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Exam 16: Kinetics: Rates and Mechanisms of Chemical Reactions

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Question 21

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Ammonia will react with oxygen in the presence of a copper catalyst to form nitrogen and water. From 164.5°C to 179.0°C, the rate constant increases by a factor of 4.27. What is the activation energy of this oxidation reaction?

A) 24.5 kJ/mol
B) 165 kJ/mol
C) 242 kJ/mol
D) 1630 kJ/mol
E) > 10⁴ kJ/mol

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Question 22

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A reaction has the following rate law: Rate = k[A][B]² In experiment 1, the concentrations of A and B are both 0.10 mol L^-1; in experiment 2, the concentrations are both 0.30 mol L^-1. If the temperature stays constant, what is the value of the ratio, Rate(2) /Rate(1) ?

A) 3.0
B) 6.0
C) 9.0
D) 18
E) 27

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Question 23

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A first-order reaction has a half-life of 20.0 minutes. Starting with 1.00 × 10²⁰ molecules of reactant at time t = 0, how many molecules remain unreacted after 100.0 minutes?

The gas-phase reaction CH₃NC  CH₃CN has been studied in a closed vessel, and the rate equation was found to be: Rate = -[CH₃NC]/t = k[CH₃NC]. Which one of the following actions is least likely to cause a change in the rate of the reaction?

Chlorine atoms act as heterogeneous catalysts in the destruction of ozone in the stratosphere.

The rate law for the rearrangement of CH₃NC to CH₃CN at 800 K is Rate = (1300 s^-1) [CH₃NC]. What is the half-life for this reaction?

At 25.0°C, a rate constant has the value 5.21 × 10^-8 L mol^-1 s^-1. If the activation energy is 75.2 kJ/mol, calculate the rate constant when the temperature is 50.0°C.

For each of the following terms/concepts, give a brief explanation or definition. Where possible, use examples. a. order of a reaction b. elementary reaction c. reaction intermediate

The elementary reaction HBr(g) + Br(g)  H(g) + Br₂(g) is endothermic. a. Would you expect the rate constant for the back reaction to be smaller or larger than that for the forward reaction? Explain, briefly. b. Draw a fully-labeled reaction energy diagram for this reaction, showing the locations of the reactants, products, and transition state.

Reaction intermediates differ from activated complexes in that

Sulfuryl chloride, SO₂Cl₂(g) , decomposes at high temperature to form SO₂(g) and Cl₂(g) . The rate constant at a certain temperature is 4.68 × 10^-5s^-1. What is the order of the reaction?

The rate constant for the reaction 3A  4B is 6.00 × 10^-3 L mol^-1min^-1. How long will it take the concentration of A to drop from 0.75 M to 0.25 M?

In the gas phase at 500.°C, cyclopropane reacts to form propene in a first-order reaction. The figure below shows the concentration of cyclopropane plotted versus time. Use the graph to calculate approximate values of a. the rate of the reaction, 600. seconds after the start. b. the half-life of the reaction, t_1/2.

The half-life of a second-order reaction does not depend on the initial concentration of reactant.

Consider the general reaction 5Br^-(aq) + BrO₃^-(aq) + 6H⁺(aq)  3Br₂(aq) + 3H₂O(aq) For this reaction, the rate when expressed as [Br₂]/t is the same as

Tetrafluoroethylene, C₂F₄, can be converted to octafluorocyclobutane which can be used as a refrigerant or an aerosol propellant. A plot of 1/[C₂F₄] vs. time gives a straight line with a slope of 0.0448 L mol^-1s^-1. What is the rate law for this reaction?

An increase in temperature increases the reaction rate because

All bimolecular reactions are second-order reactions.

Butadiene, C₄H₆ (used to make synthetic rubber and latex paints) reacts to C₈H₁₂ with a rate law of rate = 0.014 L/(mol·s) [C₄H₆]². What will be the concentration of C₄H₆ after 3.0 hours if the initial concentration is 0.025 M?

Dinitrogen tetraoxide, N₂O₄, decomposes to nitrogen dioxide, NO₂, in a first-order process. If k = 2.5 × 10³ s^-1 at -5°C and k = 3.5 × 10⁴ s^-1 at 25°C, what is the activation energy for the decomposition?