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Unformatted text preview: MECH 6120: Combustion Notes on Chemical Kinetics and Chain Reactions 1 Chemical Kinetics Thermodynamics is all one needs to predict the equi- librium state that a system reaches following a pro- cess and the heat/work transfers that occur during the process. Thermodynamics, however, cannot pre- dict the rate at which a process will occur, i.e., the speed at which a system reaches a new equilibrium state. For example, we could have a coffee cup ini- tially at 50 C , surrounded by an ambient at 22 C . Thermodynamics will tell us that heat will flow from the cup to the environment and that the final state has the cup in equilibrium with the environment (2nd law). It can also tell us how much heat was trans- ferred (1st law). It cannot tell us how long it took the coffee to cool down. To predict this, we would need to know how the rate of heat transfer depended on the temperature difference between the cup and the environment (i.e., Newtons law of cooling). Likewise, Thermodynamics can predict the chemi- cal equilibrium state when all reactions (forward and backwards) are in balance. For example, say we have a system of O and O 2 in equilibrium. The equilib- rium condition implies that the rate of O formation by the forward reaction; O 2 2O is balanced by the rate of destruction of O by the reverse reaction; 2O O Thermodynamics cannot predict how fast either of these reactions will occur it can only predict the state when they are in balance. The topic of chemical kinetics deals with reaction rates. Obviously, we would need to know something about reaction rates to predict many types of com- bustion phenomena, i.e., the rate at which gasoline vapor is consumed in a cylinder, the speed of flame spread over a combustible surface, etc. By reaction rate, we mean Rate = change in moles of species unit time unit volume Since the number of moles per unit volume is equiv- alently a concentration C of the species, the reaction rate of species i , denoted R i , corresponds to R i = dC i dt , moles / cm 3 s From the reaction, we can gather how the reaction rates for the various species are related. For example, consider CH 4 + 2O 2- CO 2 + 2H 2 O The reaction rates for the products would be positive, and they would be negative for the reactants. And by the stoichiometry of the reaction; R CH 4 = 1 2 R O 2 =- R CO 2 =- 1 2 R H 2 O which follows because for every mole of methane, we consume 2 moles of O 2 and create 1 mole of CO 2 and 2 moles of H 2 O. 1.1 Elementary vs. Overall Reactions Overall (or, equivalently, global ) reactions describe the overall conversion of a set of reactants to a set of products. These are the type of reactions we have been dealing with so far, i.e., CH 4 + 2O 2- CO 2 + 2H 2 O describes the stoichiometric, completecombustion oxidation of methane. This reaction does not rep- resent the actual molecular event that takes place in the process. That is, a molecule of methane does not run into a molecule of oxygen to form one CO...
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- Summer '09