6-section - 1 6. Macroscopic Chemical Kinetics (Chapter 18)...

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1 6. Macroscopic Chemical Kinetics (Chapter 18) Chemical kinetics is concerned with understanding how fast chemical reactions occur, and why and how those rates vary with temperature, pressure, and the amounts of the component reagent species present. How do we characterize reaction rates ? Consider the combustion of ethylene: C 2 H 4 (g) + O 2 (g) CO 2 (g) + H 2 O(g) [ begin by balancing the reaction ! ] We can describe the rate in terms of: how fast the C 2 H 4 disappears, or how fast the O 2 is used up, or how fast the CO 2 is produced, or how fast the H 2 O is produced. We usually characterize rates in terms of the time derivative of the concentration of some selected reagent or product species. For reagents being produced (species on the R.H.S. of the equation), those derivatives have numerical values, since For reagents being consumed (species on the L.H.S. of the equation), those derivatives have values,
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2 e.g., for the ethylene combustion reaction considered above, occurring in a closed container, consider a plot of the concentrations of all species vs. time. 0 20 40 60 80 100 time concentration We want to deFne ‘reaction rate’ as a positive quantity, so we write: { R ate } = d [C 2 H 4 ] dt = d [O 2 ] dt =+ d [C O 2 ] dt d [H 2 O ] dt More generally, for an arbitrary reaction n 1 A+ n 2 B n 3 C+ n 4 D+ n 5 E in which n i are the stoichiometric coefficients of the balanced equation { R ate } = d [A] dt = d [B] dt = d [C] dt = d [D] dt = d [E] dt
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3 e.g., for the reaction 2 Fe 3+ (aq) + Sn 2+ (aq) 2Fe 2+ (aq) + Sn 4+ (aq) In general, the rate of reaction changes with time.
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This note was uploaded on 09/28/2011 for the course CHEM 350 taught by Professor Prof.djasd during the Winter '10 term at Waterloo.

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6-section - 1 6. Macroscopic Chemical Kinetics (Chapter 18)...

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