transformation

Reaction progress

Figure 6. Energy scheme for the formation of an activated complex.

Activation is the communication of such an amount of energy to molecules that, when they are effectively converted, substances are formed in an activated state.

Transformation is the formation of reaction products from substances in an activated state.

If the system cannot pass through this energy barrier, chemical transformations cannot occur in it. This means that this system is chemically inactive and needs some additional energy for activation. The amount of this extra energy depends on how much energy the system already has.

The energy of the initial system cannot be less than its zero energy (ie at 0 0 K). To activate any system, it is enough to give it additional energy. This energy is called the true activation energy.

The true activation energy of an elementary chemical act is the minimum energy that the initial system must have in excess of zero energy (ie, at 0 0 K) in order for chemical transformations to occur in it. The difference between the true activation energy of the reverse and direct reactions is equal to the thermal effect of the reaction at absolute zero.

Within the framework of the theory of absolute reaction rates, E. a. - the difference between the values ​​of the average energy of the activated complexes and the average energy of the original molecules.

Representations about E. and. originated in the 70s and 80s. 19th century as a result of the work of J. van't Hoff and S. Arrhenius, devoted to the study of the effect of temperature on the rate of a chemical reaction. Reaction rate constant k associated with E. a. ( E)the equation m Arrhenius:

k = k o e -E/RT

where R is the gas constant, T is the absolute temperature in K, k o is a constant, called the pre-exponential factor of the rate constant. This equation, based on the molecular-kinetic theory, was later obtained in statistical physics, taking into account a number of simplifying assumptions, one of which is the independence of the E. a. from temperature. For practice and for theoretical calculations in relatively narrow temperature ranges, this assumption is valid.

E. a. can be found experimentally in several ways. According to one of them, they study the kinetics of the reaction at several temperatures (for methods, see the article. The rate of a chemical reaction) and build a graph in the coordinates In k - 1/T; the tangent of the slope of the straight line on this graph, in accordance with the Arrhenius equation, is equal to E. For one-step reversible reactions (see Reversible and irreversible reactions) E. a. reactions in one of the directions (forward or reverse) can be calculated if the E. a. reactions in another and the temperature dependence of the equilibrium constant (from thermodynamic data). For more accurate calculations, one should take into account the dependence of E. a. from temperature.

E. a. complex reactions is a combination of E. a. elementary stages. Sometimes, in addition to the true E. a., determined by the Arrhenius equation, the concept of "apparent" E. a. is used. For example, if the rate constants of heterogeneous catalytic reactions are determined by changes in the volume concentrations of the starting materials and products, then the apparent E. a. differs from the true one by the magnitude of the thermal effects accompanying the processes of adsorption and desorption of reactants on the catalyst surface. In non-equilibrium systems, such as plasma-chemical systems (see Plasma chemistry) , definition of E. a. is a very difficult task. In some cases, however, a formal application of the Arrhenius equation is possible.

Energy diagram of the reaction.

The activation energy significantly affects the value of the reaction rate constant and its dependence on temperature: the larger E a, the lower the rate constant and the more significantly the change in temperature affects it.

Fig.5. Energy diagram of the reaction A + B = C + D

Chemical catalysis is the acceleration of chemical reactions under the action of small amounts of substances (catalysts). After a complete cycle of intermediate chemical interactions, the catalyst restores its chemical composition.

Catalysts are divided into homogeneous and heterogeneous. A homogeneous catalyst is in the same phase with the reactants, a heterogeneous one forms an independent phase, separated by an interface from the phase in which the reactants are located. Typical homogeneous catalysts are acids and bases. Metals, their oxides and sulfides are used as heterogeneous catalysts.

Reactions of the same type can proceed with both homogeneous and heterogeneous catalysts. Thus, along with acid solutions, solid Al 2 O 3 , TiO 2 , ThO 2 , aluminosilicates, and zeolites with acidic properties are used. Heterogeneous catalysts with basic properties: CaO, BaO, MgO.

Heterogeneous catalysts, as a rule, have a highly developed surface, for which they are distributed on an inert support (silica gel, alumina, activated carbon, etc.).

For each type of reaction, only certain catalysts are effective. In addition to the acid-base ones already mentioned, there are oxidation-reduction catalysts; they are characterized by the presence of a transition metal or its compound (Co +3, V 2 O 5 +, MoO 3). In this case, catalysis is carried out by changing the oxidation state of the transition metal.

