How does the rate of chemical reactions depend? Reaction speed, its dependence on various factors

The concept of “speed” is found quite often in the literature. It is known from physics that the greater the distance a material body (a person, a train, a spaceship) covers in a certain period of time, the higher the speed of this body.

How to measure the speed of a chemical reaction that “goes nowhere” and does not cover any distance? In order to answer this question, you need to find out what Always changes in any chemical reaction? Since any chemical reaction is a process of changing a substance, the original substance disappears in it, turning into reaction products. Thus, during a chemical reaction, the amount of a substance always changes, the number of particles of the starting substances decreases, and therefore its concentration (C).

Unified State Examination task. The rate of a chemical reaction is proportional to the change:

1. concentration of a substance per unit time;
2. amount of substance per unit volume;
3. mass of a substance per unit volume;
4. volume of substance during the reaction.

Now compare your answer with the correct one:

the rate of a chemical reaction is equal to the change in the concentration of the reactant per unit time

Where C 1 And From 0- concentrations of reactants, final and initial, respectively; t 1 And t 2- the time of the experiment, the final and initial period of time, respectively.

Question. Which value do you think is greater: C 1 or From 0? t 1 or t 0?

Since reactants are always consumed in a given reaction, then

Thus, the ratio of these quantities is always negative, and speed cannot be a negative quantity. Therefore, a minus sign appears in the formula, which simultaneously indicates that the speed any reactions over time (under constant conditions) are always decreases.

So, the rate of the chemical reaction is:

The question arises: in what units should the concentration of reactants (C) be measured and why? In order to answer it, you need to understand what condition is main for any chemical reaction to occur.

In order for particles to react, they must at least collide. That's why the higher the number of particles* (number of moles) per unit volume, the more often they collide, the higher the probability of a chemical reaction.

* Read about what a “mole” is in lesson 29.1.

Therefore, when measuring the rates of chemical processes, they use molar concentration substances in reacting mixtures.

The molar concentration of a substance shows how many moles of it are contained in 1 liter of solution

So, the greater the molar concentration of the reacting substances, the more particles there are per unit volume, the more often they collide, and the higher (all other things being equal) the rate of the chemical reaction. Therefore, the basic law of chemical kinetics (this is the science of the rate of chemical reactions) is law of mass action.

The rate of a chemical reaction is directly proportional to the product of the concentrations of the reactants.

For a reaction of type A + B →... mathematically this law can be expressed as follows:

If the reaction is more complex, for example, 2A + B → or, which is the same, A + A + B → ..., then

Thus, an exponent appeared in the speed equation « two» , which corresponds to the coefficient 2 in the reaction equation. For more complex equations, large exponents are usually not used. This is due to the fact that the probability of a simultaneous collision of, say, three molecules A and two molecules B is extremely small. Therefore, many reactions occur in several stages, during which no more than three particles collide, and each stage of the process proceeds at a certain speed. This speed and the kinetic equation of speed for it are determined experimentally.

The above chemical reaction rate equations (3) or (4) are valid only for homogeneous reactions, i.e. for such reactions when the reacting substances are not separated by the surface. For example, a reaction occurs in an aqueous solution, and both reactants are highly soluble in water or any mixture of gases.

It's another matter when it happens heterogeneous reaction. In this case, there is an interface between the reacting substances, for example, carbon dioxide gas reacts with water solution alkalis. In this case, any gas molecule is equally likely to react, since these molecules move quickly and chaotically. What about particles of liquid solution? These particles move extremely slowly, and those alkali particles that are “at the bottom” have virtually no chance of reacting with carbon dioxide unless the solution is constantly stirred. Only those particles that “lie on the surface” will react. So for heterogeneous reactions -

the reaction rate depends on the size of the interface surface, which increases with grinding.

Therefore, very often the reacting substances are crushed (for example, dissolved in water), the food is thoroughly chewed, and during the cooking process - ground, passed through a meat grinder, etc. A food product that is not crushed is practically not digestible!

Thus, at maximum speed (other things being equal), homogeneous reactions occur in solutions and between gases (if these gases react at ambient conditions), moreover, in solutions where the molecules are located “side by side”, and grinding is the same as in gases (and even more!), the reaction rate is higher.

