The rate of chemical reactions, its dependence on various factors

Homogeneous and heterogeneous chemical reactions

Chemical reactions proceed at different rates: at a low rate - during the formation of stalactites and stalagmites, at an average rate - during cooking, instantly - during an explosion. Reactions in aqueous solutions take place very quickly, almost instantly. We mix solutions of barium chloride and sodium sulfate - barium sulfate in the form of a precipitate is formed immediately. Sulfur burns quickly, but not instantly, magnesium dissolves in hydrochloric acid, ethylene discolors bromine water. Rust slowly forms on iron objects, plaque on copper and bronze products, foliage slowly decays, teeth are destroyed.

Predicting the rate of a chemical reaction, as well as finding out its dependence on the conditions of the process is a task chemical kinetics- the science of the laws governing the course of chemical reactions in time.

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

The rate of a homogeneous reaction ($ v_ (homogeneous) $) is defined as the change in the amount of a substance per unit time per unit volume:

$ υ_ (homogeneous) = (∆n) / (∆t V), $

where $ ∆n $ is the change in the number of moles of one substance (most often the initial one, but there may also be a reaction product); $ ∆t $ - time interval (s, min.); $ V $ - volume of gas or solution (l).

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

$ (∆n) / (V) = ∆C. $

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

$ υ_ (hom.) = (∆C) / (∆t) [(mol) / (l · s)] $

if the volume of the system does not change. If the reaction takes place between substances in different states of aggregation (for example, between a solid and a gas or liquid), or between substances that are unable to form a homogeneous medium (for example, between immiscible liquids), then it takes place only on the contact surface of the substances. Such reactions are called heterogeneous.

Heterogeneous reaction rate is defined as the change in the amount of a substance per unit time per unit surface:

$ υ_ (hom.) = (∆C) / (∆t · S) [(mol) / (s · m ^ 2)] $

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

If, during any ongoing reaction, the concentration of the starting substance is experimentally measured at different points in time, then its change can be graphically displayed using the kinetic curve for this reagent.

The reaction rate is not constant. We have indicated only a certain average rate of this reaction in a certain time interval.

Imagine that we determine the reaction rate

$ H_2 + Cl_2 → 2HCl $

a) by the change in the concentration of $ Н_2 $;

b) by the change in the concentration of $ HCl $.

Will we get the same values? After all, $ 2 $ mol $ HCl $ is formed from $ 1 $ mol $ H_2 $, so the rate in case b) will be twice as high. Consequently, the value of the reaction rate also depends on what substance it is determined by.

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

Calculated data show that, for example, in gases, collisions of molecules at atmospheric pressure are estimated in billions per $ 1 $ second, i.e. all reactions should have been instantaneous. But this is not the case. It turns out that only a very small fraction of the molecules have the necessary energy to effectively collide.

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

Thus, there is an energy barrier on the path of all particles that enter into the reaction, 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 up a match to light the alcohol lamp, you are imparting the extra energy $ E_a $ needed to effectively collide the alcohol molecules with the oxygen molecules (breaking the barrier).

In conclusion, we conclude: many possible reactions practically do not go, because high activation energy.

This makes a huge difference in our lives. Imagine what would happen if all thermodynamically allowed reactions could proceed without any energy barrier (activation energy). The oxygen in the air would react with anything that could burn or simply oxidize. All organic matter would suffer, they would turn into carbon dioxide $ CO_2 $ and water $ H_2O $.

The rate of a chemical reaction depends on many factors. The main ones are: the nature and concentration of reactants, pressure (in reactions involving gases), temperature, the effect of catalysts and the surface of reactants in the case of heterogeneous reactions. Let's consider the influence of each of these factors on the rate of a chemical reaction.

Temperature

As you know, when the temperature rises, in most cases, the rate of a chemical reaction increases significantly. In the XIX century. Dutch chemist J. H. Van't Hoff formulated the rule:

An increase in temperature for every $ 10 ° C $ leads to an increase in the reaction rate by 2-4 times (this value is called the temperature coefficient of reaction).

