volume, which significantly improves its quality.

If the initial substances are in different phase states, then the maximum interaction is observed at the interface. For example, when an active metal is dissolved in an acid, the formation of a product (salt) is observed only on the surface of their contact.

Mathematical relationship between process speed and various factors

What is the equation for the rate of a chemical reaction as a function of temperature? For a homogeneous process, the rate is determined by the amount of a substance that interacts or is formed during the reaction in the volume of the system per unit time.

For a heterogeneous process, the rate is determined through the amount of a substance that reacts or is obtained in the process per unit area for a minimum period of time.

Factors affecting the rate of a chemical reaction

The nature of the reactants is one of the reasons for the different rates of processes. For example, alkali metals room temperature form alkalis with water, and the process is accompanied by intense evolution of gaseous hydrogen. Noble metals (gold, platinum, silver) are not capable of such processes either at room temperature or when heated.

The nature of the reactants is a factor that is taken into account in the chemical industry in order to increase the profitability of production.

The relationship between the concentration of reagents and the speed of the chemical reaction is revealed. The higher it is, the more particles will collide, therefore, the process will proceed faster.

The law of mass action in mathematical form describes a directly proportional relationship between the concentration of the initial substances and the speed of the process.

It was formulated in the middle of the nineteenth century by the Russian chemist N. N. Beketov. For each process, a reaction constant is determined, which is not related to temperature, concentration, or the nature of the reactants.

In order to speed up the reaction in which a solid is involved, it is necessary to grind it to a powder state.

In this case, an increase in the surface area occurs, which positively affects the speed of the process. For diesel fuel, a special injection system is used, due to which, when it comes into contact with air, the rate of the combustion process of a mixture of hydrocarbons increases significantly.

The heating

The dependence of the rate of a chemical reaction on temperature is explained by molecular kinetic theory. It allows you to calculate the number of collisions between the molecules of the reagents under certain conditions. Armed with such information, then normal conditions all processes must be instantaneous.

But if we consider specific example dependence of the reaction rate on temperature, it turns out that for interaction it is necessary to first break chemical bonds between atoms to form new substances. It requires significant costs energy. What is the dependence of the reaction rate on temperature? The activation energy determines the possibility of rupture of molecules, it characterizes the reality of the processes. Its units of measurement are kJ/mol.

With an insufficient energy index, the collision will be ineffective, so it is not accompanied by the formation of a new molecule.

Graphical representation

The dependence of the rate of a chemical reaction on temperature can be represented graphically. When heated, the number of collisions between particles increases, which contributes to the acceleration of interaction.

What is a graph of reaction rate versus temperature? The energy of molecules is plotted horizontally, and the number of particles with a high energy reserve is indicated vertically. A graph is a curve by which one can judge the speed of a particular interaction.

The greater the energy difference from the average, the further the curve point is from the maximum, and a smaller percentage of molecules have such an energy reserve.

Important Aspects

Is it possible to write an equation for the dependence of the reaction rate constant on temperature? Its increase is reflected in the increase in the speed of the process. Such a dependence is characterized by a certain value, called the temperature coefficient of the process rate.

For any interaction, the dependence of the reaction rate constant on temperature was revealed. In the case of its increase by 10 degrees, the process speed increases by 2-4 times.

The dependence of the rate of homogeneous reactions on temperature can be represented mathematically.

For most interactions at room temperature, the coefficient is in the range from 2 to 4. For example, with a temperature coefficient of 2.9, a temperature increase of 100 degrees speeds up the process by almost 50,000 times.

The dependence of the reaction rate on temperature can be easily explained by different values ​​of the activation energy. It has a minimum value during ionic processes, which are determined only by the interaction of cations and anions. Numerous experiments testify to the instantaneous occurrence of such reactions.

At a high value of the activation energy, only a small number of collisions between particles will lead to the implementation of the interaction. With an average activation energy, the reactants will interact at an average rate.