Unified State Examination task. Which reaction occurs at the fastest rate at room temperature:

1. carbon with oxygen;
2. iron with hydrochloric acid;
3. iron with acetic acid solution
4. solutions of alkali and sulfuric acid.

In this case, you need to find which process is homogeneous.

It should be noted that the rate of a chemical reaction between gases or a heterogeneous reaction in which a gas participates also depends on pressure, since with increasing pressure the gases are compressed and the concentration of particles increases (see formula 2). The rate of reactions in which gases are not involved is not affected by changes in pressure.

Unified State Examination task. The rate of chemical reaction between the acid solution and iron is not affected

1. acid concentration;
2. iron grinding;
3. reaction temperature;
4. increase in pressure.

And finally, the speed of the reaction also depends on the reactivity of the substances. For example, if oxygen reacts with a substance, then, other things being equal, the reaction rate will be higher than when the same substance interacts with nitrogen. The fact is that the reactivity of oxygen is noticeably higher than that of nitrogen. We will look at the reason for this phenomenon in the next part of the Self-Teacher (Lesson 14).

Unified State Examination task. The chemical reaction between hydrochloric acid and

1. copper;
2. iron;
3. magnesium;
4. zinc

It should be noted that not every collision of molecules leads to their chemical interaction (chemical reaction). In a gas mixture of hydrogen and oxygen, under normal conditions, several billion collisions occur per second. But the first signs of the reaction (water droplets) will appear in the flask only after a few years. In such cases they say that the reaction practically doesn't work. But she possible, otherwise how to explain the fact that when this mixture is heated to 300 °C, the flask quickly fogs up, and at a temperature of 700 °C a terrible explosion will occur! It’s not for nothing that a mixture of hydrogen and oxygen is called “explosive gas.”

Question. Why do you think the reaction rate increases so sharply when heated?

The reaction rate increases because, firstly, the number of particle collisions increases, and secondly, the number of active collisions. It is the active collisions of particles that lead to their interaction. In order for such a collision to occur, the particles must have a certain amount of energy.

The energy that particles must have in order for a chemical reaction to occur is called activation energy.

This energy is spent on overcoming the repulsive forces between the outer electrons of atoms and molecules and on the destruction of “old” chemical bonds.

The question arises: how to increase the energy of reacting particles? The answer is simple - increase the temperature, since with increasing temperature the speed of movement of particles increases, and, consequently, their kinetic energy.

Rule van't Hoff*:

with every 10 degree increase in temperature, the reaction rate increases by 2–4 times.

VANT-HOFF Jacob Hendrik(08/30/1852–03/1/1911) - Dutch chemist. One of the founders of physical chemistry and stereochemistry. Nobel Prize in Chemistry No. 1 (1901).

It should be noted that this rule (not a law!) was established experimentally for reactions that were “convenient” for measurement, that is, for such reactions that proceeded neither too quickly nor too slowly and at temperatures accessible to the experimenter (not too high and not too low).

Question. What do you think is the fastest way to cook potatoes: boil them or fry them in a layer of oil?

In order to properly understand the meaning of the phenomena described, you can compare the reacting molecules with a group of students who are about to jump high. If they are given a barrier 1 m high, then the students will have to run up (increase their “temperature”) in order to overcome the barrier. Nevertheless, there will always be students (“inactive molecules”) who will not be able to overcome this barrier.

What to do? If you adhere to the principle: “A smart person won’t climb a mountain, a smart person will bypass a mountain,” then you should simply lower the barrier, say, to 40 cm. Then any student will be able to overcome the barrier. At the molecular level this means: in order to increase the reaction rate, it is necessary to reduce the activation energy in a given system.

In real chemical processes, this function is performed by a catalyst.

Catalyst is a substance that changes the rate of a chemical reaction while remaining unchanged towards the end of the chemical reaction.

Catalyst participates in a chemical reaction, interacting with one or more starting substances. In this case, intermediate compounds are formed and the activation energy changes. If the intermediate is more active (active complex), then the activation energy decreases and the reaction rate increases.

