Relaxation time of substance molecules- this is the time it takes for the 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 breaking 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.
Story
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 the water hard with your palm, then the water will behave like a solid body (you can feel it if you wish!). If you hit a jet of thick liquid with a hammer, then with the help of a slow shutter speed of the camera you can fix that the jet will scatter into many small sharp fragments (drops) from the impact, which indicates some properties of a brittle solid 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 are flowing, and this can be easily determined by applying a load to them! (Of course, glass is a more viscous substance and requires more individual conditions for the manifestation of flow properties, for example, heating or a long time of application of the load).Relaxation time at the glass
Glass has a rather long relaxation time, therefore, as a result of rapid cooling, the molecules do not have time to take their metastable state in the structure. Subsequently, they freeze in their chaotic structure. And it is the disorder of the structure of the substance that causes the excess Gibbs energy, in contrast to its crystalline form, in which the molecules are with minimal energy. As a consequence, this gives the molecules in the glassy state an incentive to evolve the structure towards more order (i.e., towards a structure with less energy).This spontaneous evolution, which is a property common to all glasses, has received the collective name of structural relaxation. In the case of metallic glasses, this is a large-scale phenomenon that noticeably or even very strongly changes all of their physical properties. Despite numerous studies of this phenomenon, it still remains largely unexplored, and its mechanisms remain incomprehensible.
– Complex reactions – Effect of temperature on the rate constant – Reversible and heterogeneous reactions – Photochemical reactions – Catalysis
2.1.7 Complex reactions
Complex reactions are chemical reactions that occur in more than one stage. 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 has the following form:
The study of the reaction mechanism showed that it is a two-stage (it proceeds in two stages):
1) HI + H 2 O 2 -–> HIO + H 2 O
2) HIO + HI -–> I 2 + H 2 O
The rate of the first stage V 1 is much greater than the rate of the second stage V 2 and the overall reaction rate is determined by the rate of the slower stage, therefore called rate-determining or limiting .
It is possible to conclude whether the reaction is elementary or complex based on the results of studying its kinetics. A reaction is complex if the experimentally determined partial orders of the reaction do not match 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 concentrations of not only the starting substances, but also the reaction products.
2.1.8 Classification of complex reactions
successive reactions.
Sequential are complex reactions that proceed 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. Schematically, a sequential reaction can be depicted as follows:
A ––> B ––> C ––> ...
The number of stages and substances involved in each of the stages may be different.
parallel reactions.
Parallel chemical reactions are called, in which the same starting substances can simultaneously form different reaction products, for example, two or more isomers:
Associated reactions.
Conjugated reactions are called complex reactions that proceed 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 into 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 proceed with the participation of a common intermediate free radical OH.
Chain reactions.
Chain reactions are called reactions consisting of a number of interrelated stages, when the particles formed as a result of each stage generate subsequent stages. As a rule, chain reactions proceed with the participation of free radicals. All chain reactions are characterized by three typical stages, which we will consider using the photochemical reaction of the formation of hydrogen chloride as an example.
1. The origin of the chain (initiation):
Сl 2 + hν ––> 2 Сl
2. Chain development:
H 2 + Cl -–> HCl + H
H + Cl 2 ––> HCl + Cl
The stage of chain development is characterized by the number of molecules of the reaction product per one active particle - the length of the chain.
3. Open circuit (recombination):
H + H -–> H 2
Cl + Cl ––> Cl 2
H + Cl ––> HCl
Chain termination is also possible when active particles interact with the wall material 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 there is no more than one newly emerging one for one reacted active particle. Branched chain reactions are called, in which for each reacted active particle there is more than one newly emerging, i.e. the number of active particles in the course of the reaction is constantly increasing. An example of a branched chain reaction is the reaction of the interaction of hydrogen with oxygen:
1. Initiation:
H 2 + O 2 -–> H 2 O + O
2. Chain development:
O + H 2 ––> H + OH
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 the course of physical chemistry
Teaching aid
Novosibirsk
The teaching aid contains lecture material on the section "Chemical Kinetics" of the course "Physical Chemistry", read to 1st year students of the Faculty of Natural Sciences of NSU.
