Chemical Kinetics

Instructions

Answer all assigned questions and problems, and show all work.

1. Write the reaction rate expressions for the following reactions in terms of the disappearance of the reactants and the appearance of products. (6 points)

1. 1. H₂(g) + I₂(g) → 2HI(g)
   2. 5Br^–(aq) + BrO₃^–(aq) + 6H⁺(aq) → 3Br₂(aq) + 3H₂O(l)

1. Consider the reaction

2NO(g)  +  O₂(g) → 2NO₂(g)

Suppose that at a particular moment during the reaction nitric oxide (NO) is reacting at the rate of 0.066 M/s.

1. 1. At what rate is NO₂ being formed? (3 points)
   2. At what rate is molecular oxygen reacting? (3 points)

3. The rate law for the reaction

NH₄⁺(aq)  +  NO₂^–(aq) →  N₂(g)  +  2H₂O(l)

is given by rate = k[NH₄⁺][NO₂^–]. At 25ºC, the rate constant is 3.0 × 10^–4/ M · s.  Calculate the rate of the reaction at this temperature if [NH₄⁺] = 0.26 M and [NO₂^–] = 0.080 M. (5 points)

4. Consider the reaction

A + B →  products

From the following data obtained at a certain temperature, determine the order of the reaction and calculate the rate constant. (5 points)

5. Determine the overall orders of the reactions to which the following rate laws apply. (8 points)

1. 1. rate = k[NO₂]²
   2. rate = k
   3. rate = k[H₂][Br₂]^1/2
   4. rate = k[NO]²[O₂]

6. What is the half-life of a compound if 75 percent of a given sample of the compound decomposes in 60 minutes? Assume first-order kinetics. (5 points)

7. The thermal decomposition of phosphine (PH₃) into phosphorus and molecular hydrogen is a first-order reaction:

4PH₃(g) →  P₄(g)  +  6H₂(g)

The half-life of the reaction is 35.0 s at 680°C. Calculate (a) the first-order rate constant for the reaction and (b) the time required for 95 percent of the phosphine to decompose. (10 points)

8. The rate constant for the second-order reaction

2NOBr(g) → 2NO(g)  +  Br₂(g)

is 0.80/M · s at 10ºC.

1. 1. Starting with a concentration of 0.086 M, calculate the concentration of NOBr after 22 s. (5 points)
   2. Calculate the half-lives when [NOBr]₀ = 0.072 M and [NOBr]₀ = 0.054 M.
      (5 points)

9. Define activation energy. What role does activation energy play in chemical kinetics? (5 points)

10. Given the same reactant concentrations, the reaction

CO(g)  +  Cl₂(g) → COCl₂(g)

at 250°C is 1.50 × 10³ times as fast as the same reaction at 150°C. Calculate the activation energy for this reaction. Assume that the frequency factor is constant. (Hint: use equation 13.14.) (10 points)

11. Most reactions, including enzyme-catalyzed reactions, proceed faster at higher temperatures. However, for a given enzyme, the rate drops off abruptly at a certain temperature. Account for this behavior. (5 points)

12. The bromination of acetone is acid-catalyzed:

CH₃COCH₃ + Br₂ → CH₃COCH₂Br  +  H⁺  +  Br^–

The rate of disappearance of bromine was measured for several different concentrations of acetone, bromine, and H⁺ ions at a certain temperature:

1. What is the rate law for the reaction? (5 points)
2. Determine the rate constant. (5 points)

13. Briefly comment on the effect of a catalyst on each of the following: (a) activation energy, (b) reaction mechanism, (c) enthalpy of reaction, (d) rate of forward step, (e) rate of reverse step. (15 points)