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. So, to break bonds in molecules H 2 and N 2 high energies are required; such molecules are not very reactive. To break bonds in highly polar molecules ( HCl, H 2 O) requires less energy and the reaction rate is much faster. 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)

One of the basic laws of physical chemistry; sets speed dependency chemical reaction on the concentrations of the reacting substances and the ratio between the concentrations (or activities) of the reaction products and starting substances in a state of chemical equilibrium. The Norwegian scientists K. Guldberg and P. Vaage, who formulated the D. m. in 1864-67, they called the “acting mass” of a substance its quantity per unit volume, i.e. concentration, hence the name of the law.

At a constant temperature, the rate of a chemical reaction is directly proportional to the product of the concentrations of the reactants, taken in powers equal to the stoichiometric coefficients in the reaction equation.

For a monomolecular reaction the reaction rate  is determined by the concentration of molecules of substance A:

where k is the coefficient of proportionality, which is called rate constant reaction; [A] - molar concentration of substance A.

In the case of a bimolecular reaction, its speed is determined by the concentration of molecules not only of substance A, but also of substance B:

In the case of a trimolecular reaction, the reaction rate is expressed by the equation:

In general, if the reaction takes place at the same time t molecules of substance A and n molecules of substance B, i.e.

tA + pV = C,

the reaction rate equation is:

The form of the equation is determined by the fact that a necessary condition for an elementary act of a reaction is the collision of the molecules of the starting substances, i.e., their meeting in a certain small volume (on the order of the size of the molecules). The probability of finding a molecule A at a given moment in a given small volume is proportional to [A], i.e., the greater the concentration of reacting substances, the greater the reaction rate at a given time.

Reaction rate constant k depends on the nature of the reactants, temperature and catalyst, and in the case of a liquid solution, also on pressure; the latter dependence is significant only at high pressures, but does not depend on the concentrations of the reagents.

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.

Example

Write down the expression for the law of mass action for the following reactions:

a) N 2(d) + 3 H 2(d) = 2 NH 3(d)

b) 2 C (to) + O 2(d) = 2 CO (G)

Sections: Chemistry

The purpose of the lesson

• educational: continue the formation of the concept of "rate of chemical reactions", derive formulas for calculating the rate of homogeneous and heterogeneous reactions, consider what factors the rate of chemical reactions depends on;
• developing: learn to process and analyze experimental data; be able to find out the relationship between the rate of chemical reactions and external factors;
• educational: to continue the development of communication skills in the course of pair and collective work; to focus students' attention on the importance of knowledge about the rate of chemical reactions occurring in everyday life (metal corrosion, milk souring, rotting, etc.)

Teaching aids: D. multimedia projector, computer, slides on the main issues of the lesson, CD-ROM "Cyril and Methodius", tables on the tables, protocols of laboratory work, laboratory equipment and reagents;

Teaching methods: reproductive, research, partially search;

Form of organization of classes: conversation, practical work, independent work, testing;

Form of organization of work of students: frontal, individual, group, collective.

1. Class organization

Class readiness for work.

2. Preparation for the main stage of mastering the educational material. Activation of basic knowledge and skills(Slide 1, see the presentation for the lesson).

The topic of the lesson is “The rate of chemical reactions. Factors affecting the rate of a chemical reaction.

Task: to find out what is the rate of a chemical reaction, and on what factors does it depend. In the course of the lesson, we will get acquainted with the theory of the question on the above topic. In practice, we will confirm some of our theoretical assumptions.

Predicted student activity

The active work of students shows their readiness to perceive the topic of the lesson. Students need knowledge about the rate of a chemical reaction from the 9th grade course (intra-subject communication).

Let's discuss the following questions (frontally, slide 2):

1. Why do we need knowledge about the rate of chemical reactions?
2. What examples can confirm that chemical reactions proceed at different rates?
3. How is speed determined? mechanical movement? What is the unit for this speed?
4. How is the rate of a chemical reaction determined?
5. What conditions must be created for a chemical reaction to start?

