Pseudo First Order Reactions - UNSOLVED PRACTICE SET
Chapter: Chemical Kinetics | Topic: Pseudo First Order Reactions
PSEUDO FIRST ORDER REACTIONS - UNSOLVED PRACTICE SET
Topic: Pseudo First Order Reactions
Multiple Choice Questions
Q1. A pseudo-first-order reaction is:
- A reaction that is actually second order but appears first order
- A reaction that is actually zero order
- A reaction with negative order
- A reaction with fractional order
Q2. The hydrolysis of an ester in excess water is a pseudo-first-order reaction because:
- Water acts as a catalyst
- The concentration of water remains effectively constant
- The reaction is catalyzed by an acid
- The ester is in excess
Q3. In a pseudo-first-order reaction, the rate law becomes:
- Rate = k'[A]²
- Rate = k'[A]
- Rate = k'[A][B]
- Rate = k'
Q4. The unit of the pseudo-first-order rate constant is:
- mol L⁻¹ s⁻¹
- L mol⁻¹ s⁻¹
- s⁻¹
- L² mol⁻² s⁻¹
Q5. The inversion of cane sugar is an example of:
- Zero-order reaction
- True first-order reaction
- Pseudo-first-order reaction
- Second-order reaction
Q6. In the hydrolysis of CH₃COOC₂H₅ by dilute NaOH, if NaOH is in large excess, the reaction follows:
- Zero-order kinetics
- Pseudo-first-order kinetics with respect to the ester
- Second-order kinetics
- Third-order kinetics
Short Answer Questions
Q7. What is a pseudo-first-order reaction? Explain with a suitable example.
Q8. The hydrolysis of methyl acetate in aqueous solution follows pseudo-first-order kinetics. Explain why and write the rate law.
Q9. Distinguish between a true first-order reaction and a pseudo-first-order reaction.
Q10. In the hydrolysis of ethyl acetate, water is present in large excess. How does this affect the order of the reaction? Write the rate law before and after the approximation.
Q11. Why is the term "pseudo" used in pseudo-first-order reactions? What would happen if the excess reagent were not in large excess?
Q12. The hydrolysis of t-butyl chloride in water follows first-order kinetics: (CH₃)₃CCl + H₂O → (CH₃)₃COH + HCl. Is this a true first-order or pseudo-first-order reaction? Explain.
Long Answer Questions
Q13. Explain the concept of pseudo-first-order reactions with a detailed example. How is the pseudo-first-order rate constant (k') related to the actual second-order rate constant (k)? Derive the relationship.
Q14. (a) The hydrolysis of ethyl acetate is represented as:
CH₃COOC₂H₅ + H₂O → CH₃COOH + C₂H₅OH
Explain why this is a pseudo-first-order reaction when carried out in dilute aqueous solution.
(b) If the actual second-order rate constant is 2.0 × 10⁻⁴ L mol⁻¹ s⁻¹ and the concentration of water is 55.5 mol L⁻¹, calculate the pseudo-first-order rate constant.
Q15. (a) Define pseudo-first-order reaction and explain the conditions under which a second-order reaction becomes pseudo-first-order.
(b) The saponification of ethyl acetate:
CH₃COOC₂H₅ + NaOH → CH₃COONa + C₂H₅OH
is second order overall. Under what conditions does it become pseudo-first-order? Write the rate law in both cases.
Numerical / Application-Based Problems
Q16. The hydrolysis of an ester in aqueous solution follows pseudo-first-order kinetics. The initial concentration of the ester is 0.01 mol L⁻¹. After 30 minutes, the concentration drops to 0.0025 mol L⁻¹.
(a) Calculate the pseudo-first-order rate constant.
(b) Calculate the half-life of the reaction.
(c) If the actual second-order rate constant is 1.5 × 10⁻⁵ L mol⁻¹ s⁻¹, calculate the effective concentration of water in the solution.
Q17. In a school laboratory experiment, students study the inversion of cane sugar:
C₁₂H₂₂O₁₁ + H₂O → C₆H₁₂O₆ (glucose) + C₆H₁₂O₆ (fructose)
The reaction is carried out in dilute aqueous solution with 0.10 M sucrose.
(a) Explain why this reaction follows pseudo-first-order kinetics.
(b) If the optical rotation changes from +34° to +17° in 20 minutes, calculate the rate constant.
(c) Calculate the time required for the rotation to become +8.5°.
(d) What would be the observed order if the reaction were carried out in concentrated acid where water is not in excess?
Q18. The reaction between acetone and iodine in acidic medium is:
CH₃COCH₃ + I₂ → CH₃COCH₂I + H⁺ + I⁻
The rate law is found to be: Rate = k[CH₃COCH₃][H⁺]
(a) What is the overall order of this reaction?
(b) If the reaction is carried out with acetone and H⁺ in large excess, what order would you observe experimentally? Explain.
(c) If [CH₃COCH₃] = 2.0 M, [H⁺] = 0.5 M, and k = 3.0 × 10⁻⁵ L mol⁻¹ s⁻¹, calculate the pseudo-first-order rate constant with respect to iodine.