Substances that slow down the rate of a chemical reaction are called. Reaction speed, its dependence on various factors

Chemical methods

Physical methods

Methods for measuring the reaction rate

In the above example, the rate of reaction between calcium carbonate and acid was measured by examining the volume of evolved gas versus time. Experimental data on reaction rates can be obtained by measuring other quantities.

If the total amount of gaseous substances changes during the course of the reaction, then its progress can be observed by measuring the gas pressure at a constant volume. In cases where one of the starting materials or one of the reaction products is colored, the progress of the reaction can be followed by observing the color change of the solution. Another optical method is to measure the rotation of the plane of polarization of light (if the starting materials and reaction products have different rotational capacities).

Some reactions are accompanied by a change in the number of ions in solution. In such cases, the reaction rate can be studied by measuring the electrical conductivity of the solution. The next chapter will look at some of the other electrochemical methods that can be used to measure the rates of reactions.

The progress of the reaction can be monitored by measuring the concentration of one of the participants in the reaction over time using a variety of methods of chemical analysis. The reaction is carried out in a thermostated vessel. At regular intervals, a sample of the solution (or gas) is taken from the vessel and the concentration of one of the components is determined. To obtain reliable results, it is important that no reaction occurs in the sample taken for analysis. This is achieved by chemically binding one of the reagents, by quenching or diluting the solution.

Experimental studies show that the reaction rate depends on several factors. Let us consider the influence of these factors first at the qualitative level.

1.The nature of the reacting substances. We know from laboratory practice that acid neutralization with a base

H + + OH - ® H 2 O

interaction of salts with the formation of a poorly soluble compound

Ag + + Cl - ® AgCl

and other reactions in electrolyte solutions are very fast. The time it takes for such reactions to complete is measured in milliseconds or even microseconds. This is quite understandable, since the essence of such reactions is the approach and combination of charged particles with charges of the opposite sign.

In contrast to ionic reactions, interactions between covalently bound molecules are usually much slower. Indeed, in the course of the reaction between such particles, a rupture of bonds in the molecules of the starting substances must occur. For this, the colliding molecules must have a certain amount of energy. In addition, if the molecules are complex enough for a reaction to occur between them, they must be oriented in a certain way in space.

2. Concentration of reactants... The rate of a chemical reaction, other things being equal, depends on the number of collisions of reacting particles per unit time. The collision probability depends on the number of particles per unit volume, i.e. from concentration. Therefore, the reaction rate increases with increasing concentration.

3. Physical state of substances... In homogeneous systems, the reaction rate depends on the number of particle collisions in volume of solution (or gas). In heterogeneous systems, chemical interaction occurs at the interface... The increase in the surface area of \u200b\u200bthe solid during its grinding facilitates the access of the reacting particles to the solid particles, which leads to a significant acceleration of the reaction.

4... Temperature has a significant effect on the rate of various chemical and biological processes. As the temperature rises, the kinetic energy of the particles increases, and, consequently, the fraction of particles whose energy is sufficient for chemical interaction increases.

5. Steric factor characterizes the need for mutual orientation of the reacting particles. The more complex the molecules are, the less the probability of their proper orientation, the less the collision efficiency.

6. Availability of catalysts. Catalysts are substances in the presence of which the rate of a chemical reaction changes. Introduced into the reaction system in small amounts and remaining unchanged after the reaction, they are capable of extremely changing the rate of the process.

The main factors on which the reaction rate depends will be discussed in more detail below.

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

The concept of speed in chemistry

The reaction rate is usually called a certain change in the concentrations of the reacting compounds (ΔС) per unit time (Δt). The mathematical formula for the rate of a chemical reaction is as follows:

ᴠ \u003d ± ΔC / Δt.

The reaction rate is measured in mol / l ∙ s if it occurs in the entire volume (that is, the reaction is homogeneous) and in mol / m 2 ∙ s if the interaction occurs on the surface separating the phases (that is, the reaction is heterogeneous). The “-” sign in the formula refers to the change in the concentration values \u200b\u200bof the initial reacting substances, and the “+” sign - to the changing values \u200b\u200bof the concentrations of the products of the same reaction.

Examples of reactions with different rates

Chemical interactions can occur at different rates. So, the rate of growth of stalactites, that is, the formation of calcium carbonate, is only 0.5 mm per 100 years. Some biochemical reactions are slow, such as photosynthesis and protein synthesis. Corrosion of metals proceeds at a rather low rate.