For example, the reaction between SO 2 and O 2 occurs very slowly under normal conditions practically doesn't work. But in the presence of NO, the reaction rate increases sharply. First NO very quickly reacts with O2:

resulting nitrogen dioxide fast reacts with sulfur(IV) oxide:

Task 5.1. Using this example, show which substance is a catalyst and which is an active complex.

Conversely, if more passive compounds are formed, the activation energy may increase so much that the reaction practically does not occur under these conditions. Such catalysts are called inhibitors.

In practice, both types of catalysts are used. So special organic catalysts - enzymes- participate in absolutely all biochemical processes: food digestion, muscle contraction, breathing. Life cannot exist without enzymes!

Inhibitors are necessary to protect metal products from corrosion and fat-containing foods from oxidation (rancidity). Some medications also contain inhibitors that inhibit the vital functions of microorganisms and thereby destroy them.

Catalysis can be homogeneous or heterogeneous. An example of homogeneous catalysis is the effect of NO (this is a catalyst) on the oxidation of sulfur dioxide. An example of heterogeneous catalysis is the action of heated copper on alcohol:

This reaction occurs in two stages:

Task 5.2. Determine which substance is the catalyst in this case? Why is this type of catalysis called heterogeneous?

In practice, heterogeneous catalysis is most often used, where solid substances serve as catalysts: metals, their oxides, etc. On the surface of these substances there are special points (crystal lattice nodes), where the catalytic reaction actually occurs. If these points are covered with foreign substances, then catalysis stops. This substance, detrimental to the catalyst, is called catalytic poison. Other substances - promoters- on the contrary, they enhance catalytic activity.

A catalyst can change the direction of a chemical reaction, that is, by changing the catalyst, you can obtain different reaction products. Thus, from alcohol C 2 H 5 OH in the presence of zinc and aluminum oxides, butadiene can be obtained, and in the presence of concentrated sulfuric acid, ethylene can be obtained.

Thus, during a chemical reaction, the energy of the system changes. If during the reaction energy is released in the form of heat Q, this process is called exothermic:

For endo thermal processes heat is absorbed, i.e. thermal effect Q< 0 .

Task 5.3. Determine which of the proposed processes is exothermic and which is endothermic:

The equation of a chemical reaction in which thermal effect, is called the thermochemical equation of the reaction. In order to create such an equation, it is necessary to calculate the thermal effect per 1 mole of the reactant.

Task. When 6 g of magnesium is burned, 153.5 kJ of heat is released. Write a thermochemical equation for this reaction.

Solution. Let's create an equation for the reaction and indicate ABOVE the formulas that are given:

Having made up the proportion, we find the desired thermal effect of the reaction:

The thermochemical equation for this reaction is:

Such tasks are given in the assignments majority Unified State Exam options! For example.

Unified State Examination task. According to the thermochemical reaction equation

the amount of heat released when burning 8 g of methane is equal to:

Reversibility of chemical processes. Le Chatelier's principle

* LE CHATELIER Henri Louis(8.10.1850–17.09.1936) - French physical chemist and metallurgist. Formulated the general law of equilibrium displacement (1884).

Reactions can be reversible or irreversible.

Irreversible These are reactions for which there are no conditions under which the reverse process is possible.

An example of such reactions are reactions that occur when milk sours, or when a delicious cutlet is burnt. Just as it is impossible to put minced meat back through a meat grinder (and get a piece of meat again), it is also impossible to “reanimate” a cutlet or make milk fresh.

But let’s ask ourselves a simple question: is the process irreversible?

In order to answer this question, let's try to remember, is it possible to carry out the reverse process? Yes! The decomposition of limestone (chalk) to obtain quicklime CaO is used on an industrial scale:

Thus, the reaction is reversible, since there are conditions under which both process:

Moreover, there are conditions under which the speed of the forward reaction is equal to the speed of the reverse reaction.

Under these conditions, chemical equilibrium is established. At this time, the reaction does not stop, but the number of particles obtained is equal to the number of decomposed particles. That's why in a state of chemical equilibrium, the concentrations of reacting particles do not change. For example, for our process at the moment of chemical equilibrium

sign means equilibrium concentration.

