The rate of a chemical reaction depends on the following factors:
1) The nature of the reactants.
2) The contact surface of the reagents.
3) The concentration of reactants.
4) Temperature.
5) The presence of catalysts.
The rate of heterogeneous reactions also depends on:
a) the magnitude of the phase separation surface (with an increase in the phase separation surface, the rate of heterogeneous reactions increases);
b) the rate of supply of reactants to the interface and the rate of removal of reaction products from it.
Factors affecting the rate of a chemical reaction:
1. The nature of the reagents. An important role is played by the nature of chemical bonds in compounds, the structure of their molecules. For example, the release of hydrogen by zinc from a solution of hydrochloric acid occurs much faster than from a solution of acetic acid, since the polarity of the H-C1 bond is greater than the O-H bond in the CH 3 COOH molecule, in other words, due to the fact that Hcl - a strong electrolyte, and CH 3 COOH is a weak electrolyte in an aqueous solution.
2. Reagent contact surface. The larger the contact surface of the reactants, the faster the reaction proceeds. The surface of solids can be increased by grinding them, and for soluble substances by dissolving them. Reactions in solutions proceed almost instantaneously.
3. The concentration of reagents. For an interaction to occur, the particles of reactants in a homogeneous system must collide. With an increase reactant concentrations the rate of reactions increases. This is explained by the fact that with an increase in the amount of a substance per unit volume, the number of collisions between the particles of the reacting substances increases. The number of collisions is proportional to the number of particles of reactants in the volume of the reactor, i.e., their molar concentrations.
Quantitatively, the dependence of the reaction rate on the concentration of the reactants is expressed law of acting masses (Guldberg and Waage, Norway, 1867): the rate of a chemical reaction is proportional to the product of the concentrations of the reactants.
For reaction:
aA + bB ↔ cC + dD
the reaction rate in accordance with the law of mass action is equal to:
υ = k[A]υ a[B]υ b ,(9)
where [A] and [B] are the concentrations of the initial substances;
k-reaction rate constant, which is equal to the reaction rate at concentrations of reactants [A] = [B] = 1 mol/l.
The reaction rate constant depends on the nature of the reactants, temperature, but does not depend on the concentration of substances.
Expression (9) is called the kinetic equation of the reaction. The kinetic equations include the concentrations of gaseous and dissolved substances, but do not include the concentrations of solids:
2SO 2 (g) + O 2 (g) \u003d 2SO 3 (g); υ = k 2 · [O 2 ];
CuO (tv.) + H 2 (g) \u003d Cu (tv) + H 2 O (g); υ = k.
According to the kinetic equations, it is possible to calculate how the reaction rate changes with a change in the concentration of the reactants.
Influence of the catalyst.
5. Reaction temperature. Theory of active collisions
In order for an elementary act of chemical interaction to take place, the reacting particles must collide with each other. However, not every collision results in a chemical interaction. Chemical interaction occurs when particles approach at distances at which the redistribution of electron density and the emergence of new chemical bonds are possible. Interacting particles must have enough energy to overcome the repulsive forces that arise between their electron shells.
transition state- the state of the system, in which the destruction and creation of a connection are balanced. The system is in the transition state for a short (10 -15 s) time. The energy required to bring the system into a transition state is called activation energy. In multistep reactions that include several transition states, the activation energy corresponds to the highest energy value. After overcoming the transition state, the molecules fly apart again with the destruction of old bonds and the formation of new ones or with the transformation of the original bonds. Both options are possible, as they occur with the release of energy. There are substances that can reduce the activation energy for a given reaction.
Active molecules A 2 and B 2 upon collision combine into an intermediate active complex A 2 ... B 2 with weakening and then breaking of the A-A and B-B bonds and strengthening of the A-B bonds.
The "activation energy" of the NI formation reaction (168 kJ/mol) is much less than the energy required to completely break the bond in the initial H 2 and I 2 molecules (571 kJ/mol). Therefore, the reaction path through the formation active (activated) complex energetically more favorable than the path through the complete breaking of bonds in the original molecules. The vast majority of reactions occur through the formation of intermediate active complexes. The provisions of the active complex theory were developed by G. Eyring and M. Polyani in the 30s of the XX century.
Activation energy represents the excess of the kinetic energy of the particles relative to the average energy required for the chemical transformation of the colliding particles. Reactions are characterized by different values of activation energy (E a). In most cases, the activation energy of chemical reactions between neutral molecules ranges from 80 to 240 kJ/mol. For biochemical processes values E a often lower - up to 20 kJ / mol. This can be explained by the fact that the vast majority of biochemical processes proceed through the stage of enzyme-substrate complexes. Energy barriers limit the reaction. Due to this, in principle, possible reactions (at Q< 0) практически всегда не протекают или замедляются. Реакции с энергией активации выше 120 кДж/моль настолько медленны, что их протекание трудно заметить.
In order for a reaction to occur, the molecules must be oriented in a certain way and have sufficient energy upon collision. The probability of proper orientation in a collision is characterized by activation entropy ∆S a. The redistribution of the electron density in the active complex is favored by the condition that, upon collision, the molecules A 2 and B 2 are oriented, as shown in Fig. 3a, while with the orientation shown in Fig. 3b, the reaction probability is still much less - in Fig. 3c.
