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15 February 2012

15 February 2012. Objective : You will be able to: define “kinetics” and identify factors that affect the rate of a reaction. write rate expressions for balanced chemical reactions. Agenda. Do now Kinetics notes Reaction Rates Demonstrations Rate constant and reaction rates problems.

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15 February 2012

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  1. 15 February 2012 • Objective: You will be able to: • define “kinetics” and identify factors that affect the rate of a reaction. • write rate expressions for balanced chemical reactions.

  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, 16, 18: Thurs.

  3. Chemical Kinetics

  4. Aspects of Chemistry • 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

  5. 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.

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

  7. 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.

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

  9. 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

  10. Factors that Affect Reaction Rates • Concentration of reactants: higher concentrations = faster reactions • as concentration increases, the frequency of collisions increases, increasing reaction rate • Temperature: increasing temperature increases reaction rate because of increased KE • Physical state of reactants: homogeneous mixtures of either liquids or gases react faster than heterogeneous mixtures • Presence of a catalyst: affects the kinds of collisions that lead to a reaction.

  11. Question and Demo • Mine explosions from the ignition of powdered coal dust are relatively common, yet lumps of coal burn without exploding. Explain.

  12. 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.

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

  14. 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)

  15. 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)

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

  17. 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?

  18. 16 February 2012 • Objective: You will be able to: • solve rate expressions. • determine the order of a reaction from experimental data Homework Quiz: N2(g) + 3H2(g) → 2NH3(g) Suppose that at a particular moment during the reaction, hydrogen is reacting at the rate of 0.074 M/s. • At what rate is NH3 being formed? • At what rate is nitrogen reacting?

  19. Agenda • Do now • Iodine clock reaction. • Solving rate equations • Determining order of reactions Homework: p. 602 #15, 16, 18, 19, 20: Mon after break Hint: Use pressure just like concentration. Diagnostic test (Tues after break)

  20. Example 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?

  21. Problem • Consider the reaction between gaseous hydrogen and gaseous nitrogen to produce ammonia gas. • At a particular time during the reaction, H2(g) disappears at the rate of 3.0 M/s. • What is the rate of disappearance of N2(g)? • What is the rate of appearance of NH3(g)?

  22. If ammonia appears at 2.6 M/s, how fast does hydrogen disappear?

  23. aA + bB cC + dD The Rate Law The rate law is a mathematical relationship that shows how rate of reaction depends on the concentrations of reactants Rate = k [A]x[B]y x and y are small whole numbers that relate to the number of molecules of A and B that collide and are determined experimentally!

  24. aA + bB cC + dD The Rate Law 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 Rate = k [A]1[B]2

  25. Example • What is the numerical value of the rate constant for the reaction described in the table above? Specify units.

  26. 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]

  27. 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)

  28. 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?

  29. 27 February 2012 • Take Out: p. 602 #15, 16, 18, 19, 20 • Objective: You will be able to determine the rate of a reaction given experimental data and reactant concentrations. • Homework Quiz: What is the rate law for the reaction shown below? • What is the rate when [A]=1.50 M and [B]=0.50 M?

  30. Agenda • Homework Quiz • Homework answers • Determining and solving rate laws • Hand back tests and assignments Homework: Diagnostic test revisit/correct p. 603 #15, 16, 18

  31. 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]

  32. 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)

  33. 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

  34. 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

  35. 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)

  36. 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

  37. 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

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

  39. 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:

  40. 28 February 2012 • Take Out: Diagnostic test • Objective: You will be able to determine order of a reaction and k graphically. • Homework Quiz: What is the rate law for the reaction shown below? • What is the rate when [A]=1.50 M and [B]=0.50 M?

  41. Agenda • Homework Quiz • 1st order reactions graphically • Half life calculations Homework: p. 603 #19, 20 (use Excel!), 24, 26

  42. First Order (Overall) Reactions • rate depends on the concentration of a single reactant raised to the first power. rate = k[A] = • Using calculus, this rate law is transformed into an equation for a line: ln[A] = ln[A]0 - kt

  43. 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 =

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

  45. A non-graphical example • 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 ?

  46. 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 =

  47. 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?

  48. 29 February 2012 • Objective: You will be able to: • calculate the half-life of a first order reaction • explore the relationship between time and concentration of a second order reaction Homework Quiz: 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?

  49. 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.

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