5 Solved Problems on Chemical Reaction Engineering - Final Exam | CHE 34800, Exams of Chemistry

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Purdue University — Department of Chemical Engineering Final Examination CHE 348 Name: Closed book and notes Friday, May 2, 2008 Col ak ons I. (15 points) Decomposition of ozone catalyzed by atomic chlorine The following two-step cycle is believed to occur for the destruction of ozone by chlorine in the stratosphere: Cl +0; 02+ ClO a) clo+O > 02+Cl 2) The overall reaction is O + 03 = 202. Note that C] and ClO are very reactive species and that Cl is regenerated at the end of the cycle (Cl is a catalyst for this reaction). ‘The reaction constants measured experimentally are: ki=5x 107! exp(-140/T) cm? s and kp = 1.1 x 107° exp(-220/T) cm? s? a) Using the steady-state approximation and the fact that the number of active centers ([L]=[Cl] + [CIO]) is a constant show that the rate of ozone decomposition is re = - (Kr kp [L] [0] {Os} ki [05] + kz [0] ) b) Since [O]<<[O3}, simplify the denominator for the expression in a). The non- catalyzed rate of ozone decomposition (nc) is elementary (0 + O; — 20,) with = 1.9 x 107! exp(-2300/T) cm? s? Calculate the ratio of the catalyzed to the non-catalyzed reaction {felTne) a8 a function of temperature using [L]/[O3] = 10°, which is the value measured in the stratosphere. Calculate the ratio r/tye for a temperature in the ozone layer of 200 K. Does that explain why Cl destroys the ozone layer? (remember that CI is not consumed in the cycle, it catalyzes the reaction indefinitely)