The question arises, what will happen to the equilibrium if the temperature is increased or decreased or other conditions are changed? This question can be answered by knowing Le Chatelier's principle:

if you change the conditions (t, p, c) under which the system is in a state of equilibrium, then the equilibrium will shift towards the process that resists change.

In other words, an equilibrium system always resists any influence from the outside, just as a capricious child who does “the opposite” resists the will of his parents.

Let's look at an example. Let equilibrium be established in the reaction producing ammonia:

Questions. Is the number of moles of reacting gases the same before and after the reaction? If a reaction occurs in a closed volume, when is the pressure greater: before or after the reaction?

It is obvious that this process occurs with a decrease in the number of gas molecules, which means pressure decreases during the direct reaction. IN reverse reactions - on the contrary, the pressure in the mixture increases.

Let us ask ourselves what will happen if in this system increase pressure? According to Le Chatelier’s principle, the reaction that “does the opposite” will proceed, i.e. lowers pressure. This is a direct reaction: fewer gas molecules - less pressure.

So, at increase pressure, the equilibrium shifts towards the direct process, where the pressure drops, as the number of molecules decreases gases

Unified State Examination task. At increase pressure balance shifts right in the system:

If as a result of the reaction number of molecules gases does not change, then a change in pressure does not affect the equilibrium position.

Unified State Examination task. A change in pressure affects the shift in equilibrium in the system:

The equilibrium position of this and any other reaction depends on the concentration of the reacting substances: by increasing the concentration of the starting substances and decreasing the concentration of the resulting substances, we always shift the equilibrium towards the direct reaction (to the right).

Unified State Examination task.

will shift to the left when:

1. increased blood pressure;
2. decrease in temperature;
3. increasing CO concentration;
4. decreasing CO concentration.

The process of ammonia synthesis is exothermic, that is, accompanied by the release of heat, that is temperature rise in the mixture.

Question. How will the equilibrium shift in this system when temperature drop?

Arguing similarly, we do conclusion: when decreasing temperature, the equilibrium will shift towards the formation of ammonia, since in this reaction heat is released, and the temperature rises.

Question. How does the rate of a chemical reaction change as the temperature decreases?

Obviously, as the temperature decreases, the rate of both reactions will sharply decrease, i.e., you will have to wait a very long time for the desired equilibrium to be established. What to do? In this case it is necessary catalyst. Although he does not affect the equilibrium position, but accelerates the onset of this state.

Unified State Examination task. Chemical equilibrium in the system

shifts towards the formation of the reaction product when:

1. increased blood pressure;
2. temperature increase;
3. decrease in pressure;
4. use of a catalyst.

Conclusions

The rate of a chemical reaction depends on:

• the nature of the reacting particles;
• concentration or interface area of ​​reactants;
• temperature;
• presence of a catalyst.

Equilibrium is established when the rate of the forward reaction is equal to the rate of the reverse process. In this case, the equilibrium concentration of the reactants does not change. The state of chemical equilibrium depends on conditions and obeys Le Chatelier's principle.

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

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

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

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

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

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

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

if the volume of the system does not change.

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

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

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

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

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

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

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

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

Temperature

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

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

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

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

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

Endothermic and exothermic reactions

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

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

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

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

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

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

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

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

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

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

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

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

Concentration of reactants

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

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

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

This law is also called law of mass action.

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

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

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

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

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

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

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

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

Action of catalysts

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

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

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

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

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

There are two types of catalysis.

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

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

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

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

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

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

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

For example, hydrogen peroxide solutions are stabilized using inhibitors.

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

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

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

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

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

Contact surface of reacting substances

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

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

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

Reference material for taking the test:

Periodic table

Solubility table

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

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

Factors that can influence the rate of a chemical reaction

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

• concentration of substances;
• nature of reagents;
• temperature;
• presence of a catalyst;
• pressure (for reactions in a gas environment).

Thus, by changing certain conditions of a chemical process, you can influence how quickly the process will proceed.

In the process of chemical interaction, particles of 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 therefore proportional to the molar concentrations of the reagents.

Law of mass action describes the dependence of the reaction rate on the molar concentrations of the substances that interact.