The average speed can be characterized by reactions requiring from one to several hours. An example would be the preparation of food, which is accompanied by the decomposition and transformation of compounds contained in foods. The synthesis of individual polymers requires heating the reaction mixture for a certain time.

An example of chemical reactions, the rate of which is quite high, can serve as neutralization reactions, the interaction of sodium bicarbonate with a solution of acetic acid, accompanied by the release of carbon dioxide. You can also mention the interaction of barium nitrate with sodium sulfate, in which the precipitation of insoluble barium sulfate is observed.

A large number of reactions are capable of proceeding with lightning speed and are accompanied by an explosion. A classic example is the interaction of potassium with water.

Factors affecting the rate of a chemical reaction

It is worth noting that the same substances can react with each other at different rates. So, for example, a mixture of gaseous oxygen and hydrogen may not show signs of interaction for a rather long time, however, when the container is shaken or struck, the reaction becomes explosive. Therefore, chemical kinetics and identified certain factors that have the ability to influence the rate of chemical reaction. These include:

the nature of the interacting substances;
concentration of reagents;
temperature change;
the presence of a catalyst;
pressure change (for gaseous substances);
the area of \u200b\u200bcontact of substances (if we talk about heterogeneous reactions).

Influence of the nature of matter

Such a significant difference in the rates of chemical reactions is explained by different values \u200b\u200bof the activation energy (E a). It is understood as a certain excess amount of energy in comparison with its average value required for a molecule in a collision in order for a reaction to occur. It is measured in kJ / mol and values \u200b\u200bare usually in the range of 50-250.

It is generally accepted that if E a \u003d 150 kJ / mol for any reaction, then at n. at. it practically does not leak. This energy is spent on overcoming the repulsion between the molecules of substances and on weakening the bonds in the original substances. In other words, the activation energy characterizes the strength of chemical bonds in substances. By the value of the activation energy, one can preliminary estimate the rate of the chemical reaction:

E a< 40, взаимодействие веществ происходят довольно быстро, поскольку почти все столкнове-ния частиц при-водят к их реакции;
40-<Е а <120, предполагается средняя реакция, поскольку эффективными будет лишь половина соударений молекул (например, реакция цинка с соляной кислотой);
E a\u003e 120, only a very small part of particle collisions will lead to a reaction, and its speed will be low.

Effect of concentration

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

The rate of a chemical reaction is directly proportional to the product of the concentrations of the reacting substances, the values \u200b\u200bof which are taken in powers corresponding to their stoichiometric coefficients.

This law is suitable for elementary one-stage reactions, or for any stage of the interaction of substances characterized by a complex mechanism.

If you want to determine the rate of a chemical reaction, the equation of which can be conventionally written as:

αА + bB \u003d ϲС, then,

in accordance with the above formulation of the law, the speed can be found by the equation:

V \u003d k · [A] a · [B] b, where

a and b are stoichiometric coefficients,

[A] and [B] are the concentrations of the starting compounds,

k is the rate constant of the considered reaction.

The meaning of the rate coefficient of a chemical reaction is that its value will be equal to the rate if the concentrations of the compounds are equal to unity. It should be noted that for a correct calculation using this formula, it is worth considering the state of aggregation of the reagents. The concentration of the solid is taken to be unity and is not included in the equation, since it remains constant during the reaction. Thus, only the concentration of liquid and gaseous substances is included in the calculation for the ZDM. So, for the reaction of obtaining silicon dioxide from simple substances, described by the equation

Si (tv) + Ο 2 (g) \u003d SiΟ 2 (tv),

speed will be determined by the formula:

Typical task

How would the rate of the chemical reaction of nitrogen monoxide with oxygen change if the concentrations of the starting compounds were doubled?

Solution: This process corresponds to the reaction equation:

2ΝΟ + Ο 2 \u003d 2ΝΟ 2.

Let us write expressions for the initial (ᴠ 1) and final (ᴠ 2) reaction rates:

ᴠ 1 \u003d k · [ΝΟ] 2 · [Ο 2] and

ᴠ 2 \u003d k · (2 \u200b\u200b· [ΝΟ]) 2 · 2 · [Ο 2] \u003d k · 4 [ΝΟ] 2 · 2 [Ο 2].

ᴠ 1 / ᴠ 2 \u003d (k · 4 [ΝΟ] 2 · 2 [Ο 2]) / (k · [ΝΟ] 2 · [Ο 2]).

