Not necessarily true for a reaction mechanism series

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of elementary reactions always increase with temperature. Not necessarily true for a reaction mechanism (series of reactions). Example: Observed rate law: rate = kobs[NO]2[O2] where kobsdecreaseswith temperature. Mechanism for reaction involves rapid equilibrium in 1st step (K1), and slow 2nd step (k2), so that kobs= k2K1. k2increases with temperature, but K1decreasesmore rapidly with temperature since reaction is quite exothermic. NO +NO N2O2(fast equilibrium)Hence, can have negativeactivatjon energies for some overall reactions. 2 NO(g)+O2(g)2 NO2(g)
Reactions in Solution Reactions in solution more complicated than in gas phase. Reactant molecules must diffuse past solvent molecules to react. Consider the mechanism: A+B k–1k1ABAB k2productsSteady-state approximation rate=k2[AB]d[AB]dt=k1[A][B]k1[AB]k2[AB]=0Hence, rate=k1k2[AB]k1+k2Two limits: (1) k2>> k–1. Rate given by k1[A][B]. Step 1 is rate determining. k1limited by rate of reactant encounter. Diffusion controlled. (2) k2<< k–1. Rate given by k2K1[A][B]. Step 2 rate determining. k2limited by rate of reactant encounter. Activation energy controlled
Kinetics of Catalysis
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Kinetics of Catalysis Addition of Ag+ion speeds up reaction by following mechanism: Ag++Ce4

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