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

υ = k ∙С A ∙С B,

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

If one of the reacting substances is in a solid state, then the interaction occurs at the interface; therefore, the concentration of the solid substance is not included in the equation of the kinetic law of mass action. 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 reactant concentrations equal to unity.

Nature of the reagents

Since during the interaction the chemical bonds of the reacting substances are destroyed and new bonds of the reaction products are formed, the nature of the bonds involved 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 affects the course of the reaction, sometimes quite significantly. Grinding a solid allows you to increase the surface area of ​​​​contact of the reagents, and therefore speed up the process. The contact area of ​​soluble substances is easily increased by dissolving 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 itself will accelerate. A clear example of how an increase in temperature affects the process of interaction of substances can be considered the data given in the table.

Table 1. Effect of temperature changes on the rate of water formation (O 2 +2H 2 →2H 2 O)

To quantitatively describe 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 increases by 10 degrees, an acceleration occurs by 2-4 times.

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

Where γ is the temperature coefficient of the rate of the chemical reaction (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 multiplier determined by the type of reaction, E, A is the activation energy.

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

The activation energy does not depend on external factors, but depends on the nature of the substance. The activation energy value of 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 are 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 a chemical transformation) is divided into homogeneous and heterogeneous. If the reagents and the catalyst are in the same states of aggregation, then the catalysis is called homogeneous; if in different states, then it is heterogeneous. The mechanisms of action of catalysts are varied and quite complex. In addition, it is worth noting that catalysts are characterized by selectivity of action. That is, the same catalyst, while accelerating one reaction, may not change the rate of another.

Pressure

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

Experimental determination of the rate of a chemical reaction

The speed of a chemical transformation can be determined experimentally by obtaining data on how the concentration of substances entering the reaction or products changes per unit time. Methods for obtaining such data are divided into

• chemical,
• physico-chemical.

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

If the reaction proceeds quickly, then it has to be stopped in order to take a sample. This can be done using 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 changes in the concentrations of substances in real time. In this case, there is no need to stop the reaction and take samples.

Physicochemical methods are based on the measurement of a physical property that depends on the quantitative content of a certain compound in the system and changes over time. For example, if gases are involved in a reaction, then pressure may be such a property. Electrical conductivity, refractive index, and absorption spectra of substances are also measured.

Chemical reactions occur at different rates. Some of them are completely completed in small fractions of a second, others are carried out in minutes, hours, days; reactions are known that require several years to occur. In addition, the same reaction can proceed quickly under some conditions, for example, at elevated temperatures, and slowly under others, for example, upon cooling; Moreover, the difference in the speed of the same reaction can be very large.

When considering the rate of a chemical reaction, it is necessary to distinguish between reactions occurring in a homogeneous system (homogeneous reactions) and reactions occurring in a heterogeneous system (heterogeneous reactions).

DEFINITION

System in chemistry it is customary to call the substance or collection of substances in question. In this case, the system is contrasted with the external environment - the substances surrounding the system.

There are homogeneous and heterogeneous systems. Homogeneous a system consisting of one phase is called heterogeneous- a system consisting of several phases. Phase is a part of a system separated from its other parts by an interface, during the transition through which the properties change abruptly.

An example of a homogeneous system is any gas mixture (all gases at not very high pressures dissolve in each other without limit) or a solution of several substances in one solvent.

Examples of heterogeneous systems include the following systems: water with ice, a saturated solution with sediment, coal and sulfur in an air atmosphere.

If a reaction occurs in a homogeneous system, then it occurs throughout the entire volume of this system. If a reaction occurs between substances forming a heterogeneous system, then it can only occur at the interface between the phases forming the system. In this regard, the rate of a homogeneous reaction and the rate of a heterogeneous reaction are defined differently.

DEFINITION

Speed ​​of homogeneous reaction is the amount of a substance that reacts or is formed during a reaction per unit time per unit volume of the system.

Speed ​​of heterogeneous reaction is the amount of substance that reacts or is formed during a reaction per unit time per unit surface area of ​​the phase.