Designed for 1st year students of the Faculty of Natural Sciences of Novosibirsk State University.
Compiled by:
cand. chem. Sciences, Assoc. D. V. Kozlov, Ph.D. chem. Sciences P. A. Kolinko
The manual has been prepared as part of the implementation
Development Programs 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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Irreversible first order 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 elementary act theory |
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chemical reaction |
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Thermodynamic approach in theory |
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transition complex |
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Impact theory |
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Complex reactions and reactions involving |
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intermediate particles. Classification |
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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 chemical processes in time. This science is called
chemical kinetics. Chemical kinetics considers and establishes the dependence of the rate of chemical reactions on the concentrations of reagents,
temperature and other external conditions.
Chemical kinetics is the cornerstone on which the modern chemical industry and, in particular, petrochemistry, stands.
oil refining and polymer production.
In the first year of FEN NGU, 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 of more than 30 lectures, students get tired, this part of the lectures is not absorbed well enough. The second reason, then,
that it is in chemical kinetics that there are the most mathematical calculations and formulas, when compared with other parts of the Physical Chemistry course.
The purpose of this manual is to give students the opportunity to get acquainted with the basic concepts of chemical kinetics, formal kinetics, the theory of an elementary act of a chemical reaction, the theory of collisions and many others. At the same time, readers have the opportunity to compare the material read by the lecturer at the university with the material of the training manual and ask questions on incomprehensible topics to the lecturer and seminarians. We hope this will allow students to better understand the material.
For ease of understanding, the basic concepts
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 questions about how this reaction is carried out and at what speed. These questions, namely, questions about the mechanism and rate of a chemical reaction fall within the scope of chemical kinetics.
Chemical kinetics or kinetics of chemical reactions (from Greek κίνησις - movement) - section
physical chemistry, studying the patterns of chemical reactions in time, the dependence of these patterns 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 (practically with 100% yield) turn into carbon dioxide. However, experience shows that a piece of coal can lie in the air for years, with free access to oxygen, without undergoing any changes. The same can be said about many other known reactions. Thus, knowledge of the kinetic regularities 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 food, drugs, 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 the reactants and their phase composition. Used to describe stoichiometric equation–
this is an equation showing the quantitative ratios of reactants and products of a chemical reaction . The simplest example of such an equation is
stoichiometric coefficients. А i – reagents, B j – reaction products.
The stoichiometric equation obeys the increments of the amounts of reactants and products, and on its basis is determined material balance substances during chemical transformations. Amounts of substances are 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 quite complex. Its stoichiometric equation, as a rule, does not take into account the complexity of elementary processes.
2.2. Reaction depth
AT in such a reacting system (1) the masses of individual substances are not independent variables. Change in the number of moles dn i proportionally
stoichiometric coefficients in the reaction equation. That is, you can write
or in integral form
where ni 0 is the initial amount of the reagent or product (mol); ni is the current amount of the reagent or product (mol); yi is the stoichiometric coefficient. Recall that for reaction products yi >0, and for reactants 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. A chemical variable is measured in moles
and can take on a variety of values.
AT 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. The rate of a chemical reaction
The study of the kinetics of specific chemical reactions begins, as a rule, with the construction of experimentally determined dependencies Ci = f(t), which are called kinetic curves. Then the analysis of these data and the study of the reaction mechanism begins. But this requires long and complex studies, so after the kinetic curves are obtained, it is possible to process these
RELAXATION
(from Latin relaxatio-weakening), the process of establishing in the system thermodynamic equilibrium. macroscopic state. system is defined by many. parameters, and the processes of achieving equilibrium in different parameters can proceed with decomp. speeds. Allocate a period of linear R., when a certain parameter of the state i differs only slightly 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. It follows that at time t the deviation exp (Тt/t i). During the time t i small parameter deviation x i decreases from the equilibrium value by a factor of e times. Quantities = 1/t i, reciprocal times R., called. frequencies R.
