Chemical Kinetics. Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Transcription

1 Chemical Kinetics Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

2 Chemical Kinetics Thermodynamics does a reaction take place? Kinetics how fast does a reaction proceed? Reaction rate is the change in the concentration of a reactant or a product with time (M/s). A B rate = - Δ[A] rate = Δ[B] Δ[A] = change in concentration of A over time period Δ[B] = change in concentration of B over time period Because [A] decreases with time, Δ[A] is negative. 2

3 A B rate = - Δ[A] rate = Δ[B] 3

4 red-brown Br 2 (aq) + HCOOH (aq) 2Br - (aq) + 2H + (aq) + CO 2 (g) time t < t 2 < t nm light Detector Δ[Br 2 ] Δ Absorption 4

5 Br 2 (aq) + HCOOH (aq) 2Br - (aq) + 2H + (aq) + CO 2 (g) slope of tangent slope of tangent slope of tangent average rate = - Δ[Br 2] = - [Br 2] final [Br 2 ] initial t final - t initial instantaneous rate = rate for specific instance in time 5

6 2H 2 O 2 (aq) 2H 2 O (l) + O 2 (g) P = PV = nrt n RT = [O 2 ]RT V [O 2 ] = P RT rate = Δ[O 2 ] = ΔP RT measure ΔP over time 6

7 7

8 Reaction Rates and Stoichiometry 2A B Two moles of A disappear for each mole of B that is formed. rate = - 2 Δ[A] rate = Δ[B] aa + bb cc + dd rate = - a Δ[A] = - b Δ[B] = c Δ[C] = d Δ[D] 8

9 Write the rate expression for the following reaction: CH 4 (g) + 2O 2 (g) CO 2 (g) + 2H 2 O (g) rate = - Δ[CH 4 ] = - 2 Δ[O 2 ] = Δ[CO 2 ] = 2 Δ[H 2 O] 9

10 The Rate Law The rate law expresses the relationship of the rate of a reaction to the rate constant and the concentrations of the reactants raised to some powers. aa + bb cc + dd Rate = k [A] x [B] y Reaction is xth order in A Reaction is yth order in B Reaction is (x + y)th order overall 0

11 F 2 (g) + 2ClO 2 (g) 2FClO 2 (g) rate = k [F 2 ] x [ClO 2 ] y Double [F 2 ] with [ClO 2 ] constant Rate doubles x = Quadruple [ClO 2 ] with [F 2 ] constant Rate quadruples y = rate = k [F 2 ][ClO 2 ]

12 Rate Laws Rate laws are always determined experimentally. Reaction order is always defined in terms of reactant (not product) concentrations. The order of a reactant is not related to the stoichiometric coefficient of the reactant in the balanced chemical equation. F 2 (g) + 2ClO 2 (g) 2FClO 2 (g) rate = k [F 2 ][ClO 2 ] 2

13 Determine the rate law and calculate the rate constant for the following reaction from the following data: S 2 O 2-8 (aq) + 3I - (aq) 2SO 2-4 (aq) + I - 3 (aq) Experiment [S 2 O 8 2- ] [I - ] Initial Rate (M/s) x x x 0-4 rate = k [S 2 O 2-8 ] x [I - ] y y = x = rate = k [S 2 O 2-8 ][I - ] Double [I - ], rate doubles (experiment & 2) Double [S 2 O 8 2- ], rate doubles (experiment 2 & 3) k = rate [S 2 O 2-8 ][I - ] = 2.2 x 0-4 M/s (0.08 M)(0.034 M) = 0.08/M s 3

14 First-Order Reactions A product rate = - Δ[A] k = rate [A] = M/s M = /s or s - - rate = k [A] Δ[A] = k [A] [A] is the concentration of A at any time t [A] 0 is the concentration of A at time t=0 [A] = [A] 0 e kt ln[a] = ln[a] 0 - kt 4

15 First-Order Reactions The half-life, t ½, is the time required for the concentration of a reactant to decrease to half of its initial concentration. t ½ = t when [A] = [A] 0 /2 [A] 0 ln [A] 0 /2 ln 2 t ½ = = k k = k 5

