Reaction kinetics – zero order MCQs With Answer

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}