Consider two examples (the experiment is conducted by the teacher).

On the table are two test tubes, in one is a solution of alkali (KOH), in the other is a nail; Add CuSO4 solution to both tubes. What are we seeing?

Predicted student activity

Using examples, students judge the speed of reactions and draw appropriate conclusions. Recording on the board the reactions done (two students).

In the first test tube, the reaction occurred instantly, in the second - there are no visible changes yet.

Compose the reaction equations (two students write equations on the board):

1. CuSO 4 + 2KOH \u003d Cu (OH) 2 + K 2 SO 4; Cu 2+ + 2OH - \u003d Cu (OH) 2
2. Fe + CuSO 4 \u003d FeSO 4 + Cu; Fe 0 + Cu 2+ = Fe 2+ + Cu 0

What conclusion can we draw from the reactions carried out? Why is one reaction instant and the other slow? To do this, it is necessary to remember that there are chemical reactions that occur throughout the entire volume of the reaction space (in gases or solutions), and there are others that occur only on the contact surface of substances (combustion solid body in a gas, the interaction of a metal with an acid, a salt of a less active metal).

Predicted student activity

Based on the results of the demonstrated experiment, students conclude: reaction 1 is homogeneous, and reaction

2 - heterogeneous.

The rates of these reactions will be mathematically determined in different ways.

The study of the rates and mechanisms of chemical reactions is called chemical kinetics.

3. Assimilation of new knowledge and ways of action(Slide 3)

The reaction rate is determined by the change in the amount of a substance per unit time

In unit V

(for homogeneous)

Per unit contact surface of substances S (for heterogeneous)

Obviously, with this definition, the value of the reaction rate does not depend on the volume in a homogeneous system and on the area of ​​contact of the reagents - in a heterogeneous one.

Predicted student activity

Active actions of students with the object of study. Entering the table in a notebook.

From this follows two important moments(slide 4):

2) the calculated value of the velocity will depend on what substance it is determined by, and the choice of the latter depends on the convenience and ease of measuring its quantity.

For example, for the reaction 2H 2 + O 2 \u003d 2H 2 O: υ (for H 2) \u003d 2 υ (for O 2) \u003d υ (for H 2 O)

4. Consolidation of primary knowledge about the rate of a chemical reaction

To consolidate the considered material, we will solve the calculation problem.

Predicted student activity

Primary comprehension of the acquired knowledge about the reaction rate. The correctness of the solution of the problem.

A task (slide 5). The chemical reaction proceeds in solution according to the equation: A + B = C. Initial concentrations: substances A - 0.80 mol / l, substances B - 1.00 mol / l. After 20 minutes, the concentration of substance A decreased to 0.74 mol/l. Determine: a) the average reaction rate for this period of time;

b) the concentration of substance C after 20 minutes. Solution (Appendix 4, slide 6).

5. Assimilation of new knowledge and ways of action(carrying out laboratory work in the course of repetition and study of new material, step by step, Appendix 2).

We know that different factors affect the rate of a chemical reaction. Which?

Predicted student activity

Reliance on the knowledge of grades 8-9, writing in a notebook in the course of studying the material. List (slide 7):

The nature of the reactants;

Temperature;

The concentration of reactants;

The action of catalysts;

Contact surface of reactants (in heterogeneous reactions).

The influence of all these factors on the reaction rate can be explained using simple theory – collision theory (slide 8). Its main idea is this: reactions occur when particles of reactants that have a certain energy collide.

From this we can draw the following conclusions:

1. The more reagent particles, the closer they are to each other, the more likely they are to collide and react.
2. Only lead to a reaction effective collisions, those. those in which "old ties" are destroyed or weakened and therefore "new" ones can form. But for this, the particles must have sufficient energy.

The minimum energy excess (over the average energy of particles in the system) required for efficient collision of particles in the system) required for efficient collision of reactant particles is calledactivation energy E a.

Predicted student activity

Understanding the concept and writing the definition in a notebook.

