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Chemical Kinetics    Chapter 13
Chemical KineticsThermodynamics – does a reaction take place?Kinetics – how fast does a reaction proceed?Reaction rate is ...
A          B                    time         ∆[A]rate = -          ∆t       ∆[B]rate =        ∆t                          ...
Br2 (aq) + HCOOH (aq)          2Br- (aq) + 2H+ (aq) + CO2 (g)                                time                         ...
Br2 (aq) + HCOOH (aq)        2Br- (aq) + 2H+ (aq) + CO2 (g)                            slope of                           ...
rate α [Br2] rate = k [Br2]    ratek=        = rate constant    [Br2]  = 3.50 x 10-3 s-1                            13.1
2H2O2 (aq)       2H2O (l) + O2 (g)                                    PV = nRT                                n           ...
2H2O2 (aq)   2H2O (l) + O2 (g)                                 13.1
Reaction Rates and Stoichiometry                        2A       BTwo moles of A disappear for each mole of B that is form...
Write the rate expression for the following reaction:CH4 (g) + 2O2 (g)      CO2 (g) + 2H2O (g)         ∆[CH4]    1 ∆[O2]  ...
The Rate LawThe rate law expresses the relationship of the rate of a reactionto the rate constant and the concentrations o...
F2 (g) + 2ClO2 (g)     2FClO2 (g)   rate = k [F2]x[ClO2]yDouble [F2] with [ClO2] constantRate doublesx=1                  ...
Rate Laws•   Rate laws are always determined experimentally.•   Reaction order is always defined in terms of reactant    (...
Determine the rate law and calculate the rate constant for        the following reaction from the following data:        S...
First-Order Reactions                                       ∆[A]     A         product        rate = -              rate =...
The reaction 2A         B is first order in A with a rate     constant of 2.8 x 10-2 s-1 at 800C. How long will it take fo...
First-Order ReactionsThe half-life, t½, is the time required for the concentration of areactant to decrease to half of its...
First-order reaction        A    product  # ofhalf-lives   [A] = [A]0/n    1             2    2             4    3        ...
Second-Order Reactions                                ∆[A] A         product     rate = -               rate = k [A]2     ...
Zero-Order Reactions                                 ∆[A] A        product       rate = -               rate = k [A]0 = k ...
Summary of the Kinetics of Zero-Order, First-Order         and Second-Order Reactions                            Concentra...
A+B        C+D    Exothermic Reaction            Endothermic ReactionThe activation energy (Ea) is the minimum amount ofen...
Temperature Dependence of the Rate Constant                         k = A • exp( -Ea/RT )                         (Arrheni...
Ea 1lnk = -      + lnA        R T                     13.4
Reaction MechanismsThe overall progress of a chemical reaction can be representedat the molecular level by a series of sim...
Intermediates are species that appear in a reactionmechanism but not in the overall balanced equation.An intermediate is a...
Rate Laws and Elementary StepsUnimolecular reaction     A      products      rate = k [A]Bimolecular reaction    A+B      ...
The experimental rate law for the reaction between NO 2      and CO to produce NO and CO2 is rate = k[NO2]2. The      reac...
A catalyst is a substance that increases the rate of achemical reaction without itself being consumed.         k = A • exp...
In heterogeneous catalysis, the reactants and the catalystsare in different phases.     •   Haber synthesis of ammonia    ...
Haber Process                   Fe/Al2O3/K2ON2 (g) + 3H2 (g)                  2NH 3 (g)                     catalyst      ...
Ostwald Process                     Pt catalyst4NH3 (g) + 5O2 (g)                 4NO (g) + 6H2O (g)    2NO (g) + O2 (g)  ...
Catalytic Converters                                   catalyticCO + Unburned Hydrocarbons + O2   converter      CO2 + H2O...