Both of these definitions can be written in mathematical form. Let us introduce the following notation: υ homogen - reaction rate in a homogeneous system; υ h etero gen - reaction rate in a heterogeneous system; n - number of moles of any of the substances resulting from the reaction; V is the volume of the system; t-time; S is the surface area of ​​the phase on which the reaction occurs; Δ - sign of increment (Δn = n 2 -n 1; Δt = t 2 -t 1). Then

υ homogen = Δn / (V× Δt);

υ heterogen = Δn / (S× Δt).

The first of these equations can be simplified. The ratio of the amount of a substance (n) to the volume (V) of the system is the molar concentration (c) of the substance: c=n/V, from where Δc=Δn/V and finally:

υ homogene = Δc / Δt.

Examples of problem solving

EXAMPLE 1

Exercise	Make up the formulas of two iron oxides if the mass fractions of iron in them are 77.8% and 70.0%.
Solution	Let's find the mass fraction in each of the copper oxides: ω 1 (O) = 100% - ω 1 (Fe) = 100% - 77.8% = 22.2%; ω 2 (O) = 100% - ω 2 (Fe) = 100% - 70.0% = 30.0%. Let us denote the number of moles of elements included in the compound by “x” (iron) and “y” (oxygen). Then, the molar ratio will look like this (we will round the values ​​of relative atomic masses taken from D.I. Mendeleev’s Periodic Table to whole numbers): x:y = ω 1 (Fe)/Ar(Fe) : ω 1 (O)/Ar(O); x:y = 77.8/56: 22.2/16; x:y = 1.39: 1.39 = 1:1. This means that the formula of the first iron oxide will be FeO. x:y = ω 2 (Fe)/Ar(Fe) : ω 2 (O)/Ar(O); x:y = 70/56: 30/16; x:y = 1.25: 1.875 = 1: 1.5 = 2: 3. This means that the formula of the second iron oxide will be Fe 2 O 3.
Answer	FeO, Fe2O3

EXAMPLE 2

Exercise	Write a formula for the compound of hydrogen, iodine and oxygen if the mass fractions of the elements in it are: ω(H) = 2.2%, ω(I) = 55.7%, ω(O) = 42.1%.
Solution	The mass fraction of element X in a molecule of the composition NX is calculated using the following formula: ω (X) = n × Ar (X) / M (HX) × 100%. Let us denote the number of moles of elements included in the compound as “x” (hydrogen), “y” (iodine), “z” (oxygen). Then, the molar ratio will look like this (the values ​​of relative atomic masses taken from D.I. Mendeleev’s Periodic Table are rounded to whole numbers): x:y:z = ω(H)/Ar(H) : ω(I)/Ar(I) : ω(O)/Ar(O); x:y:z= 2.2/1: 55.7/127: 42.1/16; x:y:z= 2.2: 0.44: 2.63 = 5: 1: 6. This means that the formula for the compound of hydrogen, iodine and oxygen will be H 5 IO 6 .
Answer	H5IO6

Topics of the Unified State Examination codifier:Reaction speed. Its dependence on various factors.

The rate of a chemical reaction shows how quickly a particular reaction occurs. Interaction occurs when particles collide in space. In this case, the reaction does not occur at every collision, but only when the particle has the appropriate energy.

Reaction speed – the number of elementary collisions of interacting particles ending in a chemical transformation per unit of time.

Determining the rate of a chemical reaction is related to the conditions under which it is carried out. If the reaction homogeneous– i.e. products and reagents are in the same phase - then the rate of a chemical reaction is defined as the change in substance per unit time:

υ = ΔC / Δt.

If the reactants or products are in different phases, and the collision of particles occurs only at the phase boundary, then the reaction is called heterogeneous, and its speed is determined by the change in the amount of substance per unit time per unit of reaction surface:

υ = Δν / (S·Δt).

How to make particles collide more often, i.e. How increase the rate of a chemical reaction?

1. The easiest way is to increase temperature . As you probably know from your physics course, temperature is a measure of the average kinetic energy of motion of particles of a substance. If we increase the temperature, then particles of any substance begin to move faster and, therefore, collide more often.

