RELAXATION
(from Latin relaxatio-weakening), the process of establishing in the system thermodynamic equilibrium. The state is macroscopic. system is determined by pl. parameters, and the processes of achieving equilibrium in different parameters can proceed with different. speeds. A period of linear R. is distinguished, when a certain parameter of state i only slightly differs from its equilibrium value. During this period, the rate of change of the parameter i/dt proportional to the deviation x i from:
where t i-time P. Hence it follows that at time t, the deviation exp (T / t i). During t i small parameter deviation x i from the equilibrium value decreases by a factor. The quantities = 1 / t i, opposite to the times of R., is called. frequencies R.
R.'s times are determined by sv-you systems and the type of the process under consideration. In real systems, they can vary from negligible values to values of the order of the age of the Universe. The system can achieve equilibrium in some parameters and remain non-equilibrium in others (partial equilibrium). All R.'s processes are nonequilibrium and irreversible and are accompanied by energy dissipation, i.e., it is produced in the system (see. Thermodynamics of irreversible processes).
In gases, R. is caused by the exchange of energy and the amount of motion in collisions of particles, and R. time is determined by the free time. range (average time between two successive collisions of molecules) and the efficiency of energy exchange between all degrees of freedom of colliding particles. In monatomic gases, the stage of fast R. is distinguished, when, in a short period of time of the order of the time of collision of molecules, the initial (strongly nonequilibrium) state becomes chaotic to such an extent that to describe it, it is sufficient to know how the distribution of coordinates and momenta of just one particle changes with time (the so-called . single-particle distribution function). At the second stage R. for a time of the order of time free. mileage as a result of just a few. collisions in macroscopically small volumes moving with an average velocity of mass transfer (mass velocity), local thermodynamics is established. equilibrium. It is characterized by state parameters (t-swarm, chemical potential, etc.), which depend on spaces. coordinates and time and slowly tend to equilibrium values as a result of a large number of collisions (processes of heat conduction, diffusion, viscosity, etc.). R. time depends on the size of the system and is large in comparison with the average free time. mileage.
In polyatomic gases (with internal degrees of freedom) m. B. the exchange of energy between enter. and int. degrees of freedom (rotat., oscillate.) and R. arises, associated with this phenomenon. Equilibrium is established the fastest by acting. degrees of freedom, a cut is characterized by the corresponding t-swarm. The balance between arrive. and rotate. degrees of freedom is set much more slowly. The excitement is shaking. degrees of freedom is possible only at high t-p. Therefore, in polyatomic gases, multistage R. processes are possible (see Sec. Non-equilibrium kinetics). If the gas consists of components with molecules of very different masses, the exchange of energies between the components slows down, as a result of which states with decomp. t-rami component. For example, ionic and electronic t-ry differ in plasma and slow processes of their R. occur (see. Plasma chemistry).
In fluids R. is described with the help of spatio-temporal correlations. f-tions, characterizing the attenuation in time and space of the mutual influence of molecules (correlations). These correlations are the cause of irreversible processes - thermal conductivity and viscosity (see. Liquid). R.'s time to full thermodynamic. equilibrium can be estimated using kinetic. coefficients. For example, in a binary solution, R.'s time of concentration is t! 2
/D, where L is the size of the system, D is the coefficient. diffusion; R. time t-ry t! L 2 / x, where x is the coefficient. thermal diffusivity, etc. (for details, see. Macrokinetics).
In solids, R. is described as R. in a gas of certain quasiparticles. For example, in crystalline. lattice at low m-pax elastic vibrations are interpreted as a gas of phonons (acoustic R.). In the system of spin magnets. moments of a ferromagnet, quasiparticles are magnons (magnetic R.).
At phase transitions R. can be complex. If the transition from a nonequilibrium state to an equilibrium one is a first-order transition, the system can first pass into a metastable state and then relax extremely slowly (see Sec. Glassy state). Relaxation is especially difficult. transitions in polymers, where there is a set (spectrum) of relaxation. phenomena, each of which is conditioned by its own mechanism. In the vicinity of the second-order phase transition point, the degree of ordering of the phases is characterized by the order parameter, which tends to zero, and its R. time increases greatly. Even more complex is the character of R. from states that are very far from thermodynamic. balance. In open systems, in this case, the following phenomena are possible self-organization.