ᴠ 2 / ᴠ 1 \u003d 4 2/1 \u003d 8.

Answer: increased by 8 times.

Influence of temperature

The dependence of the chemical reaction rate on temperature was determined empirically by the Dutch scientist J. H. Van't Hoff. He found that the rate of many reactions increases by a factor of 2-4 with an increase in temperature for every 10 degrees. There is a mathematical expression for this rule, which looks like:

ᴠ 2 \u003d ᴠ 1 γ (Τ2-Τ1) / 10, where

ᴠ 1 and ᴠ 2 - corresponding speeds at temperatures Τ 1 and Τ 2;

γ - temperature coefficient, equal to 2-4.

At the same time, this rule does not explain the mechanism of the effect of temperature on the value of the rate of one or another reaction and does not describe the entire set of regularities. It is logical to conclude that as the temperature rises, the chaotic motion of particles increases and this provokes a greater number of their collisions. However, this does not particularly affect the efficiency of collision of molecules, since it depends mainly on the activation energy. Also, a significant role in the efficiency of particle collisions is played by their spatial correspondence to each other.

The temperature dependence of the rate of a chemical reaction, taking into account the nature of the reactants, obeys the Arrhenius equation:

k \u003d A 0 e -Ea / RΤ, where

And about is a factor;

E a - activation energy.

An example of a problem for Van't Hoff's law

How should the temperature be changed so that the rate of a chemical reaction, for which the temperature coefficient is numerically equal to 3, grows 27 times?

Decision. Let's use the formula

ᴠ 2 \u003d ᴠ 1 γ (Τ2-Τ1) / 10.

From the condition ᴠ 2 / ᴠ 1 \u003d 27, and γ \u003d 3. It is necessary to find ΔΤ \u003d Τ 2 -Τ 1.

Transforming the original formula, we get:

V 2 / V 1 \u003d γ ΔΤ / 10.

Substitute the values: 27 \u003d 3 ΔΤ / 10.

Hence it is clear that ΔΤ / 10 \u003d 3 and ΔΤ \u003d 30.

Answer: the temperature should be increased by 30 degrees.

Effect of catalysts

In physical chemistry, the rate of chemical reactions is also actively studied by the section called catalysis. He is interested in how and why relatively small amounts of certain substances significantly increase the rate of interaction of others. Such substances that can accelerate the reaction, but are not consumed in it themselves, are called catalysts.

It has been proven that catalysts change the mechanism of the chemical interaction itself, contribute to the appearance of new transition states, which are characterized by lower energy barrier heights. That is, they contribute to a decrease in the activation energy, and hence to an increase in the number of effective particle strikes. The catalyst cannot cause a reaction that is energetically impossible.

So hydrogen peroxide is able to decompose to form oxygen and water:

H 2 Ο 2 \u003d H 2 Ο + Ο 2.

But this reaction is very slow and in our first-aid kits it has existed unchanged for quite a long time. Opening only very old vials of peroxide, you will notice a slight popping caused by the oxygen pressure on the walls of the vessel. The addition of just a few grains of magnesium oxide will provoke active gas evolution.

The same reaction of the decomposition of peroxide, but under the action of catalase, occurs when treating wounds. Living organisms contain many different substances that increase the rate of biochemical reactions. They are called enzymes.

Inhibitors have the opposite effect on the course of reactions. However, this is not always a bad thing. Inhibitors are used to protect metal products from corrosion, to extend the shelf life of food, for example to prevent fat oxidation.

Contact area of \u200b\u200bsubstances

In the event that the interaction takes place between compounds having different states of aggregation, or between substances that are not able to form a homogeneous medium (immiscible liquids), then this factor also affects the rate of the chemical reaction significantly. This is due to the fact that heterogeneous reactions are carried out directly at the interface between the phases of the interacting substances. Obviously, the wider this boundary, the more particles have the opportunity to collide, and the faster the reaction goes.

For example, it goes much faster in the form of small chips than in the form of a log. For the same purpose, many solids are ground into a fine powder before being added to the solution. So, powdered chalk (calcium carbonate) acts faster with hydrochloric acid than a piece of the same mass. However, in addition to increasing the area, this technique also leads to a chaotic rupture of the crystal lattice of a substance, which means it increases the reactivity of particles.

Mathematically, the rate of a heterogeneous chemical reaction is found as the change in the amount of a substance (Δν) that occurs per unit of time (Δt) per unit surface

(S): V \u003d Δν / (S Δt).

