Within the framework of the theory of absolute reaction rates, E. a. - the difference between the average energy of activated complexes and the average energy of the original molecules.
Ideas about E. a. arose in the 70-80s. 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 chemical reactions. 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 absolute temperature VC, k o is a constant called the pre-exponential factor of the rate constant. This equation, based on molecular kinetic theory, was later obtained in statistical physics taking into account a number of simplifying assumptions, one of which is the independence of E. a. on temperature. For practice and for theoretical calculations in relatively narrow temperature ranges, this assumption is valid.
E. a. can be found from experimental data in several ways. According to one of them, the kinetics of the reaction is studied at several temperatures (for methods, see the article. Rate of a chemical reaction) and a graph is plotted in coordinates In k - 1/T; the tangent of the angle of inclination 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 (direct or reverse) can be calculated if the E. a is known. reactions in another and the temperature dependence of the equilibrium constant (from thermodynamic data). For more accurate calculations the dependence of E. should be taken into account. on 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 substances and products, then the apparent E. a. differs from the true value by the magnitude of the thermal effects accompanying the processes of adsorption and desorption of reactants on the surface of the catalyst. In nonequilibrium systems, for example plasma-chemical (see Plasma chemistry) , definition of E. a. is a very difficult task. In some cases, however, it is possible formal application Arrhenius equations.
Energy diagram of a reaction.
The activation energy significantly affects the value of the reaction rate constant and its dependence on temperature: the greater E a, the smaller the rate constant and the more significantly the temperature change affects it.
Fig.5. Energy diagram of the reaction A + B = C + D
20. Catalysis and catalysts (classification and mechanism of action). Features of catalytic reactions.Chemical catalysis is the acceleration of chemical reactions under the influence of small amounts of substances (catalysts). After a full 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 reacting substances, a heterogeneous catalyst forms an independent phase, separated by an interface from the phase in which the reacting substances 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 occur 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 carrier (silica gel, aluminum oxide, activated carbon, etc.).
For each type of reaction, only certain catalysts are effective. In addition to the already mentioned acid-base catalysts, 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 using catalysts that act through the coordination of reactants at a transition metal atom or ion (Ti, Rh, Ni). This type of 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. A catalyst can also increase the rate of one stage of the catalytic cycle carried out by another catalyst. Here, “catalysis of catalysis”, or second-level catalysis, takes place.
In biochemical reactions, enzymes play the role of catalysts.
There are homogeneous and heterogeneous catalysis, but for any of them the main principles boil down to the following:
1. The catalyst actively participates in the elementary act of the reaction, forming either intermediate compounds with one of the reaction participants, 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 the 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 along a different path, and at a faster rate than it would in the absence of a catalyst.
The speed can be increased by reducing 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 activation energy by 55 kJ/mol. This decrease causes the rate constant to increase by approximately 10,000 times.
5. The catalyst does not affect the position of thermodynamic equilibrium. It changes the rate of both forward and reverse reactions to the same extent.
6. By adding certain substances called promoters, the activity of the catalyst increases; adding inhibitors reduces the reaction rate.
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 SON 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 10,000 times. This happens because the reaction occurs in two stages:
CH 3 SON + I 2 = CH 3 I + HI + CO
CH 3 I + HI = CH 4 + I 2
The activation energy of each stage is less than the activation energy of the non-catalytic reaction.
Homogeneous catalysis includes many acid-base reactions, complexation reactions, redox reactions, numerous hydrogenation reactions, sulfide reactions, etc.
3. Acid and base catalysis
Acids and bases in many reactions perform the functions of a catalyst, i.e., while 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
Reduction, hydrogenation, oxidation, isomerization, polymerization reactions 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 electron donors or acceptors. Chemical interaction between reacting molecules coordinated around the central metal ion is facilitated due to the polarization of molecules and a decrease in the energy of individual bonds. The central metal ion acts as a bridge, facilitating electronic transitions between reacting molecules.
5. Enzyme catalysis
Enzymes are the most amazing catalysts. They are associated with many reactions in living organisms, 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 enzymatic reactions are called reactants.
6. Heterogeneous catalysis
Heterogeneous catalysis occurs at the interface. The first observed heterogeneous catalytic reaction was the dehydration of ethyl alcohol on active clay carried out by Priestley (1778):
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 in which the catalyst is in the solid phase and the reactants are in the liquid phase;
2) processes in which the catalyst is in the solid phase, and the reactants are in the gas phase. The reaction, as a rule, occurs (and in some multi-stage processes begins) at the phase boundary, i.e. on a surface solid-- catalyst.
61. general characteristics elements of group II-A. Biological role of S-elements of group II-A.
