Brown - Chemistry: The Central Science 14th Edition - Chapter 14

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Ch.14 - Chemical Kinetics

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Brown 14th Edition

Ch.14 - Chemical Kinetics

Problem 68b
What is the molecularity of each of the following elementary reactions? Write the rate law for each. (b)
Problem 69a
(a) Based on the following reaction profile, how many intermediates are formed in the reaction A→D?
Problem 69c
(c) Which step is the fastest?
Problem 70a
Consider the following energy profile.
(a) How many elementary reactions are in the reaction mechanism?
Problem 70b
Consider the following energy profile.
(b) How many intermediates are formed in the reaction?
Problem 70c
Consider the following energy profile.
(c) Which step is rate limiting?
Problem 71
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). (c) If the first step is slow and the second one is fast, which rate law do you expect to be observed for the overall reaction?
Problem 72a
The decomposition of hydrogen peroxide is catalyzed by iodide ion. The catalyzed reaction is thought to proceed by a two-step mechanism:
H₂O₂(aq) + I^-(aq) → H₂O(l) + IO^-(aq) (slow)
IO^-(aq) + H₂O₂(aq) → H₂O(l) + O₂(g) + I^-(aq) (fast)
(a) Write the chemical equation for the overall process.
Problem 72b
The decomposition of hydrogen peroxide is catalyzed by iodide ion. The catalyzed reaction is thought to proceed by a two-step mechanism:
H₂O₂(aq) + I^-(aq) → H₂O(l) + IO^-(aq) (slow)
IO^-(aq) + H₂O₂(aq) → H₂O(l) + O₂(g) + I^-(aq) (fast)
(b) Identify the intermediate, if any, in the mechanism.
Problem 72c
The decomposition of hydrogen peroxide is catalyzed by iodide ion. The catalyzed reaction is thought to proceed by a two-step mechanism:
H₂O₂(aq) + I^-(aq) → H₂O(l) + IO^-(aq) (slow)
IO^-(aq) + H₂O₂(aq) → H₂O(l) + O₂(g) + I^-(aq) (fast)
(c) Assuming that the first step of the mechanism is rate determining, predict the rate law for the overall process.
Problem 73a
The reaction 2 NO1g2 + Cl21g2¡2 NOCl1g2 was performed and the following data were obtained under conditions of constant 3Cl24:
(a) Is the following mechanism consistent with the data? NO1g2 + Cl21g2ΔNOCl21g2 1fast2 NOCl21g2 + NO1g2¡2 NOCl1g2 1slow2
Problem 74
(c) What are the intermediates in this mechanism? (d) If you are unable to detect HOBr or HOOBr among the products, does this disprove your mechanism?
Problem 74a
You have studied the gas-phase oxidation of HBr by O₂: 4 HBr(g) + O₂(g) → 2 H₂O(g) + 2 Br₂(g)
You find the reaction to be first order with respect to HBr and first order with respect to O₂. You propose the following mechanism:
HBr(g) + O₂(g) → HOOBr(g)
HOOBr(g) + HBr(g) → 2 HOBr(g)
HOBr(g) + HBr(g) → H₂O(g) + Br₂(g)
(a) Confirm that the elementary reactions add to give the overall reaction.
Problem 74b
You have studied the gas-phase oxidation of HBr by O₂: 4 HBr(g) + O₂(g) → 2 H₂O(g) + 2 Br₂(g)
You find the reaction to be first order with respect to HBr and first order with respect to O₂. You propose the following mechanism:
HBr(g) + O₂(g) → HOOBr(g)
HOOBr(g) + HBr(g) → 2 HOBr(g)
HOBr(g) + HBr(g) → H₂O(g) + Br₂(g)
(b) Based on the experimentally determined rate law, which step is rate determining?
Problem 75a,b
(a) What is a catalyst? (b) What is the difference between a homogeneous and a heterogeneous catalyst?
Problem 75c
(c) Do catalysts affect the overall enthalpy change for a reaction, the activation energy, or both?
Problem 76
b) What role does adsorption play in the action of a heterogeneous catalyst?
Problem 76a
(a) Most commercial heterogeneous catalysts are extremely finely divided solid materials. Why is particle size important?
Problem 78
In solution, chemical species as simple as H+ and OH- can serve as catalysts for reactions. Imagine you could measure the [H+] of a solution containing an acid-catalyzed reaction as it occurs. Assume the reactants and products themselves are neither acids nor bases. Sketch the [H+] concentration profile you would measure as a function of time for the reaction, assuming t = 0 is when you add a drop of acid to the reaction.