Effect of pressure

The change in pressure in the system has an effect only when gases are involved in the reaction. An increase in pressure is accompanied by an increase in the molecules of the substance per unit volume, that is, its concentration increases proportionally. Conversely, lowering the pressure results in an equivalent decrease in reagent concentration. In this case, the formula corresponding to the ZDM is suitable for calculating the rate of a chemical reaction.

A task. How will the rate of the reaction described by the equation

2ΝΟ + Ο 2 \u003d 2ΝΟ 2,

if the volume of a closed system is reduced by three times (T \u003d const)?

Decision. As the volume decreases, the pressure increases proportionally. We write expressions for the initial (V 1) and final (V 2) reaction rates:

V 1 \u003d k · 2 · [Ο 2] and

V 2 \u003d k · (3 ·) 2 · 3 · [Ο 2] \u003d k · 9 [ΝΟ] 2 · 3 [Ο 2].

To find how many times the new speed is greater than the initial one, you should separate the left and right parts of the expressions:

V 1 / V 2 \u003d (k · 9 [ΝΟ] 2 · 3 [Ο 2]) / (k · [ΝΟ] 2 · [Ο 2]).

The concentration values \u200b\u200band rate constants decrease, and it remains:

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

Answer: the speed has increased 27 times.

Summing up, it should be noted that the speed of interaction of substances, or rather, the quantity and quality of collisions of their particles, is influenced by many factors. First of all, this is the activation energy and the geometry of molecules, which are almost impossible to correct. As for the other conditions, for an increase in the reaction rate it follows:

increase the temperature of the reaction medium;
increase the concentration of starting compounds;
increase the pressure in the system or reduce its volume when it comes to gases;
to bring dissimilar substances to the same state of aggregation (for example, by dissolving in water) or to increase the area of \u200b\u200btheir contact.

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

Some reactions require fractions of a second, while some interactions take days and weeks.

Let's try to identify the dependence of the reaction rate on temperature, concentration, and other factors. The new educational standard assigns a minimum amount of study time to this issue. In the tests of the unified state exam there are tasks for the dependence of the reaction rate on temperature, concentration, and even calculation problems are proposed. 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 is of great practical and scientific importance. For example, in a specific production of substances and products, the performance 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, it is customary to mean a system as a substance or their combination.

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, 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 course of the process throughout the volume, which significantly improves its quality.

If the initial substances are in different phase states, then the maximum interaction is observed at the interface. For example, when an active metal is dissolved in 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 enters into interaction or is formed during the reaction in the volume of the system per unit time.

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

Factors affecting the rate of a chemical reaction

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

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

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

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

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

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

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

Heating

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

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

With an insufficient energy index, the collision will be ineffective, therefore 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 the graph of reaction rate versus temperature look like? The energy of the molecules is deposited horizontally, and the vertical indicates the number of particles with a high energy reserve. The graph is a curve by which one can judge the rate of a particular interaction.

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

Important aspects

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

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

The temperature dependence of the rate of homogeneous reactions 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, a temperature rise of 100 degrees accelerates the process by almost 50,000 times.

The temperature dependence of the reaction rate can easily be explained by the different values \u200b\u200bof the activation energy. It has a minimum value when carrying out 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 value of the activation energy, only a small number of collisions between particles will lead to the implementation of interaction. With an average value of activation energy, the reagents will interact at an average rate.

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

Measuring the speed of the process

Those processes that require significant activation energy imply an initial rupture or weakening of bonds between atoms in the initial substances. At the same time, their transition to a certain intermediate state, called an activated complex, occurs. It is an unstable state, decomposes rather quickly into reaction products, the process is accompanied by the release of additional energy.

In the simplest version, the 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 completion of the process. If catalysts increase the rate of reaction, 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 rate of the process is observed.

Conclusion

Various chemical interactions occur in the world every minute. How to establish the dependence of the reaction rate on temperature? The Arrhenius equation is a mathematical explanation for the relationship between the rate constant and temperature. It gives an idea of \u200b\u200bthose values \u200b\u200bof the activation energy at which the destruction or weakening of bonds between atoms in molecules, the distribution of particles into new chemical substances is possible.

