Regularities of changes in the properties of atoms of simple substances. Patterns of changes in the chemical properties of elements and their compounds by periods and groups

Speed chemical reaction depends on many factors, including the nature of the reactants, the concentration of the reactants, temperature, and the presence of catalysts. Let's consider these factors.

1). Nature of reactants. If there is an interaction between substances with an ionic bond, then the reaction proceeds faster than between substances with a covalent bond.

2.) Concentration of reactants. For a chemical reaction to take place, the molecules of the reacting substances must collide. That is, the molecules must come so close to each other that the atoms of one particle experience the action of the electric fields of the other. Only in this case will electron transitions and corresponding rearrangements of atoms be possible, as a result of which molecules of new substances are formed. Thus, the rate of chemical reactions is proportional to the number of collisions that occur between molecules, and the number of collisions, in turn, is proportional to the concentration of the reactants. Based on experimental material, the Norwegian scientists Guldberg and Waage and, independently of them, the Russian scientist Beketov in 1867 formulated the fundamental law chemical kinetics – law of mass action(ZDM): at a constant temperature, the rate of a chemical reaction is directly proportional to the product of the concentrations of the reacting substances to the power of their stoichiometric coefficients. For the general case:

the law of mass action has the form:

The recording of the law of mass action for a given reaction is called basic kinetic equation of the reaction. In the basic kinetic equation, k is the reaction rate constant, which depends on the nature of the reactants and temperature.

Most chemical reactions are reversible. During such reactions, their products, as they accumulate, react with each other to form the starting substances:

Forward reaction rate:

Feedback speed:

At the moment of equilibrium:

Hence the law of mass action in a state of equilibrium takes the form:

where K is the reaction equilibrium constant.

3) Effect of temperature on reaction rate. The rate of chemical reactions, as a rule, increases when the temperature is exceeded. Let's consider this using the example of the interaction of hydrogen with oxygen.

2H 2 + O 2 = 2H 2 O

At 20 0 C, the reaction rate is practically zero and it would take 54 billion years for the interaction to progress by 15%. At 500 0 C, it will take 50 minutes to form water, and at 700 0 C the reaction occurs instantly.

The dependence of the reaction rate on temperature is expressed van't Hoff's rule: with an increase in temperature by 10 o, the reaction rate increases by 2–4 times. Van't Hoff's rule is written:

4) Effect of catalysts. The rate of chemical reactions can be controlled using catalysts– substances that change the rate of a reaction and remain unchanged after the reaction. Changing the rate of a reaction in the presence of a catalyst is called catalysis. Distinguish positive(reaction speed increases) and negative(reaction rate decreases) catalysis. Sometimes a catalyst is formed during a reaction; such processes are called autocatalytic. There are homogeneous and heterogeneous catalysis.

At homogeneous In catalysis, the catalyst and reactants are in the same phase. For example:

At heterogeneous In catalysis, the catalyst and reactants are in different phases. For example:

Heterogeneous catalysis is associated with enzymatic processes. All chemical processes occurring in living organisms are catalyzed by enzymes, which are proteins with certain specialized functions. In solutions in which enzymatic processes take place, there is no typical heterogeneous environment, due to the absence of a clearly defined phase interface. Such processes are referred to as microheterogeneous catalysis.

One of the areas of physical chemistry, chemical kinetics, studies the rate of a chemical reaction and the conditions affecting its change. 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 during 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 compounds that entered the reaction (ΔC) 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 changes in the concentrations of the initial reactants, and the “+” sign refers to changing concentrations of the products of the same reaction.

Examples of reactions at different rates

Interactions chemical substances can be carried out at different speeds. Thus, the growth rate of stalactites, that is, the formation of calcium carbonate, is only 0.5 mm per 100 years. Some biochemical reactions occur slowly, such as photosynthesis and protein synthesis. Corrosion of metals occurs at a fairly low rate.

Medium speed can be used to describe reactions that require one to several hours. An example would be cooking, which involves the decomposition and transformation of compounds contained in foods. Synthesis of individual polymers requires heating the reaction mixture for a certain time.