Enzyme Catalysis                   13.6
enzymeuncatalyzed   catalyzed                          13.6
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Chemical kinetics

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Chemical kinetics

  1. 1. Chemical Kinetics Chapter 13
  2. 2. Chemical KineticsThermodynamics – does a reaction take place?Kinetics – how fast does a reaction proceed?Reaction rate is the change in the concentration of areactant or a product with time (M/s). A B ∆[A] ∆[A] = change in concentration of A over rate = - ∆t time period ∆t ∆[B] ∆[B] = change in concentration of B over rate = ∆t time period ∆t Because [A] decreases with time, ∆[A] is negative. 13.1
  3. 3. A B time ∆[A]rate = - ∆t ∆[B]rate = ∆t 13.1
  4. 4. Br2 (aq) + HCOOH (aq) 2Br- (aq) + 2H+ (aq) + CO2 (g) time Br2 (aq) 393 nm393 nm Detector light ∆[Br2] α ∆Absorption 13.1
  5. 5. Br2 (aq) + HCOOH (aq) 2Br- (aq) + 2H+ (aq) + CO2 (g) slope of tangent slope of tangent slope of tangent ∆[Br2] [Br2]final – [Br2]initial average rate = - =- ∆t tfinal - tinitialinstantaneous rate = rate for specific instance in time 13.1
  6. 6. rate α [Br2] rate = k [Br2] ratek= = rate constant [Br2] = 3.50 x 10-3 s-1 13.1
  7. 7. 2H2O2 (aq) 2H2O (l) + O2 (g) PV = nRT n P= RT = [O2]RT V 1 [O2] = P RT ∆[O2] 1 ∆P rate = = ∆t RT ∆tmeasure ∆P over time 13.1
  8. 8. 2H2O2 (aq) 2H2O (l) + O2 (g) 13.1
  9. 9. Reaction Rates and Stoichiometry 2A BTwo moles of A disappear for each mole of B that is formed. 1 ∆[A] ∆[B] rate = - rate = 2 ∆t ∆t aA + bB cC + dD 1 ∆[A] 1 ∆[B] 1 ∆[C] 1 ∆[D] rate = - =- = = a ∆t b ∆t c ∆t d ∆t 13.1
  10. 10. Write the rate expression for the following reaction:CH4 (g) + 2O2 (g) CO2 (g) + 2H2O (g) ∆[CH4] 1 ∆[O2] ∆[CO2] 1 ∆[H2O]rate = - =- = = ∆t 2 ∆t ∆t 2 ∆t 13.1
  11. 11. The Rate LawThe rate law expresses the relationship of the rate of a reactionto the rate constant and the concentrations of the reactantsraised 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 13.2
  12. 12. F2 (g) + 2ClO2 (g) 2FClO2 (g) rate = k [F2]x[ClO2]yDouble [F2] with [ClO2] constantRate doublesx=1 rate = k [F2][ClO2]Quadruple [ClO2] with [F2] constantRate quadruplesy=1 13.2
  13. 13. 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. F2 (g) + 2ClO2 (g) 2FClO2 (g) rate = k [F2][ClO2] 1 13.2
  14. 14. Determine the rate law and calculate the rate constant for the following reaction from the following data: S2O82- (aq) + 3I- (aq) 2SO42- (aq) + I3- (aq) [S2O82-] Initial RateExperiment [I-] (M/s) rate = k [S2O82-]x[I-]y 1 0.08 0.034 2.2 x 10-4 y=1 2 0.08 0.017 1.1 x 10-4 x=1 3 0.16 0.017 2.2 x 10-4 rate = k [S2O82-][I-]Double [I-], rate doubles (experiment 1 & 2)Double [S2O82-], rate doubles (experiment 2 & 3) rate 2.2 x 10-4 M/s k= = = 0.08/M•s [S2O8 ][I ] 2- - (0.08 M)(0.034 M) 13.2
  15. 15. First-Order Reactions ∆[A] A product rate = - rate = k [A] ∆t ∆[A] rate M/s - = k [A]k= = = 1/s or s-1 ∆t [A] M [A] is the concentration of A at any time t [A]0 is the concentration of A at time t=0 [A] = [A]0exp(-kt) ln[A] = ln[A]0 - kt 13.3
  16. 16. The reaction 2A B is first order in A with a rate constant of 2.8 x 10-2 s-1 at 800C. How long will it take for A to decrease from 0.88 M to 0.14 M ? [A]0 = 0.88 Mln[A] = ln[A]0 - kt [A] = 0.14 M kt = ln[A]0 – ln[A] [A]0 0.88 M ln ln ln[A]0 – ln[A] [A] 0.14 M t= = = = 66 s k k 2.8 x 10 s-2 -1 13.3
  17. 17. First-Order ReactionsThe half-life, t½, is the time required for the concentration of areactant to decrease to half of its initial concentration. t½ = t when [A] = [A]0/2 [A]0 ln [A]0/2 ln2 0.693 t½ = = = k k k What is the half-life of N2O5 if it decomposes with a rate constant of 5.7 x 10-4 s-1? t½ = ln2 = 0.693 = 1200 s = 20 minutes k 5.7 x 10 s -4 -1How do you know decomposition is first order? units of k (s-1) 13.3
  18. 18. First-order reaction A product # ofhalf-lives [A] = [A]0/n 1 2 2 4 3 8 4 16 13.3