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

Chemical reaction rate

The topic "The rate of chemical reaction" is perhaps the most difficult and controversial in the school curriculum. This is due to the complexity of chemical kinetics itself, one of the branches of physical chemistry. The very definition of the “rate of a chemical reaction” is ambiguous (see, for example, an article by L.S.Guzei in the newspaper “Khimiya”, 2001, No. 28,
from. 12). Even more problems arise when trying to apply the law of mass action for the reaction rate to any chemical systems, because the range of objects for which a quantitative description of kinetic processes within the school curriculum is possible is very narrow. I would like to especially note the incorrectness of using the law of mass action for the rate of a chemical reaction at chemical equilibrium.
At the same time, it would be wrong to refuse to consider this topic at school altogether. The idea of \u200b\u200bthe rate of a chemical reaction is very important in the study of many natural and technological processes, without them it is impossible to talk about catalysis and catalysts, including enzymes. Although, when discussing the transformations of substances, mainly qualitative concepts of the rate of a chemical reaction are used, the introduction of the simplest quantitative ratios is still desirable, especially for elementary reactions.
The published article discusses in sufficient detail the issues of chemical kinetics that can be discussed at school chemistry lessons. The exclusion of controversial and controversial aspects of this topic from the course of school chemistry is especially important for those students who are going to continue their chemistry education at a university. After all, the knowledge acquired at school often conflicts with scientific reality.

Chemical reactions can vary significantly in their duration. A mixture of hydrogen and oxygen at room temperature can remain practically unchanged for a long time, but an explosion will occur on impact or ignition. The iron plate slowly rusts, and a piece of white phosphorus ignites spontaneously in air. It is important to know how quickly this or that reaction proceeds in order to be able to control its course.

Basic concepts

A quantitative characteristic of how quickly a given reaction proceeds is the rate of a chemical reaction, i.e., the rate at which reagents are consumed or the rate at which products appear. In this case, it does not matter which of the substances participating in the reaction we are talking about, since they are all interconnected through the reaction equation. By changing the amount of one of the substances, one can judge the corresponding changes in the quantities of all the others.

The rate of chemical reaction () called the change in the amount of a substance of a reactant or product () per unit of time () per unit volume (V):

= /(V ).

The reaction rate in this case is usually expressed in mol / (l s).

The above expression refers to homogeneous chemical reactions occurring in a homogeneous medium, for example, between gases or in solution:

2SO 2 + O 2 \u003d 2SO 3,

BaCl 2 + H 2 SO 4 \u003d BaSO 4 + 2HCl.

Heterogeneous chemical reactions occur at the contact surfaces of a solid substance and a gas, a solid substance and a liquid, etc. Heterogeneous reactions include, for example, reactions of metals with acids:

Fe + 2HCl \u003d FeCl 2 + H 2.

In this case the reaction rate is the change in the amount of a reagent substance or product () per unit of time() per unit surface (S):

= /(S ).

The rate of the heterogeneous reaction is expressed in mol / (m 2 s).

To control chemical reactions, it is important not only to be able to determine their rates, but also to find out what conditions influence them. The section of chemistry that studies the rate of chemical reactions and the influence of various factors on it is called chemical kinetics.

Impact frequency of reacting particles

The most important factor determining the rate of a chemical reaction is concentration.

As the concentration of reactants increases, the reaction rate usually increases. In order to react, two chemical particles must move closer together, so the reaction rate depends on the number of collisions between them. An increase in the number of particles in a given volume leads to more frequent collisions and to an increase in the reaction rate.

For homogeneous reactions, increasing the concentration of one or more reactants will increase the reaction rate. With decreasing concentration, the opposite effect is observed. The concentration of substances in solution can be changed by adding or removing reactants or solvent from the reaction sphere. In gases, the concentration of one of the substances can be increased by introducing an additional amount of this substance into the reaction mixture. The concentration of all gaseous substances can be increased simultaneously by decreasing the volume occupied by the mixture. In this case, the reaction rate will increase. The increase in volume leads to the opposite result.

The rate of heterogeneous reactions depends on surface area of \u200b\u200bcontact of substances, i.e. on the degree of grinding of substances, the completeness of mixing of reagents, as well as on the state of the crystal structures of solids. Any disturbances in the crystal structure cause an increase in the reactivity of solids, because additional energy is required to break down a solid crystal structure.

Consider wood burning. A whole log burns relatively slowly in air. If you increase the surface of contact of wood with air, splitting the log into chips, the burning rate will increase. At the same time, wood burns in pure oxygen much faster than in air, which contains only about 20% oxygen.

