Questions on Molecularity
Multiple-Choice Questions: Molecularity
1. What does molecularity refer to in a chemical reaction?
A) Number of moles of product formed
B) Number of molecules or ions involved in an elementary step
C) Number of atoms in a molecule
D) Number of electrons transferred
E) Number of steps in a reaction mechanism
2. Which of the following is NOT a possible molecularity of an elementary reaction?
A) Unimolecular
B) Bimolecular
C) Termolecular
D) Quadrimolecular
E) Zero molecularity
3. A unimolecular reaction involves:
A) One molecule breaking down or rearranging
B) Two molecules colliding
C) Three molecules colliding simultaneously
D) Zero molecules
E) A catalyst reacting
4. Which molecularity is the most common in elementary reactions?
A) Unimolecular
B) Bimolecular
C) Termolecular
D) Quadrimolecular
E) Zero molecularity
5. What is the molecularity of the reaction: A + B → products?
A) Unimolecular
B) Bimolecular
C) Termolecular
D) Zero molecularity
E) Not an elementary step
6. Termolecular reactions involve:
A) One molecule
B) Two molecules
C) Three molecules colliding simultaneously
D) Four molecules colliding
E) None of the above
7. Why are termolecular elementary reactions rare?
A) Because three molecules rarely collide simultaneously with proper orientation and energy
B) Because molecules repel each other
C) Because they require catalysts
D) Because they are always slow
E) Because they do not involve energy transfer
8. Which of the following is an example of a unimolecular elementary step?
A) Isomerization of a single molecule
B) A + B → AB
C) A + B + C → products
D) Decomposition of AB in the presence of a catalyst
E) None of the above
9. The molecularity of the elementary reaction 2NO → N2 + O2 is:
A) Unimolecular
B) Bimolecular
C) Termolecular
D) Zero molecularity
E) Cannot be determined
10. In a reaction mechanism, if the rate law depends on the concentration of one reactant only, the rate-determining step is likely:
A) Unimolecular
B) Bimolecular
C) Termolecular
D) Zero molecularity
E) Catalyst-controlled
11. Which of the following statements is true about molecularity?
A) Molecularity applies to overall reactions
B) Molecularity applies only to elementary steps
C) Molecularity can be fractional
D) Molecularity equals reaction order always
E) Molecularity is independent of molecular collisions
12. If an elementary reaction involves three reactant molecules, its molecularity is:
A) Unimolecular
B) Bimolecular
C) Termolecular
D) Quadrimolecular
E) None of these
13. How does molecularity relate to the complexity of a reaction step?
A) Higher molecularity steps are usually faster
B) Higher molecularity steps are easier to occur
C) Higher molecularity steps are less probable and generally slower
D) Molecularity does not affect reaction speed
E) Molecularity affects product stability
14. Which molecularity corresponds to a reaction step where a single molecule rearranges or decomposes?
A) Unimolecular
B) Bimolecular
C) Termolecular
D) Quadrimolecular
E) None of the above
15. For the elementary step: A + A → products, what is the molecularity?
A) Unimolecular
B) Bimolecular
C) Termolecular
D) Zero molecularity
E) Quadrimolecular
16. What is the molecularity of the following elementary step? A + B + C → products
A) Unimolecular
B) Bimolecular
C) Termolecular
D) Quadrimolecular
E) Cannot be determined
17. Molecularity is always:
A) A whole number greater than or equal to 1
B) A whole number less than 1
C) A fraction
D) Equal to the overall reaction order
E) Independent of reaction steps
18. Which molecularity is impossible for an elementary reaction?
A) Unimolecular
B) Bimolecular
C) Termolecular
D) Zero molecularity
E) None of these
19. The rate law for a unimolecular elementary reaction A → products is:
A) Rate = k[A]^2
B) Rate = k[A]
C) Rate = k[A][B]
D) Rate = k
E) Rate = k[A]^3
20. Which of the following is true for bimolecular elementary reactions?
A) They involve collisions between two molecules or ions
B) They always require a catalyst
C) They are slower than unimolecular reactions because of lower activation energy
D) They never occur in the gas phase
E) They involve rearrangement within a single molecule
- Questions about Reaction Order
- Questions about Reaction Mechanisms
- Questions about Reaction Intermediates
Answer Key with Explanations
1. B – Molecularity refers to the number of reactant molecules or ions in an elementary step.
2. E – Zero molecularity is not possible; molecularity must be ≥ 1.
3. A – Unimolecular means one molecule reacts by itself.
4. B – Bimolecular reactions are the most common elementary steps.
5. B – A + B indicates two molecules collide, so bimolecular.
6. C – Termolecular involves three molecules colliding simultaneously.
7. A – It is rare because simultaneous collision of three molecules with proper energy/orientation is unlikely.
8. A – Isomerization or decomposition of a single molecule is unimolecular.
9. B – 2NO means two molecules collide → bimolecular.
10. A – Rate depending on one reactant suggests a unimolecular rate-determining step.
11. B – Molecularity applies only to elementary steps.
12. C – Three reactants mean termolecular.
13. C – Higher molecularity steps are less probable and generally slower.
14. A – Single molecule rearrangement is unimolecular.
15. B – Two molecules react → bimolecular.
16. C – Three molecules → termolecular.
17. A – Molecularity is a whole number ≥ 1.
18. D – Zero molecularity is impossible.
19. B – Unimolecular rate law is first order: Rate = k[A].
20. A – Bimolecular steps involve collisions between two species.
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