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Chemical kinetics, also known as reaction kinetics and chemical reaction kinetics, is a branch of physical chemistry and a branch of physical chemistry that studies the rate and reaction 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, macro-dynamics, microdynamics, etc. It can also be classified into organic reaction kinetics and inorganic reaction kinetics according to different chemical branches. Chemical kinetics is often the determining factor in chemical production.
Basic Introduction of Chemical Kinetics:
Time is an important variable in chemical kinetics. Classical chemical kinetics experiments cannot prepare reactants in a single quantum state, nor can they detect primary ecological products produced by a single reaction collision.
The thermodynamic equilibrium properties of the system cannot give information on chemical kinetics. A comprehensive understanding of a chemical reaction process and its realization must be carried out without chemical kinetics research.
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 processing methods of classical statistical mechanics.
Such processing must make some form of equilibrium assumptions, making these rate theories unsuitable for very fast reactions. Although there have been many studies on the applicability of the equilibrium hypothesis, it is still immature to completely deal with the problem of reaction rate using non- equilibrium theory.
The application of molecular beam (ie, molecular scattering), especially the cross molecular beam method, to the study of chemical element reaction kinetics makes it possible to experimentally study a single reaction collision. Molecular beam experiments have obtained a lot of microscopic information about chemical element reactions that cannot be obtained by classical chemical kinetics. Molecular reaction dynamics is a frontier of modern chemical kinetics.
The thermodynamic equilibrium properties of the system cannot give information on chemical kinetics. For example, for the following reaction:
2H2(gas) + O2(gas) ─ → 2H2O (gas)
Although all thermodynamic properties of H2, O2, and H2O are known accurately, only H2 and The possibility of O2 generating H2O, but it cannot predict what reaction rate H2 and O2 can generate H2O under given conditions nor can it provide the steps through which H2 molecules and O2 molecules are combined into H2O molecule information.
Therefore, to fully understand a chemical reaction process and implement it, chemical kinetics research cannot be lacked.
In the first half of the 20th century, a lot of research work was on the determination of these parameters, theoretical analysis and the use of parameters to study the reaction mechanism. However, confirmation of the reaction mechanism mainly depends on the ability to detect and analyze reaction intermediates.
In the late 20th century, the general research on the kinetics of free radical chain reactions has brought two development trends to chemical kinetics: first, extensive research on the kinetics of elementary reactions. This requirement and the development of electronics and laser technology have promoted the development of fast reaction kinetics. The time resolution for the detection of transiently active intermediates 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 the 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 chemical reaction mechanism and reveal the nature of the chemical reaction rate;
3) Explore the relationship and laws between material structure and reaction ability.
Chemical Kinetics Notes
Through the study of chemical kinetics, you can know how to control the reaction conditions, increase the rate of the main reaction, increase product yield, suppress 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 problems such as explosions of dangerous goods, corrosion of materials, and deterioration and aging of products. Therefore, the study of chemical kinetics has great theoretical and practical significance.
The research methods of chemical kinetics are:
① Phenomenological kinetics 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 obtains certain reaction kinetic parameters through 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 relations are:
Where r is the reaction rate; A, B, C, and D are the concentrations of each substance: α, β, γ, and δ are referred to the number of stages relative to substance A, B, C, and D. R is the gas constant. T is Thermodynamic temperature.
The chemical kinetic parameters are valid data to explore the reaction mechanism. The time resolution for the detection of transiently active intermediates has changed from milliseconds to picoseconds in the 1950s.
②Molecular reaction dynamics research method. From the microscopic molecular level, a meta-chemical reaction is a collision of reactant molecules with a certain quantum state, the rearrangement of atoms, the production of a certain quantum state product molecules and even separation from each other.
Single response 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 the theory of quantum chemistry, and the law of mechanics is used to calculate the motion trajectory of a representative point, 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 so far failed to obtain a reliable and complete potential energy surface of the reaction system.
Therefore, current reaction rate theories (such as bimolecular reaction collision theory and transition state theory) still have to borrow the methods of classical statistical mechanics. Such processing must make some form of equilibrium assumptions, making these rate theories unsuitable for very fast reactions.
