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Chapter 14-Chemical Kinetics

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  1. Chapter 14-Chemical Kinetics Tour of the Ozone Hole http://www.atm.ch.cam.ac.uk/tour/

  2. 16 September INTERNATIONAL DAY FOR THE PRESERVATION OF THE OZONE LAYER The Ozone Hole of 2008 is larger than in 2007 Geneva, 16 September 2008 (WMO) - “After decades of chemical attack, it may take another 50 years or so for the ozone layer to recover fully. As the Montreal Protocol has taught us, when we degrade our environment too far, nursing it back to health tends to be a long journey, not a quick fix”, said Ban Ki-moon, Secretary-General of the United Nations, on the occasion of the International Day for the Preservation of Ozone Layer today. According to the World Meteorological Organization (WMO), the 2008 Antarctic ozone hole will be larger than the one of 2007. The observed changes in the stratosphere could delay the expected recovery of the ozone layer. It is therefore vital that all Member States with stratospheric measurement programmes continue to support and enhance these measurements.

  3. O3 concentration is reported in Dobson Units (DU) 1DU =2.69E15 O3 molecules/cm2 In September, 2006 the largest ozone hole ever was observed.

  4. Temperature inversions and wind direction changes dramatically influence air quality in cities like Salt Lake…

  5. Photo-chemical Smog NO(g) + O3(g) → NO2(g) + O2(g)

  6. In 1998, a total of 24.5 megatons of nitrogen oxide (NOx) compounds were released to the atmosphere. NOx compound are produced primarily in the combustion of hydrocarbons N2(g) + O2(g) → 2 NO(g) DH° = + 180.6 kJ 2 NO(g) + O2(g) → 2 NO2(g) DH° = - 114.2 kJ NO2(g) + hn→ NO(g) + O(g)photochemical O2(g) + O(g) → O3(g) Ozone production

  7. Chemical Kinetics looks at both the chemical mechanism as well as the rate at which reactions occur.

  8. Chemical Reactions usually involve molecular collisions that result in bonds being broken or made. Both of these reactions are bimolecular…i.e. two molecules collide in order for the reaction to occur.

  9. The Lewis structure of NO2 indicates that the nitrogen atom has an unpaired electron. Two NO2 molecules combine by using their unpaired electrons to form an N-N bond.

  10. Chemical Reaction Mechanism In the reverse of the previous reaction, a unimolecular elementary reaction may involve bond breakage. If an N2O4 molecule possesses enough energy (from heat or light), molecular vibrations can break the N-N bond to produce two NO2 molecules. When 3 molecules collide to form chemical product the reaction mechanism would be called ter-molecular.

  11. The reaction between O3 and NO is believed to occur by a mechanism that consists of the single bimolecular step illustrated here in a molecular view.

  12. A chemical reaction rate is the change in concentration of a reactant or product during a particular time interval for the reaction N2(g) + O2(g) → 2 NO(g)

  13. N2(g) + O2(g) → 2 NO(g)

  14. »PC version Learn to calculate the average and instantaneous rate from the rate expression and concentration vs. time data. Includes practice exercises. Reaction Rate Tutorial

  15. (a) To calculate the average rate of reaction, determine how many moles are consumed during the time interval and divide by the time: • n = (0.25 mol/L) – (0.50 mol/L) = – 0.25 mol/L; • Rate = M/s = 0.25 M/30. s = 8.33E-3 • Round to two significant figures: Rate = 8.3E-3 M/s • NOTE UNITS of REACTION RATES • (b) The rate for any particular reagent is the coefficient for that reagent times the rate of reaction: Rate(NH3) = (2/3)(8.3-3 M/s) = 5.6 x 10-3 M/s • (c) The concentration of any particular reagent is its initial concentration minus the change during the time interval: • Change = (Coeff)(Rate)(time) = • –(1/3)(8.3E-3M/s)(30 s) = –8.3 x 10-2 M • Concentration = (1.25 M) – (0.083 M) = 1.17 M

  16. Problem An engineer is studying the rate of the Haber synthesis: N2(g) + 3 H2(g) → 2 NH3(g) Starting with a closed reactor containing 1.25 mol/L of N2 and 0.5 mol/L of H2, the engineer find that the H2 concentration has fallen to 0.25 M after 30 seconds. (a) What is the average rate of reaction over this time. (b) What is the ave. rate of production of NH3. (c) What is the N2 concentration after 30s?