Measurements of R.'s times are used in chemical. kinetics for the study of processes in which equilibrium is quickly established (see. Relaxation methods). Mechanical R. manifests itself in a decrease in the stress over time, which has created deformation in the body. Mechanical R. is associated with viscoelasticity, it leads to creep, hysteresis phenomena during deformation (see. Rheology). With regard to biol. systems, the term "R." sometimes used to characterize the lifetime of a system, edges by the time of physiological death comes into a state of partial equilibrium (quasi-equilibrium) with the environment. In nature. systems R. times are divided by strong inequalities; their arrangement in ascending or descending order allows us to consider the system as a hierarchical sequence. levels with diff. the degree of ordering of the structure (see. Thermodynamics of hierarchical systems).
Lit .: Zubarev D.N., Non-equilibrium, M., 1971; Lifshits E. M., Pitaevsky L. P., Physical kinetics, in the book: Theoretical physics, vol. 10, M., 1979; Gladyshev GP, Thermodynamics and natural hierarchical processes, M., 1988; Denisov E.T., Kinetics of homogeneous chemical reactions, 2nd ed., M., 1988.
Chemical encyclopedia. - M .: Soviet encyclopedia. Ed. I. L. Knunyants. 1988 .
Synonyms:See what "RELAXATION" is in other dictionaries:
- (from Lat. relaxatio weakening, decreasing), the process of establishing the equilibrium of the thermodynamic in the macroscopic. physical systems (gases, liquids, solids). The state is macroscopic. the system is determined by a large number of parameters, and the establishment ... ... Physical encyclopedia
relaxation- (from Lat. relahatio tension reduction, relaxation) a state of rest, relaxation, arising in the subject as a result of stress relief, after strong feelings or physical efforts. R. can be involuntary (relaxation when leaving ... ... Big psychological encyclopedia
Relaxation- - the process of gradual transition of a thermodynamic system from a nonequilibrium state caused by external influences into a state of thermodynamic equilibrium. Examples of relaxation processes: gradual change in tensions in the body ... ... Encyclopedia of terms, definitions and explanations of building materials
- [lat. relaxatio tension reduction, relaxation] honey. relaxation of skeletal muscles; removal of mental stress. Dictionary of foreign words. Komlev N.G., 2006. relaxation (lat. Relaxatio, tension reduction, relaxation) 1) physical. process … Dictionary of foreign words of the Russian language
relaxation- and, w. relaxation, it. Relaxation relaxatio tension reduction, relaxation. 1.phys. The process of gradually returning to a state of equilibrium of what l. a system brought out of such a state, after the cessation of the action of the factors that brought it out ... Historical Dictionary of Russian Gallicisms
RELAXATION, the process of establishing thermodynamic equilibrium in a macroscopic physical system consisting of a large number of particles. Characteristics of the relaxation time process. For example: for a system of electrons in a metal, the relaxation time is t 10 ... ... Modern encyclopedia
In physiology, relaxation or a sharp decrease in the tone of skeletal muscles up to complete immobilization. May occur as a pathological condition; artificial relaxation is achieved by using muscle relaxants ... Big Encyclopedic Dictionary
Relaxation, thermal relaxation, attenuation, weakening Dictionary of Russian synonyms. relaxation n., number of synonyms: 6 autorelaxation (1) ... Synonym dictionary
- (from the Latin relaxatio relaxation, relaxation, rest), 1) relaxation or a sharp decrease in skeletal muscle tone. Artificial relaxation achieved by the use of muscle relaxant preparations is used in surgical interventions. ... ... Modern encyclopedia
- (from Lat. relaxatio relief, relaxation) a state of rest associated with complete or partial muscle relaxation. They share long-term relaxation that occurs during sleep, hypnosis, with pharmacological influences, and ... ... Psychological Dictionary
Let's say you want to get the maximum amount of ammonia from a given amount of hydrogen and nitrogen. From the material in the previous section, you already know how you can influence the course of the reaction by shifting the chemical equilibrium in one direction or another. However, in order to solve the problem in the most efficient way, the reaction rate should also be taken into account. Knowledge of the rates of chemical reactions is of great scientific and practical importance. For example, in the chemical industry, in the production of a substance, the size and performance of the apparatus, the amount of the product produced, depend on the reaction rate.
