CH 16 Smartbook Part 2
Which elementary step in the proposed mechanism is the rate-limiting step?
(2) X (g) + C (g) → Y (g); [slow]
For the reaction of A (g) + B (g) → AB (g), how many unique collisions between A and B are possible if there are four particles of A and three particles of B present in the vessel?
12
Which term represents the frequency factor in the Arrhenius equation? k = Ae-Ea/RT
A
Which of the following statements correctly describe a reaction mechanism? Select all that apply.
A reaction mechanism describes a series of single reaction steps that sum to the overall chemical equation. A reaction mechanism is a hypothesis.
Select the correct statements regarding activation energy and the heat of a reaction. Select all that apply.
A reversible reaction has two activation energies. ΔH is negative when Ea(fwd) < Ea(rev) ΔHrxn = Ea(fwd) - Ea(rev)
Which of the following conditions must be satisfied for a collision between reactants to be effective? Select all that apply.
Appropriate molecular orientation Enough energy to overcome Ea
Transition theory focuses on the formation of the ___ complex, which exists at the moment of highest ___ energy in a reaction.
activated potential
The ___ energy of a reaction is the energy threshold that colliding molecules must exceed in order to react.
activation
For the reaction A2 + B2 → 2AB, Ea(fwd) = 125 kJ/mol and Ea(rev) = 85 kJ/mol. Assuming the reaction occurs in one step, which of the following statements are correct? Select all that apply.
In the reaction energy diagram, the products will appear at higher energy than the reactants. ΔH°rxn = 40 kJ/mol.
The individual steps that make up a reaction mechanism are called _____ reactions or steps.
elementary
For a collision to be effective, the colliding particles must have enough ___ and the appropriate molecular ___.
energy orientation
A reaction energy diagram plots potential ___ versus reaction ___.
energy progress
For the reaction represented by the accompanying reaction energy diagram, the _____ step is _____ and is rate limiting.
first; slower
The frequency factor, A, is the product of collision ___ (Z) and a(n) ___ probability factor (p).
frequency orientation
An increase in temperature causes a _____ collision frequency and energy and therefore _____ the rate dramatically.
greater increases
A reaction _____ is a species that is formed and then used up during the course of a reaction.
intermediate
For a multistep reaction, the overall rate law ______.
is the same as the rate law for the slowest step includes the concentrations of reactants only
Which form of the Arrhenius equation can be conveniently used to calculate Ea for a reaction?
lnk1k2 = -EaR(1T2-1T1)
The _____ the activation energy, or the _____ the temperature, the larger the fraction of sufficiently energetic collisions, the greater the value of k, and the greater the reaction rate
lower; higher
The _____ the activation energy, or the _____ the temperature, the larger the fraction of sufficiently energetic collisions, the greater the value of k, and the greater the reaction rate.
lower; higher
Which of the following species will not appear in the overall rate law for the proposed mechanism? 2NO(g) ⇌ N2O2(g) [fast] N2O2(g) + O2(g) → 2NO2(g) [slow]
NO2 N2O2
Which of the following criteria must be met for a reaction mechanism to be considered valid? Select all that apply.
The mechanism must correlate with the experimentally determined rate law. The elementary steps must be physically reasonable. The elementary steps must add up to the overall balanced reaction.
What is molecularity?
The number of reactant particles that react in an elementary step
What do the dashed lines represent in the illustration of the transition state of a reaction between between BrCH3 and OH-?
The partial bonds forming and breaking in the reactants
Which of the following statements correctly describe the reaction energy diagram in the illustration? Select all that apply.
The reaction mechanism consists of 2 elementary steps. The reaction is exothermic.
Which of the following statements correctly describe the transition state of a reaction? Select all that apply.
The transition state only exists at the instant of highest potential energy in the reaction. The transition state species is highly unstable.
A reaction ___ is a hypothesis about how a reaction occurs. This hypothesis is tested to see if it fits with the experimentally observed ___ law.
mechanism rate
An elementary step is characterized by its _____, which is equal to the number of reactant particles in the step.
molecularity
Collision frequency provides an upper limit on how fast a reaction can take place. The laws of probability tell us that the number of collisions depends on the _____ of the number of reactant particles, not their _____.
product; sum
The ___ - ___ step in a reaction is the one that limits how fast the overall reaction proceeds.
rate determining
Consider the mechanism shown. If the observed rate law is rate = k[NO]2[Br2] is the proposed mechanism valid? What is the rate law for the proposed mechanism? Select all that apply.
rate = k2 k1k−1 [NO]2[Br2] or rate = k[NO]2[Br2] Yes, this mechanism is valid.
The rate law for an overall reaction is equal to the rate law for the ___ - ___ step.
rate determining
For any mechanism, only ___ involved up to and including the ___ step appear in the overall rate law.
reactants slow
In the Arrhenius equation, k = Ae-Ea/RT. if T increases, the value of the negative exponent becomes _____, which means k becomes _____ and the rate _____.
smaller; larger; increases
True or false: The rate law for an elementary step can be deduced from its reaction stoichiometry (i.e., from the balanced equation for the step).
t
For a reaction mechanism to be considered valid, ______..
the rate law for the slowest step must match the experimentally observed rate law
Select the parameters that are required for proposing a valid reaction mechanism. Select all that apply.
Correlation of rate law with experimental rate law Physically reasonable elementary steps Elementary steps sum to overall balanced equation
The illustration depicts the fraction of collisions versus the collision energy for two of the same gas samples at different temperatures. Choose the statement(s) that accurately describe the illustration.
Curve b represents the sample that will exhibit a higher reaction rate. Curve b represents the sample at higher temperature.
What is an elementary step?
Each step that makes up a reaction mechanism
Which of the following statements correctly describe the kinetics of an elementary step? Select all that apply.
For an elementary step, reaction order equals molecularity. The rate law for an elementary step can be deduced from its balanced chemical equation. The elementary step 2HI (g) → H2 (g) + I2 (g) follows second-order kinetics.
Which of the following statements correctly describe the key aspects of drawing a reaction energy diagram? Select all that apply.
For an exothermic reaction, the energy of the products is less than the energy of the reactants. Intermediates will be higher in energy than any reactants or products.
Which of the following statements correctly describe reaction intermediates? Select all that apply.
Reaction intermediates are formed and used up over the course of a reaction. Reaction intermediates do not appear in the overall equation for the reaction.
The reaction NO2 (g) + CO (g) → NO (g) + CO2(g) has ΔH°overall = -226 kJ/mol. A proposed reaction mechanism is shown. Choose the statement(s) that accurately describe the reaction energy diagram for the above reaction.
The Ea of the first step will be larger than the second or third step. There will be three peaks. The energy of the products will be lower than the energy of the reactants.
The rate law for the reaction NO2 (g) + CO (g) → NO (g) + CO2 (g) is given by rate = k[NO2]2. Consider the mechanism depicted in the illustration. Which options correctly describe the validity of this mechanism? Select all that apply.
The mechanism does agree with the experimental rate law. The elementary steps add up to the overall balanced equation.
Which of the following statements correctly describes the activation energy of a reaction?
The energy threshold that the colliding particles must exceed in order to react
Transition state theory focuses on which of the following?
The high-energy species that exists at the moment of an effective collision