Rice. Fig. 3. Favorable (a) and unfavorable (b, c) orientations of A 2 and B 2 molecules upon collision
The equation characterizing the dependence of the rate and reaction on temperature, activation energy and activation entropy has the form:
(10)
where k- reaction rate constant;
BUT- in the first approximation, the total number of collisions between molecules per unit time (second) per unit volume;
e- base of natural logarithms;
R- universal gas constant;
T- absolute temperature;
E a- activation energy;
∆S a- change in entropy of activation.
Equation (11) was derived by Arrhenius in 1889. Preexponential multiplier BUT proportional to the total number of collisions between molecules per unit time. Its dimension coincides with the dimension of the rate constant and depends on the total order of the reaction.
Exhibitor is equal to the fraction of active collisions from their total number, i.e. the colliding molecules must have sufficient interaction energy. The probability of their desired orientation at the moment of impact is proportional to .
When discussing the law of mass action for velocity (9), it was specially stipulated that the rate constant is a constant value that does not depend on the concentrations of reagents. It was assumed that all chemical transformations proceed at a constant temperature. At the same time, the rate of chemical transformation can change significantly with a decrease or increase in temperature. From the point of view of the law of mass action, this change in velocity is due to the temperature dependence of the rate constant, since the concentrations of the reactants change only slightly due to thermal expansion or contraction of the liquid.
The most well known fact is that the rate of reactions increases with increasing temperature. This type of temperature dependence of velocity is called normal (Fig. 3a). This type of dependence is characteristic of all simple reactions.
Rice. 3. Types of temperature dependence of the rate of chemical reactions: a - normal;
b - abnormal; c - enzymatic
However, at present, chemical transformations are well known, the rate of which decreases with increasing temperature; this type of temperature dependence of the rate is called anomalous . An example is the gas-phase reaction of nitrogen (II) oxide with bromine (Fig. 3b).
Of particular interest to physicians is the temperature dependence of the rate of enzymatic reactions, i.e. reactions involving enzymes. Almost all reactions occurring in the body belong to this class. For example, in the decomposition of hydrogen peroxide in the presence of the enzyme catalase, the rate of decomposition depends on temperature. In the range 273-320 To temperature dependence is normal. As the temperature increases, the speed increases, and as the temperature decreases, it decreases. When the temperature rises above 320 To there is a sharp anomalous drop in the peroxide decomposition rate. A similar picture takes place for other enzymatic reactions (Fig. 3c).
From the Arrhenius equation for k it is clear that, since T included in the exponent, the rate of a chemical reaction is very sensitive to changes in temperature. The dependence of the rate of a homogeneous reaction on temperature can be expressed by the van't Hoff rule, according to which with an increase in temperature for every 10 °, the reaction rate increases by 2-4 times; the number showing how many times the rate of a given reaction increases with an increase in temperature by 10 ° is called temperature coefficient of the reaction rate -γ.
This rule is mathematically expressed by the following formula:
(12)
where γ is the temperature coefficient, which shows how many times the reaction rate increases with an increase in temperature by 10 0; υ 1 -t 1 ; υ 2 - reaction rate at temperature t2.
As the temperature rises in an arithmetic progression, the speed increases exponentially.
For example, if γ = 2.9, then with an increase in temperature by 100 ° the reaction rate increases by a factor of 2.9 10, i.e. 40 thousand times. Deviations from this rule are biochemical reactions, the rate of which increases tenfold with a slight increase in temperature. This rule is valid only in a rough approximation. Reactions involving large molecules (proteins) are characterized by a large temperature coefficient. The rate of protein denaturation (ovalbumin) increases 50 times with a temperature increase of 10 °C. After reaching a certain maximum (50-60 °C), the reaction rate decreases sharply as a result of thermal denaturation of the protein.
For many chemical reactions, the law of mass action for velocity is unknown. In such cases, the following expression can be used to describe the temperature dependence of the conversion rate:
pre-exponent A with does not depend on temperature, but depends on concentration. The unit of measure is mol/l∙s.
The theoretical dependence makes it possible to pre-calculate the velocity at any temperature if the activation energy and the pre-exponential are known. Thus, the effect of temperature on the rate of chemical transformation is predicted.
Complex reactions
The principle of independence. Everything discussed above referred to relatively simple reactions, but so-called complex reactions are often encountered in chemistry. These reactions include those discussed below. When deriving the kinetic equations for these reactions, the principle of independence is used: if several reactions take place in the system, then each of them is independent of the others and its rate is proportional to the product of the concentrations of its reactants.
Parallel Reactions are reactions that take place simultaneously in several directions.
The thermal decomposition of potassium chlorate occurs simultaneously in two reactions: 
Successive reactions are reactions that proceed in several stages. There are many such reactions in chemistry.
.
Associated reactions. If several reactions take place in the system and one of them cannot occur without the other, then these reactions are called conjugated , and the phenomenon itself by induction .
2HI + H 2 CrO 4 → I 2 + Cr 2 O 3 + H 2 O.
This reaction is practically not observed under normal conditions, but if FeO is added to the system, then the following reaction occurs:
FeO + H 2 CrO 4 → Fe 2 O 3 + Cr 2 O 3 + H 2 O
and the first reaction goes along with it. The reason for this is the formation in the second reaction of intermediate products involved in the first reaction:
FeO 2 + H 2 CrO 4 → Cr 2 O 3 + Fe 5+;
HI + Fe 5+ → Fe 2 O 3 + I 2 + H 2 O.
Chemical induction- a phenomenon in which one chemical reaction (secondary) depends on another (primary).
A+ AT- primary reaction,
A + C- secondary reaction,
then A is an activator, AT- inductor, C - acceptor.