  17. (a) What is the average rate of reaction over this time. (b) What is the ave. rate of production of NH3.

  18. (c) What is the N2 concentration after 30s?

  19. 2 NO2→ 2 NO + O2 A plot of [NO2], [O2], and [NO] as a function of time (seconds) for the decomposition reaction of NO2. The concentration data are shown in the table.

  20. Initial Rate

  21. Instantaneous Rates The rate at t=0 is the instantaneous initial rate. This is the most common way of reporting rates in the laboratory

  22. Instantaneous Rates Tropospheric ozone is rapidly consumed in many reactions, including the following. O3 + NO → NO2 + O2 Use the following data to calculate the instantaneous rate of the preceding reaction at t = 0.000 s and t = 0.102 s.

  23. The rate of a chemical reaction increases with increasing concentration because the reactants (NO + O2) are more likely to collide. It follows: reaction rate ~ [reactants] Or that reaction rate = k[reactants]n Note that the units of “k” (the rate constant) are different depending on n, the “order” of the reaction

  24. 14.48. Compounds A and B react to give a single product, C. Write the rate law for each of the following cases and determine the units of the rate constant by using the units M for concentration and s for time: a. The reaction is first order in A and second order in B. b. The reaction is first order in A and second order overall. c. The reaction is independent of the concentration of A and second order overall. d. The reaction is second order in both A and B.

  25. 14.53.Rate Laws for Destruction of Tropospheric Ozone The reaction of NO2 with ozone produces NO3 in a second-order reaction overall: NO2(g) + O3(g) → NO3(g) + O2(g) a. Write the rate law for the reaction if the reaction is first order in each reactant. b. The rate constant for the reaction is 1.93  104M–1s–1 at 298 K. What is the rate of the reaction when [NO2] = 1.8  10–8M and [O3] = 1.4  10–7M? c. What is the rate of the appearance of NO3 under these conditions? d. What happens to the rate of the reaction if the concentration of O3(g) is doubled?

  26. b. The rate constant for the reaction is 1.93  104M–1s–1 at 298 K. What is the rate of the reaction when [NO2] = 1.8  10–8M and [O3] = 1.4  10–7M? c. What is the rate of the appearance of NO3 under these conditions? d. What happens to the rate of the reaction if the concentration of O3(g) is doubled?

  27. What is the rate equation (rate law) for this reaction?

  28. The single experiment approach For reactions such as: O3(g) → O2(g) + O(g) The rate is found to be first order in ozone rate = k[O3] The mathematical solution can be found by integrating this expression. What are the units for “k”?

  29. The First Order integrated rate law is: ln([O3]/[O3]0) = - kt

  30. Problem The decomposition of N2O5 (g) is 1st Order. (a) Write the rate equation. (b) If 2.56 mg of N2O5 is present initially, and 2.50 mg remain after 4.26 min at 55 C. Calculate the rate constant, k, for this process.

  31. N2O(g) → N2(g) + 1/2 O2(g) Rate = k[N2O] t½ = 1 s

  32. 14C →14N + 0-1e t½ = 5730 yr 14_10.jpg

  33. Half-life Problem The half-life for a first order reaction at 550°C is 85 seconds. How long would it take for 23% of the reactant to decompose?

  34. At high temperatures, the reaction: 2 NO2(g) → 2NO(g) + O2(g) is second-order in NO2, i.e. Rate = k[NO2]2 The integrated rate law for a 2nd order reaction is: [NO2]-1t = kt + [NO2]-10 What are the units for “k”?

  35. 14.64. Two structural isomers of ClO2 are shown: The isomer with the Cl–O–O skeletal arrangement is unstable and rapidly decomposes according to the reaction 2ClOO(g) → Cl2(g) + 2O2(g). The following data were collected for the decomposition of ClOO at 298 K: Determine the rate law for the reaction and the value of the rate constant at 298 K. What is the half-life for the reaction?

  36. Is the reaction 1st or 2nd order in Cl-O-O? Plot of Concentration (M) versus Time [COO] Time (us)

  37. Is the reaction 1st or 2nd order in Cl-O-O? Second-Order Plot… [COO]-1 Time (us)

  38. Is the reaction 1st or 2nd order in Cl-O-O? First-Order Plot… ln[COO] Time (us)