Chemical reactions take place at different rates. Some of them end in a split second, others last minutes, hours and even days. Therefore, in the practical use of chemical reactions, it is very important to know at what rate a given reaction will proceed under certain conditions and how to change these conditions so that the reaction proceeds at the required rate.
Fast and Slow Reactions: Chemical Kinetics
The branch of chemistry that studies the rate of chemical reactions is called chemical kinetics.
The most important factors affecting the reaction rate include the following:
S nature of the reactants;
S size of reagent particles;
S concentration of reactants;
S pressure of gaseous reagents;
S temperature;
S the presence of catalysts in the system.
The nature of the reactants
As already noted, a necessary condition for chemical interaction between the particles of the initial substances is their collision with each other (collision), moreover, in the region of the molecule with high reactivity (see the section "How reactions occur: collision theory" above in the chapter ). The larger and more complex the reagent molecules, the lower the likelihood that the collision will occur precisely on the site
highly reactive molecules. Often, in rather complex molecules, the highly reactive site is completely blocked by other parts of the molecule, and no reaction occurs. In this case, of the multitude of collisions, only those that occur at the reactive site are effective (i.e., leading to chemical interaction).
In other words, the larger and more complex the reactant molecules, the slower the reaction rate.
Particle size of reagents
The rate of the reaction depends on the number of collisions between the molecules of the reacting substances. Thus, the larger the surface area on which collisions occur, the higher the reaction rate. For example, if you bring a burning match to a large piece of coal, then no reaction will occur. However, if you grind this lump of coal into powder, spray it in the air, and then strike it with a match, it will explode. The cause of the explosion (i.e., the high rate of the reaction) is a significant increase in the surface area of the coal.
Concentration of reactants
An increase in the number of collisions of reactants leads to an increase in the reaction rate. Thus, the reaction rate is proportional to the number of collisions that the molecules of the reacting substances undergo. The number of collisions, in turn, is the greater, the higher the concentration of each of the initial substances. For example, a wooden plank burns quite well in ordinary air (which is 20% oxygen), but in pure oxygen, combustion is more intense, that is, at a higher rate.
In most simple reactions, increasing the concentration of the reactants increases the reaction rate. However, in complex reactions proceeding in several stages, this dependence is not observed. In fact, by determining the effect of concentration on the rate of a reaction, you will be able to find out which reactant is influencing the stage of the reaction that determines its rate. (This information will help to calculate the reaction mechanism.) This can be done by performing reactions at several different concentrations and observing their effect on the reaction rate. If, for example, a change in the concentration of one reagent does not affect the reaction rate, then you will know that at the slowest stage of the reaction mechanism (and the reaction rate is precisely determined by this stage) this reagent is not involved.
Pressure of gaseous reactants
The pressure of the gaseous reactants has the same effect on the reaction rate as the concentration. The higher the pressure of the reagents in the gaseous state, the higher the reaction rate. This is (you guessed it!) Due to the increased number of collisions. However, if the reaction has a complex mechanism, then the change in pressure may not lead to the expected result.
Temperature
Why does every housewife rush to refrigerate the rest of the turkey after the Thanksgiving dinner? Because if you don't, the turkey can go bad. And what does “spoil” mean? This means increased bacterial growth. Now, when the turkey is in the refrigerator, it will slow down the rate of bacterial growth due to the lower temperature.
An increase in the number of bacteria is a common biochemical reaction, that is, a chemical reaction involving living organisms. In most cases, an increase in temperature leads to an increase in the rate of such reactions. In organic chemistry, there is a rule: an increase in temperature by 10 ° C leads to a doubling of the reaction rate.
Why is this happening? Partly (you guessed it by now!) Due to the increased number of collisions. As the temperature rises, the molecules move faster, thus increasing the likelihood of their collisions with each other, and hence their chemical interaction. However, this is not all. As the temperature rises, the average kinetic energy of the molecules also increases. Pay attention to fig. 8.7 for an example of how an increase in temperature affects the kinetic energy of the reactants and the reaction rate.