During chemical induction, in contrast to catalysis, the concentrations of all participants in the reaction decrease.
Induction factor is determined from the following equation:
.
Depending on the value of the induction factor, the following cases are possible.
I> 0 - fading process. The reaction rate decreases with time.
I < 0 - ускоряющийся процесс. Скорость реакции увеличивается со временем.
The phenomenon of induction is important because in some cases the energy of the primary reaction can compensate for the energy expended in the secondary reaction. For this reason, for example, it is thermodynamically possible to synthesize proteins by polycondensation of amino acids.
Chain reactions. If a chemical reaction proceeds with the formation of active particles (ions, radicals), which, entering into subsequent reactions, cause the appearance of new active particles, then such a sequence of reactions is called chain reaction.
The formation of free radicals is associated with the expenditure of energy to break bonds in a molecule. This energy can be imparted to molecules by illumination, electric discharge, heating, irradiation with neutrons, α- and β-particles. To carry out chain reactions at low temperatures, initiators are introduced into the reacting mixture - substances that easily form radicals: sodium vapor, organic peroxides, iodine, etc.
The reaction of the formation of hydrogen chloride from simple compounds, activated by light.
Total reaction:
H 2 + C1 2 2HC1.
Separate stages:
Сl 2 2Сl∙ photoactivation of chlorine (initiation)
Cl ∙ + H 2 \u003d Hcl + H ∙ chain development
H ∙ + Cl 2 \u003d Hcl + Cl ∙, etc.
H ∙ + Cl ∙ \u003d Hcl open circuit
Here H∙ and Сl∙ are active particles (radicals).
Three groups of elementary steps can be distinguished in this reaction mechanism. The first is a photochemical reaction chain origin. Chlorine molecules, having absorbed a quantum of light, dissociate into free atoms with a high reactivity. Thus, when a chain is nucleated, free atoms or radicals are formed from valence-saturated molecules. The chain generation process is also called initiation. Chlorine atoms, having unpaired electrons, are able to react with molecular hydrogen, forming molecules of hydrogen chloride and atomic hydrogen. Atomic hydrogen, in turn, interacts with a chlorine molecule, as a result of which a hydrogen chloride molecule and atomic chlorine are again formed, etc.
These processes, characterized by the repetition of the same elementary stages (links) and proceeding with the preservation of free radicals, lead to the consumption of starting substances and the formation of reaction products. These groups of reactions are called reactions of development (or continuation) of the chain.
The step in the chain reaction in which free radicals are destroyed is called chain break. Chain termination can occur as a result of the recombination of free radicals, if the energy released in this case can be given to some third body: the vessel wall or molecules of inert impurities (stages 4, 5). That is why the rate of chain reactions is very sensitive to the presence of impurities, to the shape and dimensions of the vessel, especially at low pressures.
The number of elementary links from the moment the chain is born to its break is called the chain length. In the example under consideration, up to 10 5 HCl molecules are formed for each light quantum.
Chain reactions, during which there is no "multiplication" of the number of free radicals, are called unbranched or simple chain reactions . In each elementary stage of the unbranched chain process, one radical "gives birth" to one molecule of the reaction product and only one new radical (Fig. 41).
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Other examples of simple chain reactions: a) chlorination of paraffinic hydrocarbons Cl ∙ + CH 4 → CH 3 ∙ + HC1; CH 3 ∙ + Cl - → CH 3 Cl + Cl ∙ etc.; b) radical polymerization reactions, for example, polymerization of vinyl acetate in the presence of benzoyl peroxide, which easily decomposes into radicals; c) the interaction of hydrogen with bromine, proceeding according to a mechanism similar to the reaction of chlorine with hydrogen, only with a shorter chain length due to its endothermicity.
If two or more active particles appear as a result of the act of growth, then this chain reaction is branched.
In 1925, N. N. Semenov and his collaborators discovered reactions containing elementary stages, as a result of which not one, but several chemically active particles, atoms, or radicals, arise. The appearance of several new free radicals leads to the appearance of several new chains, i.e. one chain forks. Such processes are called branched chain reactions (Fig. 42).
An example of a highly branched chain process is the oxidation of hydrogen at low pressures and a temperature of about 900°C. The reaction mechanism can be written as follows.
1. H 2 + O 2 OH∙ + OH∙ chain initiation
2. OH ∙ + H 2 → H 2 O + H ∙ chain development
3. H ∙ + O 2 → OH ∙ + O: chain branching
4. O: + H 2 → OH ∙ + H ∙
5. OH ∙ + H 2 → H 2 O + H ∙ chain continuation
6. H∙ + H∙ + wall → H 2 open circuit on the vessel wall
7. H ∙ + O 2 + M → HO 2 ∙ + M chain termination in bulk.
M is an inert molecule. The HO 2 ∙ radical, which is formed during a triple collision, is inactive and cannot continue the chain.
At the first stage of the process, hydroxyl radicals are formed, which provide the development of a simple chain. In the third stage, as a result of interaction with the initial molecule of one radical, two radicals are formed, and the oxygen atom has two free valences. This provides branching of the chain.
As a result of chain branching, the reaction rate rapidly increases in the initial period of time, and the process ends with chain ignition-explosion. However, branched chain reactions end in an explosion only when the branching rate is greater than the chain termination rate. Otherwise, the process is slow.
When the reaction conditions change (changes in pressure, temperature, mixture composition, size and condition of the walls of the reaction vessel, etc.), a transition from a slow reaction to an explosion can occur and vice versa. Thus, in chain reactions there are limiting (critical) states in which chain ignition occurs, from which one should distinguish thermal ignition that occurs in exothermic reactions as a result of ever-increasing heating of the reacting mixture with poor heat removal.