R.'s times are determined by the St. you system and the type of process under consideration. In real systems, they can vary from negligibly small values to values on the order of the age of the Universe. A system can achieve equilibrium in some parameters and remain non-equilibrium in others (partial equilibrium). All R. processes are non-equilibrium 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 due to the exchange of energy and the amount of motion during particle collisions, and R.'s time is determined by the time of free. run (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, a stage of fast R. is distinguished, when, over a short period of time on the order of the time of collision of molecules, the initial (strongly nonequilibrium) state becomes so chaotic that to describe it, it is sufficient to know how the distribution over coordinates and momenta of just one particle (so-called .single-particle distribution function). At the second stage R. during the time of the order of time free. run as a result of just several. collisions in macroscopically small volumes moving at an average mass transfer rate (mass velocity), a local thermodynamic 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.). Time R. depends on the size of the system and is large in comparison with the average time free. run.
In polyatomic gases (with internal degrees of freedom) m. the exchange of energy between and int. degrees of freedom (rotate, oscillate) and R. arises, associated with this phenomenon. Equilibrium is established most quickly by actions. degrees of freedom, which is characterized by the corresponding t-swarm. Balance between acts. and rotate. degrees of freedom is established much more slowly. Excitation oscillate. degrees of freedom is possible only at high temperatures. Therefore, in polyatomic gases, multistage R. processes are possible (see. non-equilibrium kinetics). If the gas consists of components with molecules that differ greatly in mass, the energy exchange 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 liquids, R. is described with the help of spatio-temporal correlations. functions that characterize 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 Fig. Liquid). R.'s time to full thermodynamic. equilibrium can be assessed using kinetic. coefficients. For example, in a binary solution, the time R. concentration t! 2
/D, where L is the size of the system, D is the coefficient. diffusion; time R. t-ry t! L 2 /x, where x-coefficient. thermal diffusivity, etc. (for details, see macrokinetics).
In solids, R. is described as R. in a gas of certain quasi-particles. For example, in crystalline lattice at low temperatures, elastic vibrations are interpreted as a gas of phonons (acoustic radiation). In the system of spin magnets. moments of a ferromagnet, quasiparticles are magnons (magnetic radiation).
At phase transitions The river can have difficult character. If the transition from a non-equilibrium state to an equilibrium state is a first-order transition, the system can first go into a metastable state and then relax extremely slowly (see Fig. glassy state). Relaxation is especially difficult. transitions in polymers where there is a set (spectrum) of relaxation. phenomena, each of which is due to its own mechanism. In the vicinity of a phase transition point of the second kind, the degree of phase ordering is characterized by an order parameter, which tends to zero, and its time R. increases greatly. Even more difficult is the nature of R. from states that are very far from thermodynamic. balance. In open systems, phenomena are possible self-organization.