An example of chemical reactions whose speed is quite high is 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 release of a precipitate of insoluble barium sulfate is observed.

A large number of reactions can occur at 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. For example, a mixture of gaseous oxygen and hydrogen may not show signs of interaction for quite a long time, but when the container is shaken or hit, the reaction becomes explosive. Therefore, chemical kinetics identifies 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;
presence of a catalyst;
pressure change (for gaseous substances);
area of contact of substances (if we are talking about heterogeneous reactions).

Influence of the nature of the substance

Such a significant difference in the rates of chemical reactions is explained different meanings activation energy (Ea). It is understood as a certain excess amount of energy in comparison with its average value required by a molecule during a collision in order for a reaction to occur. It is measured in kJ/mol and 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. u. it practically does not leak. This energy is spent on overcoming repulsion between the molecules of substances and on weakening the bonds in the original substances. In other words, the activation energy characterizes the strength chemical bonds in substances. Based on the value of activation energy, you can preliminary 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 (LMA), which states:

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 need to determine the rate of a chemical reaction, the equation of which can be conditionally written as:

αA+ bB = ϲС, then

in accordance with the above formulation of the law, the speed can be found using 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 reaction under consideration.

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 units. It should be noted that for correct calculation using this formula, it is worth taking into account the state of aggregation of the reagents. The solid concentration is taken to be unity and is not included in the equation because it remains constant during the reaction. Thus, only the concentrations of liquid and gaseous substances are included in the calculations according to the ZDM. So, for the reaction of producing silicon dioxide from simple substances, described by the equation

Si (tv) + Ο 2(g) = SiΟ 2(tv) ,

the 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's write down the 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 8 times.

Effect of temperature

The dependence of the rate of a chemical reaction on temperature was determined experimentally by the Dutch scientist J. H. Van't Hoff. He found that the rate of many reactions increases 2-4 times with every 10 degree increase in temperature. There is a mathematical expression for this rule that 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 influence of temperature on the rate of a particular reaction and does not describe the entire set of patterns. It is logical to conclude that with increasing temperature, the chaotic movement of particles intensifies and this provokes a greater number of collisions. However, this does not particularly affect the efficiency of molecular collisions, since it depends mainly on the activation energy. Also, their spatial correspondence to each other plays a significant role in the efficiency of particle collisions.

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

k = A 0 e -Ea/RΤ, where

A o is a multiplier;

E a - activation energy.

An example of a problem using van't Hoff's law

How should the temperature be changed so that the rate of a chemical reaction, whose temperature coefficient is numerically equal to 3, increases by 27 times?

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.

We substitute the values: 27 = 3 ΔΤ/10.

From this 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 a section called catalysis. He is interested in how and why relatively small amounts of certain substances significantly increase the rate of interaction of others. Substances that can speed up a 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 and contribute to the emergence of new transition states, which are characterized by lower energy barrier heights. That is, they help reduce the activation energy, and therefore increase the number of effective particle impacts. A catalyst cannot cause a reaction that is energetically impossible.

Thus, hydrogen peroxide can decompose to form oxygen and water:

Н 2 Ο 2 = Н 2 Ο + Ο 2 .

But this reaction is very slow and in our medicine cabinets it exists unchanged for quite a long time. When opening only very old bottles of peroxide, you may notice a slight popping sound caused by the pressure of oxygen on the walls of the vessel. Adding just a few grains of magnesium oxide will provoke active gas release.

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

Substance contact area

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

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. Thus, 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, and therefore increases the reactivity of the particles.

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

(S): V = Δν/(S·Δt).

Effect of pressure

A 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 a substance per unit volume, that is, its concentration increases proportionally. Conversely, a decrease in 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.

Task. How will the rate of the reaction described by the equation increase?

2ΝΟ + Ο 2 = 2ΝΟ 2,

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

Solution. As volume decreases, pressure increases proportionally. Let's write down the 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 sides 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 what remains is:

V 2 /V 1 = 9 3/1 = 27.