As the temperature rises, the average velocity of molecules, their energy, and the number of collisions increase insignificantly, but the fraction of active molecules participating in effective collisions that overcome the energy barrier of the reaction increases sharply.

Mathematically, this dependence is expressed by the ratio:

$ υ_ (t_2) = υ_ (t_1) γ ^ ((t_2-t_1) / (10)), $

where $ υ_ (t_1) $ and $ υ_ (t_2) $ are the reaction rates at the final $ t_2 $ and initial $ t_1 $ temperatures, respectively, and $ γ $ is the temperature coefficient of the reaction rate, which shows how many times the reaction rate increases with an increase in temperature for every $ 10 ° C $.

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

Concentration of reactants

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

For chemical interaction between particles to take place, they must effectively collide. The greater the concentration of reactants, the more collisions and, accordingly, the higher the reaction rate. For example, in pure oxygen, acetylene burns out very quickly. This develops a temperature sufficient to melt the metal. On the basis of a large experimental material in 1867 by the Norwegians K. Guldenberg and P. Vaage and independently of them in 1865 by the Russian scientist N.I.Beketov, the basic law of chemical kinetics was formulated, establishing the dependence of the reaction rate on the concentration of reacting substances.

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.

This law is also called the law of the masses at work.

For the reaction $ A + B = D $, this law is expressed as follows:

$ υ_1 = k_1 C_A C_B $

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

$ υ_2 = k_2 C_A ^ 2 C_B $

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 easy to establish - it is numerically equal to the reaction rate, in which the concentrations of the reacting substances are equal to $ 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.

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

For example, for the reaction of burning coal

the expression for the reaction rate should be written as follows:

$ υ = k C_ (O_2) $,

that is, the reaction rate is proportional only to the oxygen concentration.

If the reaction equation describes only the total chemical reaction, which 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 relationship is determined experimentally or theoretically based on the proposed reaction mechanism.

The action of catalysts

It is possible to increase 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. They are called catalysts(from lat. katalysis- destruction).

The catalyst acts as an experienced guide, directing a group of tourists not through a high pass in the mountains (overcoming it requires a lot of effort and time and is not available to everyone), but along the roundabout paths known to him, along which it is possible to overcome the mountain much easier and faster. True, by a roundabout route you can get not quite where the main pass leads. But sometimes this is exactly what is required! This is how catalysts act, which are called selective... It is clear that there is no need to burn ammonia and nitrogen, but nitric oxide (II) is used in the production of nitric acid.

Catalysts are substances that take part in a chemical reaction and change its rate 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 with the help of a catalyst is called catalysis... Catalysts are widely used in various industries and in transport (catalytic converters that convert nitrogen oxides from vehicle 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:

$ 2H_2O_2 (→) ↖ (MnO_2 (I)) 2H_2O _ ((f)) + O_2 (g) $

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 inoperative, 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 the contact method, a solid catalyst is used - vanadium (V) oxide $ V_2O_5 $:

$ 2SO_2 + O_2⇄2SO_3 $

In the production of methanol, a solid zinc-chromium catalyst ($ 8ZnO Cr_2O_3 × CrO_3 $) is used:

$ CO _ ((g)) + 2H_ (2 (g)) ⇄CH_3OH _ ((g)) $

Biological catalysts work very effectively - enzymes... By chemical nature, these are proteins. Thanks to them, complex chemical reactions proceed at a high speed in living organisms at low temperatures. Enzymes are very specific, each of them accelerates only its own reaction, which takes place at the right time and in the right place with a yield close to $ 100% $. The creation of artificial catalysts similar to enzymes is a chemists' dream!

You, of course, have heard about other interesting substances - inhibitors(from lat. inhibere- to detain). They react at a high rate with active particles to form low-active compounds. As a result, the reaction slows down dramatically and then stops. Inhibitors are often specially added to various substances to prevent unwanted processes.

