Reaction Mechanisms

AP Chemistry Name Reaction Mechanisms Period Date UNIT 4—CHEMICAL KINETICS 1. Assuming the following reactions are elementary, 4. The following questions refer to the What is the molecularity of each of the following? reaction 2A2 + B2 →2C. the following mechanism has been proposed: Write the rate law for each. Step 1 (very slow) A2 + B2 →R + C a. Cl2 (g)  2 Cl (g) Step 2 (slow) A2 + R →C a. What is the molecularity of step 2 1 Rate = k [Cl2] b. 2 NO (g)  N2O2 2 Bimolecular/2 b. Which step is the “rate determining”? Step 1 (very slow) Rate = k [NO]2 c. 2 NO (g) + O2 (g)  2 NO2 (g) 3 Rate = k [NO]2[O2] c. According to the proposed mechanism, what should the overall rate law be? Rate = k [A2][B2] 2. The rate for reaction c above is found to be extremely slow. Explain why this is. It is termolecular/requires collision of 3 molecules—the chances of this happening with enough energy/proper orientation is very low. 3. The following mechanism has been proposed for the gas phase reaction of H2 with ICl: H2 (g) + ICl (g)  HI (g) + HCl (g) HI (g) + ICl (g)  I2 (g) + HCl (g) a. Write the balanced equation for the overall reaction. H2 + 2 ICl  I2 + 2 HCl b. Identify any intermediates and catalysts in the mechanism. HI—it is made and then used c. If the first step is slow and the second step is fast, what rate law do you expect to be observed for the overall reaction? Rate = k[H2][ICl] 5. The decomposition of ozone in the upper atmosphere to dioxygen occurs by a two-step mechanism. The first step is a fast reversible step and the second is a slow reaction between an oxygen atom and an ozone molecule: (1) O3 (g)  O2 (g) + O (g) Fast equil (2) O3 (g) + O (g)  2O2 (g) Slow a. Which is the rate determining step? Step 2—slow step b. Write the rate equation for the ratedetermining step. Rate = k[O3][O] c. Write the rate law for the overall reaction. Step 1 Forward Rate = Reverse Rate k[O2][O] = k[O3]  [O] = [O3]/[O2] Rate = k[O3][O] = k[O3]( [O3]/[O2]) Rate = k[O3]2/[O2] 8. 2 NO (g) + O2 (g)  2 NO2 (g) The rate law for a reaction is found to be Rate = k[A]2[B]. Which of the following mechanisms gives the rate law. a) A + B ↔ E (fast) E + B → C + D (slow) a. Write the equation for the overall reaction. b) A + B ↔ E (fast) E + A →C + D (slow) c) A + A →E (slow) E + B →C + D (fast) 6. The oxidation of SO2 to SO3 with NO2 proceeds by the following mechanism. NO2 (g) + SO2 (g)  NO (g) + SO3 (g) SO2 + O2 + NO  SO3 + NO2 b. Is NO2 a catalyst or an intermediate? Justify your answer. A catalyst—it is used and then reformed Explain your choice. 7. The reaction 2A +B → C Has the following proposed mechanism: Step 1: A + B ↔ D (fast equilibrium) Step 2: D + B →E Step 3: E + A → C + B If step 2 is the rate-determining step, then the rate of formation of C should equal what? Rate = k [A][B]2 AP Chemistry 2013 #5 It is proposed that the formation of C2H5Cl(g) proceeds via the following two-step reaction mechanism. The reaction has a ΔH value of -450 kJ. Step 1: Step 2: C2H4 (g) + HCl (g) → C2H5+ (g) + Cl– (g) C2H5+ (g) + Cl– (g) → C2H5Cl (g) rate-determining step fast step (a) Write the rate law for the reaction that is consistent with the reaction mechanism above. Rate = k[C2H4]1[HCl]1 (b) Identify an intermediate in the reaction mechanism above. C2H5+, Cl– AP Chemistry 2002 #7 An environmental concern is the depletion of O3 in Earth's upper atmosphere, where O3 is normally in equilibrium with O2 and O. A proposed mechanism for the depletion of O3 in the upper atmosphere is shown below. O3 + Cl → O2 + ClO ClO + O → Cl + O2 Step I Step II (a) Write a balanced equation for the overall reaction represented by Step I and Step II above. O3 + O → 2 O2 (b) Clearly identify the catalyst in the mechanism above. Justify your answer. Cl. It is consumed in step I and regenerated at the end of step II. (c) Clearly identify the intermediate in the mechanism above. Justify your answer. ClO. It is formed in step I and used in step II. (d) If the rate law for the overall reaction is found to be Rate = k[O3] [Cl], determine the following. (i) The overall order of the reaction 2nd Order overall (ii) Appropriate units for the rate constant, k. M−1 s–1 (iii) The rate-determining step of the reaction, along with justification for your answer The rate law for the elementary step I is rate = k[O3][Cl], which is consistent with the experimentally determined rate law, so step I must be the rate determining step.