According to the branched chain mechanism, oxidized vapors of sulfur, phosphorus, carbon monoxide (II), carbon disulfide, etc. occur.
The modern theory of chain processes was developed by the Nobel Prize winners (1956) Soviet academician N. N. Semenov and the English scientist Hinshelwood.
Chain reactions should be distinguished from catalytic reactions, although the latter are also cyclic in nature. The most significant difference between chain reactions and catalytic ones is that with a chain mechanism, the reaction can proceed in the direction of increasing the energy of the system due to spontaneous reactions. A catalyst does not cause a thermodynamically impossible reaction. In addition, in catalytic reactions there are no such process steps as chain nucleation and chain termination.
polymerization reactions. A special case of a chain reaction is the polymerization reaction.
Polymerization is a process in which the reaction of active particles (radicals, ions) with low molecular weight compounds (monomers) is accompanied by the sequential addition of the latter with an increase in the length of the material chain (the length of the molecule), i.e., with the formation of a polymer.
Monomers are organic compounds, as a rule, containing unsaturated (double, triple) bonds in the composition of the molecule.
The main stages of the polymerization process:
1. Initiation(under the action of light, heat, etc.):
A: A→A" + A"- homolytic decomposition with the formation of radicals (active valence-unsaturated particles).
A: B→ A - + B +- heterolytic decomposition with the formation of ions.
2. Chain growth: A "+ M→ AM"
(or A - + M→ AM", or AT + + M→ VM +).
3. Open circuit: AM" + AM"→ polymer
(or AM" + B +→ polymer, VM + + A"→ polymer).
The speed of a chain process is always greater than that of a non-chain process.
Objective: the study of the rate of a chemical reaction and its dependence on various factors: the nature of the reactants, concentration, temperature.
Chemical reactions proceed at different rates. The rate of a chemical reaction is called the change in the concentration of the reactant per unit time. It is equal to the number of interaction acts per unit time per unit volume for a reaction occurring in a homogeneous system (for homogeneous reactions), or per unit interface for reactions occurring in a heterogeneous system (for heterogeneous reactions).
Average reaction rate v cf. in the time interval from t1 before t2 is determined by the relation:
where From 1 and From 2 is the molar concentration of any participant in the reaction at time points t1 and t2 respectively.
The “–“ sign in front of the fraction refers to the concentration of the starting substances, Δ FROM < 0, знак “+” – к концентрации продуктов реакции, ΔFROM > 0.
The main factors affecting the rate of a chemical reaction are: the nature of the reactants, their concentration, pressure (if gases are involved in the reaction), temperature, catalyst, interface area for heterogeneous reactions.
Most chemical reactions are complex processes that occur in several stages, i.e. consisting of several elementary processes. Elementary or simple reactions are reactions that occur in one stage.
For elementary reactions, the dependence of the reaction rate on concentration is expressed by the law of mass action.
At a constant temperature, the rate of a chemical reaction is directly proportional to the product of the concentrations of reactants, taken in powers equal to stoichiometric coefficients.
For a general reaction
a A + b B ... → c C,
according to the law of mass action v is expressed by the relation
v = K∙s(A) a ∙ c(B) b,
where c(A) and c(B) are the molar concentrations of reactants A and B;
To is the rate constant of this reaction, equal to v, if c(A) a=1 and c(B) b=1, and depending on the nature of the reactants, temperature, catalyst, surface area of the interface for heterogeneous reactions.
Expressing the dependence of the reaction rate on concentration is called the kinetic equation.
In the case of complex reactions, the law of mass action applies to each individual step.
For heterogeneous reactions, the kinetic equation includes only the concentrations of gaseous and dissolved substances; yes, for burning coal
C (c) + O 2 (g) → CO 2 (g)
the velocity equation has the form
v \u003d K s (O 2)
A few words about the molecularity and kinetic order of the reaction.
concept "molecularity of the reaction" apply only to simple reactions. The molecularity of a reaction characterizes the number of particles participating in an elementary interaction.
There are mono-, bi- and trimolecular reactions, in which one, two and three particles, respectively, participate. The probability of simultaneous collision of three particles is small. The elementary process of interaction of more than three particles is unknown. Examples of elementary reactions:
N 2 O 5 → NO + NO + O 2 (monomolecular)
H 2 + I 2 → 2HI (bimolecular)
2NO + Cl 2 → 2NOCl (trimolecular)
The molecularity of simple reactions coincides with the overall kinetic order of the reaction. The order of the reaction determines the nature of the dependence of the rate on the concentration.
The overall (total) kinetic order of a reaction is the sum of the exponents at the concentrations of the reactants in the reaction rate equation, determined experimentally.
As the temperature rises, the rate of most chemical reactions increases. The dependence of the reaction rate on temperature is approximately determined by the van't Hoff rule.
For every 10 degrees increase in temperature, the rate of most reactions increases by a factor of 2–4.
where and are the reaction rates, respectively, at temperatures t2 and t1 (t2>t1);
γ is the temperature coefficient of the reaction rate, this is a number showing how many times the rate of a chemical reaction increases with an increase in temperature by 10 0.
Using the van't Hoff rule, it is only possible to approximately estimate the effect of temperature on the reaction rate. A more accurate description of the dependence of the temperature reaction rate is feasible within the framework of the Arrhenius activation theory.