Thus, on the way of all particles entering into the reaction, there is some energy barrier equal to the activation energy. If it is small, then there are many particles that successfully overcome it. With a large energy barrier, additional energy is needed to overcome it, sometimes a good “push” is enough. I light the spirit lamp - I give additional energy E a, necessary to overcome the energy barrier in the reaction of the interaction of alcohol molecules with oxygen molecules.

Consider factors, that affect the rate of the reaction.

1) The nature of the reactants(slide 9). The nature of reacting substances is understood as their composition, structure, mutual influence of atoms in inorganic and organic substances.

The magnitude of the activation energy of substances is a factor through which the influence of the nature of the reacting substances on the reaction rate is affected.

Briefing.

Self-formulation of conclusions (Appendix 3 at home)

There is an area in physical chemistry that deals with the analysis of the rate of chemical processes, determining the conditions that affect the increase or decrease in speed. This area is called chemical kinetics. This area of ​​science studies the mechanisms of conduction and thermodynamic features of processes. The knowledge gained is used for scientific purposes, in the production of chemicals, when it is important to control the interaction of ingredients in reactors.

The term "reaction rate" means the equivalent change in the concentrations of the ingredients involved in the reaction, for the designated time unit. A special formula is used for speed calculations: ᴠ = ±C/t.

The unit of measurement of speed is mol/l*s during a homogeneous process, when the entire volume reacts. For a multi-stage reaction, when there are clearly defined phases, another unit of measurement is used - mol / m2 * s.

Different speed chemical processes

Chemical components can interact with each other at different rates. For example, stalactites are formed as a result of an increase in calcium carbonate. The growth rate of education is half a millimeter in 100 years. Other biochemical reactions take place just as slowly. The processes of protein synthesis and corrosion of metals are distinguished by a rather low speed.

Other processes that take one or more hours to complete are faster. These reactions include cooking, when the decomposition and transformation of compounds present in food products occurs.

For a certain period of time, the reaction composition, which is used for the synthesis of certain polymers, should be heated.

Fast chemical reactions include neutralization, contacting dilute acetic acid with ordinary baking soda, resulting in the release of carbon dioxide. The reaction of sodium salts with barium nitrate can be added to the list, after which insoluble barium sulfate precipitates.

Important! There are a huge number of chemical processes that proceed very quickly and end with an explosion. A striking example is the combination of potassium and water.

Factors affecting the reaction rate

By combining chemicals, a reaction is obtained, which in different conditions flows at different speeds. For example, with a combination of hydrogen and oxygen in the gaseous state, the mixture is inactive for a long time, but if the container is shaken or hit, the reaction will end with an explosion.

In connection with such features, specialists in chemical kinetics have identified a number of factors that have the ability to affect the speed of a chemical reaction.

Among these conditions are:

• natural properties of reacting components;
• concentration of reagents;
• temperature fluctuations;
• the use of a catalyst;
• pressure surges (when using gaseous components);
• the area of ​​interaction of substances.

Natural properties of reagents

A large difference in the rates of chemical reactions is due to different activation energy coefficients - an excess amount of energy, the amount of which exceeds the average values ​​\u200b\u200bnecessary for particles to collide and interact. The parameter has its own unit kJ/mol. These values ​​range from 50 to 250.

With an excess of energy in the amount of 150 kJ/mol, the reaction does not proceed under normal conditions. The amount of released energy will be spent on preventing the repulsion of molecules, minimizing bonds within the substance. The strength of chemical bonds in the ingredients depends on the activation energy. The energy value reflects the activity of the reaction:

• less than 40 - the interaction is fast, all molecular impacts end in a reaction;
• more than 40, but less than 120 - average speed, only half of the collisions are effective;
• more than 120 - slow interaction, since an insignificant part of particle collisions ends in a reaction.

Substance concentration

The number of molecules per unit volume affects the speed of interaction. The process falls under the law of mass action. The law applies to the simplest reactions that take place in one stage. Also suitable for multi-step reactions where the process takes place at a specific step.