Many reactions are carried out with the help of catalysts that act through the coordination of reactants at the atom or ion of the transition metal (Ti, Rh, Ni). Such catalysis is called coordination catalysis.

If the catalyst has chiral properties, then an optically active product is obtained from an optically inactive substrate.

In modern science and technology, systems of several catalysts are often used, each of which accelerates different stages of the reaction. The catalyst can also increase the speed of one of the stages of the catalytic cycle carried out by another catalyst. This is where "catalysis of catalysis" or second-level catalysis takes place.

Enzymes play the role of catalysts in biochemical reactions.

There are homogeneous and heterogeneous catalysis, but for any of them the main regularities are as follows:

1. The catalyst actively participates in the elementary act of the reaction, forming either intermediate compounds with one of the participants in the reaction, or an activated complex with all reactants. After each elementary act, it is regenerated and can interact with new molecules of reacting substances.

2. The rate of a catalytic reaction is proportional to the amount of catalyst.

3. The catalyst has selectivity of action. It can change the rate of one reaction and not affect the rate of another.

4. The catalyst allows the reaction to proceed in a different way, and at a faster rate than it does in the absence of a catalyst.

The speed can be increased by lowering the activation energy, increasing the pre-exponential factor, or both. For example, the thermal decomposition of acetaldehyde CH 3 CHO CH 4 + CO is catalyzed by iodine vapor, which causes a decrease in the activation energy by 55 kJ/mol. This decrease causes an increase in the rate constant by a factor of about 10,000.

5. The catalyst does not affect the position of thermodynamic equilibrium. It equally changes the rate of both forward and reverse reactions.

6. When certain substances, called promoters, are added, the activity of the catalyst increases; the addition of inhibitors reduces the rate of the reaction.

homogeneous catalysis.

In homogeneous catalysis, the catalyst is a molecule or ion in a homogeneous solution. In the case of homogeneous catalysis, the catalyst and all reactants form one common phase.
An example of homogeneous catalysis is the reaction of thermal decomposition of acetaldehyde CH 3 CH 4 + CO, catalyzed by iodine vapor. In the absence of iodine vapor E a=191.0 kJ/mol, in their presence E a= 136.0 kJ/mol. The rate constant increases by a factor of 10,000. This is because the reaction proceeds in two stages:

CH 3 SON + I 2 \u003d CH 3 I + HI + CO

CH 3 I + HI \u003d CH 4 + I 2

The activation energy of each step is less than the activation energy of the non-catalytic reaction.

Homogeneous catalysis includes many acid-base reactions, complex formation reactions, redox reactions, numerous hydrogenation, sulfation reactions, etc.

3. Acid and base catalysis

Acids and bases in many reactions act as a catalyst, i.e., participating in the reaction, they themselves are not consumed (reactions of hydrolysis, alkylation, esterification, etc. There are three types of acid-base catalysis:

4. Homogeneous catalytic reactions catalyzed by complex compounds

The reactions of reduction, hydrogenation, oxidation, isomerization, polymerization under industrial conditions are carried out in the presence of catalysts - complex compounds (metal ions of group VIII of the periodic table Fe, Co, Ni, Ru, as well as Cu, Fg, Hg, Cr, Mn). The essence of the catalytic action is that metal ions act as donors or acceptors of electrons. The chemical interaction between reacting molecules coordinated around the central metal ion is facilitated by polarization of the molecules and a decrease in the energy of individual bonds. The central metal ion is a bridge that facilitates electronic transitions between reacting molecules.

5. Enzymatic catalysis

Enzymes are the most amazing catalysts. Many reactions in living organisms are associated with them, and therefore they are often called biological catalysts. Enzymatic catalysis is a more complex phenomenon than conventional catalysis. The high organization of the processes of enzymatic catalysis is determined by the peculiarity of interaction in a living organism, associated with a special combination of the molecular structure of enzymes and substrates, which are called reactants in enzymatic reactions.

6. Heterogeneous catalysis

Heterogeneous catalysis is carried out at the interface. The first observed heterogeneous catalytic reaction was carried out by Priestley (1778) dehydration of ethyl alcohol on active clay:

C 2 H 5 OH -- C 2 H 4 + H 2 O

In practice, two types of heterogeneous catalysis are most often encountered:

1) processes, the catalyst of which is in the solid phase, and the reactants are in the liquid phase;

2) processes, the catalyst of which is in the solid phase, and the reactants are in the gas phase. The reaction, as a rule, occurs (and in some multistage processes begins) at the phase boundary, i.e. on the surface of a solid body - a catalyst.