Tasks on the dependence of the reaction rate on concentration and temperature are considered only at the senior level of education, and often cause serious difficulties for children.

Measuring the speed of the process

Those processes that require a significant activation energy involve an initial break or weakening of bonds between atoms in the original substances. In this case, they pass into a certain intermediate state, called the activated complex. It is an unstable state, rather quickly decomposes into reaction products, the process is accompanied by the release of additional energy.

In its simplest form, the activated complex is a configuration of atoms with weakened old bonds.

Inhibitors and Catalysts

Let us analyze the dependence of the enzymatic reaction rate on the temperature of the medium. Such substances act as process accelerators.

They themselves are not participants in the interaction, their number after the completion of the process remains unchanged. If catalysts increase the reaction rate, then inhibitors, on the contrary, slow down this process.

The essence of this lies in the formation of intermediate compounds, as a result of which a change in the speed of the process is observed.

Conclusion

Various chemical interactions take place every minute in the world. How to establish the dependence of the reaction rate on temperature? The Arrhenius equation is a mathematical explanation of the relationship between the rate constant and temperature. It gives an idea of ​​those activation energies at which the destruction or weakening of bonds between atoms in molecules, the distribution of particles into new chemical substances is possible.

Thanks to the molecular kinetic theory, it is possible to predict the probability of interactions between the initial components, to calculate the rate of the process. Among those factors that affect the reaction rate, the change in the temperature index, the percentage concentration of interacting substances, the contact surface area, the presence of a catalyst (inhibitor), and the nature of the interacting components are of particular importance.

Some chemical reactions occur almost instantly (explosion of an oxygen-hydrogen mixture, ion exchange reactions in an aqueous solution), the second - quickly (combustion of substances, the interaction of zinc with acid), and others - slowly (rusting of iron, decay of organic residues). So slow reactions are known that a person simply cannot notice them. For example, the transformation of granite into sand and clay takes place over thousands of years.

In other words, chemical reactions can proceed with different speed.

But what is speed reaction? What is precise definition given value and, most importantly, its mathematical expression?

The rate of a reaction is the change in the amount of a substance in one unit of time in one unit of volume. Mathematically, this expression is written as:

Where n 1 andn 2 - the amount of substance (mol) at time t 1 and t 2, respectively, in a system with a volume V.

Which plus or minus sign (±) will stand before the expression of speed depends on whether we are looking at a change in the amount of which substance - a product or a reactant.

Obviously, in the course of the reaction, the consumption of reagents occurs, that is, their number decreases, therefore, for the reagents, the expression (n 2 - n 1) always has a value less than zero. Since the speed cannot be a negative value, in this case, a minus sign must be placed before the expression.

If we are looking at the change in the amount of the product, and not the reactant, then the minus sign is not required before the expression for calculating the rate, since the expression (n 2 - n 1) in this case is always positive, because the amount of product as a result of the reaction can only increase.

The ratio of the amount of substance n to the volume in which this amount of substance is, called the molar concentration WITH:

Thus, using the concept of molar concentration and its mathematical expression, we can write another way to determine the reaction rate:

The reaction rate is the change in the molar concentration of a substance as a result of a chemical reaction in one unit of time:

Factors affecting the reaction rate

It is often extremely important to know what determines the rate of a particular reaction and how to influence it. For example, the oil refining industry literally fights for every additional half a percent of the product per unit of time. After all, given the huge amount of oil processed, even half a percent flows into a large annual financial profit. In some cases, it is extremely important to slow down any reaction, in particular, the corrosion of metals.

So what does the rate of a reaction depend on? It depends, oddly enough, on many different parameters.

In order to understand this issue, first of all, let's imagine what happens as a result of a chemical reaction, for example:

A + B → C + D

The equation written above reflects the process in which the molecules of substances A and B, colliding with each other, form molecules of substances C and D.

