Topics of the USE codifier:Speed ​​reaction. Its dependence on various factors.

Speed chemical reaction shows how fast a reaction occurs. Interaction occurs when particles collide in space. In this case, the reaction does not occur with every collision, but only when the particles have the appropriate energy.

Speed ​​reaction is the number of elementary collisions of interacting particles, ending in a chemical transformation, per unit of time.

Determination of the rate of a chemical reaction is associated with the conditions for its implementation. If the reaction homogeneous– i.e. products and reactants are in the same phase - then the rate of a chemical reaction is defined as the change in substance per unit time:

υ = ∆C / ∆t.

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

υ = Δν / (S Δt).

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

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

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

Note! This rule works the same for all chemical reactions (including exothermic and endothermic ones). The reaction rate does not depend on the thermal effect. The rate of exothermic reactions increases with increasing temperature and decreases with decreasing temperature. The rate of endothermic reactions also increases with increasing temperature, and decreases with decreasing temperature.

Moreover, back in the 19th century, the Dutch physicist van't Hoff experimentally found that most reactions increase in approximately the same rate (by about 2-4 times) with an increase in temperature by 10 ° C. Van't Hoff's rule sounds like this: an increase in temperature by 10 ° C leads to an increase in the rate of a chemical reaction by 2-4 times (this value is called the temperature coefficient of the chemical reaction rate γ). Exact value temperature coefficient is determined for each reaction.

Here v 2 - reaction rate at temperature T 2, v 1 - reaction rate at temperature T 1, γ is the temperature coefficient of the reaction rate, the van't Hoff coefficient.

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

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

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

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

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

here v is the rate of the chemical reaction,

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

k is the coefficient of proportionality, the rate constant of the reaction.

For example, for the ammonia formation reaction:

N 2 + 3H 2 ↔ 2NH 3

The law of mass action looks like this:

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

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

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

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

pV=νRT

Thus, if among the reactants there is a gaseous substance, then at When pressure is increased, the rate of a chemical reaction increases; when pressure is reduced, it decreases. .

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

CaCO 3 + SiO 2 ↔ CaSiO 3 + CO 2

with increasing pressure?

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

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

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

A+K=AK

AK + B = AB + K

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

According to the phase state of the catalyst, homogeneous and heterogeneous catalysis are distinguished.

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

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

Example heterogeneous catalysis - ammonia synthesis:

N 2 + 3H 2 ↔ 2NH 3

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

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

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

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

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

5. Area of ​​contact of reactants. For heterogeneous reactions, one way to increase the number of effective collisions is to increase reaction surface area . How more area the contact surface of the reacting phases, the greater the rate of a heterogeneous chemical reaction. Powdered zinc dissolves much faster in acid than granular zinc of the same weight.

In industry, to increase the area of ​​the contacting surface of the reactants, they use fluidized bed method. For example, in the production of sulfuric acid by the boiling layer method, pyrite is roasted.

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

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

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

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

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

Let's define the basic concept of chemical kinetics - the rate of a chemical reaction:

The rate of a chemical reaction is the number of elementary acts of a chemical reaction occurring per unit time per unit volume (for homogeneous reactions) or per unit surface (for heterogeneous reactions).

The rate of a chemical reaction is the change in the concentration of reactants per unit time.

The first definition is the most rigorous; it follows from it that the rate of a chemical reaction can also be expressed as a change in time of any parameter of the state of the system, depending on the number of particles of any reactant, referred to a unit of volume or surface - electrical conductivity, optical density, permittivity etc. etc. However, most often in chemistry, the dependence of the concentration of reagents on time is considered. In the case of unilateral (irreversible) chemical reactions (hereinafter, only unilateral reactions are considered), it is obvious that the concentrations of the starting substances are constantly decreasing with time (ΔС ref< 0), а концентрации продуктов реакции увеличиваются (ΔС прод >0). The reaction rate is assumed to be positive, so the mathematical definition is average reaction rate in the time interval Δt is written as follows:

(II.1)

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

(II.2)

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

Rice. 2.1 Kinetic curves for starting materials (A) and reaction products (B).

The true reaction rate can be determined graphically by drawing a tangent to the kinetic curve (Fig. 2.2); the true rate of reaction in this moment time is equal in absolute value to the tangent of the slope of the tangent:

Rice. 2.2 Graphic definition V ist.

