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22 Nov. 2010

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  1. 22 Nov. 2010 • Objective: SWBAT write rate expressions and calculate reaction rates for chemical reactions. • Do now: Describe one very slow reaction that you’ve seen, and one very fast reaction.

  2. Agenda • Do now • Kinetics notes • Reaction Rates Demonstrations • Rate constant and reaction rates problems. Homework: p. 602 #2, 3, 5, 7, 12, 13, 15, 17, 19

  3. How can we predict whether or not a reaction will take place? • Thermodynamics • Once started, how fast does the reaction proceed? • Chemical kinetics: this unit! • How far will the reaction go before it stops? • Equilibrium: next unit

  4. Chemical Kinetics • The area of chemistry concerned with the speeds, or rates, at which a chemical reaction occurs. • reaction rate: the change in the concentration of a reactant or product with time (M/s) • Why do reactions have such very different rates? • Steps in vision: 10-12 to 10-6 seconds! • Graphite to diamonds: millions of years! • In chemical industry, often more important to maximize the speed of a reaction, not necessarily yield.

  5. A B rate = D[A] D[B] rate = - Dt Dt Chemical Kinetics Reaction rate is the change in the concentration of a reactant or a product with time (M/s). D[A] = change in concentration of A over time period Dt D[B] = change in concentration of B over time period Dt Because [A] decreases with time, D[A] is negative.

  6. A B rate = D[A] D[B] rate = - Dt Dt

  7. Br2(aq) + HCOOH (aq) 2Br-(aq) + 2H+(aq) + CO2(g) time 393 nm Detector light red-brown t1< t2 < t3 D[Br2] aD Absorption

  8. Br2(aq) + HCOOH (aq) 2Br-(aq) + 2H+(aq) + CO2(g) slope of tangent slope of tangent slope of tangent [Br2]final – [Br2]initial D[Br2] average rate = - = - Dt tfinal - tinitial instantaneous rate = rate for specific instance in time

  9. rate k = [Br2] rate a [Br2] rate = k [Br2] = rate constant = 3.50 x 10-3 s-1

  10. 2H2O2 (aq) 2H2O (l) + O2 (g) [O2] = P n V 1 1 D[O2] P = RT = [O2]RT RT RT DP rate = = Dt Dt measure DP over time PV = nRT

  11. 2A B aA + bB cC + dD rate = - = = rate = - = - D[C] D[B] D[A] D[B] D[D] D[A] rate = 1 1 1 1 1 Dt Dt Dt Dt Dt Dt c d a 2 b Reaction Rates and Stoichiometry Two moles of A disappear for each mole of B that is formed.

  12. Example • Write the rate expression for the following reaction: • CH4 (g) + 2O2 (g) CO2 (g) + 2H2O (g)

  13. D[CO2] = Dt D[CH4] rate = - Dt D[H2O] = Dt D[O2] = - 1 1 Dt 2 2 Write the rate expression for the following reaction: CH4(g) + 2O2(g) CO2(g) + 2H2O (g)

  14. Practice Problems • Write the rate expressions for the following reactions in terms of the disappearance of the reactants and appearance of products. • I-(aq) + OCl-(aq)  Cl-(aq) + OI-(aq) • 4NH3(g) + 5O2(g)  4NO(g) + 6H2O(g)

  15. Using Rate Expressions Consider the reaction: • 4NO2(g) + O2(g)  2N2O5(g) Suppose that, at a particular moment during the reaction, molecular oxygen is reacting at the rate of 0.024 M/s. • At what rate is N2O5 being formed? • At what rate is NO2 reacting?

  16. Consider the reaction: 4PH3(g)  P4(g) + 6H2(g) Suppose that, at a particular moment during the reaction, molecular hydrogen is being formed at the rate of 0.078 M/s. • At what rate is P4 being formed? • At what rate is PH3 reacting?

  17. aA + bB cC + dD 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. 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 x and y are determined experimentally!

  18. F2(g) + 2ClO2(g) 2FClO2(g) • rate = k [F2]x[ClO2]y • Double [F2] with [ClO2] constant • Rate doubles • x = 1 • Quadruple [ClO2] with [F2] constant • Rate quadruples • y = 1 rate = k [F2][ClO2]

  19. F2(g) + 2ClO2(g) 2FClO2(g) 1 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. rate = k [F2][ClO2]

  20. 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)

  21. 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) rate k = 2.2 x 10-4 M/s = [S2O82-][I-] (0.08 M)(0.034 M) rate = k [S2O82-]x[I-]y y = 1 x = 1 rate = k [S2O82-][I-] Double [I-], rate doubles (experiment 1 & 2) Double [S2O82-], rate doubles (experiment 2 & 3) = 0.08/M•s

  22. Practice Problems • The reaction of nitric oxide with hydrogen at 1280oC: 2NO(g) + 2H2(g)  N2(g) + 2H2O(g) From the following data collected at this temperature, determine (a) the rate law, (b) the rate constant and (c) the rate of the reaction when [NO] = 12.0x10-3 M and [H2] = 6.0x10-3 M

  23. 23 Nov. 2010 • Objective: SWBAT write rate expressions and calculate reaction rates for chemical reactions. • Do now: Calculate the rate constant (k) from the data below:

  24. Agenda • Do now • Practice Problems Homework: Revisit last night’s assignment • #19 hint: Graph lnP vs. time. If linear, it is 1st order. slope = -k • Excel works great.

  25. Write the reaction rate expressions for the following in terms of the disappearance of the reactants and the appearance of products: • 2H2(g) + O2(g)  2H2O(g) • 4NH3(g) + 5O2(g)  4NO(g) + 6H2O(g)

  26. Consider the reaction N2(g) + 3H2(g)  2NH3(g) Suppose that at a particular moment during the reaction molecular hydrogen is reacting at a rate of 0.074 M/s. • At what rate is ammonia being formed? • At what rate is molecular nitrogen reacting?