The speed of the chemical process, taking into account the conditions of the law, is determined by the formula V=k·[A]a·[B]b. In the mathematical equation, a and b act as stoichiometric coefficients, [A] and [B] are the concentrations of the reactants, k is the rate constant.

The rate value displays an identical coefficient if the concentrations of the reacting ingredients are equal to one. For correct calculations using the formula, the aggregate state of the components must be taken into account. The concentration of the solid ingredient is equal to one, therefore it is not included in the equation, since it does not change during the reaction.

To determine the velocity coefficient, only the components of the liquid and gaseous states are included in the formula.

Temperature regime

The course of the chemical reaction also depends on temperature conditions. It has been experimentally revealed that the activity of some chemical processes increases several times if the temperature is raised by at least 10 degrees. The next 10 degrees also provoke an increase in activity by 2-4 times.

Unfortunately, the mechanism of the effect of temperature on the rate of a single reaction has not been studied. Also, the list of regularities is not defined. It can be assumed from the point of view of logic that an increase in temperature contributes to an increase in the chaotic movement of molecules and atoms, as a result of which the number of their collisions increases significantly.

However, this feature of the impact does not increase the efficiency of particle collision, because the main catalyst for this process is the activation energy. Also, for the effectiveness of the interaction of molecules, their spatial correspondence is necessary.

Application of catalysts

The activity of a reaction in chemistry is also studied by another direction, which is called catalysis. Its task is to find out how and by what algorithm small volumes of designated substances increase the rate of reagent withdrawal. These substances are called catalysts. Moreover, the catalyst itself, accelerating the reaction, is practically not consumed.

Accelerators are able to change the mechanism of a chemical process and provoke the formation of such transition states of a substance that have a lower energy barrier. The catalyst can reduce the activation energy, increase the number of effective collisions of molecules. If the interaction is energetically impossible, then the use of an accelerator is pointless.

Component contact area

When mixing substances that are in different states of aggregation, or components that are not able to combine into a homogeneous mixture, the chemical reaction rate is largely affected by the interaction area of ​​the ingredients.

This is due to the occurrence of a heterogeneous reaction at the contact boundary of the reagents. That is, the wider this boundary, the more particles collide and provoke a rapid reaction.

Vivid examples of such properties:

• small chips burn much more actively than whole logs;
• crushed solids dissolve better in a liquid than a whole piece.

Important! When grinding a solid ingredient, arbitrary destruction occurs crystal lattice reagent, so the reactive properties of the particles increase.

Pressure influence

Pressure drops during a reaction can affect activity only when gaseous substances are used as reactants. High pressure contributes to an increase in the number of component molecules per unit volume, the density of the reagent increases. At low pressure, the number of particles decreases, hence the concentration decreases.

The rate of chemical interaction is the amount and efficiency of the collision of reactant molecules. This process can be faster or slower if the right conditions are created. The increase in speed can be influenced by factors that can be controlled to varying degrees:

• temperature regime;
• concentration level of reacting components;
• increase or decrease in pressure;
• transformation of heterogeneous components into the same state of aggregation.

Useful video

Summing up

To a large extent, the response speed of substances depends on the starting energy and the geometric features of the molecules. These two parameters cannot be controlled and adjusted. The study of response reactions of chemical components to the impact of these factors is of great value for many industries. The data is used in laboratory research, in the pharmacological field, metallurgy, cooking, at nuclear enterprises, in the production of paints and varnishes, polymers.

In contact with

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.

In contact with

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 is 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 in the process of interaction they are destroyed chemical bonds reactants and new bonds of reaction products are formed, then the nature of the bonds participating in the reaction of the compounds and the structure of the molecules of the reacting substances will play a large 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 the colliding particles will increase, it is obvious that with an increase in temperature, the chemical process itself 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 number of 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.

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. ten

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 (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 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 up 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, 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?

t1 = 120 s = 2 min; t1 = 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 BUT - 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.