(II.3)

It should be noted that in the event that the stoichiometric coefficients in the chemical reaction equation are not the same, the reaction rate will depend on the change in the concentration of which reagent was determined. Obviously, in the reaction

2H 2 + O 2 → 2H 2 O

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

ΔC (H 2) \u003d ΔC (H 2 O) \u003d 2 ΔC (O 2).

The rate of a chemical reaction depends on many factors: the nature of the reactants, their concentration, temperature, the nature of the solvent, etc.

One of the tasks facing chemical kinetics is to determine the composition of the reaction mixture (i.e., the concentrations of all reactants) at any time, for which it is necessary to know the dependence of the reaction rate on concentrations. In general, the greater the concentration of the reactants, the greater the rate of the chemical reaction. The basis of chemical kinetics is the so-called. basic postulate of chemical kinetics:

The rate of a chemical reaction is directly proportional to the product of the concentrations of the reactants, taken to some extent.

i.e. for the reaction

AA + bB + dD + ... → eE + ...

Can be written

(II.4)

The coefficient of proportionality k is chemical reaction rate constant. The rate constant is numerically equal to the reaction rate at concentrations of all reactants equal to 1 mol/l.

The dependence of the reaction rate on the concentrations of reactants is determined experimentally and is called kinetic equation chemical reaction. Obviously, in order to write the kinetic equation, it is necessary to experimentally determine the rate constant and exponents at the concentrations of the reactants. The exponent at the concentration of each of the reactants in the kinetic equation of a chemical reaction (in equation (II.4) x, y and z, respectively) is private order reaction for this component. The sum of the exponents in the kinetic equation for a chemical reaction (x + y + z) is general reaction order . It should be emphasized that the reaction order is determined only from experimental data and is not related to the stoichiometric coefficients of the reactants in the reaction equation. The stoichiometric reaction equation is a material balance equation and in no way can determine the nature of the course of this reaction in time.

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

The rate of chemical reactions. Chemical equilibrium

Plan:

1. The concept of the rate of a chemical reaction.

2. Factors affecting the rate of a chemical reaction.

3. Chemical balance. Factors affecting the shifting balance. Le Chatelier's principle.

Chemical reactions take place with different speeds. Reactions are very fast in aqueous solutions. For example, if solutions of barium chloride and sodium sulfate are drained, then a white precipitate of barium sulfate immediately precipitates. Ethylene decolorizes bromine water quickly, but not instantly. Rust slowly forms on iron objects, plaque appears on copper and bronze products, foliage rots.

Science is engaged in the study of the rate of a chemical reaction, as well as the identification of its dependence on the conditions of the process - chemical kinetics.

If the reactions proceed in a homogeneous medium, for example, in a solution or gas phase, then the interaction of the reacting substances occurs in the entire volume. Such reactions are called homogeneous.

If a reaction occurs between substances that are in different states of aggregation (for example, between a solid and a gas or liquid) or between substances that are not capable of forming a homogeneous medium (for example, between two immiscible liquids), then it takes place only on the contact surface of the substances. Such reactions are called heterogeneous.

υ of a homogeneous reaction is determined by the change in the amount of substance per unit per unit volume:

υ \u003d Δ n / Δt ∙ V

where Δ n is the change in the number of moles of one of the substances (most often the initial, but may also be the reaction product), (mol);

V - volume of gas or solution (l)

Since Δ n / V = ​​ΔC (change in concentration), then

υ \u003d Δ C / Δt (mol / l ∙ s)

υ of a heterogeneous reaction is determined by the change in the amount of a substance per unit of time per unit of the contact surface of the substances.

υ \u003d Δ n / Δt ∙ S

where Δ n is the change in the amount of a substance (reagent or product), (mol);

Δt is the time interval (s, min);

S - surface area of ​​​​contact of substances (cm 2, m 2)

Why are the rates of different reactions not the same?

In order for a chemical reaction to start, the molecules of the reactants must collide. But not every collision results in a chemical reaction. In order for a collision to lead to a chemical reaction, the molecules must have a sufficiently high energy. Particles that collide with each other to undergo a chemical reaction are called active. They have an excess energy compared to the average energy of most particles - the activation energy E act. There are much fewer active particles in a substance than with an average energy, therefore, in order to start many reactions, the system must be supplied with some energy (a flash of light, heating, mechanical shock).

Energy barrier (value E act) of different reactions is different, the lower it is, the easier and faster the reaction proceeds.