At a given temperature, not all molecules have the same kinetic energy. Some of them can move extremely slowly (that is, they have low kinetic energy), while others are quite fast (that is, they have high kinetic energy). However, in the overwhelming majority of cases, the value of the speed of movement of molecules is somewhere in the middle between these two speeds.
In reality, temperature is a measure of the average kinetic energy of molecules. As seen in Fig. 8.7, an increase in temperature leads to an increase in the average kinetic energy of the reactants, while the curve shifts to the right, towards higher values of kinetic energy. Pay attention also to the minimum amount of kinetic energy that molecules must have in order for their collision to lead to the formation of a new substance, i.e. on the activation energy of this reaction. Molecules with this energy are called active molecules.
Reagents need not only to collide at the reactive site, while the amount of energy must be transferred, sufficient to break existing bonds and form new ones. If this energy is not enough, then upon collision of the initial molecules, the reaction will still not occur.
Note that at a lower temperature (T1), a small number of reagent molecules have the required activation energy. At a higher temperature (T2)
the activation energy (the minimum amount of kinetic energy required for the formation of a new substance) will already be possessed by many more molecules, i.e., many more collisions will be effective.
Thus, an increase in temperature increases not only the number of collisions, but also the number of effective collisions, as a result of which chemical interaction of particles occurs.
Catalysts
Substances that are not consumed as a result of the reaction, but affect its rate, are called catalysts. The phenomenon of a change in the rate of reaction under the influence of such substances is called catalysis. In most cases, the effect of a catalyst is explained by the fact that it reduces the activation energy of the reaction.
Take a look, for example, in fig. 8.1. If the value of the activation energy corresponding to the maximum on the graph were less, then the number of effective collisions of reagent molecules would be higher, which means that the reaction rate would also be higher. The same can be seen in Fig. 8.7. If you move the dotted line to the left, which denotes the minimum kinetic energy required to reach the activation energy, then a much larger number of molecules will have activation energy, and therefore the reaction will be faster.
In the chemical industry, catalysts are widely used. Under the influence of catalysts, reactions can be accelerated by a factor of millions or more.
Distinguish between homogeneous and heterogeneous catalysis. In homogeneous catalysis, the catalyst and reactants form one phase (gas or solution). In heterogeneous catalysis, the catalyst is in the system as an independent phase.
Heterogeneous catalysis
In How Reactions Occur: Collision Theory, when discussing how molecules interact, the formula below was used as an example.
To break the A-B bond and form the C-A bond shown in the equation, reactant C must collide with the part of the A-B molecule where A. is located. Whether the collision occurs in this way depends largely on the case. However, according to the theory of probability, sooner or later it will happen. To increase the likelihood of such a collision, you should "bind" the A-B molecule so that its A site is "oriented" towards the reagent C.
This can be done using a heterogeneous catalyst: it "binds" a molecule of one reactant to its surface, orienting it in such a way as to accelerate the reaction. The heterogeneous catalysis process is shown in Fig. 8.8.
The catalyst is called heterogeneous ("heterogeneous"), because it is in a state of aggregation that is different from the state of aggregation of the reacting substances. Such a catalyst is usually a finely divided solid metal or its oxide, while the reactants are gases or solutions. In heterogeneous catalysis, the reaction proceeds on the catalyst surface. Hence it follows that the activity of the catalyst depends on the size and properties of its surface. In order to have a large surface, the catalyst must have a porous structure or be in a fragmented state.
In heterogeneous catalysis, the reaction proceeds through active intermediate compounds - surface compounds of the catalyst with reactants. Passing through a series of stages in which these intermediates are involved, the reaction ends with the formation of final products, and the catalyst is not consumed as a result.
Many of us are confronted with the work of a heterogeneous catalyst almost every day. It is a catalytic converter for a vehicle. This converter consists of crushed metals (platinum and / or palladium) used to accelerate the reaction, which decomposes harmful gases from the combustion of gasoline (such as carbon monoxide and unburned hydrocarbons) into harmless products (such as water and carbon dioxide ).
