8 - Reaction Mechanisms - CHE4473 Kinetics Spring 2015...

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Unformatted text preview: CHE4473 Kinetics Spring 2015 Homework N°8: Reaction Mechanisms Due Tuesday MAR 10th 2015 IMPORTANT: You must include copies of the graphs of all the numerical solutions (not the whole spreadsheet, please !). 1. Using the Pseudo Steady State Hypothesis (PSSH) approximation, propose a mechanism (series of elementary steps) for the following reaction 2A + B è A2B that is consistent with the behavior of the experimental rate law, rA2B = k CB. 2. The mechanism for the reaction 2 NO + O2 → 2 NO2 is thought to involve the formation of a very reactive intermediate (NO3*) which is always in such a low concentration that cannot be detected, however, its presence can be inferred kinetically. (a) Write the corresponding elementary steps that describe the proposed mechanism (b) Find the rate expression that should result from such a mechanism (c) Compare this rate expression with the one that would result by assuming that the overall reaction is elementary (that is, 1 step, as written). What experiments should be run to show that the proposed mechanism describes the observed behavior better than the 1-­‐step assumption? 3. The pyrolysis of acetaldehyde (CH3CHO) is believed to take place according to the sequence: CH3CHO è CH3* + CHO* r1 = k1 CA CH3* + CH3CHO è CH3* + CO + CH4 r2 = k2 CA CB* CHO* + CH3CHO è CH3* + 2CO + H2 r3 = k3 CA CC* 2 C H * è C H r4 = k4 CB*2 3 2 6 Using the Pseudo Steady State Hypothesis (PSSH) approximation, (a) derive the rate expression for the rate of acetaldehyde disappearance. (b) Under what conditions the measured reaction order with respect to acetaldehyde will be 3/2 ? 4. The homogeneous gas phase decomposition of ClNO is an uncatalyzed homogeneous bimolecular reaction that can be described by a single elementary step 2 ClNO è 2 Cl + 2 NO. The second-­‐order rate constant for this reaction has been measured as a function of temperature, as shown in the following table. Using the transition state theory, determine the enthalpy and entropy of activation. Additional Data T(K) k(exp) 600 7.82 E+08 700 1.56 E+10 900 8.49 E+11 1000 3.43 E+12 kB = 1.38 E-­‐23 m2 kg s-­‐2 K-­‐1 h = 6.63 E-­‐34 m2 kg s-­‐1 ...
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