Time is an important variable in chemical kinetics.The classical chemical kinetics experiment method cannot be prepared singlyquantum stateThe reactants produced by a single reaction collision cannot be detectedPrimary ecologyproduct。SystematicthermodynamicsThe equilibrium property cannot give information about chemical kineticsChemical reaction processAnd put into practice, can not lack of chemical kinetics research.
Up to now, the calculation of quantum chemistry has not been able to obtain the reliable and complete potential energy surface of the reaction system.Therefore, the current reaction rate theory still has to borrow the classicstatistical mechanicsTreatment method of.Such treatment must be in some formEquilibrium hypothesis, thus making theseRate theoryNot suitable for very fast reactions.Although there have been many studies on the applicability of Yu Heng hypothesisEquilibrium stateThe theoretical treatment of reaction rate is not yet mature.
molecular beam(i.e. molecular scattering), especiallyCross molecular beam methodThe application of the study of chemical elementary reaction kinetics makes it possible to study single reaction collision in experiments.Molecular beam experiments have obtained many microscopic information about chemical elementary reactions that cannot be obtained by classical chemical dynamics,Molecular reaction dynamicsIt is a frontier of modern chemical dynamics.
The thermodynamic equilibrium properties of the system can not give chemical kinetics information.For example, for the following reaction: 2Htwo(Gas)+Otwo(Gas) → 2HtwoO (gas) Although Htwo、OtwoAnd HtwoAll of OThermodynamic propertiesThey all know exactly, but can only predict HtwoAnd OtwoGenerate HtwoO, but cannot predict HtwoAnd OtwoWhat kind ofreaction rateGenerate HtwoO，Also cannot provide HtwoMolecule and OtwoWhat steps do molecules combine into HtwoO molecule information.So, fully understand aChemical reaction processAnd put into practice, can not lack of chemical kinetics research.[1]
Research History
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In the first half of the 20th century, a lot of research work was devoted to the determination of these parameters, theoretical analysis and the use of parameters to study the reaction mechanism.However, the confirmation of reaction mechanism mainly depends on detection and analysisreaction intermediateAbility.In the late 20th century, free radicalschain reactionThe widespread development of kinetic research has brought two development trends to chemical kinetics: one is the extensive study of elementary reaction kinetics;Second, it is urgent to establish a method to detect active intermediates, which, together with the development of electronics and laser technology, has promotedRapid reaction kineticsDevelopment of.Detection of transient active intermediatestime resolution It has increased from the millisecond level in the 1950s topicosecondLevel.[1]
Research object
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Chemical kinetics is researchChemical reaction rate(rate of reaction) and mechanism of reaction.The main contents of chemical kinetics include the following:
1) Determine the rate of chemical reaction and the influence of external factors such as temperature, pressure, catalyst, solvent and light on the reaction rate;
2) Study chemical reaction mechanism and reveal the essence of chemical reaction rate;
3) Explore the relationship and law between material structure and reaction ability.
research meaning
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Through the study of chemical kinetics, we can know how to control the reaction conditions, increase the rate of main reaction, increase the product output, inhibit the rate of side reaction, reduce the consumption of raw materials, reduce the by-products, improve the purity and improve the product quality.Chemical kinetics also studies how to avoid explosion of dangerous goods, corrosion of materials, deterioration and aging of products, etc.So the study of chemical kinetics has great significance in theory and practice.[1]
research method
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The research methods of chemical kinetics include:
Common relations
① Phenomenological dynamicsresearch method, also known as classical chemical kinetics research method, it is based on the original experimental data of chemical kinetics concentrationcAnd timetThe relationship between——Reaction rate constantk、activation energyEa、Pre exponential factorA。These parameters can be used to characterize the rate characteristics of the reaction system. The commonly used relations are:rIs the reaction rate;A. B, C, D are the concentration of each substance;α、β、γ, δ is the series relative to substance A, B, C, D;RIs the gas constant;TbyThermodynamic temperature。
Chemical kinetics parameters are effective data for exploring reaction mechanism. The time resolution of transient active intermediates detection has changed from millisecond level in 1950s to millisecond levelpicosecondLevel.