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– Complex reactions - Influence of temperature on the rate constant - Reversible and heterogeneous reactions - Photochemical reactions - Catalysis
2.1.7 Complex reactions
Chemical reactions that occur in more than one stage are called complex. Consider, as an example, one of the complex reactions, the kinetics and mechanism of which are well studied:
2НI + Н 2 О 2 ––> I 2 + 2Н 2 О
This reaction is a second order reaction; its kinetic equation is as follows:
The study of the reaction mechanism has shown that it is a two-stage (occurs in two stages):
1) НI + Н 2 О 2 ––> НIО + Н 2 О
2) НIО + НI ––> I 2 + Н 2 О
The speed of the first stage V 1 is much higher than the speed of the second stage V 2 and the overall reaction rate is determined by the speed of the slower stage, which is therefore called speed-determining or limiting .
It is possible to draw a conclusion about whether the reaction is elementary or complex based on the results of studying its kinetics. The reaction is complex if the experimentally determined partial orders of the reaction do not coincide with the coefficients for the starting materials in the stoichiometric reaction equation; partial orders of a complex reaction can be fractional or negative, the kinetic equation of a complex reaction can include the concentrations of not only the starting substances, but also the reaction products.
2.1.8 Classification of complex reactions
Consecutive reactions.
Sequential complex reactions are called, proceeding in such a way that the substances formed as a result of one stage (i.e., the products of this stage) are the starting materials for another stage. A schematic of a sequential reaction can be depicted as follows:
A ––> B ––> C ––> ...
The number of stages and the substances participating in each of the stages can be different.
Parallel reactions.
Chemical reactions are called parallel in which the same starting materials can simultaneously form different reaction products, for example, two or more isomers:
Conjugate reactions.
It is customary to call conjugate complex reactions proceeding as follows:
1) A + B ––> C
2) A + D ––> E ,
moreover, one of the reactions can proceed independently, and the second is possible only in the presence of the first. Substance A, common to both reactions, is called actor, substance B - inductor, substance D, interacting with A only in the presence of the first reaction - acceptor... For example, benzene in an aqueous solution is not oxidized by hydrogen peroxide, but when Fe (II) salts are added, it is converted to phenol and diphenyl. The reaction mechanism is as follows. In the first stage, free radicals are formed:
Fe 2+ + H 2 O 2 ––> Fe 3+ + OH - + OH
which react with Fe 2+ ions and benzene:
Fe 2+ + OH ––> Fe 3+ + OH -
C 6 H 6 + OH ––> C 6 H 5 + H 2 O
Recombination of radicals also occurs:
C 6 H 5 + OH ––> C 6 H 5 OH
C 6 H 5 + C 6 H 5 ––> C 6 H 5 –C 6 H 5
Thus, both reactions involve a common intermediate free radical OH.
Chain reactions.
Chain reactions are called reactions consisting of a number of interconnected stages, when the particles formed as a result of each stage generate subsequent stages. As a rule, chain reactions take place with the participation of free radicals. All chain reactions are characterized by three typical stages, which we will consider using the example of the photochemical reaction of the formation of hydrogen chloride.
1. The origin of the chain (initiation):
Сl 2 + hν ––> 2 Сl
2. Development of the chain:
Н 2 + Сl ––> НСl + Н
Н + Сl 2 ––> НСl + Сl
The stage of chain development is characterized by the number of reaction product molecules per one active particle - the chain length.
3. Open circuit (recombination):
Н + Н ––> Н 2
Сl + Сl ––> Сl 2
Н + Сl ––> НСl
The chain can also be broken when active particles interact with the material of the wall of the vessel in which the reaction is carried out; therefore, the rate of chain reactions may depend on the material and even on the shape of the reaction vessel.
The reaction of the formation of hydrogen chloride is an example of an unbranched chain reaction - a reaction in which one reacted active particle has no more than one newly emerging one. Chain reactions are called branched, in which for each reacted active particle there is more than one newly arising, i.e. the number of active particles constantly increases during the reaction. An example of a branched chain reaction is the reaction between hydrogen and oxygen:
1. Initiation:
Н 2 + О 2 ––> Н 2 О + О
2. Development of the chain:
О + Н 2 ––> Н + ОН
H + O 2 ––> O + OH
OH + H 2 ––> H 2 O + H
Copyright © S. I. Levchenkov, 1996 - 2005.