That is, undoubtedly, in order for the reaction to take place, at least a collision of the molecules of the starting substances is necessary. Obviously, if we increase the number of molecules per unit volume, the number of collisions will increase in the same way that the frequency of your collisions with passengers in a crowded bus increases compared to a half-empty one.

In other words, the reaction rate increases with increasing concentration of the reactants.

In the case when one or several of the reactants are gases, the reaction rate increases with increasing pressure, since the pressure of a gas is always directly proportional to the concentration of its constituent molecules.

However, the collision of particles is a necessary but not sufficient condition for the reaction to proceed. The fact is that, according to calculations, the number of collisions of the molecules of the reacting substances at their reasonable concentration is so large that all reactions must proceed in an instant. However, this does not happen in practice. What's the matter?

The fact is that not every collision of reactant molecules will necessarily be effective. Many collisions are elastic - molecules bounce off each other like balls. In order for the reaction to take place, the molecules must have sufficient kinetic energy. The minimum energy that the molecules of the reactants must have in order for the reaction to take place is called the activation energy and is denoted as E a. In a system consisting of a large number of molecules, there is an energy distribution of molecules, some of them have low energy, some have high and medium energy. Of all these molecules, only a small fraction of the molecules have an energy greater than the activation energy.

As is known from the course of physics, temperature is actually a measure of the kinetic energy of the particles that make up the substance. That is, the faster the particles that make up the substance move, the higher its temperature. Thus, obviously, by raising the temperature, we essentially increase the kinetic energy of the molecules, as a result of which the proportion of molecules with energies exceeding E a increases, and their collision will lead to a chemical reaction.

The fact of the positive effect of temperature on the reaction rate was empirically established as early as the 19th century by the Dutch chemist Van't Hoff. Based on his research, he formulated a rule that still bears his name, and it sounds like this:

The rate of any chemical reaction increases by 2-4 times with an increase in temperature by 10 degrees.

The mathematical representation of this rule is written as:

where V 2 and V 1 is the speed at temperature t 2 and t 1, respectively, and γ is the temperature coefficient of the reaction, the value of which most often lies in the range from 2 to 4.

Often the rate of many reactions can be increased by using catalysts.

Catalysts are substances that speed up a reaction without being consumed.

But how do catalysts manage to increase the rate of a reaction?

Recall the activation energy E a . Molecules with energies less than the activation energy cannot interact with each other in the absence of a catalyst. Catalysts change the path along which the reaction proceeds, similar to how an experienced guide will pave the route of the expedition not directly through the mountain, but with the help of bypass paths, as a result of which even those satellites that did not have enough energy to climb the mountain will be able to move to another her side.

Despite the fact that the catalyst is not consumed during the reaction, nevertheless it takes an active part in it, forming intermediate compounds with reagents, but by the end of the reaction it returns to its original state.

In addition to the above factors affecting the reaction rate, if there is an interface between the reacting substances (heterogeneous reaction), the reaction rate will also depend on the contact area of ​​the reactants. For example, imagine a granule of aluminum metal that is thrown into a test tube with aqueous solution of hydrochloric acid. Aluminum is an active metal that can react with non-oxidizing acids. With hydrochloric acid, the reaction equation is as follows:

2Al + 6HCl → 2AlCl 3 + 3H 2

Aluminum is a solid, which means it only reacts with hydrochloric acid on its surface. Obviously, if we increase the surface area by first rolling the aluminum granule into foil, we thereby provide large quantity aluminum atoms available for reaction with acid. As a result, the reaction rate will increase. Similarly, an increase in the surface of a solid can be achieved by grinding it into a powder.

Also, the rate of a heterogeneous reaction, in which a solid reacts with a gaseous or liquid, is often positively affected by mixing, which is due to the fact that as a result of mixing, the accumulating molecules of the reaction products are removed from the reaction zone and a new portion of the reagent molecules is “brought up”.

The last thing to note is also the huge influence on the rate of the reaction and the nature of the reagents. For example, the lower the alkali metal is in the periodic table, the faster it reacts with water, fluorine reacts most quickly with hydrogen gas among all halogens, etc.