Group IIA elements have 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 pvalence of 2. When a covalent bond is formed, s excitation of the electron and sp hybridization of the AO occur. Group IIA elements can be divided into three parts: 1) alkaline earth metals Ca, Sr, Ba, Ra, whose bases are alkalis, 2) Mg, whose base is slightly soluble in water, 3) Be, whose base is an amphoteric base. In nature, group IIA elements are found in the form of salts: sulfates, carbonates, phosphates, silicates. These elements are obtained by electrolysis of molten salts. Group IIA elements are light silvery metals that are harder than the 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 ordinary temperatures. Mg and Be are covered with oxide films and are oxidized by oxygen only when heated:
2Ca + O2 = 2CaO
2Mg + O2 = 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= Ca(OH) 2 + H 2
Alkaline earth metal hydrides are ionic salt-like compounds and react 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 to form alkalis. Magnesium oxide is slightly soluble in water and has only basic properties. Beryllium oxide, insoluble in water, has amphoteric properties.
CaClCaO + 2HCl 2 + H 2 O
Ca, Sr, Ba, Ra hydroxides are alkalis, Mg hydroxide is a poorly soluble basic hydroxide, Be hydroxide is an amphoteric hydroxide.
Carbonates and sulfates of Group IIA elements are slightly soluble in water. Carbonates dissolve in acids:
CWater hardness (W) is measured in millimoles of salt equivalents in 1 liter of water: W = 1000 oe, 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, and is part of chlorophyll. Slaked lime 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 and also in animal bodies. The beryllium content in living organisms is 10 -7 %, i.e. it is an impurity ultramicroelement. The biological role of beryllium has not been sufficiently studied. Beryllium compounds are toxic and cause a number of diseases (beryllium rickets, berylliosis, etc.). Volatile beryllium compounds are especially toxic. Negative influence of Ve 2 + on physiological processes is explained by its chemical properties.
Magnesium is formally classified as a macronutrient. General content it 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 enamel of teeth and 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 protein-bound state in the amount of which the hydrogen phosphate ion HPO is formed 2- and stands out a large number of energy, occurs 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 - hydroxyapatite Ca 10 (PO 4) 6 (OH) 2, the formation of which occurs through the interaction of Ca ions with phosphate ions. In the blood and lymph, calcium is found in both ionized and non-ionized states - in combination with proteins, carbohydrates, etc. The blood clotting mechanism consists of a number of stages depending on the presence of ionized Ca. Ca ions take part in the transfer nerve impulses, muscle contraction, regulation of the heart muscle.
The concentration of Ca ions inside and outside the cell is 10 -6 and (2.25-2.8) 10 -3 mol/l, respectively. Since calcium is practically not used inside the cell, it acts as a building material in the body - in bones and teeth. The skeleton is the main store of calcium in the body.
Calculations show that for many chemical reactions, if they proceed through the mechanism of direct conversion of molecules of starting substances into products, the energy imparted to the molecules during thermal activation is not enough to overcome the energy barrier. In other words, with such a mechanism, the activation energy even at very high temperatures so great that the reactions should not proceed at any appreciable rate. However, chemical reactions in nature, industrial and laboratory installations They go and often go very quickly. Consequently, the theory of active collisions alone is not enough to explain the causes and mechanisms of reactions.
In the 1930s E. Wigner, M. Polyani, G. Eyring and M. Evans created a theory that allows one to explain the occurrence of reactions at low thermal velocities of molecules. It's called transition state theories(or the theory of absolute reaction rates). The main provisions of this theory:
1) The interaction of molecules does not immediately lead to the formation of product molecules. First, the so-called “transition state” or activated complex.
2) Activated complex is an unstable formation that includes all the atoms of colliding and interacting molecules. The lifetime of the activated complex is very short; it is measured in small (millionths, ten-millionths, etc.) fractions of a second. The distances between atoms in an activated complex are somewhat greater than in ordinary molecules, so additional energy is required for its formation.
3) The activation energy in this regard is considered as the energy required for the formation of an activated complex.
4) Some time after its formation, the activated complex disintegrates with the formation of product molecules; this releases energy.
5) The energy released during the decomposition of the activated complex can be fully or partially spent on the activation of other molecules of the starting substances.
A visual representation of the course of a reaction over time in accordance with the transition state theory can give energy profile reaction, for example, exothermic (Fig. 12.6).