Answer: the speed has increased 27 times.

To summarize, it should be noted that the speed of interaction of substances, or more precisely, the quantity and quality of collisions of their particles, is influenced by many factors. First of all, these are the activation energy and the geometry of the molecules, which are almost impossible to correct. As for the remaining conditions, to increase the reaction rate one should:

increase the temperature of the reaction medium;
increase the concentrations of the starting compounds;
increase the pressure in the system or reduce its volume if we are talking about gases;
bring dissimilar substances to the same state of aggregation (for example, by dissolving them in water) or increase the area of their contact.

Main concepts studied:

Rate of chemical reactions

Molar concentration

Kinetics

Homogeneous and heterogeneous reactions

Factors affecting the rate of chemical reactions

Catalyst, inhibitor

Catalysis

Reversible and irreversible reactions

Chemical equilibrium

Chemical reactions are reactions as a result of which other substances are obtained from one substance (new substances are formed from the original substances). Some chemical reactions occur in a fraction of a second (explosion), while others take minutes, days, years, decades, etc.

For example: the combustion reaction of gunpowder occurs instantly with ignition and explosion, and the reaction of darkening of silver or rusting of iron (corrosion) occurs so slowly that its result can be monitored only after a long time.

To characterize the speed of a chemical reaction, the concept of chemical reaction speed - υ is used.

Chemical reaction rate is the change in the concentration of one of the reactants of a reaction per unit time.

Formula for calculating the rate of a chemical reaction:

υ =	from 2 – from 1	=	∆s
t 2 – t 1	∆t

c 1 – molar concentration of the substance at the initial time t 1

c 2 – molar concentration of the substance at the initial time t 2

since the rate of a chemical reaction is characterized by a change in the molar concentration of the reactants (starting substances), then t 2 > t 1, and c 2 > c 1 (the concentration of the starting substances decreases as the reaction proceeds).

Molar concentration (s)– is the amount of substance per unit volume. The unit of measurement for molar concentration is [mol/l].

The branch of chemistry that studies the rate of chemical reactions is called chemical kinetics. Knowing its laws, a person can control chemical processes and set them at a certain speed.

When calculating the rate of a chemical reaction, it is necessary to remember that reactions are divided into homogeneous and heterogeneous.

Homogeneous reactions– reactions that occur in the same environment (i.e., the reactants are in the same state of aggregation; for example: gas + gas, liquid + liquid).

Heterogeneous reactions– these are reactions occurring between substances in a heterogeneous medium (there is a phase interface, i.e. the reacting substances are in different states of aggregation; for example: gas + liquid, liquid + solid).

The above formula for calculating the rate of a chemical reaction is valid only for homogeneous reactions. If the reaction is heterogeneous, then it can only occur at the surface of the reactants.

For a heterogeneous reaction, the rate is calculated using the formula:

∆ν – change in the amount of substance

S – interface area

∆ t – time period during which the reaction took place

The rate of chemical reactions depends on various factors: the nature of the reactants, the concentration of the substances, temperature, catalysts or inhibitors.

Dependence of reaction rates on the nature of the reacting substances.

Let's analyze this dependence of the reaction rate using an example: let’s drop metal granules of equal area into two test tubes containing the same amount of hydrochloric acid (HCl) solution: an iron (Fe) granule into the first test tube, and a magnesium (Mg) granule into the second. As a result of observations, based on the rate of hydrogen release (H2), it can be noted that magnesium reacts with hydrochloric acid at the highest speed than iron. The rate of this chemical reaction is influenced by the nature of the metal (i.e. magnesium is more chemically active metal than iron, and therefore reacts more vigorously with acid).

Dependence of the rate of chemical reactions on the concentration of reactants.

The higher the concentration of the reacting (starting) substance, the faster the reaction proceeds. Conversely, the lower the concentration of the reactant, the slower the reaction.