MINISTRY OF EDUCATION AND SCIENCE OF RUSSIA
Federal State Budgetary Educational Institution
higher professional education
"NOVOSIBIRSK NATIONAL RESEARCH STATE UNIVERSITY"
FACULTY OF NATURAL SCIENCES
P.A. Kolinko, D. V. Kozlov
Chemical Kinetics in Physical Chemistry Course
Study guide
Novosibirsk
The teaching aid contains material of lectures on the section "Chemical kinetics" of the course "Physical chemistry", read to the 1st year students of the FEN NSU.
Intended for 1st year students of the Faculty of Natural Sciences of Novosibirsk State University.
Compiled by:
Cand. chem. Sciences, Assoc. D. V. Kozlov, Cand. chem. Sciences P. A. Kolinko
The manual was prepared as part of the implementation
Development programs of NRU - NSU
© Novosibirsk State
university, 2013
FOREWORD |
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Chemical kinetics as a section |
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physical chemistry |
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Basic concepts of chemical kinetics |
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Methods for measuring the rate of chemical |
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The concept of the mechanism of a chemical reaction |
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Elementary chemical reactions |
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Kinetic equation of a chemical reaction |
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Methods for Finding the Order of a Reaction |
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Chemical reaction rate constant |
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Arrhenius law |
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Formal kinetics as a branch of chemical |
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kinetics |
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First-order irreversible reactions |
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Irreversible second order reactions |
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Irreversible third-order reactions |
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Effective reaction time |
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Reversible reactions |
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The concept of the path of a chemical reaction |
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General concepts of the theory of an elementary act |
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chemical reaction |
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Thermodynamic approach in theory |
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transition complex |
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Collision theory |
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Complex reactions and reactions involving |
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intermediate particles. Classification |
|
FOREWORD
In chemical science in general and in physical chemistry, in
in particular, there is a special area that studies the mechanisms and patterns of the course of chemical processes in time. This science is called -
chemical kinetics... Chemical kinetics examines and establishes the dependence of the rate of chemical reactions on the concentration of reagents,
temperature and other external conditions.
Chemical kinetics is the cornerstone on which the modern chemical industry and, in particular, petrochemistry,
oil refining and polymer production.
In the first year of FEN NSU, chemical kinetics is read at the end of the course "Physical chemistry" in the last five lectures. Perhaps due to the fact that by the end of the course, out of more than 30 lectures, students get tired, this part of the lectures is not mastered well enough. The second reason is
what exactly is in chemical kinetics most of all mathematical calculations and formulas, when compared with other parts of the course "Physical Chemistry".
The purpose of this manual is to provide students with an opportunity to familiarize themselves with the basic concepts of chemical kinetics, formal kinetics, the theory of the elementary act of a chemical reaction, the theory of collisions, and much more. At the same time, readers have the opportunity to compare the material read by the lecturer at the university with the material of the manual and ask questions on incomprehensible topics to the lecturer and seminarians. We hope this will allow students to better assimilate the material.
For ease of understanding, the basic concepts,
those mentioned in the text for the first time are in bold italics, their definitions are in bold.
1. Chemical kinetics as a branch of physical chemistry
composition and energy effect chemical reaction.
However, this science cannot answer the questions about how this reaction is carried out and at what speed. These questions, namely, questions about the mechanism and chemical reaction rate fall within the competence of chemical kinetics.
Chemical kinetics or kinetics of chemical reactions (from the Greek κίνησις - movement) - share
physical chemistry, studying the laws of the course of chemical reactions in time, the dependence of these laws on external conditions, as well as the mechanisms of chemical transformations ... Unlike thermodynamics, chemical kinetics studies the course of chemical reactions in time. Those. thermodynamics studies the initial and final states of a system, while chemical kinetics studies the change in a system during the transition from the initial state to the final state. For example, the reaction
from the point of view of thermodynamics, it is very favorable, in any case, at temperatures below 1000 ° C (at
At higher temperatures, the decomposition of CO2 molecules occurs), i.e. carbon and oxygen should (almost 100% yield) be converted to carbon dioxide. However, experience shows that a piece of coal can lie in the air for years, with free access of oxygen, without undergoing any changes. The same can be said for many other known reactions. Thus, knowledge of the kinetic laws is also important in the storage and operation of chemical products, when it is necessary to slow down their destruction. This is important, for example, when storing foodstuffs, medicines, fuels, polymers.
