CMH 411: REACTIONS AND MECHANISMS
Below is a set of 100 multiple‐choice questions on “Reactions and Mechanisms,” covering a
range of topics from general reaction concepts and kinetics to specific organic reaction
mechanisms, redox reactions, rearrangements, radical reactions, and catalysis.
Note: These questions are intended as a study aid for courses covering reaction mechanisms in
organic and general chemistry. They assume a basic familiarity with chemical kinetics, reaction
intermediates, and common organic reaction types.
Section 1: General Concepts, Kinetics, and Mechanisms (Questions 1–10)
1. What is a reaction mechanism?
A. The overall balanced chemical equation
B. The detailed step-by-step sequence of elementary reactions that occur during a
chemical reaction
C. The rate law of a reaction
D. The stoichiometric coefficients in a balanced reaction
Correct Answer: B
Rationale: A reaction mechanism explains the step-by-step sequence of events (elementary
steps) that lead from reactants to products.
2. An elementary reaction is defined as one that:
A. Occurs in multiple steps
B. Occurs in a single step with a single transition state
C. Requires a catalyst to proceed
D. Has a complex rate law
Correct Answer: B
Rationale: An elementary reaction is a single-step process with no intermediates; its rate law can
be written directly from the stoichiometry of that step.
Page 1 of 31
, 3. The rate-determining step in a reaction mechanism is:
A. The fastest step
B. The slowest step that limits the overall reaction rate
C. The step that forms the most intermediates
D. The last step in the mechanism
Correct Answer: B
Rationale: The rate-determining step is the slowest step in a reaction mechanism and thus
controls the overall reaction rate.
4. Which of the following best describes a reaction intermediate?
A. A reactant that is present in excess
B. A species that is formed in one step and consumed in a subsequent step
C. The final product of a reaction
D. A catalyst that remains unchanged
Correct Answer: B
Rationale: Intermediates are short-lived species that are produced in one elementary step and
consumed in another.
5. A catalyst works by:
A. Increasing the equilibrium constant of a reaction
B. Providing an alternative pathway with a lower activation energy
C. Being consumed during the reaction
D. Changing the thermodynamic favorability of the reaction
Correct Answer: B
Rationale: A catalyst lowers the activation energy without altering the overall thermodynamics
(ΔG) or equilibrium constant of the reaction.
6. Activation energy is best defined as:
A. The difference in energy between reactants and products
B. The energy released during a reaction
C. The minimum energy required to convert reactants into products
D. The total energy of the reactants
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,Correct Answer: C
Rationale: Activation energy is the energy barrier that must be overcome for reactants to be
converted into products.
7. According to the Arrhenius equation, an increase in temperature will generally:
A. Decrease the reaction rate constant
B. Increase the reaction rate constant
C. Have no effect on the reaction rate constant
D. Change the reaction mechanism
Correct Answer: B
Rationale: The Arrhenius equation k=Aexp(−Ea/RT)k = A \exp(-E_a/RT) shows that as
temperature increases, the exponential factor increases, thereby increasing the rate constant.
8. In a unimolecular reaction mechanism (such as SN1), the rate law depends on:
A. The concentration of the nucleophile only
B. The concentration of the substrate only
C. The concentrations of both the substrate and the nucleophile
D. Neither concentration
Correct Answer: B
Rationale: SN1 reactions are unimolecular in the rate-determining step, so the rate depends
solely on the concentration of the substrate that forms the carbocation.
9. When a reaction mechanism involves a fast pre-equilibrium, the overall rate law is
determined by:
A. The slow, rate-determining step
B. The fast, reversible step
C. The sum of the fast and slow steps
D. The reaction’s thermodynamics
Correct Answer: A
Rationale: The overall reaction rate is controlled by the slowest (rate-determining) step, even if
earlier steps are in rapid equilibrium.
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, 10. The transition state in a chemical reaction is:
A. A stable intermediate
B. A high-energy configuration representing the maximum energy along the reaction
coordinate
C. The final product
D. A catalyst
Correct Answer: B
Rationale: The transition state is the highest energy point along the reaction pathway and is not
a stable species.
Section 2: Nucleophilic Substitution Mechanisms (SN1 and SN2) (Questions 11–25)
11. In an SN2 reaction, the nucleophile attacks the substrate:
A. From the same side as the leaving group
B. From the opposite side of the leaving group
C. Without any specific orientation
D. Simultaneously with the leaving group leaving
Correct Answer: B
Rationale: SN2 reactions occur in a concerted, backside attack, leading to inversion of
configuration at the reaction center.
12. An SN2 reaction is characterized by which type of kinetics?
A. Unimolecular (first-order)
B. Bimolecular (second-order)
C. Zero-order
D. Third-order
Correct Answer: B
Rationale: The rate of an SN2 reaction depends on both the substrate and the nucleophile
concentrations, making it bimolecular.
