Q: The decomposition of cyclopropane to propene at 1000°C follows first-order kinetics with a rate constant $k = 9.2\ \text{s}^{-1}$. What is the half-life of this reaction?

$t_{1/2} = \frac{0.693}{9.2\ \text{s}^{-1}}$

Question 1

A reaction is found to have a rate law of $	ext{Rate} = k[A]^2[B]$. If the concentration of A is doubled and the concentration of B is tripled, by what factor does the reaction rate increase?

Accepted Answer

The rate increases by a factor of 12.

Question 2

Which of the following plots would yield a straight line for a first-order reaction?

Accepted Answer

A plot of $\ln [A]$ versus time.

Question 3

The decomposition of cyclopropane to propene at 1000°C follows first-order kinetics with a rate constant $k = 9.2\ 	ext{s}^{-1}$. What is the half-life of this reaction?

Accepted Answer

$t_{1/2} = \frac{0.693}{9.2\ 	ext{s}^{-1}}$

Question 4

Which statement best describes the effect of a catalyst on a chemical reaction?

Accepted Answer

A catalyst lowers the activation energy, increasing the rate of both the forward and reverse reactions.

Question 5

For a zero-order reaction, which of the following is true about the rate and the concentration of reactant over time?

Accepted Answer

The rate is constant and independent of the concentration of the reactant.

Question 6

The Arrhenius equation is $k = Ae^{-E_a/(RT)}$. If the activation energy $E_a$ is decreased, what is the effect on the rate constant $k$?

Accepted Answer

The rate constant $k$ increases.

Question 7

A reaction has a rate law $	ext{Rate} = k[NO_2][F_2]$. Which of the following mechanisms is consistent with this rate law?

Accepted Answer

A mechanism where the slow step involves one $NO_2$ and one $F_2$ molecule.

Question 8

Which of the following best describes a reaction intermediate?

Accepted Answer

A species that is formed in one step of a reaction mechanism and consumed in a subsequent step.

Question 9

In the reaction $2HI(g) ightarrow H_2(g) + I_2(g)$, the rate constant increases from $9.51 	imes 10^{-9}\ 	ext{L/mol·s}$ at 500 K to $1.10 	imes 10^{-5}\ 	ext{L/mol·s}$ at 600 K. What does this indicate about the effect of temperature on the rate constant?

Accepted Answer

The rate constant increases exponentially with temperature.

Question 10

Which of the following is a correct statement about the half-life of a first-order reaction?

Accepted Answer

The half-life is independent of the initial concentration.

Question 11

A reaction mechanism consists of the following steps:
1. $Cl_2 \rightarrow 2Cl$
2. $Cl + CHCl_3 \rightarrow HCl + CCl_3$
3. $Cl + CCl_3 \rightarrow CCl_4$
Which species are intermediates in this mechanism?

Accepted Answer

Cl and CCl$_3$

Question 12

Which of the following best explains why termolecular elementary steps are rare in reaction mechanisms?

Accepted Answer

The probability of three particles colliding simultaneously with the correct orientation and energy is very low.

Question 13

For a reaction with the integrated rate law $\ln [A]_t = -kt + \ln [A]_0$, what is the order of the reaction?

Accepted Answer

First order

Question 14

Which of the following statements about the transition state is correct?

Accepted Answer

The transition state is a high-energy, unstable arrangement of atoms that exists momentarily as reactants are converted to products.

Question 15

A plot of $1/[A]$ versus time gives a straight line for a reaction. What is the order of the reaction?

Accepted Answer

Second order

Question 16

Which of the following factors does NOT affect the rate constant $k$ of a reaction?

Accepted Answer

The initial concentration of reactants

Question 17

In a real-world scenario, why is the decomposition of hydrogen peroxide ($H_2O_2$) in the presence of NaBr much faster than without NaBr?

Accepted Answer

NaBr acts as a catalyst, providing an alternative pathway with lower activation energy.

Question 18

Which of the following best describes the lock-and-key model of enzyme action?

Accepted Answer

The enzyme active site has a fixed shape that matches the shape of its substrate(s).

Question 19

A reaction has a rate law $	ext{Rate} = k[A]$. If the initial concentration of A is 0.50 M, how long will it take for the concentration to decrease to 0.125 M if $k = 0.20\ 	ext{min}^{-1}$?

Accepted Answer

$t = \frac{1}{0.20} \ln \left(\frac{0.50}{0.125}\right)$

Chapter 12: Chemical Kinetics – Study Guide

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