2. Factors affecting υ(number of particle collisions and their efficiency).

1) The nature of the reactants: their composition, structure => activation energy

▪ the less E act, the more υ;

If a E act < 40 кДж/моль, то это значит, 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, т.к. в этих реакциях участвуют разноименнозаряженные частицы, и энергия активации в этих случаях ничтожно мала.

If a E act> 120 kJ/mol, this means that only a negligible part of the collisions between the interacting particles leads to the reaction. The rate of such reactions is very low. For example, the rusting of iron, or

the course of the ammonia synthesis reaction at ordinary temperature is almost impossible to notice.

If a E act have intermediate values ​​(40 - 120 kJ / mol), then the rate of such reactions will be average. Such reactions include the interaction of sodium with water or ethanol, decolorization of bromine water with ethylene, etc.

2) Temperature: at t for every 10 0 C, υ 2-4 times (van't Hoff rule).

υ 2 \u003d υ 1 ∙ γ Δt / 10

At t, the number of active particles (s E act) and their active collisions.

Task 1. The rate of a certain reaction at 0 0 C is 1 mol/l ∙ h, the temperature coefficient of the reaction is 3. What will be the rate of this reaction at 30 0 C?

υ 2 \u003d υ 1 ∙ γ Δt / 10

υ 2 \u003d 1 ∙ 3 30-0 / 10 \u003d 3 3 \u003d 27 mol / l ∙ h

3) Concentration: the more, the more often collisions and υ occur. At a constant temperature for the reaction mA + nB = C according to the law of mass action:

υ = k ∙ C A m ∙ C B n

where k is the rate constant;

С – concentration (mol/l)

Law of acting masses:

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

W.d.m. does not take into account the concentration of reacting substances in the solid state, because they react on surfaces and their concentrations usually remain constant.

Task 2. The reaction proceeds according to the equation A + 2B → C. How many times and how will the reaction rate change with an increase in the concentration of substance B by 3 times?

Solution: υ = k ∙ C A m ∙ C B n

υ \u003d k ∙ C A ∙ C B 2

υ 1 = k ∙ a ∙ in 2

υ 2 \u003d k ∙ a ∙ 3 in 2

υ 1 / υ 2 \u003d a ∙ in 2 / a ∙ 9 in 2 \u003d 1/9

Answer: increase by 9 times

For gaseous substances reaction rate depends on pressure

The more pressure, the higher the speed.

4) Catalysts Substances that change the mechanism of a reaction E act => υ .

▪ Catalysts remain unchanged at the end of the reaction

▪ Enzymes are biological catalysts, proteins by nature.

▪ Inhibitors - substances that ↓ υ

5) For heterogeneous reactions, υ also depends on:

▪ on the state of the contact surface of the reactants.

Compare: equal volumes of sulfuric acid solution were poured into 2 test tubes and simultaneously lowered into one - an iron nail, into the other - iron filings. Grinding solid leads to an increase in the number of its molecules that can simultaneously react. Therefore, the reaction rate in the second test tube will be higher than in the first one.

The rate of a chemical reaction

The rate of a chemical reaction- change in the amount of one of the reacting substances per unit of time in a unit of reaction space. Is key concept chemical kinetics. The rate of a chemical reaction is always positive, therefore, if it is determined by the initial substance (the concentration of which decreases during the reaction), then the resulting value is multiplied by −1.

For example for a reaction:

the expression for speed will look like this:

. The rate of a chemical reaction at each point in time is proportional to the concentrations of the reactants, raised to powers equal to their stoichiometric coefficients.

For elementary reactions, the exponent at the concentration value of each substance is often equal to its stoichiometric coefficient; for complex reactions, this rule is not observed. In addition to concentration, the following factors influence the rate of a chemical reaction:

• the nature of the reactants,
• the presence of a catalyst
• temperature (van't Hoff rule),
• pressure,
• the surface area of ​​the reactants.

If we consider the simplest chemical reaction A + B → C, then we notice that instant the rate of a chemical reaction is not constant.

Literature

• Kubasov A. A. Chemical kinetics and catalysis.
• Prigogine I., Defey R. Chemical thermodynamics. Novosibirsk: Nauka, 1966. 510 p.
• Yablonsky G. S., Bykov V. I., Gorban A. N., Kinetic models of catalytic reactions, Novosibirsk: Nauka (Siberian Branch), 1983.- 255 p.