For example, using inhibitors, they stabilize hydrogen peroxide solutions, monomers to prevent premature polymerization, hydrochloric acid so that it can be transported in a steel container. Inhibitors are also found in living organisms, they suppress various harmful oxidation reactions in tissue cells, which can be initiated, for example, by radioactive radiation.

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

The value of the activation energy is the factor through which the influence of the nature of the reacting substances affects the reaction rate.

If the activation energy is small ($< 40$ 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 этих случаях ничтожно мала.

If the activation energy is high ($> 120 $ kJ / mol), then this means that only an insignificant part of collisions between interacting particles leads to a reaction. The speed of this 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 have intermediate values ​​($ 40-120 $ kJ / mol), then the rates of such reactions will be average. These reactions include the interaction of sodium with water or ethyl alcohol, bleaching of bromic water with ethylene, the interaction of zinc with hydrochloric acid, etc.

Contact surface of reactants

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

The increase in the reaction rate is explained, first of all, by an increase in the contact surface of the initial substances, as well as by a number of other reasons, for example, the destruction of the structure of a regular crystal lattice. This leads to the fact that particles on the surface of the formed microcrystals are much more reactive than the same particles on a smooth surface.

In industry, for carrying out heterogeneous reactions, a fluidized bed is used to increase the contact surface of the reactants, the supply of starting materials and the removal of products. For example, in the production of sulfuric acid using a fluidized bed, pyrite is roasted; in organic chemistry, using a fluidized bed, catalytic cracking of petroleum products and regeneration (recovery) of a failed (coked) catalyst are carried out.

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. She 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 = ± ΔC / Δt.

The reaction rate is measured in mol / l ∙ s, if it occurs throughout 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 values ​​of the concentrations of the initial reacting substances, and the “+” sign - to the changing values ​​of 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 that require from one to several hours. An example would be the preparation of food, which is accompanied by the decomposition and conversion 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 can proceed 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 hit, the reaction becomes explosive. Therefore, chemical kinetics and identified certain factors that have the ability to influence the rate of a 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);
• contact area 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 ​​of 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 the values ​​are usually in the range of 50-250.

It is generally accepted that if E a = 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 preliminarily estimate the rate of a 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> 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 (MLA), which reads:

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

This law is suitable for elementary one-stage reactions, or 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 = ϲС, then,

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

V = 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 taking into account 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 concentrations of liquid and gaseous substances are included in the calculation for ZDM. So, for the reaction of obtaining silicon dioxide from simple substances, described by the equation

Si (tv) + Ο 2 (g) = 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 = 2ΝΟ 2.

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

ᴠ 1 = k · [ΝΟ] 2 · [Ο 2] and

ᴠ 2 = k · (2 ​​· [ΝΟ]) 2 · 2 · [Ο 2] = k · 4 [ΝΟ] 2 · 2 [Ο 2].

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

ᴠ 2 / ᴠ 1 = 4 2/1 = 8.

Answer: increased by 8 times.

Influence of temperature

The dependence of the rate of a chemical reaction 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 = 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 a particular reaction and does not describe the entire set of regularities. It is logical to conclude that with an increase in temperature, the chaotic movement 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 dependence of the rate of a chemical reaction on temperature, taking into account the nature of the reactants, obeys the Arrhenius equation:

k = A 0 e -Ea / RΤ, where

And about is a multiplier;

E a is the activation energy.

An example of a problem on the 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 by a factor of 27?

Solution. Let's use the formula

ᴠ 2 = ᴠ 1 γ (Τ2-Τ1) / 10.

From the condition ᴠ 2 / ᴠ 1 = 27, and γ = 3. You need to find ΔΤ = Τ 2 -Τ 1.

Transforming the original formula, we get:

V 2 / V 1 = γ ΔΤ / 10.

Substitute the values: 27 = 3 ΔΤ / 10.

Hence it is clear that ΔΤ / 10 = 3 and ΔΤ = 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, promote 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 collisions of particles. 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 = H 2 Ο + Ο 2.