The energy of the system is plotted along the ordinate axis E , and the x-axis is the so-called reaction coordinate. The average energy reserve of thermal motion of the molecules of the starting substances corresponds to the level E ref, energy stored in the activated complex - level E AK. Then the difference E AK - E ref is equal to the value of the energy barrier that molecules must overcome in order to interact (activation energy). A visual representation of it is given by the curve connecting the levels E ref and E AK. The height of the energy barrier depends on the nature of the reacting substances, the energy required for the formation of the activated complex (activation energy), as well as on the average energy of thermal motion of the molecules E ref.
As the temperature rises, the level E ref rises, the energy barrier becomes smaller and a larger number of molecules can interact. This is the reason why the reaction accelerates with increasing temperature. When the temperature decreases, on the contrary, the level E ref decreases and the energy barrier increases, which leads to a decrease in the reaction rate.
During the decomposition of the activated complex with the formation of product molecules, energy is released, which corresponds to the difference E AK - E prod, where E prod - the average energy reserve of product molecules. Part of this released energy equal to the difference E AK - E ref will be used to activate new molecules of the starting substances, and the excess E ref - E prod will be allocated to environment in the form of an exothermic heat effect of the reaction DH r .
For endothermic reactions, the energy profile looks slightly different (Fig. 12.7). It can be seen that in this case the energy level E ref lower than level E cont. As a result of this energy E AK - E the product released during the disintegration of the activated complex is not enough to
to cause the activation of new reactant molecules. Therefore, to continue the reaction, it is necessary to supply energy from the outside, in the form of an endothermic thermal effect.
The existence of an activated complex is confirmed by experimental data. So, for example, for one of the simple model reactions of interaction of a hydrogen atom with a hydrogen molecule
Н 2 + Н ® Н + Н 2 ,
the activation energy value is close to 36.8 kJ/mol. If the reaction proceeded through the stage of complete dissociation of H 2 molecules, and not through the stage of formation of the activated complex H 2 ·H, then an activation energy of 435.1 kJ/mol would be required.
The theory of the transition state is based on the following provisions (postulates of the theory).
The collision of particles leads to the formation of a bond between them.
An unstable state in which there are connections between all particles is called transitional state. It is also represented as a complex temporarily formed by interacting particles, and is called active complex.
The formation and disintegration of the active complex occurs only in one direction (see Fig. 12 - 3).
The order of formation and decay of the complex is as follows. The interacting particles move towards each other until an additional bond arises between them, the formation of which leads to a weakening of the bond already existing in one of the interacting molecules. Then the particles begin to disperse. The weakened previously existing connection disappears, but the new connection that arises when the particles approach each other remains.
Rice. 12 - 3. Formation and decay of the active complex.
This postulate prohibits the decomposition of the active complex into initial particles. It can only decompose to form reaction products.The formation of an active complex does not lead to disruption of the particle velocity and Maxwell-Boltzmann energy distribution.
It is assumed that the displacement of electron orbitals in particles during the formation of an active complex occurs many times faster than the movement of atomic nuclei.
This postulate of transition state theory is called adiabatic principle. It underlies calculations of the energy of interacting particles, since it assumes that electrons always have time to take on the most stable configuration for a given distance between the centers of atoms.
Let us show how the above postulates can be used to derive the basic equation of the transition state theory.
Let it proceed as shown in Fig. 12 - 3, reaction:
XY + Z = X + YZ.
Formally, the rate of this reaction is determined by the equation:
.
(12 - 26)
On the other hand, the rate of formation of reaction products is determined by the number of active complexes decaying per unit time according to the following scheme:
X YZ X + YZ.
Since the decomposition of the complex is a monomolecular reaction, the following expression can be written for its rate:
Using equation (9 - 20), which connects the rate constant of an irreversible first-order reaction with the average lifetime of the converted substance, equality (12 - 27) can be represented as follows:
.
(12 - 28)
Comparing equalities (12 - 26) and (12 - 28), we get:
.
(12 - 29)
Equation (12 - 29) is the basic equation for calculating the reaction rate constant. However, it can obtain its final form if the quantities included in it are expressed through energy characteristics.
The average lifetime of the complex can be estimated using the second postulate of the theory.
Since the formation and disintegration of the complex occurs only in one direction, its existence can be represented as one oscillatory cycle along a new bond. The energy of such vibrations is equal to:
,
(12 - 30)
where h is Planck's constant.
The energy required to excite vibrations is equal to the kinetic component of the colliding particles. When particles move along one axis, it is equal to:
,
(12 - 31)
where is Boltzmann's constant.
From the equality of kinetic energy and vibration energy it follows:
The oscillation frequency is the reciprocal of the period of one oscillation, and taking into account that the complex exists only during one oscillatory cycle, we have:
.