  19. 19. Second-Order Reactions ∆[A] A product rate = - rate = k [A]2 ∆t rate M/s ∆[A]k= = 2 = 1/M•s - = k [A]2 [A] 2 M ∆t 1 1 [A] is the concentration of A at any time t = + kt [A] [A]0 [A]0 is the concentration of A at time t=0 t½ = t when [A] = [A]0/2 1 t½ = k[A]0 13.3
  20. 20. Zero-Order Reactions ∆[A] A product rate = - rate = k [A]0 = k ∆t rate ∆[A]k= = M/s - =k [A] 0 ∆t [A] is the concentration of A at any time t [A] = [A]0 - kt [A]0 is the concentration of A at time t=0 t½ = t when [A] = [A]0/2 [A]0 t½ = 2k 13.3
  21. 21. Summary of the Kinetics of Zero-Order, First-Order and Second-Order Reactions Concentration-Time Order Rate Law Equation Half-Life [A]0 0 rate = k [A] = [A]0 - kt t½ = 2k 1 rate = k [A] ln[A] = ln[A]0 - kt t½ = ln2 k 1 1 1 2 rate = k [A] 2 = + kt t½ = [A] [A]0 k[A]0 13.3
  22. 22. A+B C+D Exothermic Reaction Endothermic ReactionThe activation energy (Ea) is the minimum amount ofenergy required to initiate a chemical reaction. 13.4
  23. 23. Temperature Dependence of the Rate Constant k = A • exp( -Ea/RT ) (Arrhenius equation) Ea is the activation energy (J/mol) R is the gas constant (8.314 J/K•mol) T is the absolute temperature A is the frequency factor Ea 1 lnk = - + lnA R T 13.4
  24. 24. Ea 1lnk = - + lnA R T 13.4
  25. 25. Reaction MechanismsThe overall progress of a chemical reaction can be representedat the molecular level by a series of simple elementary stepsor elementary reactions.The sequence of elementary steps that leads to productformation is the reaction mechanism. 2NO (g) + O2 (g) 2NO2 (g) N2O2 is detected during the reaction! Elementary step: NO + NO N 2 O2 + Elementary step: N2O2 + O2 2NO2 Overall reaction: 2NO + O2 2NO2 13.5
  26. 26. Intermediates are species that appear in a reactionmechanism but not in the overall balanced equation.An intermediate is always formed in an early elementary stepand consumed in a later elementary step. Elementary step: NO + NO N 2 O2+ Elementary step: N2O2 + O2 2NO2 Overall reaction: 2NO + O2 2NO2The molecularity of a reaction is the number of molecules reacting in an elementary step.• Unimolecular reaction – elementary step with 1 molecule• Bimolecular reaction – elementary step with 2 molecules• Termolecular reaction – elementary step with 3 molecules 13.5
  27. 27. Rate Laws and Elementary StepsUnimolecular reaction A products rate = k [A]Bimolecular reaction A+B products rate = k [A][B]Bimolecular reaction A+A products rate = k [A]2Writing 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 thesequence of steps leading to product formation. 13.5
  28. 28. The experimental rate law for the reaction between NO 2 and CO to produce NO and CO2 is rate = k[NO2]2. The reaction is believed to occur via two steps: Step 1: NO2 + NO2 NO + NO3 Step 2: NO3 + CO NO2 + CO2What is the equation for the overall reaction? NO2+ CO NO + CO2What is the intermediate? NO3What can you say about the relative rates of steps 1 and 2? rate = k[NO2]2 is the rate law for step 1 so step 1 must be slower than step 2 13.5
  29. 29. A catalyst is a substance that increases the rate of achemical reaction without itself being consumed. k = A • exp( -Ea/RT ) Ea k uncatalyzed catalyzed ratecatalyzed > rateuncatalyzed Ea‘ < Ea 13.6
  30. 30. In heterogeneous catalysis, the reactants and the catalystsare in different phases. • Haber synthesis of ammonia • Ostwald process for the production of nitric acid • Catalytic convertersIn homogeneous catalysis, the reactants and the catalystsare dispersed in a single phase, usually liquid. • Acid catalysis • Base catalysis 13.6
  31. 31. Haber Process Fe/Al2O3/K2ON2 (g) + 3H2 (g) 2NH 3 (g) catalyst 13.6
  32. 32. Ostwald Process Pt catalyst4NH3 (g) + 5O2 (g) 4NO (g) + 6H2O (g) 2NO (g) + O2 (g) 2NO2 (g)2NO2 (g) + H2O (l) HNO2 (aq) + HNO3 (aq) Hot Pt wire Pt-Rh catalysts used over NH3 solution in Ostwald process 13.6
  33. 33. Catalytic Converters catalyticCO + Unburned Hydrocarbons + O2 converter CO2 + H2O catalytic 2NO + 2NO2 converter 2N2 + 3O2 13.6
  34. 34. Enzyme Catalysis 13.6
  35. 35. enzymeuncatalyzed catalyzed 13.6

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