Reaction kinetics – zero order MCQs With Answer provides B. Pharm students a focused review of zero-order reaction kinetics as applied to drug stability, controlled-release formulations, and saturated elimination pathways. This concise introduction covers key concepts: rate law (rate = k or rate ∝ [A]0), integrated rate equation ([A] = [A]0 − kt), linear concentration–time plots, units of the rate constant (concentration/time), and the distinctive half-life dependence on initial concentration. Emphasis is on practical calculations, graph interpretation, and pharmaceutical examples (phenytoin, ethanol, transdermal release). Clear understanding aids drug formulation, shelf-life estimation and pharmacokinetic interpretation. Now let’s test your knowledge with 30 MCQs on this topic.
Q1. What defines a zero-order reaction in chemical kinetics?
- Rate is directly proportional to reactant concentration
- Rate is inversely proportional to reactant concentration
- Rate is constant and independent of reactant concentration
- Rate changes exponentially with concentration
Correct Answer: Rate is constant and independent of reactant concentration
Q2. Which expression correctly represents the differential rate law for a zero-order process?
- rate = k[A]
- rate = k[A]^2
- rate = k
- rate = k/[A]
Correct Answer: rate = k
Q3. What is the integrated rate equation for a zero-order reaction?
- [A] = [A]0 e^{-kt}
- ln[A] = ln[A]0 − kt
- [A] = [A]0 − kt
- 1/[A] = 1/[A]0 + kt
Correct Answer: [A] = [A]0 − kt
Q4. Which plot gives a straight line for a zero-order reaction?
- ln[A] vs time
- [A] vs time
- 1/[A] vs time
- ln(rate) vs ln([A])
Correct Answer: [A] vs time
Q5. What are the correct SI units for the rate constant k in zero-order kinetics?
- s^{-1}
- L mol^{-1} s^{-1}
- concentration/time (e.g., mol L^{-1} s^{-1})
- dimensionless
Correct Answer: concentration/time (e.g., mol L^{-1} s^{-1})
Q6. Which formula gives the half-life (t1/2) for a zero-order reaction?
- t1/2 = ln 2 / k
- t1/2 = 1 / (k [A]0)
- t1/2 = [A]0 / (2k)
- t1/2 = 2k / [A]0
Correct Answer: t1/2 = [A]0 / (2k)
Q7. For a drug following zero-order elimination with k = 0.5 mg·L^{-1}·h^{-1} and initial concentration 10 mg·L^{-1}, what is the half-life?
- 5 hours
- 10 hours
- 20 hours
- 0.2 hours
Correct Answer: 10 hours
Q8. If [A]0 = 20 mg·L^{-1} and k = 2 mg·L^{-1}·h^{-1}, what is [A] after 3 hours for a zero-order process?
- 14 mg·L^{-1}
- 6 mg·L^{-1}
- 26 mg·L^{-1}
- 20 mg·L^{-1}
Correct Answer: 14 mg·L^{-1}
Q9. Which pharmaceutical drug is classically associated with zero-order elimination at therapeutic concentrations?
- Paracetamol (acetaminophen)
- Phenytoin
- Metformin
- Penicillin
Correct Answer: Phenytoin
Q10. In zero-order kinetics, the half-life of a drug depends on which factor(s)?
- Only the rate constant k
- Only the initial concentration [A]0
- Both initial concentration [A]0 and rate constant k
- Neither; half-life is constant
Correct Answer: Both initial concentration [A]0 and rate constant k
Q11. How long will it take to completely deplete a reactant in a zero-order reaction?
- t = ln 2 / k
- t = [A]0 / k
- t = 2k / [A]0
- t = 1 / (k [A]0)
Correct Answer: t = [A]0 / k
Q12. Which of the following plots would NOT be expected for a zero-order reaction?
- [A] vs time — straight line
- ln[A] vs time — straight line
- slope of [A] vs time = −k
- intercept of [A] vs time = [A]0
Correct Answer: ln[A] vs time — straight line
Q13. A kinetic study shows ln[A] vs time is linear. What is the reaction order?
- Zero order
- First order
- Second order
- Mixed order
Correct Answer: First order
Q14. Which drug-release system is designed to provide near zero-order release in formulations?
- Immediate-release tablet
- Transdermal patch (controlled-release matrix)
- Enteric-coated capsule
- Oral solution
Correct Answer: Transdermal patch (controlled-release matrix)
Q15. If initial concentration is doubled for a zero-order process (k constant), what happens to the half-life?
