EXAM PRACTIVE QUESTIONS Chapter 16 1. The gas phase reaction A + B o C has an experimentally determined rate expression that follows the relationship rate = k[A]2. What is the reaction order of B? Answer: zero 2. Which of the following reactions would you predict to occur at the faster rate? I. Ag+(aq) + Cl-(aq) o AgCl(s) II. CH4(g) + O2(g) o CO2(g) + H2O(g) Answer: I, nature of the reactants 3. The gas phase reaction A + B o C has an experimentally determined rate expression that follows the relationship rate = k[A]2[B]. If the concentration of A is tripled and the concentration of B is doubled, the reaction rate would be increased by a factor of ____________. Answer: 18 4. Determine the reaction rate-law expression for the reaction below. 2 A + B2 + C o A2B + BC Trial Initial [A] Initial [B2] Initial [C] Initial Rate of Formation of BC 1 0.20 M 0.20 M 0.20 M 2.4 u 10-6 Mmin-1 2 0.40 M 0.30 M 0.20 M 9.6 u 10-6 Mmin-1 3 0.20 M 0.30 M 0.20 M 2.4 u 10-6 Mmin-1 4 0.20 M 0.40 M 0.40 M 4.8 u 10-6 Mmin-1 Answer: rate = k[A]2[C] 5. Calculate the rate constant, k, for the reaction described in Question 4. Answer: 3.3 u 103 M-2 min-1 6. Consider a chemical reaction involving compounds A and B, which is found to be first order in A and second order in B. At what rate will the reaction occur in experiment 2? Experiment Rate (M s-1) Initial [A] Initial [B] 1 0.10 1.0 M 0.20 M 2 ? 2.0 M 0.60 M Answer: 1.8 M s-17. The rate constant for the first order reaction A o B + C is k = 3.3 u 10-2 min-1 at 57 K. What is the half-life for this reaction at 57 K? Answer: 21 min 8. Cyclopropane rearranges to form propene in a reaction that is first order. If the rate constant is 2.74 u 10-3 s-1, how long would it take for 85.6% of the cyclopropane to rearrange if the initial concentration was 0.460 M? Answer: 706 s 9. At a certain temperature the reaction 2B o C + D obeys the rate-law expression rate = 1.14 u 10-3 M-1 s-1[B]2. If 5.00 mol of B is initially present in a 1.99-L container at that temperature, how long would it take for 2.00 mol of B to be consumed at constant temperature? Answer: 58.3 s 10. Compounds A and B react to form C and D in a reaction that is found to be second-order overall and second-order in B. The rate constant at 50 qC is 2.48 L/mol min. What is the half-life of B (in min) if 0.822 M B reacts with excess A? Answer: 0.491 min 11. The specific rate constant, k, for a reaction is 2.64 u 10-2 s-1 at qC, and the activation energy is 74.0 kJ/mol. Calculate k at 50 qC. Answer: 0.266 s-1 12. The following mechanism was proposed for a reaction. Write the overall reaction and the rate law expression for the proposed mechanism. H2 + CO o H2CO slow H2 + H2CO o CH4 + O fast H2 + O o H2O fast Answer: overall reaction 3 H2 + CO o CH4 + H2O rate law expression rate =