61. General characteristics of the elements of group II-A. The biological role of S-elements of group II-A.

Group IIA elements have the electronic formula ns 2 . All of them are metals, strong reducing agents, somewhat less active than the alkali metals. They are characterized by an oxidation state of +2 and a pvalence of 2. When a covalent bond is formed, an s excitation of an electron and an sp hybridization of AO occur. Group IIA elements can be divided into three parts: 1) alkaline earth metals Ca, Sr, Ba, Ra, the bases of which are alkalis, 2) Mg, the base of which is slightly soluble in water, 3) Be, the base of which is an amphoteric base. In nature, the elements of group IIA are in the form of salts: sulfates, carbonates, phosphates, silicates. These elements are obtained by electrolysis of their salt melts. Group IIA elements are light silvery metals that are harder than alkali metals.

Chemical properties of elements

Group IIA elements are less active reducing agents than alkali metals. Their reducing properties increase from beryllium to radium. Air oxygen oxidizes Ca, Sr, Ba, Ra at normal temperature. Mg and Be are covered with oxide films and are oxidized by oxygen only when heated:

2Ca + O 2 \u003d 2CaO

2Mg + O 2 \u003d 2MgO

Active reducing agents, group IIA metals, react with non-metals (for example, chlorine), water, acids:

Ca + Cl 2= CaCl 2

Ca + 2H 2 O \u003d Ca (OH) 2 + H 2 

Alkaline earth metal hydrides are ionic salt-like compounds and interact with water and acids:

CaH 2 + 2H 2 O Ca (OH) 2 + 2H 2

CaH 2 + 2HCl 2  CaCl2 + 2H 2

Oxides of alkaline earth metals Ca, Sr, Ba, Ra dissolve in water with the formation of alkalis. Magnesium oxide is slightly soluble in water and has only basic properties. Beryllium oxide, which is insoluble in water, has amphoteric properties.

CaClCaO + 2HCl 2 + H 2 O

Ca, Sr, Ba, Ra hydroxides are alkalis, Mg hydroxide is a sparingly soluble basic hydroxide, Be hydroxide is an amphoteric hydroxide.

Carbonates and sulfates of group IIA elements are sparingly soluble in water. Carbonates dissolve in acids:

C Water hardness (W) is measured in millimoles of salt equivalents in 1 liter of water: W = 1000 e, where C e is the molar concentration of equivalents (normality) of salts in water.

Salts BaCl 2 and BaCO 3 are poisonous and are used as insecticides. Magnesium is an important structural material, is a trace element, is part of chlorophyll. Slaked lime is used in construction. Calcium salts, for example, CaSO 4 2H 2 O - gypsum - is used for gypsuming saline soils.

biological role.

Beryllium is found in plants as well as in animal organisms. The content of beryllium in living organisms is 10 -7 %, i.e. it is an impurity ultramicroelement. The biological role of beryllium has not been studied enough. Beryllium compounds are toxic and cause a number of diseases (beryllium rickets, berylliosis, etc.). Volatile compounds of beryllium are especially toxic. Negative effect of Be 2 + on physiological processes due to its chemical properties.

Magnesium is formally a macronutrient. Its total content in the body is 0.027% (about 20 g). The topography of magnesium in the human body is as follows: magnesium is concentrated to the greatest extent in dentin and tooth enamel, bone tissue. It also accumulates in the pancreas, skeletal muscles, kidneys, brain, liver and heart. In an adult, the daily requirement for magnesium is about 0.7 g. The Mg ion, like the K ion, is an intracellular cation.

In biological fluids and tissues of the body, magnesium is found both in the form of an aqua ion and in a state associated with proteins, in the amount of which hydrophosphate ion HPO is formed. 2- and a large amount of energy is released, passes with an excess of Mg 2+.

Calcium is a macronutrient. Its total content in the body is 1.4%. Calcium is found in every cell of the human body. The bulk of calcium is found in bone and dental tissues. On average, an adult should consume 1 g of calcium per day, although the need for calcium is only 0.5 g. Calcium administered with food is only 50% absorbed in the intestines. Relatively poor absorption is a consequence of the formation in the gastrointestinal tract of sparingly soluble calcium phosphate Ca 3 (PO 4) 2 and calcium salts of fatty acids. In the body, the concentration of Ca ions is regulated by hormones.