But this reaction is very slow and in our first-aid kits it exists unchanged for quite a long time. Opening only very old vials of peroxide, you will notice a slight popping caused by the pressure of oxygen 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 ​​substances

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 significantly affects the rate of the chemical reaction. 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 proceeds.

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 the substance, which means it increases the reactivity of the 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 = Δν / (S Δt).

Influence of pressure

The change in pressure in the system has an effect only when gases take part 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 leads to an equivalent decrease in the concentration of the reagent. 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 = 2ΝΟ 2,

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

Solution. As the volume decreases, the pressure increases proportionally. Let's write expressions for the initial (V 1) and final (V 2) reaction rates:

V 1 = k · 2 · [Ο 2] and

V 2 = k · (3 ·) 2 · 3 · [Ο 2] = 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 = (k · 9 [ΝΟ] 2 · 3 [Ο 2]) / (k · [ΝΟ] 2 · [Ο 2]).

The concentration values ​​and rate constants are reduced, and it remains:

V 2 / V 1 = 9 3/1 = 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 the 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 ​​their contact.

Some chemical reactions occur almost instantly (explosion of an oxygen-hydrogen mixture, ion exchange reactions in an aqueous solution), the second - quickly (combustion of substances, interaction of zinc with acid), and still others - slowly (iron rusting, decay of organic residues). Such slow reactions are known that a person simply cannot notice them. For example, the transformation of granite into sand and clay occurs over thousands of years.

In other words, chemical reactions can proceed in different ways. speed.

But what is it speed reaction? What is the exact definition of this quantity and, most importantly, its mathematical expression?

The reaction rate is the change in the amount of a substance per unit of time in one unit of volume. Mathematically, this expression is written as:

Where n 1 andn 2 Is the amount of substance (mol) at time t 1 and t 2, respectively, in a system of volume V.

Which plus or minus sign (±) will stand in front of the speed expression depends on whether we are looking at the change in the amount of what substance we are looking at - a product or a reagent.

Obviously, in the course of the reaction, the consumption of reagents occurs, that is, their amount decreases, therefore, for reagents, the expression (n 2 - n 1) always has a value less than zero. Since the speed cannot be negative, in this case a minus sign must be placed in front of the expression.

If we are looking at the change in the amount of the product, and not the reagent, then a minus sign is not required before the expression for calculating the speed, since the expression (n 2 - n 1) in this case is always positive, because the amount of product as a result of the reaction can only increase.

The ratio of the amount of substance n to the volume in which this amount of substance is located is called the molar concentration WITH:

Thus, using the concept of molar concentration and its mathematical expression, you can write another version of determining the reaction rate:

The reaction rate is the change in the molar concentration of a substance as a result of a chemical reaction in one unit of time:

Factors affecting reaction rate

It is often extremely important to know what determines the speed of a particular reaction and how to influence it. For example, the refining industry is literally beating for every additional half a percent of the product per unit of time. After all, given the huge amount of refined oil, even half a percent flows into a large financial annual profit. In some cases, it is extremely important to slow down any reaction, in particular the corrosion of metals.

So what determines the reaction rate? It depends, oddly enough, on many different parameters.

In order to understand this issue, first of all, let's imagine what happens as a result of a chemical reaction, for example:

A + B → C + D

The above equation reflects the process in which molecules of substances A and B, colliding with each other, form molecules of substances C and D.

That is, undoubtedly, in order for the reaction to take place, at least, a collision of the molecules of the initial substances is necessary. Obviously, if we increase the number of molecules per unit volume, the number of collisions will increase in the same way as the frequency of your collisions with passengers on a crowded bus increases compared to a half-empty one.

In other words, the reaction rate increases with an increase in the concentration of reactants.

In the case when one of the reagents or several at once are gases, the reaction rate increases with increasing pressure, since the gas pressure is always directly proportional to the concentration of its constituent molecules.

Nevertheless, the collision of particles is a necessary, but not at all sufficient, condition for the reaction to proceed. The fact is that, according to calculations, the number of collisions of molecules of reacting substances at their reasonable concentration is so great that all reactions should take place in an instant. However, in practice this does not happen. What's the matter?