Problem 79
The oxidation of SO2 to SO3 is accelerated by NO2. The reaction proceeds according to: NO2(g) + SO2(g) → NO(g) + SO3(g) 2 NO(g) + O2(g) → 2 NO2(g) (a) Show that, with appropriate coefficients, the two reactions can be summed to give the overall oxidation of SO2 by O2 to give SO3. (d) Is this an example of homogeneous catalysis or heterogeneous catalysis?
Problem 80b
The addition of NO accelerates the decomposition of N2O, possibly by the following mechanism: NO1g2 + N2O1g2¡N21g2 + NO21g2 2 NO21g2¡2 NO1g2 + O21g2 (b) Is NO serving as a catalyst or an intermediate in this reaction?
Problem 80c
The addition of NO accelerates the decomposition of N₂O, possibly by the following mechanism:
NO(g) + N₂O(g) → N₂(g) + NO₂(g)
2 NO₂(g) → 2 NO(g) + O₂(g)
(c) If experiments show that during the decomposition of N₂O, NO₂ does not accumulate in measurable quantities, does this rule out the proposed mechanism?
Problem 81
Many metallic catalysts, particularly the precious-metal ones, are often deposited as very thin films on a substance of high surface area per unit mass, such as alumina (Al2O3) or silica (SiO2). (a) Why is this an effective way of utilizing the catalyst material compared to having powdered metals?
Problem 81b
Many metallic catalysts, particularly the precious-metal ones, are often deposited as very thin films on a substance of high surface area per unit mass, such as alumina (Al₂O₃) or silica (SiO₂). (b) How does the surface area affect the rate of reaction?
Problem 82
(b) Automobile catalytic converters have to work at high temperatures, as hot exhaust gases stream through them. In what ways could this be an advantage? In what ways a disadvantage? (c) Why is the rate of flow of exhaust gases over a catalytic converter important?
Problem 85
The enzyme carbonic anhydrase catalyzes the reaction CO2(g) + H2O(l) ↔ HCO3⁻(aq) + H⁺(aq). In water, without the enzyme, the reaction proceeds with a rate constant of 0.039 s⁻¹ at 25 _x001E_C. In the presence of the enzyme in water, the reaction proceeds with a rate constant of 1.0 * 10⁶ s⁻¹ at 25 _x001E_C. Assuming the collision factor is the same for both situations, calculate the difference in activation energies for the uncatalyzed versus enzyme-catalyzed reaction.
Problem 86b
The enzyme urease catalyzes the reaction of urea, (NH₂CONH₂), with water to produce carbon dioxide and ammonia. In water, without the enzyme, the reaction proceeds with a first-order rate constant of 4.15 × 10^-5 s^-1 at 100°C. In the presence of the enzyme in water, the reaction proceeds with a rate constant of 3.4 × 10⁴ s^-1 at 21°C. (b) If the rate of the catalyzed reaction were the same at 100°C as it is at 21°C, what would be the difference in the activation energy between the catalyzed and uncatalyzed reactions?
Problem 86c
The enzyme urease catalyzes the reaction of urea, (NH₂CONH₂), with water to produce carbon dioxide and ammonia. In water, without the enzyme, the reaction proceeds with a first-order rate constant of 4.15 × 10^-5 s^-1 at 100°C. In the presence of the enzyme in water, the reaction proceeds with a rate constant of 3.4 × 10⁴ s^-1 at 21°C. (c) In actuality, what would you expect for the rate of the catalyzed reaction at 100°C as compared to that at 21°C?
Problem 87a
The activation energy of an uncatalyzed reaction is 95 kJ/mol. The addition of a catalyst lowers the activation energy to 55 kJ/mol. Assuming that the collision factor remains the same, by what factor will the catalyst increase the rate of the reaction at (a) 25 C
Problem 87b
The activation energy of an uncatalyzed reaction is 95 kJ/mol. The addition of a catalyst lowers the activation energy to 55 kJ/mol. Assuming that the collision factor remains the same, by what factor will the catalyst increase the rate of the reaction at (b) 125 °C?