For a chemical reaction to occur, a collision of particles - atoms, molecules or ions - must occur. As a result of collisions, atoms are rearranged and new chemical bonds appear, which leads to the formation of new substances. The probability of collision of two particles is rather high, the probability of simultaneous collision of three particles is much less. A simultaneous collision of four particles is extremely unlikely. Therefore, most reactions proceed in several stages, at each of which no more than three particles interact.

The oxidation reaction of hydrogen bromide proceeds at a noticeable rate at 400–600 ° С:

4HBr + O 2 \u003d 2H 2 O + 2Br 2.

According to the reaction equation, five molecules must collide simultaneously. However, the likelihood of such an event is practically zero. Moreover, experimental studies have shown that increasing the concentration - either oxygen or hydrogen bromide - increases the reaction rate by the same number of times. And this despite the fact that for each oxygen molecule, four molecules of hydrogen bromide are consumed.

A detailed examination of this process shows that it takes place in several stages:

1) HBr + O 2 \u003d HOOBr (slow reaction);

2) HOOBr + HBr \u003d 2HOVr (fast response);

3) HOBr + HBr \u003d H 2 O + Br 2 (fast response).

These reactions, the so-called elementary reactionsreflect reaction mechanism oxidation of hydrogen bromide with oxygen. It is important to note that only two molecules are involved in each of the intermediate reactions. Adding the first two equations and doubled the third gives the total reaction equation. The overall reaction rate is determined by the slowest intermediate reaction, in which one molecule of hydrogen bromide and one molecule of oxygen interact.

The rate of elementary reactions is directly proportional to the product of molar concentrations from (from Is the amount of substance per unit volume, from = /V) of reagents taken in powers equal to their stoichiometric coefficients ( law of mass action for the rate of chemical reaction). This is true only for the reaction equations reflecting the mechanisms of real chemical processes, when the stoichiometric coefficients in front of the reagent formulas correspond to the number of interacting particles.

According to the number of molecules interacting in the reaction, reactions are distinguished: monomolecular, bimolecular and trimolecular. For example, the dissociation of molecular iodine into atoms: I 2 \u003d 2I is a monomolecular reaction.

Interaction of iodine with hydrogen: I 2 + H 2 \u003d 2HI - bimolecular reaction. The law of mass action for chemical reactions of different molecular weight is written in different ways.

Monomolecular reactions:

A \u003d B + C,

= kc A,

where kIs the reaction rate constant.

Bimolecular reactions:

= kc A c IN.

Trimolecular reactions:

= kc 2 A c IN.

Activation energy

The collision of chemical particles leads to chemical interaction only if the colliding particles have an energy exceeding a certain certain value. Consider the interaction of gaseous substances consisting of molecules A 2 and B 2:

A 2 + B 2 \u003d 2AB.

In the course of a chemical reaction, a rearrangement of atoms occurs, accompanied by the breaking of chemical bonds in the starting materials and the formation of bonds in the reaction products. When reacting molecules collide, the so-called activated complex, in which a redistribution of electron density occurs, and only then the final reaction product is obtained:

The energy required for the transition of substances to the state of an activated complex is called activation energy.

The activity of chemicals is manifested in the low activation energy of reactions with their participation. The lower the activation energy, the higher the reaction rate. For example, in the reactions between cations and anions, the activation energy is very low, so such reactions proceed almost instantly. If the activation energy is large, then a very small part of the collisions leads to the formation of new substances. Thus, the rate of reaction between hydrogen and oxygen at room temperature is practically zero.

So, the reaction rate is influenced by nature of reactants... Consider, for example, the reaction of metals with acids. If we dip the same pieces of copper, zinc, magnesium, and iron into tubes with diluted sulfuric acid, we can see that the rate of bubbling of hydrogen gas, which characterizes the rate of the reaction, differs significantly for these metals. In a test tube with magnesium, a violent evolution of hydrogen is observed, in a test tube with zinc, gas bubbles are released somewhat more calmly. The reaction in the test tube with iron proceeds even more slowly (Fig.). Copper does not react at all with dilute sulfuric acid. Thus, the reaction rate depends on the activity of the metal.

When replacing sulfuric acid (strong acid) with acetic (weak acid), the reaction rate in all cases slows down significantly. It can be concluded that the nature of both reagents, both metal and acid, affects the rate of reaction of a metal with an acid.

Enhancement temperature leads to an increase in the kinetic energy of chemical particles, i.e. increases the number of particles with an energy higher than the activation energy. As the temperature rises, the number of particle collisions also increases, which to some extent increases the reaction rate. However, increasing the collision efficiency by increasing the kinetic energy has a greater effect on the reaction rate than an increase in the number of collisions.