(12 - 33)
While maintaining the equilibrium distribution of particle velocities and energies, the ratio between the concentrations of the starting substances and the active complex is determined by the constant K#:
.
(12 - 34)
The K# constant is not a true equilibrium constant, since the complex does not decay in the opposite direction (into the original particles). However, the ratio between the concentrations depends on the energy of the particles in the initial state and in the state of the active complex. In this case, you can use the chemical reaction isotherm equation in the following form (see Part I, page 77):
.
(12 - 35)
The change in Gibbs energy for the transition from the initial state of particles to the state of the active complex (transition state) G # is determined by the change in enthalpy H # and the change in entropy S #:
Therefore, the constant K# can be represented as follows:
.
(12 - 36)
Thus, the equation for the reaction rate constant takes the form:
.
(12 - 37)
Magnitude 
, containing the activation entropyS #, corresponds to the steric factor in the theory of active collisions. The enthalpy of activation H # in the transition state theory corresponds to the activation energy. To calculate it, it is necessary to know the energy of the system in the initial state and the energy of the activated complex.
To calculate the change in the energy of a system during the transition from the initial state to the transition state, it is necessary to find the dependence of the energy of the system on the distances between atoms. In the case under consideration of the formation of an active complex from the initial molecule XY and particle Z, the independent variables are the distances between the centers of the atoms in the pair X and Y, which we denote as r XY, and the distance between the centers of the atoms Y and Z, which we denote as r YZ. The energy of the system is a function of these variables:
In a three-coordinate system, this dependence is transmitted by the surface. To represent the dependence of energy on the distances r XY and r YZ on the plane, the same method is used as in constructing topographic maps, namely: planes equidistant from each other are drawn, perpendicular to the energy axis, and the lines of intersection of these planes with the surface are drawn on the drawing plane. In Fig. 12 - 4 shows an example of constructing an energy diagram using this method.
Rice. 12 - 4. Energy diagram of a triatomic system.
To construct a diagram, the potential energy of the system is calculated for various combinations of distances r XY and r YZ. In this case, the fourth postulate of the theory is used (the principle of adiabaticity), according to which calculations are carried out for systems with equilibrium electronic configurations. The XY molecule has a minimum energy when the distance between the atoms is equal to the bond length. An increase or decrease in this distance leads to an increase in energy in an individual molecule. The same applies to the YZ molecule. Consequently, the diagram should contain two areas with reduced energy values A and B (they are figuratively called valleys). Regions A and B are separated from each other by a section of a small increase in energy C (it is figuratively called a pass).On a typical energy diagram (Fig. 12 - 4) there are several special points. The first one a corresponds to the initial state of the system (the state before the start of the reaction). In this state, the distance between the centers of the X and Y atoms should be equal to the normal bond length in the steady state of the XY molecule. The distance between the centers of the Y and Z atoms must be very large, since the Z particle has not yet interacted with the XY molecule. Another characteristic point b reflects the final state of the system (state after the reaction). It corresponds to the distance between the centers of the Y and Z atoms, equal to the bond length in the newly formed molecule, and long distance between the separated particle X and atom Y. The third most important point on the energy diagram is the saddle point With. It is at the pass point that a fully formed active complex exists.
From the foregoing it follows that a chemical transformation, according to the transition state theory, is a transition from the point A exactly b through the point With. This transition occurs at minimum energy values (in the energy diagram it corresponds to movement from the point A along the bottom of valley A to the pass With, and then descend into valley B and move to point b). It is called by reaction and is shown with a dotted line.
If you cut the spatial energy diagram along the reaction path perpendicular to the plane r XY - r YZ, then the section will produce a line whose length corresponds to the length of the reaction path, and the ordinate corresponds to the energy of the system. Let's call the line in these coordinates reaction path profile(Fig. 12 - 5).
Rice. 12 - 5. Energy profile of the reaction pathway.
The difference between the energy of the system in the transition state and the energy in the initial state E 1, as shown in Fig. 12 - 5, is the classical activation energy of a forward reaction. The energy difference in the state of the active complex and the final state E −1 is equal to the activation energy of the reverse reaction. The difference between the activation energies of the forward and reverse reactions corresponds to thermal effect reactions H.Thus, the activation energy in the transition state theory has a clear interpretation as the value of the energy barrier equal to the energy difference in the transition and initial states .
As has been repeatedly noted, all calculations of the potential energy of a system are possible only in the case when the electrons have equilibrium configurations. During the reaction, the principle of adiabaticity is violated. Therefore, the calculated energy value turns out to be overestimated. To take into account the discrepancy between the calculated and real energy values in the state of the active complex, a correction factor is introduced, which is called transmission coefficient. With the introduction of this amendment, the basic equation of the transition state theory takes its final form:
.
