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1、COMBUSTION,2 COMBUSTION KINETICS ( 4 class hours),First and Second Laws of thermodynamics are used to predict the final equilibrium state of the products after the reaction is complete. Chemical kinetics deals with how fast the chemical reaction proceeds.,Chemical Reaction Rates and Factors,Chemical
2、 and Physical properties of Reactants and Activation Energy,Temperature,Chemical Reaction Rates,Classifying of Chemical Reactions,Chain Reactions,Concepts and Characteristics,explosion limits of branched chain reaction,Solutions for Reaction Rates,CONTENTS,OBJECTIVES,Chemical Mechanisms,Factors Infl
3、uencing the Chemical Reaction Rates,Definition,Elementary Reactions,Law of Mass Action,Arrhenius Rate Expression,Collision Theory,Pressure,Law of Mass Action; Arrhenius Rate Expression; Collision Theory; Chemical Reaction Rates; Kinetic Classifying of Chemical Reaction; Factors Influencing the Chemi
4、cal Reaction Rates; Chain Reactions; Three Limits for Explosion,Law of Mass Action; Arrhenius Rate Expression; Collision Theory; Chain Reactions; Three Limits for explosion,Collision Theory; Three Limits for Explosion,KEY POINTS OF KNOWLEDGE,EMPHASES,DIFFICULTIES,2.1 Chemical Reaction Rates,1 Defini
5、tion,How fast the fuel is consumed is of interest.,Chemical reaction rates may be expressed as the rate of decrease of the concentration of a reactant or the rate of increase of the concentration of a product.,(molm3s) Mole Concentration （generally being used） (1m3s) Molecular Concentration (1s) Rel
6、ative Concentration,The reaction rate w,Negative sign is due to the fact that the fuel is consumed. w always positive.,( ),aAbB eEfF,w denotes the chemical reaction rate of the reaction system, which is the only one for the specified system. Thus, w is defined as system reaction rate.,where a, b, e
7、and f are the stoichiometric coefficients.,2 Elementary Reactions,2H2+O2 2H2O,The global (or overall) reaction does not reflect the actual reaction process, it just describes the initial and final states. When oxygen and hydrogen molecules collide and react, they do not directly yield water, but, in
8、stead, form many intermediate species. There are many intermediate reaction steps in the overall reaction. This overall reaction is called global reaction. The use of global reactions to express the chemistry in a specific problem frequently a “black box” approach. Global reaction can not provide a
9、basis for understanding what actually happening chemically in a system.,GLOBAL REACTION,To have a complete picture of the combustion of H2 and O2,more than 20 elementary reactions can be considered. The collection of elementary reactions necessary to describe an overall reaction is called a reaction
10、 mechanism or a reaction pathway. Reaction mechanisms may involve only a few steps (i.e., elementary reactions) or as many as several hundred.,Elementary reaction is that reactants are transformed to products in molecule collision with a single-step and not like global reaction with many steps.,ELEM
11、ENTARY REACTION,3 Law of Mass Action,The law of mass action states that for an elementary reaction the reaction rate is proportional to the product of the concentrations of the reactants present raised to a power equal to the corresponding stoichiometric coefficient.,The law of mass action describes
12、 the relationship between chemical reaction rates and concentrations of the reactants.,k the reaction rate constant.In general, it is a strong function of temperature.,Mathematical Expression,（A） the fuel concentration (kmol/m3 or kg/m3),Example1 Compound reaction of hydrogen atoms is as follows:,Fi
13、nd：the reaction rate of the compound reaction of hydrogen atom?,Solution,Applying the law of mass action to the reaction :,Law of Mass Action：,Reaction Rates,Concentration of Reactants,The Reaction Rate Constant k,4 Arrhenius Rate Expression,Relationship between Reaction Rate Constant k and temperat
14、ure T,T,Vant Hoff Approximate Rule,E Activation Energy,Elementary Reaction,（most reactions）,The slopes of lines on these plots are equal to (-E/R), thus the activation energy E may be determined.,Slope=-E/R,E：k; T：k,E0，RT20,When E is constant, at a higher temperature, is smaller, T，k；,When E is cons
15、tant, at a lower temperature, is larger, T：k.,Example E=100kJ/mol ，A is constant：,In a similar way :,E=150kJ/mol ，A is constant：,(1),(2) For the same reaction:,A high value of E: T, k; A low value of temperature: T, k.,Comment,E Activation Energy,Arrhenius Rate Expression :,5 Collision Theory,Backgr
16、ound,In Arrhenius Rate Expression, physical meaning of constant A is not clear. In 1918, Lewis put forward the effective collision theory according to the concept of activation energy and kinetic theory of gases and provided insight into the form of clarified physical meaning of constant A .,Kinetic Theory of Gases,Chemical reaction rate is defined as the number of the effe