When the temperature rises by ten degrees, the speed increases by a factor equal to the temperature coefficient of the speed:

= T+10 /T .

When the temperature rises from T before T"
reaction rate ratio T"and Tequally
temperature coefficient of speed in power ( T" – T)/10:

T" /T = (T"–T)/10.

For many homogeneous reactions, the temperature coefficient of the rate is 24 (van't Hoff's rule). The dependence of the reaction rate on temperature can be traced by the example of the interaction of copper (II) oxide with dilute sulfuric acid. The reaction is very slow at room temperature. When heated, the reaction mixture quickly turns blue due to the formation of copper (II) sulfate:

CuO + H 2 SO 4 \u003d CuSO 4 + H 2 O.

Catalysts and inhibitors

Many reactions can be accelerated or slowed down by the introduction of certain substances. The added substances do not participate in the reaction and are not consumed during its course, but have a significant effect on the reaction rate. These substances change the reaction mechanism (including the composition of the activated complex) and lower the activation energy, which accelerates chemical reactions. Substances - reaction accelerators are called catalysts, and the very phenomenon of such an acceleration of the reaction is catalysis.

Many reactions in the absence of catalysts proceed very slowly or not at all. One of these reactions is the decomposition of hydrogen peroxide:

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

If you put a piece of solid manganese dioxide into a vessel with an aqueous solution of hydrogen peroxide, then the rapid evolution of oxygen will begin. After removal of manganese dioxide, the reaction practically stops. By weighing it is easy to make sure that manganese dioxide is not consumed in this process - it only catalyzes the reaction.

Depending on whether the catalyst and the reactants are in the same or different states of aggregation, homogeneous and heterogeneous catalysis are distinguished.

With homogeneous catalysis, the catalyst can accelerate the reaction by forming intermediates by reacting with one of the starting reagents. For example:

In heterogeneous catalysis, a chemical reaction usually occurs on the catalyst surface:

Catalysts are widespread in nature. Almost all transformations of substances in living organisms proceed with the participation of organic catalysts - enzymes.

Catalysts are used in chemical production to speed up certain processes. In addition to them, substances that slow down chemical reactions are also used - inhibitors... With the help of inhibitors, in particular, they protect metals from corrosion.

Factors affecting the rate of a chemical reaction

Increase speed	Reduce speed
The presence of chemically active reagents	The presence of chemically inactive reagents
Increasing the concentration of reagents	Reducing the concentration of reagents
Increasing the surface of solid and liquid reagents	Reducing the surface of solid and liquid reagents
Temperature increase	Lowering the temperature
The presence of a catalyst	Presence of an inhibitor

TASKS

1. Give a definition of the rate of a chemical reaction. Write an expression for the kinetic law of effective masses for the following reactions:

a) 2C (tv.) + O 2 (g) \u003d 2CO (g);

b) 2HI (g) \u003d H 2 (g) + I 2 (g).

2. What determines the rate of a chemical reaction? Give a mathematical expression for the dependence of the rate of a chemical reaction on temperature.

3. Indicate how it affects the reaction rate (at constant volume):

a) increasing the concentration of reagents;

b) grinding the solid reagent;
c) lowering the temperature;
d) the introduction of the catalyst;
e) reducing the concentration of reagents;
f) temperature rise;
g) the introduction of an inhibitor;
h) decrease in the concentration of products.

4. Calculate the rate of a chemical reaction

CO (g) + H 2 O (g) \u003d CO 2 (g) + H 2 (g)

in a vessel with a capacity of 1 liter, if after 1 min 30 s after its beginning the amount of hydrogen substance was 0.32 mol, and after 2 min 10 s it became 0.44 mol. How will an increase in CO concentration affect the reaction rate?

5. As a result of one reaction over a certain period of time, 6.4 g of hydrogen iodide was formed, and in another reaction under the same conditions, 6.4 g of sulfur dioxide. Compare the rates of these reactions. How will the rates of these reactions change with increasing temperature?

6. Determine the reaction rate

CO (g) + Cl 2 (g) \u003d COCl 2 (g),

if, 20 s after the start of the reaction, the initial amount of carbon monoxide (II) substance decreased from 6 mol by 3 times (the volume of the reactor is 100 l). How will the reaction rate change if less active bromine is used instead of chlorine? How will the reaction rate change when administered
a) catalyst; b) inhibitor?

