Order and Molecularity of Reaction - UNSOLVED PRACTICE SET
Chapter: Chemical Kinetics | Topic: Order and Molecularity of Reaction
ORDER AND MOLECULARITY OF REACTION - UNSOLVED PRACTICE SET
Topic: Order and Molecularity of Reaction
Multiple Choice Questions
Q1. The order of a reaction is:
- Always equal to the molecularity
- Determined experimentally
- Equal to the sum of stoichiometric coefficients
- Always a whole number
Q2. The molecularity of a reaction is:
- The number of molecules participating in the rate-determining step
- Determined experimentally
- Can be fractional
- Can be zero
Q3. For the elementary reaction: 2HI → H₂ + I₂, the molecularity is:
- 0
- 1
- 2
- 3
Q4. A reaction can have:
- Negative order with respect to a reactant
- Fractional order
- Zero order
- All of the above
Q5. The order of a reaction can be determined from:
- The balanced chemical equation
- Experimental data using the method of initial rates
- Theoretical predictions only
- The physical state of reactants
Q6. For a complex reaction, the molecularity:
- Has no meaning
- Is equal to the order
- Is always 2
- Is determined by the slowest step
Short Answer Questions
Q7. Differentiate between order and molecularity of a reaction. Write two key differences.
Q8. Why can the order of a reaction be fractional or negative, but molecularity can never be fractional or zero?
Q9. For the reaction: 2NO + O₂ → 2NO₂, the rate law is Rate = k[NO]²[O₂]. What is the order of the reaction? Is this reaction elementary? Justify.
Q10. A reaction has the rate law: Rate = k[A]^(1/2)[B]. What is the overall order? Can you determine the molecularity from this information? Explain.
Q11. The hydrolysis of sucrose is a pseudo-first-order reaction. What is its molecularity? Explain.
Q12. Can the order of a reaction be predicted from the stoichiometric equation? Give an example to support your answer.
Long Answer Questions
Q13. Explain the difference between order and molecularity of a reaction with suitable examples. Why is molecularity defined only for elementary reactions?
Q14. (a) Define order and molecularity of a reaction.
(b) For the following reactions, identify the molecularity (if it is an elementary reaction) and explain why:
(i) NH₄NO₂ → N₂ + 2H₂O
(ii) 2NO + Br₂ → 2NOBr
(iii) Cl₂ → 2Cl (in photochemical decomposition)
Q15. Discuss the following statements and determine whether they are true or false. Give reasons for each:
(a) The order of a reaction can be zero.
(b) The molecularity of a reaction can be zero.
(c) For an elementary reaction, order = molecularity.
(d) For a complex reaction, order may or may not be equal to molecularity.
Numerical / Application-Based Problems
Q16. For the decomposition of N₂O₅: 2N₂O₅ → 4NO₂ + O₂
The experimentally determined rate law is: Rate = k[N₂O₅]
(a) What is the order of the reaction?
(b) What would be the molecularity if this were an elementary reaction?
(c) Is this reaction likely to be elementary? Give reasons.
(d) What does this tell you about the mechanism of the reaction?
Q17. Consider the following data for a reaction:
| Experiment | [A] (mol L⁻¹) | [B] (mol L⁻¹) | Initial Rate (mol L⁻¹ s⁻¹) |
|---|---|---|---|
| 1 | 0.10 | 0.10 | 1.0 × 10⁻³ |
| 2 | 0.20 | 0.10 | 4.0 × 10⁻³ |
| 3 | 0.10 | 0.20 | 2.0 × 10⁻³ |
(a) Determine the order with respect to A and B, and the overall order.
(b) Write the rate law.
(c) If the proposed mechanism is:
Step 1: 2A → A₂ (slow)
Step 2: A₂ + B → Products (fast)
Does the molecularity of the slow step match the order? Explain.
18. In your chemistry class, your teacher explains that the reaction between hydrogen and iodine: H₂ + I₂ → 2HI was once thought to be elementary with molecularity 2. However, later studies showed it proceeds through a complex mechanism.
(a) What would be the expected rate law if it were elementary?
(b) What does the discovery of a complex mechanism tell you about the relationship between molecularity and order?
(c) Why is it important not to assume order = molecularity for all reactions?