(12 - 38)
The transition state theory is applicable not only to chemical transformations, but also to other kinetic processes: diffusion, viscous flow, electrical conductivity of solutions. It is assumed that the movement of particles in a liquid is associated with overcoming an energy barrier, the value of which is equal to the activation energy.
The collision theory is unsuitable for complex molecules because it assumes the existence of molecules in the form of ideal elastic spherical particles. However, for complex molecules, in addition to translational energy, other types of molecular energy must be taken into account, for example, rotational and vibrational. According to collision theory, reactions in which three or more molecules must collide are impossible. In addition, decomposition reactions like AB = A + B difficult to explain with this theory.
To overcome these difficulties, H. Eyring in 1935. proposed the activated complex theory. Any chemical reaction or any other molecular process that occurs over time (diffusion, viscous flow, etc.) consists of a continuous change in the distances between the nuclei of atoms. In this case, the configuration of nuclei corresponding to the initial state, through some intermediate configuration - an activated complex or a transition state - turns into the final configuration. It is assumed that the activated complex is formed as an intermediate state in all chemical reactions. It is viewed as a molecule that exists only temporarily and is destroyed when certain speed. This complex is formed from such interacting molecules, the energy of which is sufficient for them to come close to each other according to the scheme: reactants, activated complex, products. The activated complex has a structure intermediate between reactants and products. The activation energy of a reaction is the additional energy that the reacting molecules must acquire in order to form the activated complex necessary for the reaction to occur.
The activation energy always represents the energy absorbed, regardless of whether overall change it for the reaction to be positive (endothermic reaction) or negative (exothermic reaction). This is shown schematically in Fig. 6.
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Figure 6. Energy diagram of the formation of the activated complex.
Activation is the imparting of such an amount of energy to molecules that, during their effective transformation, 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 requires some additional energy to activate. The amount of this additional energy depends on how much energy the system already has.
The energy of the original system cannot be less than its zero energy (i.e. at 0 0 K). To activate any system, it is enough to provide it with additional energy. This energy is called true activation energy.
The true activation energy of an elementary chemical act is the minimum energy that the original system must have above its zero energy (i.e. at 0 0 K) so that chemical transformations can occur in it. The difference between the true activation energies of the reverse and forward reactions is equal to the thermal effect of the reaction at absolute zero.
The theory of an activated complex or transition state is based on the fact that the elementary act of interaction between molecules consists of a gradual rearrangement of chemical bonds, in which the initial configuration of atoms in the initial molecules transforms into the final configuration of the reaction products with a continuous change in interatomic distances.
A quantitative theory based on these ideas, using the mathematical apparatus of statistical thermodynamics, the so-called theory of absolute reaction rates, was proposed by G. Eyring and M. Polyani (1935).
Let's consider the reaction mechanism
According to the theory of the activated complex, when atom A approaches the molecule, BC weakens B-C connection and an A-B connection arises. The process ends with the formation of an AB molecule and a C atom, for which the system must pass through the activated ABC complex, when the B atom belongs equally to the BC and AB molecules:
Qualitative ideas about an elementary act as a complex process of rearrangement of chemical bonds when molecules approach each other, as well as ideas about the potential energy surface and reaction coordinate are called activated complex or transition state theory.
Strict quantity theory, based on this physical model of the mechanism of an elementary act, should consist of a theoretical calculation of the energy surface of the reaction using quantum mechanics methods and theoretical assessment on this basis the activation energy and pre-exponential factor. This has not yet been possible due to mathematical difficulties. Therefore, they use an approximate mathematical model, the so-called theory of absolute reaction rates.
According to this theory, the rate of any chemical reaction is equal to the rate of transition of the activated complex through the potential barrier, i.e. the rate of decomposition of the activated complex into reaction products. In this case, the molecule of the activated complex passes through d(Fig. 8) .
When deriving an expression for the rate constant in the activated complex theory, an elementary reaction is considered as a one-dimensional translational motion along the reaction coordinate in the direction of the reaction products. The following assumptions are made:
1. During a chemical reaction, an activated complex () is formed at the top of the potential barrier, consisting of molecules of the starting substance and reaction products.
2. The activated complex in the area (Fig. 8) performs a one-dimensional translational movement in the direction of the reaction products.
3. Movement along the reaction path can be described in terms of classical mechanics without taking into account quantum effects.
4. An elementary reaction occurs adiabatically, that is, without transferring potential energy to another surface.
Thus, the activated complex is considered as an ordinary molecule in which one vibrational degree of freedom is replaced by a translational one in the direction of the reaction products.