Reaction kinetics – pseudo-zero order MCQs With Answer
Reaction kinetics is essential in pharmaceutical sciences, especially when studying drug stability, degradation pathways, and formulation behavior. Pseudo-zero order kinetics occurs when a reaction proceeds at an approximately constant rate due to saturated catalysts, enzymes, or excess reactant, producing a rate law that appears independent of reactant concentration. B. Pharm students must master concepts like rate law, half-life, linear plots, reaction mechanism, enzyme saturation, and practical implications for drug shelf-life and dosing. These keyword-rich MCQs emphasize mechanistic understanding, data analysis, and calculation skills tied to pseudo-zero order and compare them with true zero and first-order kinetics. Now let’s test your knowledge with 30 MCQs on this topic.
Q1. Which expression represents the integrated rate law for a true zero-order reaction?
- [A] = [A]0 e^(-kt)
- [A] = [A]0 – kt
- ln[A] = ln[A]0 – kt
- 1/[A] = 1/[A]0 + kt
Correct Answer: [A] = [A]0 – kt
Q2. A reaction shows constant rate because an enzyme is saturated with substrate. This kinetic behavior is best described as:
- First-order kinetics
- Second-order kinetics
- Pseudo-zero order kinetics
- Autocatalytic kinetics
Correct Answer: Pseudo-zero order kinetics
Q3. For a pseudo-zero order process, which plot of experimental data yields a straight line?
- ln[Drug] vs time
- [Drug] vs time
- 1/[Drug] vs time
- Rate vs ln[Drug]
Correct Answer: [Drug] vs time
Q4. Units of the rate constant k for a zero-order reaction are typically:
- s^-1
- M^-1 s^-1
- M s^-1 or concentration/time
- Dimensionless
Correct Answer: M s^-1 or concentration/time
Q5. Which half-life expression is correct for a zero-order reaction?
- t1/2 = ln 2 / k
- t1/2 = [A]0 / k
- t1/2 = [A]0 / (2k)
- t1/2 = 1 / (k [A]0)
Correct Answer: t1/2 = [A]0 / (2k)
Q6. In the context of drug degradation, when is pseudo-zero order kinetics most likely observed?
- When drug concentration is extremely low
- When a catalyst or enzyme is saturated and substrate is in excess
- When the reaction follows simple bimolecular collision theory
- Only in gas-phase reactions
Correct Answer: When a catalyst or enzyme is saturated and substrate is in excess
Q7. Which experimental outcome indicates pseudo-zero order behavior?
- Rate ∝ [A]
- Rate ∝ [A]^2
- Rate ≈ constant regardless of [A]
- Rate ∝ ln[A]
Correct Answer: Rate ≈ constant regardless of [A]
Q8. A reaction follows pseudo-zero order with k = 0.02 M min^-1 and [A]0 = 0.50 M. How long to reach [A] = 0.10 M?
- 5 min
- 10 min
- 20 min
- 40 min
Correct Answer: 20 min
Q9. For pseudo-zero order kinetics under Michaelis–Menten conditions, which relationship holds when [S] >> Km?
- Rate ≈ kcat/Km · [S]
- Rate ≈ Vmax
- Rate ≈ 0
- Rate ∝ 1/[S]
Correct Answer: Rate ≈ Vmax
Q10. Which statement distinguishes pseudo-zero from true zero-order kinetics?
- Pseudo-zero order is independent of catalyst saturation.
- True zero-order arises only under surface or limiting-effect conditions; pseudo-zero appears under conditional saturation of one reactant.
- They are identical in cause and effect.
- Pseudo-zero order always has a variable rate constant.
Correct Answer: True zero-order arises only under surface or limiting-effect conditions; pseudo-zero appears under conditional saturation of one reactant.
Q11. If a concentration vs time plot is linear with slope -0.015 M h^-1, what is k for a zero-order process?
- 0.015 h
- 0.015 M h^-1
- 15 M h^-1
- 0.015 h^-1
Correct Answer: 0.015 M h^-1
Q12. How does the half-life change with initial concentration for a pseudo-zero order reaction?
- It is independent of initial concentration
- It decreases as [A]0 increases
- It increases linearly with [A]0
- It varies inversely with [A]0^2
Correct Answer: It increases linearly with [A]0
Q13. Which method helps determine if a degradation follows pseudo-zero order?
- Plot ln[Drug] vs time and check for curvature
- Compare 1/[Drug] vs time linearity
- Plot [Drug] vs time and check for straight-line behavior with constant slope
- Use NMR only
Correct Answer: Plot [Drug] vs time and check for straight-line behavior with constant slope
Q14. In forced degradation studies, observing constant degradation rate regardless of concentration suggests:
- First-order hydrolysis
- Pseudo-zero order degradation
- Second-order oxidation
- Zero degradation
Correct Answer: Pseudo-zero order degradation
Q15. For a pseudo-zero order reaction, which parameter is directly obtained from the slope of [A] vs t?