Although there have been many studies on the applicability of the equilibrium hypothesis, it is still immature to completely deal with the problem of reaction rate using non-equilibrium theory.
In the 1960s, experimental studies of chemical reactions at the molecular level were difficult to achieve. It applies the advanced analysis methods of modern physical chemistry to study the dynamic structure, reaction process and reaction mechanism of elementary chemical reactions of single molecules in different states and in different molecular systems at the atomic and molecular levels.
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It studies the rate and mechanism of the elementary reaction process from the micro-level of the molecule and focuses on the dynamic behavior of the chemical elementary process from the internal motion of the molecule and the interaction caused by the collision of the molecule.
③Network dynamics research method, which performs computer simulation and optimization of important chemical reaction processes (such as hydrocarbon pyrolysis) including dozens or even hundreds of elementary reaction steps, in order to carry out research on the optimal design of the reactor.
What does chemical kinetics study?
Chemical kinetics is a branch of physical chemistry that studies the rate and reaction mechanism of chemical processes.
Chemical kinetics is different from chemical thermodynamics. Instead of calculating the degree and conversion of the reaction when the reaction equilibrium is reached, it is to observe the chemical reaction from a dynamic point of view, study the time required for the transformation of the reaction system, and the microcosms involved. Chemical dynamics and thermodynamics are based on statistical mechanics, quantum mechanics, and molecular kinematics.
Its research object is an unbalanced dynamic system whose properties change with time. Chemical thermodynamics is a branch of physical chemistry and thermodynamics. It mainly studies the energy changes accompanying physical and chemical changes of material systems under various conditions, so as to make accurate judgments on the direction and progress of chemical reactions.
Chemical thermodynamics was developed based on three basic laws. The first law of thermodynamics is the law of conservation of energy and transformation, which is summarized by many scientists’ experiments.
Dynamics is a branch of theoretical mechanics that studies the relationship between the force acting on an object and its motion. The research object of dynamics is macro-objects that move much faster than the speed of light.
The study of the dynamics of atoms 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 basis of physics and astronomy, as well as the basis of many engineering disciplines. Many mathematical advances are often related to solving dynamics problems, so mathematicians have a strong interest in dynamics.
Use of Chemical kinetics
Using chemical kinetics principles:
(1) Study the mechanism of drug degradation;
(2) Study the factors that affect drug degradation and stabilization measures;
(3) Predict the effective period of the drug preparation.
How do you calculate the rate in chemical kinetics?
The reaction rate is a measure of the speed of a chemical reaction. In a broad sense, it is the absolute value of the amount of the substance involved in the reaction over time. It is divided into two types: average rate and instantaneous rate.
The average rate is the change of the number of substances involved in the reaction within a certain time interval (Δt) in the reaction process, which can be expressed by the decrease of the reactant or the increase of the product per unit time, the instantaneous rate is the rate of change of concentration with time, The tangent slope of the function on a concentration-time image at a specific time.
Reaction Equilibrium: Thermodynamics studies the state of a reaction when it reaches reaction equilibrium. In a reversible reaction, the reactants and products reach a dynamic equilibrium, the rates of the forward reaction and the reverse reaction are equal, and the concentrations of the reactants and products no longer change.
It can be demonstrated by multi-component reaction processes such as the Hubble synthesis of ammonia, chemical oscillation reactions such as the Belousov-Zhabotinsky reaction (BZ reaction), and the iodine clock reaction.
Reaction mechanism: Although the measurement ratio of each substance in the chemical equation seems simple, microscopically, a chemical reaction is usually completed through several steps. The chemical kinetic branch that describes the microscopic process of a chemical reaction is called the reaction mechanism.
In the reaction mechanism, each step of the reaction is called a primitive reaction, and the total number of molecules of a reactant in a primitive reaction is called a reaction molecular number. The reactor is composed of one or more elementary reactions, and the net reaction of these elementary reactions is the apparent chemical reaction.