- Half-life is halved
- Half-life doubles
- Half-life is unchanged
- Half-life becomes ln 2 / k
Correct Answer: Half-life doubles
Q16. A zero-order reaction proceeds at 5 mg·L^{-1}·h^{-1}. Starting concentration 50 mg·L^{-1}, how long to reach 20 mg·L^{-1}?
- 6 hours
- 10 hours
- 30 hours
- 5 hours
Correct Answer: 6 hours
Q17. Which statement correctly describes the concentration–time profile for zero-order kinetics?
- Exponential decay curve
- Hyperbolic decay curve
- Linear decline until depletion
- Logarithmic decline
Correct Answer: Linear decline until depletion
Q18. You determine the slope of [A] vs time plot for a zero-order reaction is −0.02 mol·L^{-1}·s^{-1}. What is k?
- k = −0.02 mol·L^{-1}·s^{-1}
- k = 0.02 mol·L^{-1}·s^{-1}
- k = 0.0004 mol·L^{-1}·s^{-1}
- k = ln(2)/0.02
Correct Answer: k = 0.02 mol·L^{-1}·s^{-1}
Q19. Which unit indicates zero-order kinetics for k?
- s^{-1}
- mol·L^{-1}·s^{-1}
- L·mol^{-1}·s^{-1}
- dimensionless
Correct Answer: mol·L^{-1}·s^{-1}
Q20. How do you obtain the rate constant k from experimental [A] vs time data for a zero-order process?
- k = intercept of ln[A] vs time
- k = slope of 1/[A] vs time
- k = negative slope of [A] vs time
- k = slope of ln rate vs ln[A]
Correct Answer: k = negative slope of [A] vs time
Q21. Which equation would you use to calculate concentration after time t in zero-order kinetics?
- [A] = [A]0 e^{-kt}
- [A] = [A]0 − kt
- 1/[A] = 1/[A]0 + kt
- ln([A]/[A]0) = −kt
Correct Answer: [A] = [A]0 − kt
Q22. Under what physiological condition might a drug exhibit zero-order elimination?
- When elimination pathways are far below saturation
- When elimination pathways are saturated (capacity-limited)
- When renal clearance is proportional to concentration
- When drug follows passive diffusion only
Correct Answer: When elimination pathways are saturated (capacity-limited)
Q23. A drug follows zero-order kinetics with [A]0 = 40 mg·L^{-1} and k = 4 mg·L^{-1}·h^{-1}. What is t1/2?
- 2 hours
- 5 hours
- 10 hours
- 20 hours
Correct Answer: 5 hours
Q24. Which graph best distinguishes zero-order from first-order kinetics?
- Plot ln[A] vs time — linear for zero-order
- Plot [A] vs time — linear for zero-order
- Plot 1/[A] vs time — linear for zero-order
- Plot rate vs ln[A] — linear for zero-order
Correct Answer: Plot [A] vs time — linear for zero-order
Q25. A formulation shows a constant drug release of 0.8 mg·h^{-1}. This is an example of which kinetic behavior?
- First-order release
- Zero-order release
- Second-order release
- Michaelis–Menten limited release
Correct Answer: Zero-order release
Q26. For a zero-order process, if k increases due to temperature rise, what is the expected effect on half-life?
- Half-life increases
- Half-life decreases
- Half-life remains unchanged
- Half-life becomes independent of [A]0
Correct Answer: Half-life decreases
Q27. A drug shows zero-order elimination at high doses but first-order at low doses. What term describes this behavior?
- Autocatalysis
- Mixed-order or dose-dependent kinetics
- Simple first-order kinetics
- Pseudo-second-order kinetics
Correct Answer: Mixed-order or dose-dependent kinetics
Q28. If [A]0 = 100 units and k = 4 units·h^{-1}, what is [A] after 12 hours in zero-order kinetics?
- 52 units
- 48 units
- 52.0 units
- Not defined
Correct Answer: 52 units
Q29. Which experimental observation suggests a zero-order degradation during stability studies?
- Concentration decreases by a constant fraction per day
- Concentration decreases by the same absolute amount each day
- ln concentration decreases linearly with time
- 1/concentration increases linearly with time
Correct Answer: Concentration decreases by the same absolute amount each day
Q30. In a kinetic experiment the concentration reaches zero at t = 8 h and initial concentration was 40 mg·L^{-1}. What is the zero-order rate constant k?
- 0.2 mg·L^{-1}·h^{-1}
- 5 mg·L^{-1}·h^{-1}
- 40 mg·L^{-1}·h^{-1}
- 8 mg·L^{-1}·h^{-1}
Correct Answer: 5 mg·L^{-1}·h^{-1}