For example: pour a concentrated solution of hydrochloric acid (HCl) into one test tube, and a dilute solution of hydrochloric acid into the other. Let's put a zinc granule (Zn) in both test tubes. We will observe, by the rate of hydrogen evolution, that the reaction will proceed faster in the first test tube, because the concentration of hydrochloric acid in it is greater than in the second test tube.

To determine the dependence of the rate of a chemical reaction, use law of action of (acting) masses : the rate of a chemical reaction is directly proportional to the product of the concentrations of the reacting substances, taken in powers that are equal to their coefficients.

For example, for a reaction proceeding according to the scheme: nA + mB → D, the rate of a chemical reaction is determined by the formula:

υ h.r. = k · C (A) n · C (B) m , Where

υ x.r - rate of chemical reaction

C (A) – A

C (B) – molar concentration of a substance IN

n and m – their coefficients

k – rate constant of a chemical reaction (reference value).

The law of mass action does not apply to substances in a solid state, because their concentration is constant (due to the fact that they react only on the surface, which remains unchanged).

For example: for reaction 2 Cu + O 2 = 2 CuO the reaction rate is determined by the formula:

υ h.r. = k C(O 2)

PROBLEM: The rate constant for the reaction 2A + B = D is 0.005. calculate the reaction rate at the molar concentration of substance A = 0.6 mol/l, substance B = 0.8 mol/l.

Dependence of the rate of a chemical reaction on temperature.

This dependence is determined van't Hoff rule (1884): with every 10°C increase in temperature, the rate of a chemical reaction increases on average by 2–4 times.

Thus, the interaction of hydrogen (H 2) and oxygen (O 2) at room temperature almost does not happen, the rate of this chemical reaction is so low. But at a temperature of 500 C o this reaction takes place in 50 minutes, and at a temperature of 700 C o it occurs almost instantly.

Formula for calculating the rate of a chemical reaction according to the Van't Hoff rule:

where: υ t 1 and υ t 2 - rates of chemical reactions at t 2 and t 1

γ is the temperature coefficient, which shows how many times the reaction rate increases with an increase in temperature by 10 C o.

Changing reaction speed:

2. Substitute the data from the problem statement into the formula:

Dependence of reaction rates on special substances - catalysts and inhibitors.

Catalyst- a substance that increases the rate of a chemical reaction, but does not itself participate in it.

Inhibitor- a substance that slows down a chemical reaction, but does not itself participate in it.

Example: into a test tube with a solution of 3% hydrogen peroxide (H 2 O 2), which has been heated, add a smoldering splinter - it will not light up, because the rate of the decomposition reaction of hydrogen peroxide into water (H 2 O) and oxygen (O 2) is very low, and the resulting oxygen is not enough to carry out qualitative reaction for oxygen (sustaining combustion). Now let’s add a little black powder of manganese (IV) oxide (MnO 2) into the test tube and see that the rapid release of gas bubbles (oxygen) has begun, and the smoldering splinter brought into the test tube flares up brightly. MnO 2 is the catalyst for this reaction; it accelerated the rate of the reaction, but did not participate in it itself (this can be proven by weighing the catalyst before and after the reaction - its mass will not change).

Knowledge of the rates of chemical reactions is of great theoretical and practical significance. For example, in the chemical industry, during the production of a substance, the size and productivity of the equipment and the amount of the resulting product depend on the reaction rate.

Different chemical reactions have different rates. Some reactions occur within a fraction of a second, while others take months or even years to complete. The speed of chemical reactions studies chemical kinetics.

The basic concepts with which chemical kinetics operates are chemical system And phase:

Chemical system- substance (a set of substances);
Chemical phase- part of a system separated from other parts interface.

Systems consisting of one phase are called homogeneous or homogeneous, for example, gas mixtures or solutions. Reactions occurring in homogeneous systems are called homogeneous reactions, such reactions occur throughout the entire volume of the mixture.

Systems consisting of several phases are called heterogeneous or heterogeneous, for example, liquid + solid. Reactions occurring in heterogeneous systems are called heterogeneous reactions, such reactions occur only at the interface.