R.'s measurements of times use in chemical. kinetics for the study of processes in which equilibrium is quickly established (see. relaxation methods). Mechanical R. is manifested in a decrease in time of stress, which 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 the system, which by the time of physiological death comes into a state of partial equilibrium (quasi-equilibrium) with the environment. In nature systems R. times are separated by strong inequalities; their arrangement in ascending or descending order allows us to consider the system as a sequence of hierarchical. levels with diff. the degree of order in the structure (cf. 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 G.P., 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, reduction), the process of establishing thermodynamic equilibrium in macroscopic. physical systems (gases, liquids, solid bodies). macroscopic state. system is determined by a large number of parameters, and the establishment ... ... Physical Encyclopedia
relaxation- (from the Latin relahatio reduction of tension, weakening) a state of rest, relaxation, which occurs in the subject as a result of stress relief, after strong experiences or physical efforts. R. may be involuntary (relaxation upon departure ... ... Great Psychological Encyclopedia
Relaxation- is the process of gradual transition of a thermodynamic system from a non-equilibrium state caused by external influences to a state of thermodynamic equilibrium. Examples of relaxation processes: a gradual change in stress in the body ... ... Encyclopedia of terms, definitions and explanations of building materials
- [lat. relaxatio decrease in tension, weakening] honey. relaxation of skeletal muscles; relieving mental stress. Dictionary of foreign words. Komlev N.G., 2006. relaxation (Latin relaxatio, stress reduction, weakening) 1) physical. process … Dictionary of foreign words of the Russian language
relaxation- and, well. relaxation, German Relaxation relaxatio reduction of tension, relaxation. 1. physical. The process of gradually returning to a state of equilibrium a system brought out of such a state, after the cessation of the factors that brought it out ... Historical Dictionary of Gallicisms of the Russian Language
RELAXATION, the process of establishing thermodynamic equilibrium in a macroscopic physical system consisting of a large number of particles. Characteristic of the relaxation time process. For example: for a system of electrons in a metal, the relaxation time 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 the use of muscle relaxants ... Big Encyclopedic Dictionary
Relaxation, thermal relaxation, attenuation, weakening Dictionary of Russian synonyms. relaxation noun, number of synonyms: 6 autorelaxation (1) … Synonym dictionary
- (from the Latin relaxatio relaxation, discharge, rest), 1) relaxation or a sharp decrease in the tone of skeletal muscles. Artificial relaxation achieved by the use of muscle relaxant preparations is used in surgical interventions. ... ... Modern Encyclopedia
- (from Latin relaxatio relief, relaxation) a state of rest associated with complete or partial muscle relaxation. They share the long-term relaxation that occurs during sleep, hypnosis, under pharmacological influences, and ... ... Psychological Dictionary
Let's say you need to get the maximum possible amount of ammonia from a given amount of hydrogen and nitrogen. From the material of the previous section, you already know how you can influence the course of a reaction by shifting the chemical equilibrium in one direction or another. However, in order to solve the problem most effectively, one should also take into account the rate of the reaction. 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 particular substance, the size and productivity of the equipment, the amount of the product produced depend on the reaction rate.
Chemical reactions proceed at different rates. Some of them end in a fraction of a 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 desired 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 that affect the rate of a reaction are:
S nature of reactants;
S particle sizes of reagents;
S concentration of reactants;
S pressure of gaseous reagents;
S temperature;
S presence in the system of catalysts.
The nature of the reactants
As already noted, a necessary condition for a chemical interaction to occur between the particles of the initial substances is their collision with each other (collision), and at the site of the molecule with high reactive activity (see the section "How reactions occur: collision theory" above in the chapter ). The larger and more complex the reactant molecules, the lower the probability that a collision will occur precisely in the area
highly reactive molecules. Often, in rather complex molecules, a site with high reactivity is completely blocked by other parts of the molecule, and the reaction does not occur. In this case, of the many collisions, only those that occur in the reactive region are effective (i.e., leading to chemical interaction).
In other words, the larger and more complex the molecules of the reactants, the slower the reaction rate.
Reagent Particle Size
The rate of a reaction depends on the number of collisions between the molecules of the reactants. 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 piece of coal into powder, spray it into the air, and then strike a match, an explosion will occur. The cause of the explosion (i.e., the high rate of the reaction) is a significant increase in the surface area of the coal.
Reactant concentration
An increase in the number of collisions of the reactants leads to an increase in the rate of the reaction. Thus, the reaction rate is proportional to the number of collisions that the molecules of the reactants undergo. The number of collisions, in turn, the greater, the higher the concentration of each of the starting substances. For example, a wooden plank burns quite well in ordinary air (which is 20% oxygen), but in pure oxygen, combustion occurs more intensely, i.e., at a faster rate.