In the bones and teeth of an adult, about 1 kg of calcium is in the form of an insoluble crystalline mineral - hydroxylapatite Ca 10 (PO 4) 6 (OH) 2, the formation of which occurs when Ca ions interact with phosphate ions. In the blood and lymph, calcium is found both in an ionized and non-ionized state - in compounds with proteins, carbohydrates, etc. The blood coagulation mechanism consists of a number of stages, depending on the presence of ionized Ca. Ca ions are involved in the transmission of nerve impulses, muscle contraction, regulation of the heart muscle.

The concentration of Ca ions inside and outside the cell, respectively, is 10 -6 and (2.25-2.8) 10 -3 mol / l. Since calcium is practically not used inside the cell, it acts as a building material in the body - in bones, teeth. The skeleton is the main storage of calcium in the body.

Activated complex

grouping of atoms at the decisive moment of the elementary act of a chemical reaction. The concept of A. to. is widely used in the theory of rates of chemical reactions.

The course of an elementary act can be considered on the example of a gaseous bimolecular reaction (See Bimolecular reactions) of the formation of hydrogen iodide from hydrogen and iodine vapor:

H 2 + I 2 \u003d 2HI (1)

As quantum mechanical theory shows, when H 2 and I 2 molecules approach at a distance comparable to their molecular dimensions, they repel each other with a force that rapidly increases with decreasing distance. The overwhelming majority of collisions of H 2 and I 2 molecules in a gas mixture do not lead to a reaction, because the energy of the thermal motion of the molecules is insufficient to overcome the repulsion. For a certain, very small fraction of molecules, the intensity of thermal motion is by chance much greater than the average; this creates the possibility of such a close approach of the H 2 and I 2 molecules that new chemical bonds arise between the H and I atoms, and the previously existing chemical bonds H-H and I-I are broken. The two formed HI molecules repel each other and therefore diverge, which completes the elementary act of the reaction. Navigate from link location

2HI \u003d H 2 + I 2 (2)

the arrangement of atoms in an A. to. will be the same as for the direct reaction (1), but the directions of movement of atoms in the activated complexes of reactions (1) and (2) are mutually opposite.

The energy relations in the elementary act of the reaction can be schematically represented using a graph on which the potential energy of the reacting system U shown as a function of the so-called. reaction coordinate X, describing the mutual arrangement of atoms.

Given some very small interval Δ X (rice. ) and assuming that the configuration of atoms corresponds to A. to., if the coordinate X has a value lying within this interval, we can introduce the concepts - the concentration of activated direct reaction complexes in a given reacting system with + and their lifetime τ. During the time τ per unit volume, with + acts of direct reaction. Since the rate of direct reaction r + . is the number of corresponding acts of reaction per unit volume per unit time, then

Since the interval Δ X is small, then both c + and τ are proportional to Δ X, so that their ratio does not depend on the value of an arbitrarily chosen quantity Δ X. Quantities with + and τ are calculated by methods of statistical mechanics, while using a number of simplifying assumptions, of which the main one is the assumption that the reaction does not violate the statistically equilibrium distribution of molecules over states.

Equation (3) expresses the main idea of ​​the theoretical interpretation of reaction rates based on the concept of A. to. It not only allows one to judge the dependence of the reaction rate on the concentrations of substances participating in the reaction, on temperature, and other factors, but also establishes the absolute value of the rate. Therefore, the method of A. to. is often called the theory of absolute reaction rates. In some comparatively few reactions, the restructuring of chemical bonds is difficult, so that the achievement of the configuration of the A. to. does not yet guarantee the implementation of the reaction. To take into account the existence of such reactions, called non-adiabatic, an additional factor, the "transmission coefficient" or "transmission coefficient" is introduced into the right side of equality (3); in the case of nonadiabatic reactions, it is much less than unity.

The basic concepts of the A. to. method were explained above using the example of a homogeneous gas reaction, but the method is also applied to the rates of reactions in solutions, heterogeneous catalytic reactions, and in general to the calculation of rates in all cases when the transformation is associated with the need for random concentration of thermal motion energy in an amount that is much higher than the average energy of the molecules at a given temperature.

A comparison of the theory of absolute reaction rates with experimental data, as well as a theoretical analysis of its premises, shows that this theory, while not quite accurate, is at the same time a successful approximation, valuable for its simplicity.

Lit.: Glesston S., Leidler K., Eyring G., Theory of absolute reaction rates, trans. from English, M., 1948.