In summary, the reaction rate depends on the following factors:

1) concentration of reagents: the higher, the greater the reaction rate

2) temperature: with increasing temperature, the rate of any reaction increases

3) the contact area of ​​the reactants: than more area contact of reagents, the higher the reaction rate

4) stirring, if the reaction occurs between a solid and a liquid or gas, stirring can accelerate it.

The mechanisms of chemical transformations and their rates are studied by chemical kinetics. Chemical processes proceed in time at different rates. Some happen quickly, almost instantly, while others take a very long time to occur.

Speed ​​reaction- the rate at which reagents are consumed (their concentration decreases) or reaction products are formed per unit volume.

Factors that can affect the rate of a chemical reaction

The following factors can affect how quickly a chemical interaction occurs:

• concentration of substances;
• the nature of the reagents;
• temperature;
• the presence of a catalyst;
• pressure (for reactions in a gaseous medium).

Thus, by changing certain conditions for the course of a chemical process, it is possible to influence how quickly the process will proceed.

In the process of chemical interaction, the particles of the reacting substances collide with each other. The number of such coincidences is proportional to the number of particles of substances in the volume of the reacting mixture, and hence proportional to the molar concentrations of the reagents.

Law of acting masses describes the dependence of the reaction rate on the molar concentrations of the reacting substances.

For an elementary reaction (A + B → ...) this law expressed by the formula:

υ \u003d k ∙С A ∙С B,

where k is the rate constant; C A and C B are the molar concentrations of the reactants, A and B.

If one of the reacting substances is in a solid state, then the interaction occurs at the phase interface, and therefore the concentration of the solid substance is not included in the equation of the kinetic law of acting masses. To understand the physical meaning of the rate constant, it is necessary to take C, A and C B equal to 1. Then it becomes clear that the rate constant is equal to the reaction rate at reagent concentrations equal to unity.

The nature of the reagents

Since the chemical bonds of the reacting substances are destroyed in the process of interaction and new bonds of the reaction products are formed, the nature of the bonds participating in the reaction of the compounds and the structure of the molecules of the reacting substances will play an important role.

Surface area of ​​contact of reagents

Such a characteristic as the surface area of ​​contact of solid reagents, sometimes quite significantly, affects the course of the reaction. Grinding a solid allows you to increase the surface area of ​​contact of the reagents, and hence speed up the process. The area of ​​contact of solutes is easily increased by the dissolution of the substance.

Reaction temperature

As the temperature increases, the energy of colliding particles will increase, it is obvious that with increasing temperature, the chemical process will accelerate. good example how an increase in temperature affects the process of interaction of substances, we can consider the data given in the table.

Table 1. Effect of temperature change on the rate of water formation (О 2 +2Н 2 →2Н 2 О)

For a quantitative description of how temperature can affect the rate of interaction of substances, the van't Hoff rule is used. Van't Hoff's rule is that when the temperature rises by 10 degrees, there is an acceleration of 2-4 times.

The mathematical formula describing the van't Hoff rule is as follows:

Where γ is the temperature coefficient of the chemical reaction rate (γ = 2−4).

But the Arrhenius equation describes the temperature dependence of the rate constant much more accurately:

Where R is the universal gas constant, A is a factor determined by the type of reaction, E, A is the activation energy.

The activation energy is the energy that a molecule must acquire in order for a chemical transformation to occur. That is, it is a kind of energy barrier that will need to be overcome by molecules colliding in the reaction volume in order to redistribute bonds.

The activation energy does not depend on external factors, but depends on the nature of the substance. The value of the activation energy up to 40 - 50 kJ / mol allows substances to react with each other quite actively. If the activation energy exceeds 120 kJ/mol, then the substances (at ordinary temperatures) will react very slowly. A change in temperature leads to a change in the amount active molecules, that is, molecules that have reached an energy greater than the activation energy, and therefore capable of chemical transformations.