In most simple reactions, increasing the concentration of reactants increases the rate of the reaction. However, in complex reactions occurring 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 has an effect on the step in the reaction that determines its rate. (This information will help calculate the reaction mechanism.) This can be done by running the reactions at several different concentrations and observing their effect on the reaction rate. If, for example, changing the concentration of one reactant does not affect the rate of the reaction, then you will know that at the slowest stage of the reaction mechanism (and the reaction rate is precisely determined by such a stage), this reactant is not involved.
Pressure of gaseous reagents
The pressure of the gaseous reactants has the same effect on the reaction rate as does the concentration. The higher the pressure of the reactants in the gaseous state, the higher the reaction rate. This is due to (you guessed it!) increased collisions. However, if the reaction has a complex mechanism, then changing the pressure may not lead to the expected result.
Temperature
Why, after Thanksgiving dinner, does every housewife rush to put the rest of the turkey in the refrigerator? Yes, because if this is not done, then the turkey may deteriorate. What does "spoil" mean? This means increased bacterial growth. Now, when the turkey is in the refrigerator, it will slow down the growth rate of bacteria 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. There is a rule in organic chemistry that a 10°C increase in temperature doubles the rate of a reaction.
Why is this happening? Partly (you guessed it!) due to the increased number of collisions. When the temperature rises, the molecules move faster, thus increasing the likelihood of their collisions with each other, and hence their chemical interaction. However, that's not all. As the temperature increases, the average kinetic energy of the molecules also increases. Pay attention to fig. 8.7, which gives an example of how an increase in temperature affects the kinetic energy of the reactants and the rate of the reaction.
At a given temperature, not all molecules have the same kinetic energy. Some of them can move extremely slowly (i.e., have low kinetic energy), while others can move quite quickly (i.e., 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 the kinetic energy. Also pay attention 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. the activation energy of this reaction. Molecules that have this energy are called active molecules.
The reactants not only need to collide in the reactive region, but also enough energy must be transferred to break the existing bonds and form new ones. If this energy is not enough, then the reaction will still not occur during the collision of the initial molecules.
Note that at a lower temperature (T1) a small number of reactant molecules have the required activation energy. At higher temperature (T2)
the activation energy (the minimum amount of kinetic energy required to form a new substance) will already have 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 the 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 changing the rate of a reaction under the action 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.
Look, for example, at fig. 8.1. If the value of the activation energy corresponding to the maximum on the graph were lower, then the number of effective collisions of the reactant molecules would be higher, which means that the reaction rate would also be higher. The same can be seen in Fig. 8.7. If we move the dotted line to the left, which indicates the minimum kinetic energy required to reach the activation energy, then many more molecules will have activation energy, and therefore the reaction will proceed faster.
Catalysts are widely used in the chemical industry. Under the influence of catalysts, reactions can be accelerated millions of times 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 present in the system as an independent phase.
heterogeneous catalysis
In the How Reactions Occur: Collision Theory section, when talking about the mechanism of interaction between molecules, the following formula was used as an example.
In order 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 chance. However, according to the theory of probability, sooner or later it will still happen. To increase the probability of such a collision, the A-B molecule should be “attached” in such a way that its A section 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 speed up the reaction. The process of heterogeneous catalysis is shown in fig. 8.8.
The catalyst is called heterogeneous ("heterogeneous"), because it is in a state of aggregation different from the state of aggregation of the reacting substances. A finely divided solid metal or its oxide usually acts as such a catalyst, while the reactants are gases or solutions. In heterogeneous catalysis, the reaction proceeds on the surface of the catalyst. It follows that the activity of a 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 crushed state.
In heterogeneous catalysis, the reaction proceeds through active intermediates - surface compounds of the catalyst with the reactants. Passing through a series of stages in which these intermediates participate, the reaction ends with the formation of end products, and the catalyst is not consumed as a result.
Many of us deal with the operation of a heterogeneous catalyst almost every day. This is a catalytic converter in a car. This converter consists of crushed metals (platinum and/or palladium) used to speed up a reaction that breaks down harmful gases from gasoline combustion (such as carbon monoxide and unburned hydrocarbons) into harmless products (such as water and carbon dioxide). ).
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