14 - GENERAL CHEMISTRY Chemical Kinetics CONTENTS 14-1 14-2...

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1 GENERAL CHEMISTRY Chemical Kinetics 14 Chemical Kinetics CONTENTS 14-1 The Rate of a Chemical Reaction 14-2 Measuring Reaction Rates 14-3 Effect of Concentration on Reaction Rates: The Rate Law 14-4 Zero-Order Reactions 14-5 First-Order Reactions 4 d d R t i 14-6 Second-Order Reactions 14-7 Reaction Kinetics: A Summary Chemical Kinetics CONTENTS 14-8 Theoretical Models for Chemical Kinetics 14-9 The Effect of Temperature on Reaction Rates 14-11 Catalysis 14-1 The Rate of a Chemical Reaction Rate of change of concentration with time. 2 Fe 3+ (aq) + Sn 2+ 2 Fe 2+ (aq) + Sn 4+ (aq) t = 38.5 s [Fe 2+ ] = 0.0010 M Δ t = 38.5 s Δ [Fe 2+ ] = (0.0010 – 0) M Rate of formation of Fe 2+ = = = 2.6 10 -5 M s -1 Δ [Fe 2+ ] Δ t 0.0010 M 38.5 s
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2 2 Fe 3+ (aq) + Sn 2+ 2 Fe 2+ (aq) + Sn 4+ (aq) Δ [Sn 4+ ] Δ t Δ [Fe 2+ ] Δ t = 1 2 Δ [Fe 3+ ] Δ t = - 1 2 General Rate of Reaction a A + b B g G + h H Rate of reaction = negative of rate of disappearance of reactants Δ [A] Δ t 1 a = - Δ [B] Δ t 1 b = - = rate of appearance of products = Δ [G] Δ t 1 g = Δ [H] Δ t 1 h 14-2 Measuring Reaction Rates H 2 O 2 (aq) H 2 O(l) + ½ O 2 (g) 2 MnO 4 - (aq) + 5 H 2 O 2 (aq) + 6 H + 2 Mn 2+ +8 H 2 O(l) + 5 O 2 (g) Measure the rate by monitoring O 2 volume or by chemical analysis of aliquots FIGURE 14-1 Experimental setup for determining the rate of decomposition of H 2 O 2 2 MnO 4 - (aq) + 5 H 2 O 2 (aq) + 6 H + 2 Mn 2+ 2 O(l) + 5 O 2 (g) or by chemical analysis of aliquots
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3 Initial Rate of Reaction -(-2.32 M / 1360 s) = 1.71 10 -3 M s -1 Rate = - Δ [H 2 O 2 ] Δ t -(-1.7 M / 2800 s) = 6.1 10 -4 M s -1 Rate of Reaction at time t FIGURE 14-2 Graphical representation of kinetic data for the reaction H 2 O 2 (aq) H 2 O(l) + ½ O 2 (g) 14-3 Effect of Concentration on Reaction Rates: The Rate Law a A + b B …. g G + h H …. rate of reaction = k [A] m [B] n …. Rate constant = k Overall order of reaction = m + n + …. Method of Initial Rates 2 HgCl 2 (aq) + C 2 O 4 2- (aq) 2 Cl - (aq) + 2 CO 2 (g) + Hg 2 Cl 2 (s) rate of reaction = k[HgCl 2 ] m [C 2 O 4 2- ] n General effect of doubling the initial concentration of a particular reactant (with other reactant concentrations held constant) . Zero order in the reactant —there is no effect on the initial rate of reaction. First order in the reactant —the initial rate of reaction doubles. Second order in the reactant —the initial rate of reaction quadruples. Third order in the reactant —the initial rate of reaction increases eightfold.
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4 14-4 Zero-Order Reactions A products R rxn = k [A] 0 R rxn = k = = l L (0 - [A] 0 ) (t f –0) [A] 0 t f FIGURE 14-3 A zero-order reaction: A products [ k ] = mol L -1 s -1 Integrated Rate Law Δ t - Δ [A] d t = k -d[A] Move to the infinitesimal = k - d t = k d[A] [A] 0 [A] t 0 t And integrate from 0 to time t -[A] t + [A] 0 = kt [A] t = - kt + [A] 0 14-5 First-Order Reactions H 2 O 2 (aq) H 2 O(l) + ½ O 2 (g) [H = - k [H 2 O 2 ] d[H 2 O 2 ] d t = - k d t [H 2 O 2 ] d[H 2 O 2 ] [A] 0 [A] t 0 t [ k ] = s -1 An Integrated rate Law for a First-Order Reaction = - kt ln [A] t [A] 0 ln[A] t = - kt + ln[A] 0 Test for a first-order reaction: Decomposition of H 2 O 2 (aq) FIGURE 14-4
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5 Half-Life, t ½ the time taken for one-half of a reactant to be consumed.
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This note was uploaded on 03/22/2011 for the course MAC AECH111 taught by Professor Cheresres during the Spring '11 term at McGill.

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14 - GENERAL CHEMISTRY Chemical Kinetics CONTENTS 14-1 14-2...

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