Catalyst action

A catalyst is a substance that can speed up a process, but is not part of its products. Catalysis (acceleration of the course of a chemical transformation) is divided into · homogeneous, · heterogeneous. If the reactants and the catalyst are in the same state of aggregation, then catalysis is called homogeneous, if in different states, then heterogeneous. The mechanisms of action of catalysts are diverse and quite complex. In addition, it should be noted that catalysts are characterized by selectivity of action. That is, the same catalyst, accelerating one reaction, may not change the rate of another in any way.

Pressure

If gaseous substances are involved in the transformation, then the rate of the process will be affected by a change in pressure in the system . This happens because that for gaseous reactants, a change in pressure leads to a change in concentration.

Experimental determination of the rate of a chemical reaction

It is possible to determine the rate of a chemical transformation experimentally by obtaining data on how the concentration of reacting substances or products changes per unit time. Methods for obtaining such data are divided into

• chemical,
• physical and chemical.

Chemical methods are quite simple, affordable and accurate. With their help, the speed is determined by directly measuring the concentration or amount of a substance of reactants or products. In the case of a slow reaction, samples are taken to monitor how the reagent is consumed. After that, the content of the reagent in the sample is determined. By sampling at regular intervals, it is possible to obtain data on the change in the amount of a substance during the interaction. The most commonly used types of analysis are titrimetry and gravimetry.

If the reaction proceeds quickly, then in order to take a sample, it has to be stopped. This can be done by cooling abrupt removal of the catalyst, it is also possible to dilute or transfer one of the reagents to a non-reactive state.

Methods of physicochemical analysis in modern experimental kinetics are used more often than chemical ones. With their help, you can observe the change in the concentrations of substances in real time. There is no need to stop the reaction and take samples.

Physico-chemical methods are based on the measurement physical property, depending on the quantitative content of a certain compound in the system and changing with time. For example, if gases are involved in the reaction, then pressure can be such a property. Electrical conductivity, refractive index, and absorption spectra of substances are also measured.

Let's 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 rigorous; 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, referred to a unit of volume or surface - electrical conductivity, optical density, dielectric constant, etc. etc. However, most often in chemistry, the dependence of the concentration of reagents on time is considered. In the case of unilateral (irreversible) chemical reactions(hereinafter, only one-way reactions are considered) it is obvious that the concentrations of the starting substances are constantly decreasing with time (ΔС ref< 0), а концентрации продуктов реакции увеличиваются (ΔС прод >0). The reaction rate is assumed to be positive, so the mathematical definition is average reaction rate in the time interval Δt is written as follows:

(II.1)

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

(II.2)

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

Rice. 2.1 Kinetic curves for starting materials (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 rate of reaction in this moment time is equal in absolute value to the tangent of the slope of the tangent:

Rice. 2.2 Graphic definition V ist.

(II.3)

It should be noted that in the event that the stoichiometric coefficients in the chemical reaction equation are not the same, 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

concentrations of hydrogen, oxygen and water vary to varying degrees:

ΔC (H 2) \u003d ΔC (H 2 O) \u003d 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 to determine the composition of the reaction mixture (i.e., the concentrations of all reactants) 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 the reactants, the greater the rate of the chemical reaction. The basis of chemical kinetics is 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 reactants, taken to some extent.

i.e. for the reaction

AA + bB + dD + ... → eE + ...

Can be written

(II.4)

The coefficient of proportionality 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 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 rate constant and exponents at the concentrations of the reactants. The exponent at 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 order reactions for this component. The sum of the exponents in the kinetic equation for 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 reaction equation is a material balance equation and in no way can determine the nature of the course of this reaction in time.

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

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:

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. On the graph (Fig. 10) is shown directly proportional dependence. The quantitative dependence of the reaction rate on the concentration of the reactants is expressed by the MMA (the 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 established empirically 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 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, a 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 sharp increase reaction rates 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.