One of the methods of accelerating a chemical reaction is catalysis, which is carried out with the help of substances (catalysts).
Catalysts- these are substances that change the rate of a chemical reaction due to repeated participation in the intermediate chemical interaction with the reaction reagents, but after each cycle of the intermediate interaction they restore their chemical composition.
The mechanism of action of the catalyst is reduced to a decrease in the activation energy of the reaction, i.e. a decrease in the difference between the average energy of the active molecules (active complex) and the average energy of the molecules of the starting substances. This increases the rate of the chemical reaction.
Chemical reactions proceed at different speeds: at a low speed - during the formation of stalactites and stalagmites, at an average speed - when cooking food, instantly - during an explosion. Reactions in aqueous solutions are very fast.
Determination of the rate of a chemical reaction, as well as elucidation of its dependence on the conditions of the process, is the task of 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 are called homogeneous.
(v homog) is defined as the change in the amount of substance per unit time per unit volume:
where Δn is the change in the number of moles of one substance (most often the initial one, but it can also be the reaction product); Δt - time interval (s, min); V is the volume of gas or solution (l).
Since the ratio of the amount of substance to volume is the molar concentration C, then
Thus, the rate of a homogeneous reaction is defined as a change in the concentration of one of the substances per unit time:
if the volume of the system does not change.
If a reaction occurs between substances in different states of aggregation (for example, between 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 substances. Such reactions are called heterogeneous.
It is defined as the change in the amount of substance per unit of time per unit of surface.
where S is the surface area of contact of substances (m 2, cm 2).
A change in the amount of a substance by which the 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 to scatter like balls in different directions, but in such a way that the “old bonds” in the particles are destroyed or weakened and “new ones” can form. ”, and for this the particles must have sufficient energy.
The calculated data show that, for example, in gases, collisions of molecules at atmospheric pressure are in the billions per 1 second, that is, all reactions should have gone instantly. But it's not. It turns out that only a very small fraction of the molecules have the necessary energy to produce an effective collision.
The minimum excess energy that a particle (or pair of particles) must have in order for an effective collision to occur is called activation energy Ea.
Thus, on the way of all particles entering into the reaction, there is an energy barrier equal to the activation energy E a . When it is small, there are many particles that can overcome it, and the reaction rate is high. Otherwise, a "push" is required. When you bring a match to light a spirit lamp, you impart additional energy, E a , necessary for the effective collision of alcohol molecules with oxygen molecules (overcoming the barrier).
The rate of a chemical reaction depends on many factors. The main ones are: the nature and concentration of the reactants, pressure (in reactions involving gases), temperature, the action of catalysts and the surface of the reactants in the case of heterogeneous reactions.
Temperature
As the temperature rises, in most cases the rate of a chemical reaction increases significantly. In the 19th century Dutch chemist J. X. Van't Hoff formulated the rule:
An increase in temperature for every 10 ° C leads to an increase inreaction speed by 2-4 times(this value is called the temperature coefficient of the reaction).
With an increase in temperature, the average velocity of molecules, their energy, and the number of collisions increase slightly, but the proportion of "active" molecules participating in effective collisions that overcome the energy barrier of the reaction increases sharply. Mathematically, this dependence is expressed by the relation:
where v t 1 and v t 2 are the reaction rates 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 each 10 ° C increase in temperature.
However, to increase the reaction rate, raising the temperature is not always applicable, since the starting materials may begin to decompose, solvents or the substances themselves may evaporate, etc.
Endothermic and exothermic reactions
The reaction of methane with atmospheric oxygen is known to be accompanied by the release of a large amount of heat. Therefore, it is used in everyday life for cooking, heating water and heating. Natural gas supplied to homes through pipes is 98% methane. The reaction of calcium oxide (CaO) with water is also accompanied by the release of a large amount of heat.
What can these facts say? When new chemical bonds are formed in the reaction products, more energy than required to break the chemical bonds in the reactants. Excess energy is released in the form of heat and sometimes light.
CH 4 + 2O 2 \u003d CO 2 + 2H 2 O + Q (energy (light, heat));
CaO + H 2 O \u003d Ca (OH) 2 + Q (energy (heat)).
Such reactions should proceed easily (as a stone easily rolls downhill).
Reactions in which energy is released are called EXOTHERMIC(from the Latin "exo" - out).
For example, many redox reactions are exothermic. One of these beautiful reactions is an intramolecular oxidation-reduction occurring inside the same salt - ammonium dichromate (NH 4) 2 Cr 2 O 7:
(NH 4) 2 Cr 2 O 7 \u003d N 2 + Cr 2 O 3 + 4 H 2 O + Q (energy).
Another thing is the backlash. They are similar to rolling a stone uphill. It is still not possible to obtain methane from CO 2 and water, and strong heating is required to obtain quicklime CaO from calcium hydroxide Ca (OH) 2. Such a reaction occurs only with a constant influx of energy from the outside:
Ca (OH) 2 \u003d CaO + H 2 O - Q (energy (heat))
This suggests that the breaking of chemical bonds in Ca(OH) 2 requires more energy than can be released during the formation of new chemical bonds in CaO and H 2 O molecules.
Reactions in which energy is absorbed are called ENDOTHERMIC(from "endo" - inside).
Reactant concentration
A change in pressure with the participation of gaseous substances in the reaction also leads to a change in the concentration of these substances.