The point is that not every collision of reagent molecules will necessarily be effective. Many collisions are elastic - molecules bounce off each other like balls. For the reaction to proceed, the molecules must have sufficient kinetic energy. The minimum energy that the molecules of the reacting substances must possess in order for the reaction to proceed is called the activation energy and is denoted as E a. In a system consisting of a large number of molecules, there is a distribution of molecules in energy, some of them have low energy, some have high and medium energy. Of all these molecules, only a small fraction of the molecules have energy that exceeds the activation energy.

As you know from the course of physics, temperature is actually a measure of the kinetic energy of the particles that make up the substance. That is, the faster the particles that make up the substance move, the higher its temperature. Thus, obviously, by increasing the temperature, we essentially increase the kinetic energy of the molecules, as a result of which the fraction of molecules with an energy exceeding E a increases and their collision will lead to a chemical reaction.

The fact of the positive influence of temperature on the rate of the reaction was established empirically by the Dutch chemist Van't Hoff back in the 19th century. Based on his research, he formulated a rule that still bears his name, and it sounds like this:

The rate of any chemical reaction increases by 2-4 times when the temperature rises by 10 degrees.

The mathematical representation of this rule is written as:

where V 2 and V 1 Is the rate at temperature t 2 and t 1, respectively, and γ is the temperature coefficient of the reaction, the value of which most often lies in the range from 2 to 4.

Often the speed of many reactions can be increased by using catalysts.

Catalysts are substances that accelerate the course of any reaction and are not consumed at the same time.

But how do catalysts manage to increase the reaction rate?

Let us recall the activation energy E a. Molecules with an energy lower than the activation energy in the absence of a catalyst cannot interact with each other. The catalysts change the path along which the reaction proceeds, just as an experienced guide will make the route of the expedition not directly through the mountain, but using detour paths, as a result of which even those satellites that did not have enough energy to climb the mountain can move to another her side.

Despite the fact that the catalyst is not consumed during the reaction, it nevertheless takes an active part in it, forming intermediate compounds with reagents, but by the end of the reaction it returns to its original state.

In addition to the above factors affecting the reaction rate, if there is an interface between the reactants (heterogeneous reaction), the reaction rate will also depend on the contact area of ​​the reactants. For example, imagine a granule of metallic aluminum thrown into a test tube containing an aqueous solution of hydrochloric acid. Aluminum is an active metal that can react with acids as non-oxidizing agents. With hydrochloric acid, the reaction equation is as follows:

2Al + 6HCl → 2AlCl 3 + 3H 2

Aluminum is a solid, which means that the reaction with hydrochloric acid takes place only on its surface. Obviously, if we increase the surface area by first rolling the aluminum granule into foil, we thereby provide a larger number of aluminum atoms available for the reaction with the acid. As a result, the reaction rate will increase. Similarly, an increase in the surface of a solid can be achieved by pulverizing it.

Also, the rate of a heterogeneous reaction, in which a solid reacts with a gaseous or liquid, is often positively influenced by stirring, which is due to the fact that as a result of stirring, the accumulating molecules of the reaction products are removed from the reaction zone and a new portion of the reagent molecules is "brought in".

The latter should also note the enormous influence on the rate of the reaction and the nature of the reagents. For example, the lower the alkali metal is in the periodic table, the faster it reacts with water, fluorine reacts most rapidly with gaseous hydrogen among all halogens, etc.

Summarizing all of the above, the reaction speed depends on the following factors:

1) the concentration of reagents: the higher, the greater the reaction rate

2) temperature: as the temperature rises, the rate of any reaction increases

3) the contact area of ​​the reactants: the larger the contact area of ​​the reactants, the higher the reaction rate

4) stirring, if the reaction occurs with a solid and a liquid or gas, stirring can accelerate it.

A chemical reaction is the transformation of some substances into others.