Homogeneous reaction rate

The rate of a homogeneous reaction is the amount of substance (ν) formed as a result of a reaction per unit time (t) per unit volume of the system (V):

ν 1 - number of moles of substance at time t 1;
ν 2 - number of moles of substance at time t 2 ;

Mole-volume concentration substance (C, mol/l) - the ratio of the number of moles of a substance (ν) to the entire volume of the reaction mixture (V): С=ν/V.

The rate of a homogeneous reaction is equal to the change in the concentration of the reactant per unit time.

In the event that we are talking about the concentration of one of the reaction products, a “plus” sign is put in the expression, if we are talking about the concentration of one of the original substances, a “minus” sign is put in the expression.

Heterogeneous reaction rate

As mentioned above, the main difference between heterogeneous reactions and homogeneous ones is that the reaction occurs at the phase boundary.

The rate of a heterogeneous reaction (v het) is the amount of substance (ν) formed per unit time (t) per unit interface surface (S).

The main factors influencing the speed of reactions:

the nature of the reacting substances;
concentration;
temperature;
catalysts;
reagent particle sizes;
pressure.

The last two points relate to heterogeneous reactions.

Nature of reactants

A necessary condition for chemical interaction between molecules of substances is their collision with each other in the “right” part of the molecule, called area with high reactivity. It’s like in boxing: if a boxer’s blow hits the opponent’s gloves, there will be no reaction; but if the blow lands on the opponent’s head, then the probability of a knockout (reaction) increases significantly; and if the impact force (the force of collisions of molecules) is high, then knockout (reaction) becomes inevitable.

Based on the above, we can conclude that the more complex the molecule, the smaller its highly reactive region. Hence, the larger and more complex the molecules of the reacting substances, the slower the reaction rate.

Reagent concentration

The rate of a reaction is directly proportional to the number of molecular collisions. The higher the concentration of reagents, the more collisions, the higher the rate of chemical reaction. For example, combustion in pure oxygen occurs much faster than in ordinary air.

However, it should be said that in complex reactions occurring in several stages; such dependence is not respected. This allows you to determine which of the reagents is not involved in the slowest stage of the reaction, which determines the reaction rate itself.

The dependence of the reaction rate on the concentration of reactants is expressed law of mass action, which was discovered in 1867 by Norwegian scientists Guldberg and Waage.

The speed (v) of the conditioned reaction described by the equation aA+bB=cC+dD, in accordance with the law of mass action, will be calculated using a formula called kinetic reaction equation:

V=k·[A] a ·[B] b

[A], [B] - concentrations of starting substances;
k is the reaction rate constant, equal to the rate of this reaction at concentrations of reactants equal to 1 mol each.

k does not depend on the concentration of the reacting substances, but depends on their nature and temperature.

Using the kinetic equation of a reaction, you can determine the rate of change of the reaction depending on the change in the concentration of the reactants.

Examples of kinetic equations:

2SO 2 (g)+O 2 (g)=2SO 3 (g) v=k 2 CuO(s)+H 2 (g)=Cu(s)+H 2 O(g) v=k

Note that the kinetic equations do not include concentrations of solids, only gaseous and dissolved ones.

Reagent temperature

As the temperature rises, the molecules move faster, hence the number of their collisions with each other increases. In addition, the kinetic energy of molecules increases, which increases the efficiency of collisions, which ultimately determine the rate of reaction.

According to activation theory, only molecules with energy that exceeds a certain average value can take part in a chemical reaction. Excess amount average energy molecules is called activation energies. This energy is necessary to weaken the chemical bonds in the molecules of the starting substances. Molecules that have the necessary excess energy to allow them to react are called active molecules. The higher the temperature, the more active molecules, the higher the reaction rate.

The dependence of the reaction rate on temperature is characterized van't Hoff's rule:

Mathematically, Van't Hoff's rule is expressed by the following formula:

γ is the temperature coefficient, showing the increase in the reaction rate with an increase in temperature by 10°C;
v 1 - reaction rate at temperature t 1;
v 2 - reaction rate at temperature t 2;

Catalysts

Catalysts- these are substances that affect the rate of reaction, but are not consumed themselves.