  27. Calculate the rate of the reaction at the time when [F2] = 0.010 M and [ClO2] = 0.020 M. • F2(g) + 2ClO2(g)  2FClO2(g)

  28. Consider the reaction X + Y  Z From the following data, obtained at 360 K, • determine the order of the reaction • determine the initial rate of disappearance of X when the concentration of X is 0.30 M and that of Y is 0.40 M

  29. Consider the reaction A B. The rate of the reaction is 1.6x10-2 M/s when the concentration of A is 0.35 M. Calculate the rate constant if the reaction is • first order in A • second order in A

  30. 24 Nov. 2010 • Objective: SWBAT determine reaction order graphically and relate concentration of 1st order reactions to time. • Do now: What is the overall order of reaction for the data shown below? • What is the rate constant?

  31. Agenda • Do now • Homework solutions • First order reactions • Time calculations for first-order reactions. Homework: p. 603 #19, 21, 22, 23, 25-29

  32. The rate laws can be used to determine the concentrations of any reactants at any time during the course of a reaction.

  33. 29 Nov. 2010 • Take Out Homework p. 603 #19, 21, 22, 23, 25-29 • Objective: SWBAT compare 1st order, 2nd order, and zero order reactions, and describe how temperature and activation energy effect the rate constant. • Do now: Calculate the half life of the reaction F2(g) + 2ClO2(g)  2FClO2(g), with rate data shown below:

  34. Agenda • Homework solutions • Review 1st order reactions • Second and zero order reactions • Activation Energy • Problem Set Quiz Weds. p. 31, 32, 35, 37, 39, 42 + Problem set

  35. A product rate = [A] M/s D[A] - M = k [A] Dt [A] = [A]0e−kt ln[A] = ln[A]0 - kt D[A] rate = - Dt First-Order Reactions rate = k [A] = 1/s or s-1 k = [A] is the concentration of A at any time t [A]0 is the concentration of A at time t=0

  36. Graphical Determination of k 2N2O5 4NO2 (g) + O2 (g)

  37. 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 ?

  38. 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 ? 0.88 M ln 0.14 M = 2.8 x 10-2 s-1 ln ln[A]0 – ln[A] = k k [A]0 [A] [A]0 = 0.88 M ln[A] = ln[A]0 - kt [A] = 0.14 M kt = ln[A]0 – ln[A] = 66 s t =

  39. The conversion of cyclopropane to propene in the gas phase is a first order reaction with a rate constant of 6.7x10-4 s-1 at 500oC. • If the initial concentration of cyclopropane was 0.25 M, what is the concentration after 8.8 minutes? • How long, in minutes, will it take for the concentration of cyclopropane to decrease from 0.25 M to 0.15 M? • How long, in minutes, will it take to convert 74% of the starting material?

  40. Practice Problem The reaction 2A → B is first order in A with a rate constant of 2.8×10-2s-1 at 80oC. How long, in seconds, will it take for A to decrease from 0.88 M to 0.14 M?

  41. The rate of decomposition of azomethane (C2H6N2) is studied by monitoring partial pressure of the reactant as a function of time: CH3-N=N-CH3(g) → N2(g) + C2H6(g) The data obtained at 300oC are shown here: Are these values consistent with first-order kinetics? If so, determine the rate constant.

  42. The following gas-phase reaction was studied at 290oC by observing the change in pressure as a function of time in a constant-volume vessel: • ClCO2CCl3(g)  2COCl2(g) • Determine the order of the reaction and the rate constant based on the following data, where P is the total pressure

  43. Ethyl iodide (C2H5I) decomposes at a certain temperature in the gas phase as follows: C2H5I(g) → C2H4(g) + HI(g) From the following data, determine the order of the reaction and the rate constant:

  44. [A]0 ln t½ [A]0/2 0.693 = = = k k ln 2 k First-Order Reactions The half-life, t½, is the time required for the concentration of a reactant to decrease to half of its initial concentration. t½ = t when [A] = [A]0/2 What is the half-life of N2O5 if it decomposes with a rate constant of 5.7 x 10-4 s-1? How do you know decomposition is first order?

  45. [A]0 ln t½ [A]0/2 0.693 = = = = k k t½ ln 2 ln 2 0.693 = k k 5.7 x 10-4 s-1 First-Order Reactions The half-life, t½, is the time required for the concentration of a reactant to decrease to half of its initial concentration. t½ = t when [A] = [A]0/2 What is the half-life of N2O5 if it decomposes with a rate constant of 5.7 x 10-4 s-1? = 1200 s = 20 minutes How do you know decomposition is first order? units of k (s-1)

  46. A product # of half-lives [A] = [A]0/n First-order reaction 1 2 2 4 3 8 4 16

  47. The decomposition of ethane (C2H6) to methyl radicals is a first-order reaction with a rate constant of 5.36x10-4 s-1 at 700oC: C2H6(g)  2CH3(g) Calculate the half-life of the reaction in minutes.

  48. Calculate the half-life of the decomposition of N2O5: 2N2O5 4NO2(g) + O2(g)

  49. A product rate = [A]2 M/s D[A] 1 1 - M2 = k [A]2 = + kt Dt [A] [A]0 t½ = D[A] rate = - Dt 1 k[A]0 Second-Order Reactions rate = k [A]2 = 1/M•s k = [A] is the concentration of A at any time t [A]0 is the concentration of A at time t=0 t½ = t when [A] = [A]0/2