In order for a chemical interaction to occur between particles, they must effectively collide. The greater the concentration of reactants, the more collisions and, accordingly, the higher the reaction rate. For example, acetylene burns very quickly in pure oxygen. This develops a temperature sufficient to melt the metal. On the basis of a large amount of experimental material, in 1867 the Norwegians K. Guldenberg and P. Waage, and independently of them in 1865, the Russian scientist N. I. Beketov formulated the basic law of chemical kinetics, which establishes 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 mass action.
For the reaction A + B \u003d D, this law will be expressed as follows:
For the reaction 2A + B = D, this law is expressed as follows:
Here C A, C B are the concentrations of substances A and B (mol / l); k 1 and k 2 - coefficients of proportionality, called the rate constants of the reaction.
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 reactants are 1 mol / l or their product is equal to one. In this case, it is clear that the rate constant of the reaction depends only on temperature and does not depend on the concentration of substances.
Law of acting masses does not take into account the concentration of reactants in the solid state, since they react on surfaces and their concentrations are usually constant.
For example, for the combustion reaction of coal, the expression for the reaction rate should be written as follows:
i.e., the reaction rate is only proportional to the oxygen concentration.
If the reaction equation describes only the overall 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 dependence 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 Latin katalysis - destruction).
The catalyst acts as an experienced guide, guiding a group of tourists not through a high pass in the mountains (overcoming it requires a lot of effort and time and is not accessible to everyone), but along the detour paths known to him, along which you can overcome the mountain much easier and faster.
True, on a detour you can get not quite where the main pass leads. But sometimes that's exactly what you need! This is how catalysts, which are called selective, work. It is clear that there is no need to burn ammonia and nitrogen, but nitric oxide (II) finds use in the production of nitric acid.
Catalysts- These are substances that participate in a chemical reaction and change its speed or direction, but at the end of the reaction 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 in car exhaust gases into harmless nitrogen).
There are two types of catalysis.
homogeneous catalysis, in which both the catalyst and the reactants are in the same state of aggregation (phase).
heterogeneous catalysis where the catalyst and reactants are in different phases. For example, the decomposition of hydrogen peroxide in the presence of a solid manganese (IV) oxide catalyst:
The catalyst itself is not consumed as a result of the reaction, but if other substances are adsorbed on its surface (they are called catalytic poisons), then the surface becomes inoperable, and catalyst regeneration 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 2 O 5:
In the production of methanol, a solid "zinc-chromium" catalyst is used (8ZnO Cr 2 O 3 x CrO 3):
Biological catalysts - enzymes - work very effectively. By chemical nature, these are proteins. Thanks to them, complex chemical reactions proceed at a high speed in living organisms at low temperatures.
Other interesting substances are known - inhibitors (from the Latin inhibere - to delay). They react with active particles at a high rate to form inactive compounds. As a result, the reaction slows down sharply and then stops. Inhibitors are often specifically added to various substances in order to prevent unwanted processes.
For example, hydrogen peroxide solutions are stabilized with inhibitors.
The nature of the reactants (their composition, structure)
Meaning activation energy is the factor through which the influence of the nature of the reacting substances on the reaction rate is affected.
If the activation energy is low (< 40 кДж/моль), то это означает, что значительная часть столкновений между частицами реагирующих веществ приводит к их взаимодействию, и скорость такой реакции очень большая. Все реакции ионного обмена протекают практически мгновенно, ибо в этих реакциях участвуют разноименно заряженные ионы, и энергия активации в данных случаях ничтожно мала.
If the activation energy is high(> 120 kJ/mol), this means that only a negligible part of the collisions between interacting particles leads to a reaction. The rate of such a reaction is therefore very slow. For example, the progress of the ammonia synthesis reaction at ordinary temperature is almost impossible to notice.
If the activation energies of chemical reactions have intermediate values (40120 kJ/mol), then the rates of such reactions will be average. Such reactions include the interaction of sodium with water or ethyl alcohol, the decolorization of bromine 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 powdered chalk dissolves much faster in hydrochloric acid than an equal mass piece of chalk.
The increase in the reaction rate is primarily due to increase in the contact surface of the starting substances, as well as a number of other reasons, for example, a violation of the structure of the "correct" crystal lattice. This leads to the fact that the 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 with the help of a "fluidized bed", pyrite is roasted.
Reference material for passing the test:
periodic table
Solubility table
When defining the concept chemical reaction rate it is necessary to distinguish between homogeneous and heterogeneous reactions. If the reaction proceeds in a homogeneous system, for example, in a solution or in a mixture of gases, then it takes place in the entire volume of the system. The rate of a homogeneous reaction called the amount of a substance that enters into a reaction or is formed as a result of a reaction per unit of time in a unit volume of the system. Since the ratio of the number of moles of a substance to the volume in which it is distributed is the molar concentration of the substance, the rate of a homogeneous reaction can also be defined as change in the concentration per unit time of any of the substances: the initial reagent or reaction product. To ensure that the result of the calculation is always positive, regardless of whether it is produced by a reagent or a product, the “±” sign is used in the formula:
Depending on the nature of the reaction, time can be expressed not only in seconds, as required by the SI system, but also in minutes or hours. During the reaction, the value of its rate is not constant, but continuously changes: it decreases, since the concentrations of the starting substances decrease. The above calculation gives the average value of the reaction rate over a certain time interval Δτ = τ 2 – τ 1 . The true (instantaneous) speed is defined as the limit to which the ratio Δ FROM/ Δτ at Δτ → 0, i.e. the true velocity is equal to the time derivative of the concentration.