Reactions that occur with the participation of catalysts are called catalytic reactions.

The main effect of a catalyst is to reduce the activation energy of the reaction, as a result of which the number of effective collisions of molecules increases.

Catalysts can speed up reactions millions of times!

There are two types of catalysis:

homogeneous (uniform) catalysis- the catalyst and reagents form one phase: gas or solution;
heterogeneous (heterogeneous) catalysis- the catalyst is in the form of an independent phase.

The mechanism of catalytic reactions is very complex and completely unknown. According to one scientific hypothesis, in catalytic reactions, a catalyst and a reagent react to form an intermediate compound, which reacts much more actively with another starting substance to form the final reaction product, while the catalyst itself is released in a free state.

Typically, catalysts are understood as substances that accelerate a reaction, but there are substances that slow down the course of a reaction - they are called inhibitors.

Biological catalysts are called enzymes. Enzymes are proteins.

Reagent particle size

Let's take a match and bring it to a piece of coal. It is unlikely that the coal will have time to ignite before the match goes out. Let's grind the coal and repeat the experiment - the coal dust will not just ignite, but will ignite very quickly - an explosion will occur (the main danger in coal mines). What's going on?

By grinding the coal, we will dramatically increase its surface area. How larger area the surface on which molecular collisions occur, the higher the reaction rate.

Reagent pressure

The pressure of gaseous reagents is similar to their concentration - the higher the pressure, the higher the concentration - the higher the reaction rate, because the number of molecular collisions increases. Like concentration, pressure of reactants does not “work” in complex reactions.

Chemical reaction rate

The topic “Rate of a chemical reaction” is perhaps the most complex 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 concept “speed of a chemical reaction” is already ambiguous (see, for example, the article by L.S. Guzey in the newspaper “Khimiya”, 2001, No. 28, With. 12). More more problems arises 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 is possible within the framework school curriculum
At the same time, it would be wrong to completely refuse to consider this topic in school. Ideas about the rate of a chemical reaction are very important when studying 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 ideas about the rate of a chemical reaction are used, the introduction of the simplest quantitative relationships is still desirable, especially for elementary reactions.
The published article discusses in sufficient detail the issues of chemical kinetics, which can be discussed in school chemistry lessons. Excluding controversial and controversial aspects of this topic from the school chemistry course is especially important for those students who are going to continue their chemical education at the university.

After all, the knowledge acquired at school often conflicts with scientific reality.

Chemical reactions can vary significantly in the time they take to occur. A mixture of hydrogen and oxygen at room temperature can remain virtually unchanged for a long time, but if struck or ignited, an explosion will occur. The iron plate slowly rusts, and a piece of white phosphorus spontaneously ignites in air. It is important to know how quickly a particular reaction occurs in order to be able to control its progress.

Basic Concepts

A quantitative characteristic of how quickly a given reaction proceeds is the speed of the chemical reaction, i.e. the rate of consumption of reagents or the rate of appearance of products. In this case, it does not matter which of the substances participating in the reaction are being discussed, 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 amounts of all the others. () Speed of chemical reaction () called a change in the amount of a reactant or product () per unit of time (per unit volume):

= /(per unit volume ).

V Reaction speed in in this case

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 = 2SO 3,

BaCl 2 + H 2 SO 4 = BaSO 4 + 2HCl.

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

Fe + 2HCl = FeCl 2 + H 2. the rate of a reaction is the change in the amount of a reactant or product () called a change in the amount of a reactant or product() per unit surface (S):

= /(S ).

The rate of a 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. chemical kinetics.

The branch of chemistry that studies the rate of chemical reactions and the influence of various factors on it is called

Collision 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 for a reaction to occur, two chemical particles must come together, so the rate of the reaction 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 an increase in the reaction rate.