However, as the temperature increases, the rate of chemical reactions increases mainly due to the fact that the number of effective collisions increases. As the temperature rises, the number of active particles that can overcome the energy barrier of the reaction sharply increases. If we lower the temperature, the particles begin to move more slowly, the number of active particles decreases, and the number of effective collisions per second decreases. Thus, When the temperature increases, the rate of a chemical reaction increases, and when the temperature decreases, it decreases..

Pay attention! This rule works the same for all chemical reactions (including exothermic and endothermic). The reaction rate is independent of the thermal effect. The rate of exothermic reactions increases with increasing temperature, and decreases with decreasing temperature. The rate of endothermic reactions also increases with increasing temperature and decreases with decreasing temperature.

Moreover, back in the 19th century, the Dutch physicist Van't Hoff experimentally established that most reactions increase their speed approximately equally (about 2-4 times) when the temperature increases by 10 o C. Van't Hoff's rule sounds like this: an increase in temperature by 10 o C leads to an increase in the rate of a chemical reaction by 2-4 times (this value is called the temperature coefficient of the rate of a chemical reaction γ). The exact value of the temperature coefficient is determined for each reaction.

Here v 2 - reaction rate at temperature T 2, v 1 - reaction rate at temperature T 1, γ — temperature coefficient of reaction rate, Van't Hoff coefficient.

In some situations, it is not always possible to increase the reaction rate using temperature, because some substances decompose when the temperature rises, some substances or solvents evaporate at elevated temperatures, etc., i.e. the conditions of the process are violated.

2. Concentration. You can also increase the number of effective collisions by changing concentration reactants . usually used for gases and liquids, because in gases and liquids, particles move quickly and actively mix. The greater the concentration of reacting substances (liquids, gases), the greater the number of effective collisions, and the higher the rate of the chemical reaction.

Based on a large number of experiments in 1867 in the works of Norwegian scientists P. Guldenberg and P. Waage and, independently of them, in 1865 by Russian scientist N.I. Beketov derived the basic law of chemical kinetics, establishing the dependence of the rate of a chemical reaction on the concentration of the reactants:

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

For a chemical reaction of the form: aA + bB = cC + dD the law of mass action is written as follows:

here v is the rate of the chemical reaction,

C A And C B — concentrations of substances A and B, respectively, mol/l

k – proportionality coefficient, reaction rate constant.

For example, for the reaction of ammonia formation:

N 2 + 3H 2 ↔ 2NH 3

The law of mass action looks like this:

The reaction rate constant shows at what speed substances will react if their concentrations are 1 mol/l, or their product is equal to 1. The rate constant of a chemical reaction depends on temperature and does not depend on the concentration of the reacting substances.

The law of mass action does not take into account the concentrations of solids, because They react, as a rule, on the surface, and the number of reacting particles per unit surface does not change.

In most cases, a chemical reaction consists of several simple steps, in which case the equation of a chemical reaction shows only the summary or final equation of the processes occurring. In this case, the rate of a chemical reaction depends in a complex way (or does not depend) on the concentration of reactants, intermediates or catalyst, therefore the exact form of the kinetic equation is determined experimentally, or based on an analysis of the proposed reaction mechanism. Typically, the rate of a complex chemical reaction is determined by the rate of its slowest step ( limiting stage).

3. Pressure. For gases, the concentration directly depends on pressure. As pressure increases, the concentration of gases increases. The mathematical expression of this dependence (for an ideal gas) is the Mendeleev-Clapeyron equation:

pV = νRT

Thus, if among the reactants there is a gaseous substance, then when As pressure increases, the rate of a chemical reaction increases; as pressure decreases, it decreases. .

For example. How will the reaction rate of the fusion of lime with silicon oxide change:

CaCO 3 + SiO 2 ↔ CaSiO 3 + CO 2

when pressure increases?

The correct answer would be - not at all, because... there are no gases among the reagents, and calcium carbonate is a solid salt, insoluble in water, silicon oxide is a solid. The product gas will be carbon dioxide. But the products do not affect the rate of the direct reaction.

Another way to increase the rate of a chemical reaction is to direct it along a different path, replacing the direct interaction, for example, of substances A and B with a series of sequential reactions with a third substance K, which require much less energy (have a lower activation energy barrier) and occur at given conditions faster than the direct reaction. This third substance is called catalyst .