For a reaction whose equation contains stoichiometric coefficients that differ from unity, the rate values expressed for different substances are not the same. For example, for the reaction A + 3B \u003d D + 2E, the consumption of substance A is one mole, substance B is three moles, the arrival of substance E is two moles. That's why υ (A) = ⅓ υ (B) = υ (D)=½ υ (E) or υ (E) . = ⅔ υ (AT) .
If a reaction proceeds between substances that are in different phases of a heterogeneous system, then it can only take place at the interface between these phases. For example, the interaction of an acid solution and a piece of metal occurs only on the surface of the metal. The rate of a heterogeneous reaction called the amount of a substance that enters into a reaction or is formed as a result of a reaction per unit of time per unit of the interface between phases:
.
The dependence of the rate of a chemical reaction on the concentration of reactants is expressed by the law of mass action: at a constant temperature, the rate of a chemical reaction is directly proportional to the product of the molar concentrations of the reactants raised to powers equal to the coefficients in the formulas of these substances in the reaction equation. Then for the reaction
2A + B → products
the ratio υ ~ · FROM A 2 FROM B, and for the transition to equality, the coefficient of proportionality is introduced k, called reaction rate constant:
υ = k· FROM A 2 FROM B = k[A] 2 [V]
(molar concentrations in formulas can be denoted as the letter FROM with the corresponding index, and the formula of the substance enclosed in square brackets). The physical meaning of the reaction rate constant is the reaction rate at concentrations of all reactants equal to 1 mol/L. The dimension of the reaction rate constant depends on the number of factors on the right side of the equation and can be from -1; s –1 (l/mol); s –1 (l 2 / mol 2), etc., that is, such that in any case, in calculations, the reaction rate is expressed in mol l –1 s –1.
For heterogeneous reactions, the equation of the law of mass action includes the concentrations of only those substances that are in the gas phase or in solution. The concentration of a substance in the solid phase is a constant value and is included in the rate constant, for example, for the combustion process of coal C + O 2 = CO 2, the law of mass action is written:
υ = k I const = k·,
where k= k I const.
In systems where one or more substances are gases, the reaction rate also depends on pressure. For example, when hydrogen interacts with iodine vapor H 2 + I 2 \u003d 2HI, the rate of a chemical reaction will be determined by the expression:
υ = k··.
If the pressure is increased, for example, 3 times, then the volume occupied by the system will decrease by the same amount, and, consequently, the concentrations of each of the reacting substances will increase by the same amount. The rate of reaction in this case will increase by 9 times
Temperature dependence of the reaction rate is described by the van't Hoff rule: for every 10 degrees increase in temperature, the reaction rate increases by 2-4 times. This means that as the temperature increases exponentially, the rate of a chemical reaction increases exponentially. The base in the progression formula is reaction rate temperature coefficientγ, showing how many times the rate of a given reaction increases (or, what is the same, the rate constant) with an increase in temperature by 10 degrees. Mathematically, the van't Hoff rule is expressed by the formulas:
or 
where and are the reaction rates, respectively, at the initial t 1 and final t 2 temperatures. Van't Hoff's rule can also be expressed as follows:
; 
; 
; 
,
where and are, respectively, the rate and rate constant of the reaction at a temperature t; and are the same values at temperature t +10n; n is the number of “ten-degree” intervals ( n =(t 2 –t 1)/10) by which the temperature has changed (can be an integer or fractional number, positive or negative).
Examples of problem solving
Example 1 How will the rate of the reaction 2СО + О 2 = 2СО 2 proceeding in a closed vessel change if the pressure is doubled?
Solution:
The rate of the specified chemical reaction is determined by the expression:
υ start = k· [CO] 2 · [O 2 ].
An increase in pressure leads to an increase in the concentration of both reagents by a factor of 2. With this in mind, we rewrite the expression for the law of mass action:
υ 1 = k 2 = k 2 2 [CO] 2 2 [O 2] \u003d 8 k[CO] 2 [O 2] \u003d 8 υ early
Answer: The reaction rate will increase by 8 times.
Example 2 Calculate how many times the reaction rate will increase if the temperature of the system is raised from 20 °C to 100 °C, assuming the value of the temperature coefficient of the reaction rate to be 3.
Solution:
The ratio of reaction rates at two different temperatures is related to the temperature coefficient and temperature change by the formula:
Calculation:
Answer: The reaction rate will increase by 6561 times.
Example 3 When studying the homogeneous reaction A + 2B = 3D, it was found that within 8 minutes of the reaction, the amount of substance A in the reactor decreased from 5.6 mol to 4.4 mol. The volume of the reaction mass was 56 liters. Calculate the average rate of a chemical reaction for the studied period of time for substances A, B and D.
Solution:
We use the formula in accordance with the definition of the concept of "average rate of a chemical reaction" and substitute the numerical values, obtaining the average rate for reagent A:
It follows from the reaction equation that, compared with the rate of loss of substance A, the rate of loss of substance B is twice as large, and the rate of increase in the amount of product D is three times greater. Consequently:
υ (A) = ½ υ (B)=⅓ υ (D)
and then υ (B) = 2 υ (A) \u003d 2 2.68 10 -3 \u003d 6. 36 10 -3 mol l -1 min -1;
υ (D)=3 υ (A) = 3 2.68 10 -3 = 8.04 10 -3 mol l -1 min -1
Answer: u(A) = 2.68 10 -3 mol l -1 min -1; υ (B) = 6.36 10–3 mol l–1 min–1; υ (D) = 8.04 10–3 mol l–1 min–1.