For homogeneous reactions, increasing the concentration of one or more reactants will increase the rate of the reaction. When the concentration decreases, the opposite effect is observed. The concentration of substances in a 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 additional amounts of this substance into the reaction mixture. The concentrations of all gaseous substances can be increased simultaneously by reducing the volume occupied by the mixture. At the same time, the reaction speed will increase. Increasing the volume leads to the opposite result. The rate of heterogeneous reactions depends on surface area of contact between substances, i.e. on the degree of grinding of substances, the completeness of mixing of reagents, as well as on the state of crystal structures

solids

For a chemical reaction to occur, particles—atoms, molecules, or ions—must collide. As a result of collisions, atoms rearrange and new chemical bonds arise, which leads to the formation of new substances. The probability of a collision of two particles is quite high, the probability of a simultaneous collision of three particles is much less. It is extremely unlikely for four particles to collide at the same time.

Therefore, most reactions occur in several stages, at each of which no more than three particles interact.

The oxidation reaction of hydrogen bromide occurs at a noticeable rate at 400–600 °C:

4HBr + O 2 = 2H 2 O + 2Br 2.

According to the reaction equation, five molecules must collide simultaneously.

However, the probability 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 every molecule of oxygen four molecules of hydrogen bromide are consumed.

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

1) HBr + O 2 = HOOBr (slow reaction);

2) HOOBr + HBr = 2HOVr (fast reaction); 3) HOWr + HBr = H 2 O + Br 2 (fast reaction). The above reactions, the so-called elementary reactions, reflect 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 twice the third gives

summary equation reactions. The overall reaction rate is determined by the slowest intermediate reaction, in which one molecule of hydrogen bromide and one molecule of oxygen interact. (reactions. 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 reactions. The overall reaction rate is determined by the slowest intermediate reaction, in which one molecule of hydrogen bromide and one molecule of oxygen interact. = /per unit volume With law of mass action is the amount of substance per unit volume,

) reagents taken in powers equal to their stoichiometric coefficients (

Interaction of iodine with hydrogen: I 2 + H 2 = 2HI – bimolecular reaction. The law of mass action for chemical reactions of different molecularities is written differently.

Monomolecular reactions:

A = B + C,

= kc A,

Where k– 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 a chemical interaction only if the colliding particles have energy exceeding some specific value.

Let's consider the interaction of gaseous substances consisting of molecules A 2 and B 2:

A 2 + B 2 = 2AB. During a chemical reaction, a rearrangement of atoms occurs, accompanied by the breaking of chemical bonds in the starting substances and the formation of bonds in the reaction products. When reacting molecules collide, a so-called activated complex

, in which the electron density is redistributed, and only then the final reaction product is obtained: The energy required for the transition of substances into the state of an activated complex is called.

activation energy

The activity of chemicals is manifested in the low activation energy of reactions involving them. The lower the activation energy, the higher the reaction rate. For example, in reactions between cations and anions, the activation energy is very small, so such reactions occur almost instantly. If the activation energy is high, 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 affected by nature of reactants) the reaction rate in all cases slows down significantly.

We can conclude that the rate of reaction of a metal with an acid is influenced by the nature of both reagents - both the metal and the acid. Promotion temperature leads to an increase in the kinetic energy of chemical particles, i.e. increases the number of particles with energy higher than the activation energy. As the temperature increases, the number of particle collisions also increases, which increases the reaction rate to some extent. However, increasing the efficiency of collisions due to increasing kinetic energy has greater influence

on reaction speed than increasing the number of collisions.

= When the temperature increases by ten degrees, the speed increases by a number of times equal to the temperature coefficient of speed:+10 /When the temperature increases by ten degrees, the speed increases by a number of times equal to the temperature coefficient of speed: .

