Questions about Reaction Mechanisms
Multiple-Choice Questions: Reaction Mechanisms
1. What is a reaction mechanism?
A) A tool for measuring reaction rate
B) A diagram showing the products formed
C) A series of elementary steps that describe how a reaction occurs
D) A device that speeds up reactions
E) The final balanced chemical equation
2. What is an elementary step in a reaction mechanism?
A) A simple catalyst
B) A single molecular-level event leading to a reaction
C) A repeated sequence in all reactions
D) The slowest reaction step
E) An equilibrium process only
3. Which of the following statements is true about elementary steps?
A) Their rate law must be determined experimentally
B) They cannot be used to find the overall reaction
C) Their rate law is based on molecularity
D) They are always the fastest step
E) They contain only catalysts
4. What is molecularity in an elementary step?
A) The number of moles of product
B) The number of reactant molecules involved in the step
C) The energy needed to activate the step
D) The total reaction order
E) The change in entropy
5. What is the rate-determining step?
A) The step involving the most reactants
B) The last step in a mechanism
C) The slowest elementary step
D) The step with the least energy
E) The step with no intermediates
6. An intermediate is a substance that:
A) Appears in the overall reaction
B) Is a product of the reaction
C) Is both produced and consumed during the reaction
D) Is the same as a catalyst
E) Only appears in the final step
7. Which of the following is a characteristic of a valid reaction mechanism?
A) It predicts the rate law exactly
B) It must include all products as intermediates
C) The sum of elementary steps equals the overall reaction
D) Each step must have the same rate
E) Only one step can occur
8. What feature in a potential energy diagram represents the activated complex?
A) The lowest point
B) The endpoint
C) The highest energy peak
D) The middle of the reactants
E) The product plateau
9. In a potential energy diagram, the activation energy is:
A) The difference between products and reactants
B) The total energy of products
C) The energy difference from reactants to the peak
D) The energy of the catalyst
E) The slope of the graph
10. If a catalyst is present, how does it affect a potential energy diagram?
A) Raises the activation energy
B) Lowers the activation energy
C) Changes the energy of reactants
D) Increases product energy
E) Has no effect
11. Which substance is never consumed during a reaction?
A) Intermediate
B) Product
C) Reactant
D) Catalyst
E) Precipitate
12. In a multi-step mechanism, what determines the observed rate law?
A) The first step
B) The last step
C) The fastest step
D) The slowest (rate-determining) step
E) The equilibrium step
13. A reaction has three peaks in a potential energy diagram. What does this indicate?
A) A single-step reaction
B) Three products
C) Three intermediates
D) Three elementary steps
E) Three catalysts
14. Which of the following is most likely an intermediate?
A) A reactant that remains unchanged
B) A compound that appears in early steps and disappears in later steps
C) The compound with the most mass
D) A product found at the end
E) A catalyst used throughout
15. What is a characteristic of a termolecular elementary step?
A) It involves one molecule
B) It involves two molecules
C) It is very common
D) It involves three molecules colliding simultaneously
E) It is the slowest type of reaction
16. What happens to the reaction rate if the rate-determining step is very slow?
A) The overall reaction becomes faster
B) The overall rate is unaffected
C) The overall reaction is also slow
D) Only the last step slows down
E) The equilibrium shifts
17. What can be concluded if a proposed mechanism agrees with the experimental rate law?
A) It must be the correct mechanism
B) It is a possible mechanism
C) It is the fastest mechanism
D) It includes all intermediates
E) It proves the products
18. In a two-step mechanism, how is the intermediate identified?
A) It is present in the overall reaction
B) It is a catalyst
C) It is produced in the first step and used in the second
D) It is present throughout
E) It increases the energy
19. How does a catalyst affect the reaction mechanism?
A) Increases the energy of reactants
B) Changes the products
C) Adds an additional step with lower activation energy
D) Makes intermediates permanent
E) Eliminates all elementary steps
20. Which of the following would suggest a step is not elementary?
A) The rate law matches the stoichiometry
B) It occurs with a single molecule
C) It involves three or more molecules colliding
D) It produces only one product
E) It occurs in the gas phase
Answer Key with Explanations
1. C – A mechanism explains the step-by-step process of a reaction.
2. B – An elementary step is a single molecular event that leads to product or intermediate.
3. C – The rate law of an elementary step is based on its molecularity.
4. B – Molecularity = number of reactant molecules in an elementary step.
5. C – The slowest step limits the overall rate.
6. C – An intermediate is formed and then used up.
7. C – The sum of all elementary steps must give the overall reaction.
8. C – The peak in the diagram = activated complex or transition state.
9. C – Activation energy is from reactants to peak.
10. B – A catalyst lowers the activation energy.
11. D – A catalyst is not consumed—it is reused.
12. D – The rate-determining step governs the overall rate law.
13. D – Three peaks = three elementary steps.
14. B – An intermediate appears and disappears during the mechanism.
15. D – Termolecular = three molecules colliding—rare and complex.
16. C – If RDS is slow, the whole reaction is slow.
17. B – Agreement with the rate law means the mechanism is plausible, not proven.
18. C – An intermediate is produced and then consumed.
19. C – Catalysts usually add a new pathway with lower activation energy.
20. C – Three or more molecules colliding is unlikely for an elementary step.
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