Questions about Reaction Intermediates
Multiple-Choice Questions: Reaction Intermediates
1. What is a reaction intermediate?
A) A catalyst that remains unchanged
B) A substance formed and consumed during the reaction
C) A final product of the reaction
D) A reactant that initiates the reaction
E) A solvent used in the process
2. Where does a reaction intermediate appear?
A) Only in the overall reaction
B) Only in the products
C) In the elementary steps but not in the overall equation
D) As a catalyst
E) In the initial reactant list
3. How can an intermediate be detected?
A) It appears in both reactants and products
B) It is always visible in the final mixture
C) Through fast-sampling techniques or spectroscopy
D) Through stoichiometric coefficients
E) By measuring the temperature
4. Which statement about intermediates is correct?
A) They are always solids
B) They never react
C) They are produced in one step and consumed in a later step
D) They act as catalysts
E) They appear in the final products
5. Which of the following is not a characteristic of a reaction intermediate?
A) It is formed during the reaction
B) It is consumed during the reaction
C) It appears in the overall balanced equation
D) It may be short-lived
E) It can often be unstable
6. In a multi-step mechanism, the intermediate is typically:
A) A reactant that initiates the first step
B) The product of the overall reaction
C) A substance that appears in an early step and disappears in a later one
D) A substance used to speed up the reaction
E) A gas formed in the final step
7. Which of the following best describes the difference between an intermediate and a catalyst?
A) Both are used and regenerated
B) Catalysts are consumed, intermediates are not
C) Intermediates are formed and consumed, catalysts are not consumed
D) Intermediates appear in the overall reaction
E) Catalysts lower the activation energy, intermediates do not
8. In the following mechanism, which is the intermediate?
Step 1: A + B → C
Step 2: C + D → E + F
Overall: A + B + D → E + F
A) A
B) B
C) C
D) D
E) F
9. What happens to a reaction intermediate by the end of the reaction?
A) It remains unchanged
B) It is converted into a catalyst
C) It is consumed
D) It becomes part of the solvent
E) It remains in equilibrium
10. Why do intermediates not appear in the overall chemical equation?
A) They are products
B) They are not involved in any chemical step
C) They cancel out in the sum of the steps
D) They are too reactive
E) They are only theoretical
11. Which of the following could be a reason why an intermediate is hard to isolate?
A) It has high polarity
B) It reacts too slowly
C) It is often highly unstable or short-lived
D) It doesn't participate in any chemical step
E) It is identical to the product
12. What is the role of an intermediate in a reaction mechanism?
A) To act as a permanent reactant
B) To facilitate energy transfer
C) To temporarily store chemical information between steps
D) To serve as a final product
E) To reduce the number of steps
13. How is an intermediate different from a transition state?
A) Intermediates have lower energy than transition states and can be isolated
B) Transition states are always stable
C) Both are permanent species
D) Intermediates do not exist
E) Intermediates and transition states are the same
14. Which term best describes the lifetime of a typical intermediate?
A) Permanent
B) Instantaneous
C) Long-lasting
D) Variable but often short
E) Unchanging
15. Which graph feature in a potential energy diagram indicates an intermediate?
A) The first peak
B) The final dip
C) A valley between two peaks
D) The tallest peak
E) A flat line at the top
16. In the reaction mechanism:
Step 1: A + B → X (slow)
Step 2: X + C → D (fast)
What is X?
A) Product
B) Catalyst
C) Intermediate
D) Solvent
E) Reactant
17. What type of chemical species is always regenerated in a reaction mechanism?
A) Intermediate
B) Product
C) Reactant
D) Catalyst
E) Precipitate
18. What is the primary difference between a reaction intermediate and a final product?
A) Intermediates have lower molecular mass
B) Products are formed permanently; intermediates are temporary
C) Intermediates are colored
D) Products are always less stable
E) Intermediates are not reactive
19. What usually follows the formation of an intermediate in a mechanism?
A) The system reaches equilibrium
B) The intermediate is consumed in the next step
C) The reaction stops
D) The catalyst is activated
E) The product decomposes
20. What happens if an intermediate accumulates significantly during a reaction?
A) The reaction becomes faster
B) The mechanism becomes one-step
C) The intermediate may become detectable
D) The catalyst is deactivated
E) The product disappears
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Answer Key with Explanations
1. B – An intermediate is formed and then consumed during the reaction.
2. C – Intermediates appear in the elementary steps, not in the overall equation.
3. C – Some advanced techniques, like spectroscopy, can detect transient intermediates.
4. C – Intermediates are produced in one step and consumed in the next.
5. C – Intermediates do not appear in the overall reaction equation.
6. C – An intermediate is a temporary species, formed and then used.
7. C – Catalysts are not consumed; intermediates are both formed and used up.
8. C – C is formed in Step 1 and consumed in Step 2 → it's the intermediate.
9. C – By the end of the reaction, an intermediate is consumed.
10. C – Intermediates cancel out when elementary steps are added up.
11. C – Most intermediates are unstable or react very quickly.
12. C – They temporarily carry information or chemical structure between steps.
13. A – Intermediates can often be isolated; transition states cannot.
14. D – Lifespan of an intermediate is short, but measurable.
15. C – Intermediates appear as valleys between two peaks in energy diagrams.
16. C – X is formed in Step 1 and used in Step 2 → it's an intermediate.
17. D – Catalysts are regenerated by the end of the reaction.
18. B – Products are stable and permanent; intermediates are temporary.
19. B – After being formed, the intermediate is used in the next step.
20. C – If intermediates accumulate, they may be detectable, which is rare.
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