M. I. Tyomkin.

Great Soviet Encyclopedia. - M.: Soviet Encyclopedia. 1969-1978 .

See what "Activated Complex" is in other dictionaries:

In chemistry, the same as the transition state ... Big Encyclopedic Dictionary

- (chem.), the same as the transition state. * * * ACTIVATED COMPLEX ACTIVATED COMPLEX, in chemistry the same as the transition state (see TRANSITION STATE) ... encyclopedic Dictionary

activated complex- aktyvintasis kompleksas statusas T sritis chemija apibrėžtis Nepatvarus, iš reaguojančiųjų medžiagų susidarantis ir skylantis į reakcijos produktus kompleksas. atitikmenys: engl. activated complex rus. activated complex... Chemijos terminų aiskinamasis žodynas

- (chem.), the same as the transition state ... Natural science. encyclopedic Dictionary

(transition state)

This theory is the simplest and historically the first version of the statistical theory of chemical reactions. Developed by E. Wigner, M. Polyani, G. Eyring, M. Evans in the 30s of the twentieth century.

The theory is also based on the idea of ​​a collision of molecules as an indispensable condition for a reaction, but at the same time, the mechanism of collision of molecules is considered.

If we consider such a reaction: A + B = C, then based on the theory of the transition state, we can say that this reaction proceeds as follows: A + B ⇄ X ¹ ® C, where A and B are the starting substances, X ¹ is the transition complex , C is the product of the reaction.

What is a transition complex? Immediately after the collision of active molecules A and B, the redistribution of chemical bonds and the formation of a transition complex begin. A transition complex is a state of interacting molecules when old bonds have not yet been broken, and new ones have not yet formed, but the redistribution of bonds has already begun. The transition complex is when the molecules A and B have lost their individuality and we have a combination of atoms intermediate between A, B and C. The transition state is characterized by a continuous change in the distances between the interacting atoms. This is the essential difference between the transition complex and an ordinary molecule, in which the average distances between atoms do not depend on time. The transition complex should also not be confused with intermediate products, in which the distances between atoms also remain unchanged.

It should be noted that the formation of the transition complex requires energy. The energy required to transform the reactants into the state of the transition complex is called the activation energy. Since the initial molecules have not yet disintegrated, but the bonds characteristic of the molecules of the reaction products have already begun to form, then, naturally, the energy of transition to the activated state (E a) is less than the bond breaking energy in the molecules of the initial substances: E a< E диссоциации. Таким образом, образование переходного комплекса – процесс энергетически более выгодный, чем полный распад вступающих в реакцию молекул. Превращение активированного комплекса в продукты реакции всегда является процессом экзотермическим.

The main postulate of the transition state theory is that the initial substances are always in equilibrium with the transition complexes: A+B ⇄ X ¹ ®C. Then the chemical equilibrium constant of the formation of the transition complex is equal to: . (26)

From this expression, the concentration of the transition complex is:

X ¹ = [A]×[B] (27)

Then the transition complex is irreversibly destroyed with the formation of the reaction product C. The quantitative characteristic of this process is the decay frequency of the transition complex - R.

It is known from statistical mechanics that the number P depends only on temperature. This dependency looks like:

where k is the Boltzmann constant; h is Planck's constant; T is the absolute temperature.

Therefore, for a given temperature, the number P is the same for all transition states, and the rate of any chemical reaction depends only on the concentration of transition complexes:

V = (29)

However, the concentration of transition states is related to the concentration of reagents by relation (27) and therefore, substituting the value of X ¹ into equation (29), we obtain an expression for the rate of formation of reaction products.

V = ×[A]×[B] (30)

To the usual interaction reaction A + B ⇄ C, the law of mass action is applicable:

V = k v [A]×[B] (31)

(The symbol k v is used for the rate constant, in contrast to the Boltzmann constant).

We equate the right parts of equations (30) and (31), we get:

kv = × or kv =P× (32).

Equation (32) shows that at a given temperature, the reaction rate constant depends on the chemical equilibrium constant of the formation of the transition complex and on the frequency of decomposition of the transition complexes.

Equation (32) is called the basic equation of the transition state theory.

In contrast to the theory of active collisions, the transition state theory compares various possible complexes, reveals their greater or lesser achievability, and as a result determines the energetically most favorable reaction path.

Thus, chemical kinetics is based on two theories that complement each other. If the transition state theory is used to calculate the absolute rates of electrode processes, diffusion processes, etc., then the theory of active collisions describes well, as a rule, reactions in the gas phase.