Whatever type of chemical reactions are, they occur at different rates. For example, geochemical transformations in the bowels of the Earth (formation of crystalline hydrates, hydrolysis of salts, synthesis or decomposition of minerals) take thousands, millions of years. And such reactions as the combustion of gunpowder, hydrogen, saltpeter, berthollet salt occur within a fraction of a second.

The rate of a chemical reaction is understood as the change in the amounts of reactants (or reaction products) per unit of time. The most commonly used concept is average reaction rate (Δc p) in the time interval.

v cf = ± ∆C / ∆t

For products ∆С> 0, for starting materials - ∆С< 0. Наиболее употребляемая единица измерения - моль на литр в секунду (моль/л*с).

The rate of each chemical reaction depends on many factors: the nature of the reactants, the concentration of the reactants, the change in the reaction temperature, the degree of fineness of the reactants, the change in pressure, and the introduction of a catalyst into the reaction medium.

The nature of the reactants significantly affects the rate of a chemical reaction. As an example, consider the interaction of some metals with a constant component - water. Let's define metals: Na, Ca, Al, Au. Sodium reacts very violently with water at ordinary temperatures, releasing a large amount of heat.

2Na + 2H 2 O = 2NaOH + H 2 + Q;

Calcium reacts less vigorously at normal temperatures with water:

Ca + 2H 2 O = Ca (OH) 2 + H 2 + Q;

Aluminum reacts with water already at elevated temperatures:

2Al + 6H 2 O = 2Al (OH) h + 3H 2 - Q;

And gold is one of the inactive metals; it does not react with water either at normal or at elevated temperatures.

The rate of a chemical reaction is in direct proportion to concentration of reactants ... So, for a reaction:

C 2 H 4 + 3O 2 = 2CO 2 + 2H 2 O;

The expression for the reaction rate is:

v = k ** [O 2] 3;

Where k is the rate constant of a chemical reaction, numerically equal to the rate of this reaction, provided that the concentrations of the reacting components are equal to 1 g / mol; the values ​​of [C 2 H 4] and [O 2] 3 correspond to the concentrations of the reactants raised to the power of their stoichiometric coefficients. The higher the concentration of [C 2 H 4] or [O 2], the more collisions of the molecules of these substances per unit time, hence the greater the rate of the chemical reaction.

The rates of chemical reactions, as a rule, are also in direct proportion on reaction temperature ... Naturally, with increasing temperature, the kinetic energy of the molecules increases, which also leads to large collisions of molecules per unit time. Numerous experiments have shown that when the temperature changes every 10 degrees, the reaction rate changes 2-4 times (van't Hoff's rule):

where V T 2 - the rate of chemical reaction at T 2; V ti - the rate of chemical reaction at T 1; g is the temperature coefficient of the reaction rate.

Influence the degree of fineness of substances the reaction rate is also directly dependent. The finer the state of the particles of the reacting substances, the more they come into contact with each other per unit of time, the greater the rate of the chemical reaction. Therefore, as a rule, reactions between gaseous substances or solutions proceed faster than in the solid state.

The change in pressure affects the rate of reaction between substances in a gaseous state. Being in a closed volume at a constant temperature, the reaction proceeds at a rate of V 1. If in this system we increase the pressure (hence, decrease the volume), the concentrations of the reactants will increase, the collision of their molecules per unit time will increase, the reaction rate will increase to V 2 (v 2 > v 1).

Catalysts are substances that change the rate of a chemical reaction, but remain unchanged after the chemical reaction ends. The effect of catalysts on the reaction rate is called catalysis. Catalysts can both accelerate the chemical-dynamic process and slow it down. When the interacting substances and the catalyst are in the same state of aggregation, we speak of homogeneous catalysis, and in the case of heterogeneous catalysis, the reactants and the catalyst are in different states of aggregation. The catalyst forms an intermediate complex with the reagents. For example, for a reaction:

Catalyst (K) forms a complex with A or B - AK, BK, which releases K when interacting with a free particle A or B:

AK + B = AB + K

VK + A = BA + K;

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