Lecture 3

Lecture 3 - University of Southern California Version 1.4...

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University of Southern California ©Hai Wang Version 1.4 1 AME599 Combustion Chemistry and Physics Lecture 3 3. Basic Chemical Kinetics In Lecture 1, we learned that thermodynamics determines the “ideal” end state of a reaction process. It does not tell us, however, how fast the reaction proceeds and whether under a particular reaction condition, the reaction can proceed to the equilibrium state. Answers to these questions are left to chemical kinetics. This is the topic of the current lecture. 3.1. Chemical Reaction Rate 3.1.1 Global Versus Elementary Reactions So far all reactions we have discussed are called global reactions. These reactions described an overall process by which reactants are converted to anticipated products at the end of the process. They do not describe the actual physical process. Take, for example, the simple reaction of hydrogen oxidation: 2 H 2 + O 2 2 H 2 O . Physically this reaction is almost impossible since it would require two H 2 molecules and one O 2 molecules to simultaneously collide, and while they collide the three molecules have align themselves in a way that two water molecules may be produced. Statistically this is a very unlikely event. The above discussion leads us to define a class of reactions known as elementary reactions. These are the reactions that take place physically, e.g., the combination of two H atoms to form H 2 H + H H 2 . Obviously the formation of the H 2 molecule requires the two H atoms to physically “touch” each other. In the gas phase this is done by atom-atom collision . The dissociation of the H 2 molecule H 2 H + H is also an elementary reaction. Since the above two reactions describe a reversible reaction process, we often write the reaction as H + H H 2 . Unlike global reactions, all elementary reactions are reversible .
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University of Southern California ©Hai Wang Version 1.4 2 3.1.2 Elementary Reaction Type and Reaction Rates As we discussed above, elementary reactions have two key characteristics. First, the reactant molecules will have to physically collide with each other for the reaction to take place. Second the reaction must be reversible. There are only a few types of reactions that satisfy these criteria. The first type of elementary reactions is known as the unimolecular reaction, which involves only one reactant: A products. (3.1) Examples include bond breaking reactions and isomerization reactions. As we will learn later, a unimolecular reaction actually requires collision so the first criterion is not violated. The rate of the above reaction is proportional to the molar concentration of A , i.e., [ ] [] dA kA dt −= . (3.2) Here the bracket [] denotes molar concentration (mol/cm 3 ) and the proportionality k (1/s) is the reaction rate constant . If A is the only species in the reactor and k is a constant during the reaction process, the concentration of A can be easily determined, [ ] [ ] 0 kt t AA e = . (3.3) where [ A ] 0 is the initial concentration ( t = 0).
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Lecture 3 - University of Southern California Version 1.4...

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