20080218 Kinetics 5 - Intro Chemistry II 030.102 - Chapter...

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Intro Chemistry II – 030.102 - Chapter 13 Spring, 2008 1 Chain Reactions Chain reaction is one that proceeds through series of elementary steps, some of which are repeated many times. Three stages: initiation (2 or more reactive intermediates generated), propagation (products formed but intermediates regenerated), and termination (two intermediates combine to form a stable product). Example: Chain reaction of methane with fluorine: CH 4 ( g ) + F 2 ( g ) CH 3 F( g ) + HF( g ) Mechanism involves chain reaction with following steps: CH 4 + F 2 CH 3 + HF + F (initiation) CH 3 + F 2 CH 3 F + F (propagation) CH 4 + F CH 3 + HF (propagation) CH 3 + F + M CH 3 F + M (termination) Reactive intermediates are CH 3 and F. Chain Reactions Chain reaction of methane with fluorine proceeds at constant rate, since each propagation step uses up and produces an intermediate. concentrations of intermediates remain approximately constant. In other type of chain reaction ( branching chain reaction ), the number of intermediates increases in one or more propagation steps. Example: Chain reaction of oxygen with hydrogen. Mechanism complex and involves several intermediates such as O, H, and OH. Some steps involve loss and formation of one intermediate each, e.g. OH + H 2 H 2 O + H Others are branching steps H + O 2 OH + O O + H 2 OH + H Production of more intermediates in a step speeds up overall rate, leading in certain conditions to an explosion.
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Intro Chemistry II – 030.102 - Chapter 13 Spring, 2008 2 Computational Modeling of Reaction Mechanisms With modern techniques, reaction constants of elementary gas-phase reactions can be measured. These can be used to solve numerically the rate equations for all species involved (reactants, intermediates, products). Example: Combustion of methane. Species included in mechanism: CH 4 , CH 3 , CH 2 , CH, CH 2 O, HCO, CO 2 , H 2 , H, O 2 , OH, HO 2 , H 2 O 2 , H 2 O. Number of reactions included in mechanism: 58.
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This note was uploaded on 04/14/2008 for the course CHEMISTRY 102 taught by Professor Dagdigian during the Spring '08 term at Johns Hopkins.

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20080218 Kinetics 5 - Intro Chemistry II 030.102 - Chapter...

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