T When the temperature increases by ten degrees, the speed increases by a number of times equal to the temperature coefficient of speed: When the temperature rises from When the temperature increases by ten degrees, the speed increases by a number of times equal to the temperature coefficient of speed:"
before When the temperature increases by ten degrees, the speed increases by a number of times equal to the temperature coefficient of speed: reaction rate ratio " And T
equals When the temperature increases by ten degrees, the speed increases by a number of times equal to the temperature coefficient of speed:" – When the temperature increases by ten degrees, the speed increases by a number of times equal to the temperature coefficient of speed:)/10:

When the temperature increases by ten degrees, the speed increases by a number of times equal to the temperature coefficient of speed:" /When the temperature increases by ten degrees, the speed increases by a number of times equal to the temperature coefficient of speed: = (When the temperature increases by ten degrees, the speed increases by a number of times equal to the temperature coefficient of speed:"–When the temperature increases by ten degrees, the speed increases by a number of times equal to the temperature coefficient of speed:)/10.

temperature coefficient of speed to the power (

For many homogeneous reactions, the temperature coefficient of rate is 24 (van't Hoff's rule). The dependence of the reaction rate on temperature can be observed using the example of the interaction of copper(II) oxide with dilute sulfuric acid. At room temperature the reaction proceeds very slowly.

When heated, the reaction mixture quickly turns blue due to the formation of copper(II) sulfate:

CuO + H 2 SO 4 = 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 that accelerate reactions are called catalysts.

, and the very phenomenon of such acceleration of the reaction is

catalysis

Many reactions in the absence of catalysts proceed very slowly or not at all. One such reaction is the decomposition of hydrogen peroxide: 2H 2 O 2 = 2H 2 O + O 2. If you put it in a vessel with

aqueous solution hydrogen peroxide, a piece of solid manganese dioxide, then a rapid release of oxygen will begin. After manganese dioxide is removed, the reaction practically stops. By weighing, it is easy to verify that manganese dioxide is not consumed in this process - it only catalyzes the reaction. the catalyst and reactants are located; homogeneous and heterogeneous catalysis are distinguished.

In homogeneous catalysis, a catalyst can speed up a reaction by forming intermediates by reacting with one of the original reactants. For example:

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

Catalysts are widespread in nature.

Almost all transformations of substances in living organisms occur 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, metals are protected from corrosion.	Factors affecting the rate of a chemical reaction
Increase speed	Reduce speed
Presence of chemically active reagents	Presence of chemically inactive reagents
Increasing the concentration of reagents	Reducing the concentration of reagents
Increasing the surface area of solid and liquid reagents	Reducing the surface area of solid and liquid reagents
Temperature increase	Temperature drop

Presence of catalyst

1. Presence of inhibitor

TASKS

Define the rate of a chemical reaction. Write an expression for the kinetic law of mass action for the following reactions:

2. a) 2C (sol) + O 2 (g) = 2CO (g);

3. b) 2НI (g.) = H 2 (g.) + I 2 (g.).

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

Indicate how it affects the reaction rate (at constant volume):
a) increasing the concentration of reagents;
b) grinding the solid reagent;
c) decrease in temperature;
d) introduction of a catalyst;
e) decreasing the concentration of reagents;
f) increase in temperature;

4. g) introduction of an inhibitor;

h) reducing the concentration of products.

Calculate the rate of a chemical reaction

5. CO (g.) + H 2 O (g.) = CO 2 (g.) + H 2 (g.)

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

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

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

7. In what case is the reaction

CaO (tv.) + CO 2 (g.) = CaCO 3 (tv.)

flows faster: when using large pieces or calcium oxide powder? Calculate:
a) amount of 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 l.

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

E) from 70 to 40 °C the reaction rate decreased 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 by 27 times.

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

ANSWERS TO TASKS

1. a) = kc(O 2); kc b) =

2. When the temperature increases by ten degrees, the speed increases by a number of times equal to the temperature coefficient of speed:+10 = When the temperature increases by ten degrees, the speed increases by a number of times equal to the temperature coefficient of speed: .

(HI)2.

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

4. 0.003 mol/(l s). As the CO concentration increases, 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 by 2 times; 4 times – a, b, f; 8 times - c, d.

10. Temperature coefficient:

2 for reactions b, e; = 2.5 – c, g; = 3 – d, h; = 3.5 – a, g.
a) 768 hours (32 days, i.e. more than 1 month);
b) 19,683 hours (820 days, i.e. more than 2 years);