- these are chemical substances participating in a chemical reaction, changing its speed and direction, but non-consumable during the reaction (at the end of the reaction, they do not change either in quantity or composition). An approximate mechanism for the operation of a catalyst for a reaction of type A + B can be chosen as follows:

A+K=AK

AK + B = AB + K

The process of changing the reaction rate when interacting with a catalyst is called catalysis. Catalysts are widely used in industry when it is necessary to increase the rate of a reaction or direct it along a specific path.

Based on the phase state of the catalyst, homogeneous and heterogeneous catalysis are distinguished.

Homogeneous catalysis – this is when the reactants and the catalyst are in the same phase (gas, solution). Typical homogeneous catalysts are acids and bases. organic amines, etc.

Heterogeneous catalysis - this is when the reactants and the catalyst are in different phases. As a rule, heterogeneous catalysts are solid substances. Because interaction in such catalysts occurs only on the surface of the substance; an important requirement for catalysts is a large surface area. Heterogeneous catalysts are characterized by high porosity, which increases the surface area of ​​the catalyst. Thus, the total surface area of ​​some catalysts sometimes reaches 500 square meters per 1 g of catalyst. Large area and porosity ensure effective interaction with reagents. Heterogeneous catalysts include metals, zeolites - crystalline minerals of the aluminosilicate group (compounds of silicon and aluminum), and others.

Example heterogeneous catalysis – ammonia synthesis:

N 2 + 3H 2 ↔ 2NH 3

Porous iron with Al 2 O 3 and K 2 O impurities is used as a catalyst.

The catalyst itself is not consumed during the chemical reaction, but other substances accumulate on the surface of the catalyst, binding the active centers of the catalyst and blocking its operation ( catalytic poisons). They must be removed regularly by regenerating the catalyst.

In biochemical reactions, catalysts are very effective - enzymes. Enzymatic catalysts act highly efficiently and selectively, with 100% selectivity. Unfortunately, enzymes are very sensitive to increased temperature, acidity of the environment and other factors, so there are a number of limitations for the implementation of processes with enzymatic catalysis on an industrial scale.

Catalysts should not be confused with initiators process and inhibitors. For example, ultraviolet irradiation is necessary to initiate the radical reaction of methane chlorination. This is not a catalyst. Some radical reactions are initiated by peroxide radicals. These are also not catalysts.

Inhibitors- These are substances that slow down a chemical reaction. Inhibitors can be consumed and participate in a chemical reaction. In this case, inhibitors are not catalysts, on the contrary. Reverse catalysis is impossible in principle - the reaction will in any case try to follow the fastest path.

5. Contact area of ​​reacting substances. For heterogeneous reactions, one way to increase the number of effective collisions is to increase reaction surface area . The larger the contact surface area of ​​the reacting phases, the greater the rate of the heterogeneous chemical reaction. Powdered zinc dissolves much faster in acid than granular zinc of the same mass.

In industry, to increase the contact surface area of ​​reacting substances, they use fluidized bed method. For example, in the production of sulfuric acid by the boiling donkey method, pyrites are fired.

6. Nature of reactants . The rate of chemical reactions, other things being equal, is also influenced by chemical properties, i.e. nature of the reacting substances. Less active substances will have a higher activation barrier, and react more slowly than more active substances. More active substances have a lower activation energy, and enter into chemical reactions much easier and more often.

At low activation energies (less than 40 kJ/mol), the reaction occurs very quickly and easily. A significant part of collisions between particles ends in a chemical transformation. For example, ion exchange reactions occur very quickly under normal conditions.

At high activation energies (more than 120 kJ/mol), only a small number of collisions result in a chemical transformation. The rate of such reactions is negligible. For example, nitrogen practically does not interact with oxygen under normal conditions.

At average activation energies (from 40 to 120 kJ/mol), the reaction rate will be average. Such reactions also occur under normal conditions, but not very quickly, so that they can be observed with the naked eye. Such reactions include the interaction of sodium with water, the interaction of iron with hydrochloric acid, etc.

Substances that are stable under normal conditions usually have high activation energies.