Example 4 To determine the rate constant of the homogeneous reaction A + 2B → products, two experiments were carried out at different concentrations of substance B and the reaction rate was measured.
The rate of a chemical reaction is understood as a change in the concentration of one of the reacting substances per unit time with a constant volume of the system.
Typically, concentration is expressed in mol/L and time in seconds or minutes. If, for example, the initial concentration of one of the reactants was 1 mol / l, and after 4 s from the start of the reaction it became 0.6 mol / l, then the average reaction rate will be equal to (1-0.6) / 4 \u003d 0, 1 mol/(l*s).
The average reaction rate is calculated by the formula:
The rate of a chemical reaction depends on:
The nature of the reactants.
Substances with a polar bond in solutions interact faster, this is due to the fact that such substances in solutions form ions that easily interact with each other.
Substances with non-polar and low-polar covalent bonds react at different rates, this depends on their chemical activity.
H 2 + F 2 = 2HF (goes very fast with an explosion at room temperature)
H 2 + Br 2 \u003d 2HBr (goes slowly, even when heated)
Surface contact values of reactants (for heterogeneous)
Reactant concentrations
The reaction rate is directly proportional to the product of the concentrations of the reactants raised to the power of their stoichiometric coefficients.
Temperatures
The dependence of the reaction rate on temperature is determined by the van't Hoff rule:
with an increase in temperature for every 10 0 the rate of most reactions increases by 2-4 times.
The presence of a catalyst
Catalysts are substances that change the rate of chemical reactions.
The change in the rate of a reaction in the presence of a catalyst is called catalysis.
Pressure
With an increase in pressure, the reaction rate increases (for homogeneous)
Question number 26. Mass action law. Speed constant. Activation energy.
Mass action law.
the rate at which substances react with each other depends on their concentration
Speed constant.
coefficient of proportionality in the kinetic equation of a chemical reaction, expressing the dependence of the reaction rate on concentration
The rate constant depends on the nature of the reactants and on the temperature, but does not depend on their concentrations.
Activation energy.
the energy that must be imparted to the molecules (particles) of reacting substances in order to turn them into active
The activation energy depends on the nature of the reactants and changes in the presence of a catalyst.
An increase in concentration increases the total number of molecules, and, accordingly, active particles.
Question number 27. Reversible and irreversible reactions. Chemical equilibrium, equilibrium constant. Le Chatelier's principle.
Reactions that proceed only in one direction and end with the complete transformation of the starting materials into the final ones are called irreversible.
Reversible reactions are those that simultaneously proceed in two mutually opposite directions.
In the equations of reversible reactions, two arrows pointing in opposite directions are placed between the left and right sides. An example of such a reaction is the synthesis of ammonia from hydrogen and nitrogen:
3H 2 + N 2 \u003d 2NH 3
Irreversible are such reactions, during the course of which:
The resulting products precipitate, or are released as a gas, for example:
BaCl 2 + H 2 SO 4 \u003d BaSO 4 + 2HCl
Na 2 CO 3 + 2HCl \u003d 2NaCl + CO 2 + H 2 O
Water formation:
HCl + NaOH = H 2 O + NaCl
Reversible reactions do not reach the end and end with the establishment chemical equilibrium.
Chemical equilibrium is the state of a system of reacting substances in which the rates of the forward and reverse reactions are equal.
The state of chemical equilibrium is influenced by the concentration of reacting substances, temperature, and for gases - pressure. When one of these parameters changes, the chemical equilibrium is disturbed.
Equilibrium constant.
The most important parameter characterizing a reversible chemical reaction is the equilibrium constant K. If we write for the considered reversible reaction A + D C + D the condition of equality of the rates of the forward and reverse reactions in the equilibrium state - k1[A]equal[B]equal = k2[C]equal[ D] equals, whence [C] equals [D] equals / [A] equals [B] equals = k1/k2 = K, then the value of K is called the equilibrium constant of a chemical reaction.
So, at equilibrium, the ratio of the concentration of reaction products to the product of the concentration of reactants is constant if the temperature is constant (the rate constants k1 and k2 and, consequently, the equilibrium constant K depend on temperature, but do not depend on the concentration of reactants). If several molecules of starting substances participate in the reaction and several molecules of the product (or products) are formed, the concentrations of substances in the expression for the equilibrium constant are raised to the powers corresponding to their stoichiometric coefficients. So for the reaction 3H2 + N2 2NH3, the expression for the equilibrium constant is written as K = 2 equal / 3 equal. The described method of deriving the equilibrium constant, based on the rates of forward and reverse reactions, cannot be used in the general case, since for complex reactions the dependence of the rate on concentration is usually not expressed by a simple equation or is not known at all. Nevertheless, in thermodynamics it is proved that the final formula for the equilibrium constant turns out to be correct.
For gaseous compounds, pressure can be used instead of concentrations when writing the equilibrium constant; Obviously, the numerical value of the constant can change in this case if the number of gaseous molecules on the right and left sides of the equation is not the same.
Principle of Le Chatelier.
If an external influence is made on a system in equilibrium, then the equilibrium is shifted in the direction of the reaction that counteracts this influence.
Chemical balance is affected by:
Temperature change. As the temperature rises, the equilibrium shifts towards an endothermic reaction. As the temperature decreases, the equilibrium shifts towards an exothermic reaction.
Change in pressure. As the pressure increases, the equilibrium shifts in the direction of decreasing the number of molecules. As the pressure decreases, the equilibrium shifts in the direction of increasing the number of molecules.