16 First-order reaction A product # of half-lives [A] = [A] 0 /n

17 Second-Order Reactions A product rate = - Δ[A] rate = k [A] 2 k = rate [A] 2 = M/s M 2 = /M s - Δ[A] = k [A] 2 [A] = [A] + kt 0 [A] is the concentration of A at any time t [A] 0 is the concentration of A at time t=0 t ½ = t when [A] = [A] 0 /2 t ½ = k[a] 0 7

18 Zero-Order Reactions A product rate = - Δ[A] rate = k [A] 0 = k k = rate [A] 0 = M/s - Δ[A] = k [A] = [A] 0 - kt [A] is the concentration of A at any time t [A] 0 is the concentration of A at time t = 0 t ½ = t when [A] = [A] 0 /2 t ½ = [A] 0 2k 8

19 Summary of the Kinetics of Zero-Order, First-Order and Second-Order Reactions Order Rate Law Concentration-Time Equation Half-Life 0 rate = k [A] = [A] 0 - kt t ½ = [A] 0 2k 2 rate = k [A] rate = k [A] 2 ln[a] = ln[a] 0 - kt [A] = [A] + kt 0 t ln 2 ½ = k t ½ = k[a] 0 9

20 Exothermic Reaction + A + B AB C + D Endothermic Reaction The activation energy (E a ) is the minimum amount of energy required to initiate a chemical reaction. 20

21 Temperature Dependence of the Rate Constant (Arrhenius equation) E a is the activation energy (J/mol) R is the gas constant (8.34 J/K mol) T is the absolute temperature A is the frequency factor Alternate format: ln k = - E a R T + lna 2

22 Alternate Form of the Arrhenius Equation At two temperatures, T and T 2 or 22

23 Reaction Mechanisms The overall progress of a chemical reaction can be represented at the molecular level by a series of simple elementary steps or elementary reactions. The sequence of elementary steps that leads to product formation is the reaction mechanism. 2NO (g) + O 2 (g) 2NO 2 (g) N 2 O 2 is detected during the reaction! Elementary step: NO + NO N 2 O 2 + Elementary step: N 2 O 2 + O 2 2NO 2 Overall reaction: 2NO + O 2 2NO 2 23

24 2NO (g) + O 2 (g) 2NO 2 (g) Mechanism: 24

25 Intermediates are species that appear in a reaction mechanism but not in the overall balanced equation. An intermediate is always formed in an early elementary step and consumed in a later elementary step. Elementary step: NO + NO N 2 O 2 + Elementary step: N 2 O 2 + O 2 2NO 2 Overall reaction: 2NO + O 2 2NO 2 The molecularity of a reaction is the number of molecules reacting in an elementary step. Unimolecular reaction elementary step with molecule Bimolecular reaction elementary step with 2 molecules Termolecular reaction elementary step with 3 molecules 25

26 Rate Laws and Elementary Steps Unimolecular reaction A products rate = k [A] Bimolecular reaction A + B products rate = k [A][B] Bimolecular reaction A + A products rate = k [A] 2 Writing plausible reaction mechanisms: The sum of the elementary steps must give the overall balanced equation for the reaction. The rate-determining step should predict the same rate law that is determined experimentally. The rate-determining step is the slowest step in the sequence of steps leading to product formation. 26

27 Sequence of Steps in Studying a Reaction Mechanism 27

28 A catalyst is a substance that increases the rate of a chemical reaction without itself being consumed. Uncatalyzed E a k Catalyzed rate catalyzed > rate uncatalyzed E a < E a 28

29 In heterogeneous catalysis, the reactants and the catalysts are in different phases. Haber synthesis of ammonia Ostwald process for the production of nitric acid Catalytic converters In homogeneous catalysis, the reactants and the catalysts are dispersed in a single phase, usually liquid. Acid catalysis Base catalysis 29

30 Haber Process N 2 (g) + 3H 2 (g) Fe/Al 2 O 3 /K 2 O catalyst 2NH 3 (g) 30

31 Ostwald Process 4NH 3 (g) + 5O 2 (g) Pt catalyst 4NO (g) + 6H 2 O (g) 2NO (g) + O 2 (g) 2NO 2 (g) + H 2 O (l) 2NO 2 (g) HNO 2 (aq) + HNO 3 (aq) Pt-Rh catalysts used in Ostwald process 3

32 Catalytic Converters CO + Unburned Hydrocarbons + O 2 catalytic converter CO 2 + H 2 O 2NO + 2NO 2 2N 2 + 3O 2 catalytic converter 32