2. Basic concepts of chemical kinetics
2.1. Stoichiometric equation of a chemical reaction
Formal kinetics makes it possible to quantitatively describe the course of a chemical process in time at a constant temperature, depending on the concentration of reactants and their phase composition. Used for description stoichiometric equation–
is an equation showing the quantitative ratios of reagents and products of a chemical reaction ... The simplest example of such an equation is
stoichiometric coefficients. And i - reagents, B j - reaction products.
The increments in the quantities of reagents and products obey the stoichiometric equation, and on its basis is determined material balance substances during chemical transformations. The amount of substances is usually measured in moles. If necessary, other mass characteristics of the system are expressed through them. The use of stoichiometric equations is the main way to describe chemical reactions in classical chemistry. However, the stoichiometric equation does not describe reaction mechanism... Any chemical reaction is difficult enough. Its stoichiometric equation, as a rule, does not take into account all the complexity of elementary processes.
2.2. Depth of reaction
V In such a reactive system (1), the masses of individual substances are not independent variables. Change in the number of moles dn i is proportional to
stoichiometric coefficients in the reaction equation. That is, you can write
or in integral form
where ni 0 is the initial amount of reagent or product (mol); ni is the current amount of reagent or product (mol); yi is the stoichiometric coefficient. Recall that for the reaction products yi> 0, and for the reagents yi<0.
Thus, the redistribution of masses in the system as a result of the reaction can be described by a single variable ξ, which is called chemical variable... The chemical variable is measured in moles
and can take on a variety of meanings.
V in particular, the initial state of the system is characterized by the value ξ = 0. If the process proceeds towards the reaction products, then ξ will be greater than 0, and if towards the reactants (reverse reaction), then ξ< 0. Вообще,
the course of the reaction.
2.3. Chemical reaction rate
The study of the kinetics of specific chemical reactions begins, as a rule, with the construction of experimentally determined dependences Ci = f (t), which are called kinetic curves... Further, the analysis of these data begins and the study of the reaction mechanism. But this requires lengthy and complex research, therefore, after the kinetic curves are obtained, it is possible to process these
Relaxation time of molecules of a substance- this is the time it takes for a molecule to move (react) to the impact. If the relaxation time of the molecules of a given substance is much longer than the time of exposure to the substance, then the molecules do not have time to rearrange (move) under load, which leads to the rupture of chemical bonds in the substance. For different substances, the relaxation time is different and can vary over a very wide range: from thousandths of a second to several millennia.
History
In the 19th century, it was even suggested that liquid and solid bodies do not have a clear boundary. If you think about it, there is nothing particularly surprising here. If, for example, you hit water hard with your palm, then the water will behave like a solid body (you can feel it if you wish!). If you hit with a hammer on a stream of thick liquid, then with the help of a short exposure of the camera, you can fix that from the impact the stream will fly into many small sharp fragments (drops), which indicates some properties of a solid fragile body. Or if you take a piece of resin, glasses are amorphous bodies. They are so viscous that the flow properties of these materials are not visible. In fact, they flow, and this can be easily determined by applying a load to them! (of course, glass is a more viscous substance and, upon manifestation of flow properties, requires more individual conditions, for example, heating or a long time of application of a load).Relaxation time by glass
Glass has a rather long relaxation time, therefore, as a result of rapid cooling, the molecules do not have time to take up their metastable state in the structure. Subsequently, they freeze in their chaotic structure. And it is the disorder of the structure of matter that determines the excess Gibbs energy, in contrast to its crystalline form, in which the molecules are found with minimal energy. As a consequence, it gives the molecules in the glassy state an incentive to evolve the structure towards more ordering (that is, towards a structure with less energy).This spontaneous evolution, which is a property common to all glasses, is collectively called structural relaxation. In the case of metallic glasses, this is a large-scale phenomenon, which noticeably or even very strongly changes all of their physical properties. Despite numerous studies of this phenomenon, it remains largely unexplored, and its mechanisms are incomprehensible.