- Initial concentration [A]0
- Rate constant k with negative sign
- Activation energy E_a
- Order of reaction as exponent
Correct Answer: Rate constant k with negative sign
Q16. A drug degrades with pseudo-zero order and k = 0.04 mg L^-1 day^-1. If allowable concentration loss is 0.12 mg L^-1, approximate time to reach that loss?
- 0.3 day
- 1.5 day
- 3.0 days
- 30 days
Correct Answer: 3.0 days
Q17. Which kinetic test can distinguish pseudo-zero from apparent first-order behavior when one reactant is in large excess?
- Vary the excess reactant concentration and observe rate change
- Measure color change only
- Use mass spectrometry exclusively
- Keep all concentrations constant
Correct Answer: Vary the excess reactant concentration and observe rate change
Q18. In a heterogeneous catalytic degradation at the solid surface, kinetic behavior resembling zero-order often arises because:
- The reactant is always in very low concentration
- The surface is saturated with reactant molecules, so rate is surface-limited
- Temperature is constant
- Pressure effects dominate
Correct Answer: The surface is saturated with reactant molecules, so rate is surface-limited
Q19. When using initial rate method, pseudo-zero order kinetics can be mistaken for zero-order unless:
- Multiple initial concentrations and saturating conditions are tested
- Only a single initial concentration is used
- Temperature is not controlled
- Reaction is too fast
Correct Answer: Multiple initial concentrations and saturating conditions are tested
Q20. Which of the following is a consequence of pseudo-zero order drug degradation for shelf-life estimation?
- Shelf-life is independent of initial drug concentration
- Shelf-life depends on initial concentration and k, requiring linear extrapolation
- Half-life follows ln 2 / k
- Degradation follows exponential decay
Correct Answer: Shelf-life depends on initial concentration and k, requiring linear extrapolation
Q21. A reaction shows linear decrease in concentration with time. What is the value of ln[A] vs time plot?
- Also linear with same slope
- Nonlinear (curved)
- Has slope equal to -k/[A]
- Gives a straight line through origin
Correct Answer: Nonlinear (curved)
Q22. Which is a typical lab cause of observing pseudo-zero order kinetics in drug degradation?
- Using extremely dilute drug solutions
- Presence of a high concentration of buffer or excipient that saturates catalytic sites
- Poor mixing only
- Only measuring at very late times
Correct Answer: Presence of a high concentration of buffer or excipient that saturates catalytic sites
Q23. For a drug following pseudo-zero order, doubling the initial concentration will:
- Double the half-life
- Halve the half-life
- Leave half-life unchanged
- Reduce rate constant by half
Correct Answer: Double the half-life
Q24. Which best describes a pseudo-zero order rate law derived from a bimolecular mechanism with excess B?
- Rate = k'[A][B]
- Rate = k[A]^2
- Rate = k_obs (constant) because [B] ≈ constant
- Rate = k/[A]
Correct Answer: Rate = k_obs (constant) because [B] ≈ constant
Q25. When plotting experimental data to confirm pseudo-zero order, the y-intercept of [A] vs t equals:
- -k
- [A]0
- k
- 0
Correct Answer: [A]0
Q26. A degradation experiment yields slope = -0.025 M h^-1 from [A] vs t. What is t1/2 if [A]0 = 0.10 M?
- 2 h
- 4 h
- 0.1 h
- 1 h
Correct Answer: 2 h
Q27. Which practical formulation factor can induce pseudo-zero order degradation in a solid dosage form?
- Very low humidity
- High concentration of catalytic excipient uniformly distributed
- Reduced particle size alone
- Using inert fillers only
Correct Answer: High concentration of catalytic excipient uniformly distributed
Q28. In comparing pseudo-zero and first-order kinetics, which is true about concentration decay?
- Pseudo-zero shows exponential decay; first-order shows linear decay
- Both show linear decay
- Pseudo-zero is linear in [A] vs t; first-order is linear in ln[A] vs t
- Both are linear in ln[A] vs t
Correct Answer: Pseudo-zero is linear in [A] vs t; first-order is linear in ln[A] vs t
Q29. Which experimental observation suggests transition from pseudo-zero to first-order as concentration decreases?
- Rate remains constant at all concentrations
- Rate begins to decrease proportionally with [A] once [A] falls below a threshold
- Rate increases as concentration decreases
- No change in slope on [A] vs t plot
Correct Answer: Rate begins to decrease proportionally with [A] once [A] falls below a threshold
Q30. Which calculation correctly gives time t to reach [A]t from [A]0 under zero-order kinetics?
- t = ln([A]0/[A]t) / k
- t = 1/k ([A]t – [A]0)
- t = ([A]0 – [A]t) / k
- t = k / ([A]0 – [A]t)
Correct Answer: t = ([A]0 – [A]t) / k