Page 4 of 31
Below is a set of 100 multiple‐choice questions on “Reactions and Mechanisms,” covering a
range of topics from general reaction concepts and kinetics to specific organic reaction
mechanisms, redox reactions, rearrangements, radical reactions, and catalysis.
Note: These questions are intended as a study aid for courses covering reaction mechanisms in
organic and general chemistry. They assume a basic familiarity with chemical kinetics, reaction
intermediates, and common organic reaction types.
Section 1: General Concepts, Kinetics, and Mechanisms (Questions 1–10)
1. What is a reaction mechanism?
A. The overall balanced chemical equation
B. The detailed step-by-step sequence of elementary reactions that occur during a
chemical reaction
C. The rate law of a reaction
D. The stoichiometric coefficients in a balanced reaction
Correct Answer: B
Rationale: A reaction mechanism explains the step-by-step sequence of events (elementary
steps) that lead from reactants to products.
2. An elementary reaction is defined as one that:
A. Occurs in multiple steps
B. Occurs in a single step with a single transition state
C. Requires a catalyst to proceed
D. Has a complex rate law
Correct Answer: B
Rationale: An elementary reaction is a single-step process with no intermediates; its rate law can
be written directly from the stoichiometry of that step.
Page 1 of 31
, 3. The rate-determining step in a reaction mechanism is:
A. The fastest step
B. The slowest step that limits the overall reaction rate
C. The step that forms the most intermediates
D. The last step in the mechanism
Correct Answer: B
Rationale: The rate-determining step is the slowest step in a reaction mechanism and thus
controls the overall reaction rate.
4. Which of the following best describes a reaction intermediate?
A. A reactant that is present in excess
B. A species that is formed in one step and consumed in a subsequent step
C. The final product of a reaction
D. A catalyst that remains unchanged
Correct Answer: B
Rationale: Intermediates are short-lived species that are produced in one elementary step and
consumed in another.
5. A catalyst works by:
A. Increasing the equilibrium constant of a reaction
B. Providing an alternative pathway with a lower activation energy
C. Being consumed during the reaction
D. Changing the thermodynamic favorability of the reaction
Correct Answer: B
Rationale: A catalyst lowers the activation energy without altering the overall thermodynamics
(ΔG) or equilibrium constant of the reaction.
6. Activation energy is best defined as:
A. The difference in energy between reactants and products
B. The energy released during a reaction
C. The minimum energy required to convert reactants into products
D. The total energy of the reactants
Page 2 of 31
,Correct Answer: C
Rationale: Activation energy is the energy barrier that must be overcome for reactants to be
converted into products.
7. According to the Arrhenius equation, an increase in temperature will generally:
A. Decrease the reaction rate constant
B. Increase the reaction rate constant
C. Have no effect on the reaction rate constant
D. Change the reaction mechanism
Correct Answer: B
Rationale: The Arrhenius equation k=Aexp(−Ea/RT)k = A \exp(-E_a/RT) shows that as
temperature increases, the exponential factor increases, thereby increasing the rate constant.
8. In a unimolecular reaction mechanism (such as SN1), the rate law depends on:
A. The concentration of the nucleophile only
B. The concentration of the substrate only
C. The concentrations of both the substrate and the nucleophile
D. Neither concentration
Correct Answer: B
Rationale: SN1 reactions are unimolecular in the rate-determining step, so the rate depends
solely on the concentration of the substrate that forms the carbocation.
9. When a reaction mechanism involves a fast pre-equilibrium, the overall rate law is
determined by:
A. The slow, rate-determining step
B. The fast, reversible step
C. The sum of the fast and slow steps
D. The reaction’s thermodynamics
Correct Answer: A
Rationale: The overall reaction rate is controlled by the slowest (rate-determining) step, even if
earlier steps are in rapid equilibrium.
Page 3 of 31
, 10. The transition state in a chemical reaction is:
A. A stable intermediate
B. A high-energy configuration representing the maximum energy along the reaction
coordinate
C. The final product
D. A catalyst
Correct Answer: B
Rationale: The transition state is the highest energy point along the reaction pathway and is not
a stable species.
Section 2: Nucleophilic Substitution Mechanisms (SN1 and SN2) (Questions 11–25)
11. In an SN2 reaction, the nucleophile attacks the substrate:
A. From the same side as the leaving group
B. From the opposite side of the leaving group
C. Without any specific orientation
D. Simultaneously with the leaving group leaving
Correct Answer: B
Rationale: SN2 reactions occur in a concerted, backside attack, leading to inversion of
configuration at the reaction center.
12. An SN2 reaction is characterized by which type of kinetics?
A. Unimolecular (first-order)
B. Bimolecular (second-order)
C. Zero-order
D. Third-order
Correct Answer: B
Rationale: The rate of an SN2 reaction depends on both the substrate and the nucleophile
concentrations, making it bimolecular.
Page 4 of 31
