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We are constantly faced with various chemical interactions. The combustion of natural gas, the rusting of iron, the souring of milk - these are not all the processes that are studied in detail in school course chemistry.

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

Let's try to identify the dependence of the reaction rate on temperature, concentration, and other factors. In the new educational standard for this question is given minimal amount school time. The tests of the Unified State Exam include tasks on the dependence of the reaction rate on temperature, concentration, and even offer calculation problems. Many high school students experience certain difficulties in finding answers to these questions, so we will analyze this topic in detail.

Relevance of the issue under consideration

Information about the reaction rate has important practical and scientific significance. For example, in the specific production of substances and products, the productivity of equipment and the cost of goods directly depend on this value.

Classification of ongoing reactions

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

In chemistry, a system usually means a substance or a combination of them.

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

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

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

A homogeneous composition is characterized by the process occurring throughout the entire volume, which significantly improves its quality.

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

Mathematical relationship between process speed and various factors

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

For a heterogeneous process, the rate is determined in terms of the amount of substance reacting or produced in the process per unit area in a minimum period of time.

Factors affecting the rate of a chemical reaction

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

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

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

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

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

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

In this case, the surface area increases, which has a positive effect on the speed of the process. Used for diesel fuel special system injection, due to which, when it comes into contact with air, the rate of combustion of the hydrocarbon mixture increases significantly.

Heating

The dependence of the rate of a chemical reaction on temperature is explained by molecular kinetic theory. It allows you to calculate the number of collisions between reagent molecules under certain conditions. If you are armed with such information, then under normal conditions all processes should proceed instantly.

But if you consider specific example depending on the reaction rate on temperature, it turns out that for interaction it is necessary to first break the chemical bonds between atoms so that new substances are formed from them. This requires significant energy expenditure. What is the dependence of the reaction rate on temperature? The activation energy determines the possibility of rupture of molecules; it is precisely this energy that characterizes the reality of the processes. Its units are kJ/mol.

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

Graphical representation

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

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

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

Important aspects

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

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

The dependence of the rate of homogeneous reactions on temperature can be represented in mathematical form.

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

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

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

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

Measuring the speed of a process

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

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

Inhibitors and catalysts

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

They themselves are not participants in the interaction; their number remains unchanged after the process is completed. While catalysts help increase the reaction rate, inhibitors, on the contrary, slow down this process.

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

Conclusion

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

Thanks to the molecular kinetic theory, it is possible to predict the probability of interactions between starting components, calculate the speed of the process. Among those factors that affect the reaction rate are special meaning has a change in temperature, percentage concentration of interacting substances, contact surface area, the presence of a catalyst (inhibitor), as well as the nature of the interacting components.

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 speeds chemical processes 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 As a rule, large exponents are 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 velocity equation 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, with maximum speed(other things being equal) homogeneous reactions take place in solutions and between gases (if these gases react at ambient conditions), and in solutions where the molecules are located “nearby”, and the grinding is the same as in gases (and even more !), - the reaction speed is higher.

Unified State Examination task. Which of the reactions occurs with the most higher speed 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 in this case we 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.

Finally, the speed of a 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 Tutorial (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 outer electrons atoms and molecules and 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 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 described phenomena, 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 fast 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 the 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 common law shifts of equilibrium (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 burnt delicious cutlet. How impossible to miss chopped meat back through the meat grinder (and get a piece of meat again), it is also impossible to “reanimate” the cutlet or make fresh milk.

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 able 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 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 reacting substances occurs throughout the entire volume. Such reactions are called homogeneous.

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

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

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

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

if the volume of the system does not change.

If a reaction occurs between substances in different states of aggregation (for example, between solid and gas or liquid), or between substances that are unable to form a homogeneous medium (for example, between immiscible liquids), then it passes 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 rate of a reaction is determined is 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 reactants, pressure (in reactions involving gases), temperature, the action of catalysts and the surface of the reactants 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 the bypass 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 work. 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 these 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 different 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:

Mendeleev table

Solubility table

DEFINITION

Chemical kinetics– the study of the rates and mechanisms of chemical reactions.

The study of reaction rates, obtaining data on factors influencing the rate of a chemical reaction, as well as the study of the mechanisms of chemical reactions are carried out experimentally.

DEFINITION

Chemical reaction rate– change in the concentration of one of the reacting substances or reaction products per unit time with a constant volume of the system.

The rates of homogeneous and heterogeneous reactions are defined differently.

The definition of a measure of the rate of a chemical reaction can be written in mathematical form. Let be the rate of a chemical reaction in a homogeneous system, n B be the number of moles of any of the substances resulting from the reaction, V be the volume of the system, and be time. Then in the limit:

This equation can be simplified - the ratio of the amount of a substance to the volume is the molar concentration of the substance n B / V = ​​c B, from where dn B / V = ​​dc B and finally:

In practice, the concentrations of one or more substances are measured at certain time intervals. Concentrations of starting substances decrease over time, and concentrations of products increase (Fig. 1).

Rice. 1. Change in the concentration of the starting substance (a) and the reaction product (b) over time

Factors affecting the rate of a chemical reaction

Factors that influence the rate of a chemical reaction are: the nature of the reactants, their concentrations, temperature, the presence of catalysts in the system, pressure and volume (in the gas phase).

The influence of concentration on the rate of a chemical reaction is associated with the basic law of chemical kinetics - the law of mass action (LMA): the rate of a chemical reaction is directly proportional to the product of the concentrations of the reacting substances raised to the power of their stoichiometric coefficients. The ZDM does not take into account the concentration of substances in the solid phase in heterogeneous systems.

For the reaction mA +nB = pC +qD the mathematical expression of the ZDM will be written:

K × C A m × C B n

K × [A] m × [B] n,

where k is the rate constant of a chemical reaction, which is the rate of a chemical reaction at a concentration of reactants of 1 mol/l. Unlike the rate of a chemical reaction, k does not depend on the concentration of the reactants. The higher k, the faster the reaction proceeds.

The dependence of the rate of a chemical reaction on temperature is determined by the Van't Hoff rule. Van't Hoff's rule: for every ten degrees increase in temperature, the rate of most chemical reactions increases by about 2 to 4 times. Mathematical expression:

(T 2) = (T 1) × (T2-T1)/10,

where is the van’t Hoff temperature coefficient, showing how many times the reaction rate increases when the temperature increases by 10 o C.

Molecularity and reaction order

The molecularity of a reaction is determined by the minimum number of molecules that simultaneously interact (participate in an elementary act). There are:

- monomolecular reactions (an example is decomposition reactions)

N 2 O 5 = 2NO 2 + 1/2O 2

K × C, -dC/dt = kC

However, not all reactions that obey this equation are monomolecular.

- bimolecular

CH 3 COOH + C 2 H 5 OH = CH 3 COOC 2 H 5 + H 2 O

K × C 1 × C 2 , -dC/dt = k × C 1 × C 2

- trimolecular (very rare).

The molecularity of a reaction is determined by its true mechanism. It is impossible to determine its molecularity by writing the equation of a reaction.

The order of the reaction is determined by the form of the kinetic equation of the reaction. It is equal to the sum of the exponents of the degrees of concentration in this equation. For example:

CaCO 3 = CaO + CO 2

K × C 1 2 × C 2 – third order

The order of the reaction can be fractional. In this case, it is determined experimentally. If the reaction proceeds in one stage, then the order of the reaction and its molecularity coincide, if in several stages, then the order is determined by the slowest stage and is equal to the molecularity of this reaction.

Examples of problem solving

EXAMPLE 1