Reaction kinetics – pseudo-zero order MCQs With Answer

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