Schematic diagram of chemical kinetics
②Molecular reaction dynamicsResearch methods, frommicrocosmicAt the molecular level ofquantum stateIt is a single reaction collision behavior that the reactant molecules collide with each other, rearrange the atoms, produce the product molecules of a certain quantum state, and even separate from each other.useTransition state theoryExplain that it is in the reaction systemSuperpotential energy surfaceLast representative systemparticleThe act of crossing a reaction barrier.In principle, if we canquantum chemistryThe correctPotential energy surfaceThe parameters of reaction rate and chemical kinetics can be calculated by applying the laws of mechanics to calculate the trajectory of representative points on it.However, except for a few very simple chemical reactions, the calculation of quantum chemistry has not yet been able to obtain a reliable and complete potential energy surface of the reaction system.Therefore, the current reaction rate theory (such asBimolecular reactionCollision theoryTransition state theory) still has to borrow the treatment method of classical statistical mechanics.Such treatment must make some form of equilibrium assumption, so these rate theories are not applicable to very fast reactions.Although toEquilibrium hypothesisThere have been many studies on the applicability of the theory of non-equilibrium state, but it is not mature to deal with the problem of reaction rate completely with the theory of non-equilibrium state.In the 1960s, it was difficult to carry out experimental research on chemical reactions at the molecular level.It applies advanced analytical methods of modern physical chemistryatomThe dynamic structure, reaction process and reaction mechanism of elementary chemical reactions of single molecules in different states and different molecular systems are studied at the molecular level.It starts from the microscopic level of moleculesElementary reactionThe rate and mechanism of the process, focusing on the internal movement of molecules and the interaction caused by collision of moleculesChemical baseThe dynamic behavior of meta process.Dalian Institute of Chemical Physics, Chinese Academy of SciencesState Key Laboratory of Molecular Reaction DynamicsIn this regard, research has made outstanding contributions.
Chemical kinetics is a branch of physical chemistry that studies the rate of chemical process and reaction mechanism.
Chemical Kinetics andChemical thermodynamicsDifferent, not calculated to reachReaction equilibriumIt is the degree or conversion rate of the reaction, but from a dynamic perspective to observe the chemical reaction, study the time required for the transformation of the reaction system, and the microscopic processes involved.Chemical kinetics and thermodynamics are based on statistical mechanicsquantum mechanicsandTheory of molecular motion。
Its research object is the non-equilibrium dynamic system whose properties change with time.Chemical thermodynamicsIt is a branch of physical chemistry and thermodynamics, which mainly studies the physics andChemical changeThe direction and extent of the chemical reaction can be accurately judged by the accompanying energy changes.Chemical thermodynamics is developed on the basis of three basic laws.First law of thermodynamicsnamelyConservation of energyAnd the transformation law, which is summarized by many scientists' experiments.
Dynamics is a branch of theoretical mechanics, which studies the relationship between the force acting on an object and its motion.The research object of dynamics isMovement speedA macroscopic object that is much smaller than the speed of light.Atoms and subatomsparticleThe research on dynamics of is quantum mechanics;Comparablelight speedThe study of high speed motion in the field of physics belongs to relativistic mechanics.Dynamics is the basis of physics and astronomy, as well as many engineering disciplines.Many mathematical advances are often related to solving dynamic problems, so mathematicians have a strong interest in dynamics.[2]
purpose
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The principle of chemical kinetics can be used to:
(1) Study the mechanism of drug degradation;
(2) Study the factors affecting drug degradation and stabilization measures;
Reaction rate is a measure of the speed of chemical reaction. Broadly speaking, it is the absolute value of the change of the amount of substances involved in the reaction with time. It can be divided into average rate and instantaneous rate.The average rate is the change of the amount of substances involved in the reaction within a certain time interval (Δ t) in the reaction process, which can be expressed by the decrease of reactants or the increase of products per unit time;Instantaneous rateIs the rate of change of concentration with time, that is, the tangent slope of the function on the concentration time image at a specific time.[2]
Reaction equilibrium
Reaction equilibrium: Thermodynamics studies the state when a reaction reaches reaction equilibrium.stayReversible reactionMedium, reactants and products reachdynamic equilibrium, positive and reverse reactiverateThe concentration of reactants and products will not change.It can be accessed throughHaber process synthetic ammonia、Chemical oscillationReactions such as Belousov Zhabotinsky reaction (B-Z reaction), iodine clock reaction and other multi-component reaction processes are demonstrated.[2]
reaction mechanism
Reaction mechanism: althoughChemical equationThe stoichiometric ratio of each substance in is seemingly simple, but microscopically, a chemical reaction is usually completed through several steps. The branch of chemical kinetics that describes the microscopic process of chemical reaction is called reaction mechanism.In the reaction mechanism, each step of reaction is calledElementary reactionThe total number of molecules of reactants in the elementary reaction is calledNumber of reaction molecules。The reaction mechanism is composed of one or more elementary reactions, and the net reaction of these elementary reactions is the apparent chemical reaction.[2]