7. In which case the reaction

CaO (tv.) + CO 2 (g.) \u003d CaCO 3 (tv.)

runs faster: when using large chunks or calcium oxide powder? Calculate:
a) the amount of the substance; b) the mass of calcium carbonate formed in 10 s, if the reaction rate is 0.1 mol / (l s), the volume of the reactor is 1 liter.

8. The interaction of a sample of magnesium with hydrochloric acid HCl makes it possible to obtain 0.02 mol of magnesium chloride 30 s after the start of the reaction. Determine how long it takes to get 0.06 mol of magnesium chloride.

E) from 70 to 40 ° C, the reaction rate decreased by 8 times;
g) from 60 to 40 ° C, the reaction rate decreased by 6.25 times;
h) from 40 to 10 ° C, the reaction rate decreased 27 times.

11. The owner of the car painted it with new paint, and then found that according to the instructions, it should dry for 3 hours at 105 ° C. How long will the paint dry at 25 ° C, if the temperature coefficient of the polymerization reaction underlying this process is: a) 2; b) 3; at 4?

ANSWERS TO QUESTIONS

1.a) \u003d kc(O 2); b) \u003d kc(HI) 2.

2. T+10 = T .

3. The reaction rate increases in cases a, b, d, f; decreases - c, d, f; does not change - h.

4. 0.003 mol / (l s). With an increase in CO concentration, the reaction rate increases.

5. The speed of the first reaction is 2 times lower.

6. 0.002 mol / (l s).

7. a) 1 mol; b) 100 g.

9. The speed of reactions d, g, h will increase 2 times; 4 times - a, b, f; 8 times - in, g.

10. Temperature coefficient:

2 for reactions b, f; \u003d 2.5 - in, g; \u003d 3 - d, h; \u003d 3.5 - a, d.

a) 768 hours (32 days, i.e. more than 1 month);
b) 19,683 hours (820 days, that is, more than 2 years);
c) 196 608 hours (8192 days, i.e. 22 years).

DEFINITION

Chemical kinetics - the doctrine of the rates and mechanisms of chemical reactions.

The study of reaction rates, obtaining data on the factors affecting 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 rate of homogeneous and heterogeneous reactions is determined 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 - the number of mole of any of the substances obtained during the reaction, V - the volume of the system, - time. Then in the limit:

This equation can be simplified - the ratio of the amount of substance to volume is the molar concentration of the substance n B / V \u003d \u200b\u200bc B, whence dn B / V \u003d \u200b\u200bdc B and finally:

In practice, the concentration of one or more substances is measured at certain intervals. The concentrations of the starting materials decrease with time, while the concentrations of the products increase (Fig. 1).

Figure: 1. Change in the concentration of the starting substance (a) and the reaction product (b) with time

Factors affecting the rate of a chemical reaction

The factors influencing the rate of a chemical reaction are: the nature of the reacting substances, their concentration, temperature, the presence of catalysts in the system, pressure and volume (in the gas phase).

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

For the reaction mA + nB \u003d pC + qD, the mathematical expression of 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 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: with an increase in temperature for every ten degrees, the rate of most chemical reactions increases by about 2 to 4 times. Mathematical expression:

(T 2) \u003d (T 1) × (T2-T1) / 10,

where is the Van't Hoff temperature coefficient, showing how many times the reaction rate increased when the temperature rises by 10 o C.

Molecularity and order of reaction

Molecularity of the reaction is determined by the minimum number of molecules simultaneously interacting (participating in an elementary act). Distinguish:

- monomolecular reactions (an example is decomposition reactions)

N 2 O 5 \u003d 2NO 2 + 1 / 2O 2

K × C, -dC / dt \u003d kC

However, not all reactions obeying this equation are monomolecular.

- bimolecular

CH 3 COOH + C 2 H 5 OH \u003d CH 3 COOC 2 H 5 + H 2 O

K × C 1 × C 2, -dC / dt \u003d 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 down the reaction equation.

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 indicators of the degrees of concentration in this equation. For example:

CaCO 3 \u003d CaO + CO 2

K × C 1 2 × C 2 - third order

The reaction order 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

The task

This reaction proceeds according to the equation 2A + B \u003d 4C. The initial concentration of substance A is 0.15 mol / l, and after 20 seconds - 0.12 mol / l. Calculate the average reaction rate.

Decision

Let's write the formula for calculating the average rate of a chemical reaction: