Chemical kinetics, also known as reaction kinetics, chemical reaction kinetics, is a branch of physical chemistry, a branch of physics and chemistry that studies the rate and mechanism of chemical processes.Its research object is an unbalanced dynamic system whose properties change with time.
Its main research areas include: molecular reaction kinetics, catalytic kinetics, elementary reaction kinetics, macroscopic kinetics, microscopic kinetics, etc., and can also be classified into organic reaction kinetics and inorganic reaction kinetics according to different chemical branches.
Chemical kinetics is often a decisive factor in the chemical production process.
Basic Introduction of Chemical Kinetics
Time is an important variable in chemical kinetics. The classical chemical kinetics experimental method cannot produce a single quantum state reactant, nor can it detect the initial ecological product produced by a single reaction collision.
The thermodynamic equilibrium properties of the system do not give information on chemical kinetics. A comprehensive understanding of a chemical reaction process and its implementation cannot be lacked.
The calculation of quantum chemistry has not yet obtained a reliable and complete potential energy surface of the reaction system.
Therefore, the current reaction rate theory still has to borrow the classical statistical mechanics processing method. Such a process must make some form of equilibrium assumptions, thus making these rate theories unsuitable for very fast reactions.
Although there have been many studies on the applicability of the hypothesis, it is not yet mature to deal with the reaction rate completely with the non- equilibrium theory.
The use of molecular beams (ie, molecular scattering), especially cross-molecular beam methods, to study the kinetics of chemical element reactions makes it possible to experimentally study single-reaction collisions.
Molecular beam experiments have obtained microscopic information about chemical element reactions that many classical chemical kinetics cannot achieve. Molecular reaction kinetics is a frontier of modern chemical kinetics.
The thermodynamic equilibrium properties of the system do not give information on chemical kinetics. For example, the following reaction:
2H2 (gas) + O2 (gas) – → 2H2O (gas)
Although all the thermodynamic properties of H2, O2 and H2O are accurately known, only H2 and the possibility of generating H2O O2, but can not predict H2 and O2 can be in what conditions at a given rate of reaction generating H2O, cannot provide H2 molecules, and O2 molecule is bound by the steps of H2O molecule information.
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Therefore, a comprehensive understanding of a chemical reaction process and implementation, there can be no lack of chemical kinetics research.
Chemical Kinetics Research History
In the first half of the 20th century, a large amount of research work was carried out on the determination of these parameters, theoretical analysis and the use of parameters to study the reaction mechanism.
However, confirmation of the reaction mechanism relies primarily on the ability to detect and analyze the reaction intermediates. In the late 20th century, the development of free radical chain reaction kinetics brought two development trends to chemical kinetics:
1) extensive research on meta-reaction kinetics.
2) the urgent need to establish methods for detecting active intermediates.
This requirement and the development of electronics and laser technology have facilitated the development of rapid reaction kinetics. The temporal resolution of transient active intermediate detection has increased from milliseconds in the 1950s to picoseconds.
Chemical kinetics is a branch of chemistry that studies the rate of reaction and the mechanism of reaction. The main contents of chemical kinetics include the following:
1) Determine the rate of the chemical reaction and the influence of external factors such as temperature, pressure, catalyst, solvent, and light on the reaction rate.
2) Study the mechanism of a chemical reaction and reveal the nature of the chemical reaction rate.
3) Explore the relationship and laws between material structure and responsiveness.
Through chemical kinetics research, we can know how to control the reaction conditions, increase the rate of main reaction, increase product yield, inhibit the rate of side reactions, reduce raw material consumption, reduce by-products, improve purity, and improve product quality.
Chemical kinetics also studies how to avoid explosions of dangerous goods, corrosion of materials, deterioration, and aging of products. Therefore, the study of chemical kinetics has great theoretical and practical significance.
The methods of chemical kinetics are:
1. Phenomenological dynamics research method, also known as classical chemical kinetics research method, is based on the original experimental data of chemical kinetics – the relationship between concentration c and time t – and some analytical kinetic parameters are obtained after analysis- reaction rate constant K, the activation energy E A, pre-exponential factor A.
These parameters can be used to characterize the rate characteristics of the reaction system. Commonly used relationships are:
Wherein r is the reaction rate; A, B, C, D is the concentration of each substance, [alpha], beta], gamma], [delta] with respect to the number of stages is called material A, B, C, D of; R & lt is the gas constant, T is Thermodynamic temperature.
The chemical kinetic parameters are effective data to explore the reaction mechanism. The time resolution of the detection of transient active intermediates has changed from milliseconds to picoseconds in the 1950s.
2. molecular reaction kinetics research method, from the microscopic molecular level, a meta-chemical reaction is a collision of reactant molecules with a certain quantum state, performing atomic rearrangement, producing product molecules of a certain quantum state and even separating from each other.
Single reaction collision behavior. With a transition state theory to explain, it is in the reaction system over potential energy surface on behalf of a system of particles once behavior over the reaction barrier.
In principle, if the correct potential energy surface of the reaction system can be calculated from quantum chemistry theory and the laws of mechanics are used to calculate the trajectories of the representative points on it, the reaction rate and chemical kinetic parameters can be calculated.
However, with the exception of a few very simple chemical reactions, the calculation of quantum chemistry has not yet achieved a reliable and complete potential energy surface of the reaction system.
Therefore, the current reaction rate theory (such as bimolecular reaction collision theory, transition state theory) still has to borrow the classical statistical mechanics processing method. Such a process must make some form of equilibrium assumptions, thus making these rate theories unsuitable for very fast reactions.
Although there have been many studies on the applicability of the equilibrium hypothesis, it is not mature to completely deal with the reaction rate with the non-equilibrium theory.
In the 1960s, experimental studies at the molecular level of chemical reactions were difficult to achieve. It uses advanced analytical methods of modern physical chemistry to study the dynamic structure, reaction process and reaction mechanism of single-molecule elementary chemical reactions in different states and different molecular systems at the atomic and molecular levels.
It studies the rate and mechanism of the elementary reaction process from the microscopic level of the molecule, focusing on the dynamic behavior of the chemical elementary process from the interaction between the internal motion of the molecule and the collision of molecules.
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The State Key Laboratory of Molecular Reaction Dynamics of the Dalian Institute of Chemical Physics of the Chinese Academy of Sciences has made outstanding contributions in this area.
3. Network dynamics research method, which simulates and optimizes important chemical reaction processes (such as hydrocarbon thermal cracking) including dozens or even hundreds of elementary reaction steps for the optimal design of the reactor.
Chemical kinetics is a branch of physical chemistry that studies the rate and mechanism of the reaction of chemical processes.
Chemical kinetics is different from chemical thermodynamics in that it does not calculate the extent of conversion of a reaction at the equilibrium of the reaction, but rather observes the chemical reaction from a dynamic point of view, studies the time required for the reaction system to change, and the microscopic involved.
The basis of chemical kinetics and thermodynamics is statistical mechanics, quantum mechanics, and molecular motion theory.
Its research object is an unbalanced dynamic system whose properties change with time. Chemical thermodynamics is a sub-discipline of physical chemistry and thermodynamics.
It mainly studies the energy changes accompanying physical and chemical changes of a material system under various conditions, thus making an accurate judgment on the direction and extent of a chemical reaction.
Chemical thermodynamics is based on three basic laws. The first law of thermodynamics is the law of conservation of energy and the law of transformation, which is summarized by many scientists.
Dynamics is a sub-discipline of theoretical mechanics, studying the relationship between the forces acting on objects and the motion of objects. The study of dynamics is a macroscopic object whose velocity is much slower than the speed of light.
The study of the dynamics of atomic and subatomic particles belongs to quantum mechanics, the study of high-speed motion comparable to the speed of light belongs to relativistic mechanics. Dynamics is the foundation of physics and astronomy and the basis of many engineering disciplines.
Many mathematical advances are often associated with solving dynamic problems, so mathematicians have a strong interest in dynamics.
Chemical Kinetics Use
Using the principles of chemical kinetics:
(1) Study the mechanism of drug degradation.
(2) Study factors affecting drug degradation and stabilization measures.
(3) Predicting the expiration date of the pharmaceutical preparation.
The reaction rate is a measure of the degree of a chemical reaction. In a broad sense, the absolute value of the amount of the substance participating in the reaction changes with time and is divided into an average rate and an instantaneous rate.
The average rate is the amount of change in the amount of the substance participating in the reaction at a certain time interval (Δt) in the course of the reaction, and can be expressed by the amount of reduction of the reactant per unit time or the amount of increase in the product; the instantaneous rate is the rate of change with time.
That is, the tangent slope of the function at a particular time on the concentration-time image.
Reaction equilibrium: Thermodynamics studies the state of the reaction as it reaches equilibrium. In the reversible reaction, the reactants and the product reach a dynamic equilibrium, the rates of the forward and reverse reactions are equal, and the concentrations of the reactants and products no longer change.
It can be demonstrated by the Haber method of synthesizing ammonia, chemical oscillation reaction such as Belousov-Zhabotinsky reaction (BZ reaction), iodine clock reaction and other multi-component reaction processes.
Reaction mechanism: Although the stoichiometric ratio of each substance in the chemical equation seems simple, microscopically, a chemical reaction is usually completed in several steps, and the chemical kinetic branch describing the microscopic process of the chemical reaction is called the Reaction Mechanism.
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In the reaction mechanism, each step of the reaction is called an elementary reaction, and the sum of the number of molecules of the reactants in the elementary reaction is called the number of reactive molecules. The reactor consists of one or more elementary reactions, and the net reaction of these elements is an apparent chemical reaction.