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The rate of a chemical reaction- change in the amount of one of the reacting substances per unit of time in a unit of reaction space.

The following factors influence the rate of a chemical reaction:

• the nature of the reactants;
• concentration of reactants;
• contact surface of reactants (in heterogeneous reactions);
• temperature;
• the action of catalysts.

Theory of active collisions allows explaining the influence of some factors on the rate of a chemical reaction. The main provisions of this theory:

• Reactions occur when particles of reactants that have a certain energy collide.
• The more reagent particles, the closer they are to each other, the more likely they are to collide and react.
• Only effective collisions lead to the reaction, i.e. those in which "old ties" are destroyed or weakened and therefore "new" ones can form. To do this, the particles must have sufficient energy.
• The minimum excess energy required for efficient collision of reactant particles is called activation energy Ea.
• Activity chemical substances manifests itself in the low activation energy of reactions with their participation. The lower the activation energy, the higher the reaction rate. For example, in reactions between cations and anions, the activation energy is very low, so such reactions proceed almost instantly.

Influence of the concentration of reactants on the reaction rate

As the concentration of the reactants increases, the rate of the reaction increases. In order to enter into a reaction, two chemical particles must approach each other, 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.

An increase in the pressure or a decrease in the volume occupied by the mixture will lead to an increase in the rate of the reaction occurring in the gas phase.

On the basis of experimental data in 1867, the Norwegian scientists K. Guldberg and P Vaage, and independently of them in 1865, the Russian scientist N.I. Beketov formulated the basic law of chemical kinetics, which establishes dependence of the reaction rate on the concentrations of the reacting substances -

Law of mass action (LMA):

The rate of a chemical reaction is proportional to the product of the concentrations of the reactants, taken in powers equal to their coefficients in the reaction equation. (“acting mass” is a synonym for modern concept"concentration")

aA +bB =cC +dd, where k is the reaction rate constant

ZDM is performed only for elementary chemical reactions occurring in one stage. If the reaction proceeds sequentially through several stages, then the total rate of the entire process is determined by its slowest part.

Expressions for speeds various types reactions

ZDM refers to homogeneous reactions. If the reaction is heterogeneous (reagents are in different states of aggregation), then only liquid or only gaseous reagents enter the MDM equation, and solid ones are excluded, affecting only the rate constant k.

Reaction molecularity- this is minimum number molecules involved in elemental chemical process. By molecularity, elementary chemical reactions are divided into molecular (A →) and bimolecular (A + B →); trimolecular reactions are extremely rare.

Rate of heterogeneous reactions

• Depends on surface area of ​​contact of substances, i.e. on the degree of grinding of substances, the completeness of mixing of reagents.
• An example is the burning of wood. 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.
• Pyrophoric iron is poured onto a sheet of filter paper. During the fall, the iron particles become hot and set fire to the paper.

The effect of temperature on the reaction rate

In the 19th century, the Dutch scientist Van't Hoff experimentally discovered that when the temperature rises by 10 ° C, the rates of many reactions increase by 2-4 times.

Van't Hoff's rule

For every 10 ◦ C increase in temperature, the reaction rate increases by a factor of 2–4.

Here γ ( Greek letter"gamma") - the so-called temperature coefficient or van't Hoff coefficient, takes values ​​from 2 to 4.

For each specific reaction, the temperature coefficient is determined empirically. It shows exactly how many times the rate of a given chemical reaction (and its rate constant) increases with every 10 degrees increase in temperature.

The van't Hoff rule is used to approximate the change in the rate constant of a reaction with an increase or decrease in temperature. A more accurate relationship between the rate constant and temperature was established by the Swedish chemist Svante Arrhenius:

How more E a specific reaction, the less(at a given temperature) will be the rate constant k (and the rate) of this reaction. An increase in T leads to an increase in the rate constant; this is explained by the fact that an increase in temperature leads to a rapid increase in the number of "energetic" molecules capable of overcoming the activation barrier E a .

Influence of a catalyst on the reaction rate

It is possible to change the reaction rate by using special substances that change the reaction mechanism and direct it along an energetically more favorable path with a lower activation energy.

Catalysts- These are substances that participate in a chemical reaction and increase its speed, but at the end of the reaction remain unchanged qualitatively and quantitatively.

Inhibitors- Substances that slow down chemical reactions.

Changing the rate of a chemical reaction or its direction with the help of a catalyst is called catalysis .