Units of basic rate constants MCQs With Answer

Units of basic rate constants MCQs With Answer is a focused resource for B. Pharm students to master kinetic units used in drug stability, degradation and reaction rate analysis. This introduction clarifies how units change with reaction order (zero, first, second, third and fractional), explains pseudo‑first‑order conditions, and links kinetics to pharmacokinetic parameters like ke, t1/2, clearance and Vd. Emphasis is on practical unit conversions (s‑1, min‑1, h‑1, L mol‑1 s‑1, mol L‑1 s‑1), integrated rate laws and plotting strategies used in the drug industry and research. Clear examples help interpret experimental data and calculate meaningful parameters. Now let’s test your knowledge with 30 MCQs on this topic.

Q1. What is the SI unit of a first‑order rate constant?

• s^-1
• mol L^-1 s^-1
• L mol^-1 s^-1
• mol L^-1

Correct Answer: s^-1

Q2. What are the units of a second‑order bimolecular rate constant in conventional concentration units?

• s^-1
• mol L^-1 s^-1
• L mol^-1 s^-1
• mol^2 L^-2 s^-1

Correct Answer: L mol^-1 s^-1

Q3. What unit is used for a zero‑order rate constant when concentration is in mol L^-1 and time in seconds?

• mol L^-1 s^-1
• s^-1
• L mol^-1 s^-1
• M^-1 s^-1

Correct Answer: mol L^-1 s^-1

Q4. For a third‑order reaction (overall order = 3), which unit correctly represents k?

• s^-1
• L mol^-1 s^-1
• L^2 mol^-2 s^-1
• mol L^-1 s^-1

Correct Answer: L^2 mol^-2 s^-1

Q5. In pharmacokinetics, the elimination rate constant ke is commonly expressed in which unit?

• h^-1
• mol L^-1 s^-1
• L mol^-1 s^-1
• M^-1 s^-1

Correct Answer: h^-1

Q6. Which reaction order shows a half‑life that is independent of the initial concentration?

• First order
• Zero order
• Second order
• Pseudo‑first order

Correct Answer: First order

Q7. For a zero‑order degradation process, how does half‑life depend on initial concentration?

• t1/2 is directly proportional to initial concentration
• t1/2 is independent of initial concentration
• t1/2 is inversely proportional to initial concentration
• t1/2 equals 0.693/k

Correct Answer: t1/2 is directly proportional to initial concentration

Q8. In a pseudo‑first‑order experiment where B is in large excess and the true reaction is second order, what are the units of the observed rate constant kobs?

• L mol^-1 s^-1
• s^-1
• mol L^-1 s^-1
• L^2 mol^-2 s^-1

Correct Answer: s^-1

Q9. To convert a rate constant from s^-1 to h^-1, which factor should you use?

• Divide by 60
• Multiply by 60
• Multiply by 3600
• Divide by 3600

Correct Answer: Multiply by 3600

Q10. If rate = k[A]^2 (A in mol L^-1, time in s), what is the correct unit for k?

• mol L^-1 s^-1
• L mol^-1 s^-1
• s^-1
• M s^-1

Correct Answer: L mol^-1 s^-1

Q11. General expression for units of k given overall order n is:

• concentration^(n-1) time^-1
• concentration^(1-n) time^-1
• time^(1-n) concentration^-1
• time^-n concentration^(1-n)

Correct Answer: concentration^(1-n) time^-1

Q12. For the bimolecular elementary rate law rate = k[A][B], which unit is appropriate for k (concentrations in M, time in s)?

• M s^-1
• s^-1
• M^-1 s^-1
• mol L^-1

Correct Answer: M^-1 s^-1

Q13. A drug follows first‑order elimination with k = 0.05 h^-1. What are the units of k?

• h
• h^-1
• mol L^-1 h^-1
• L mol^-1 h^-1

Correct Answer: h^-1

Q14. Typical units for a bimolecular rate constant measured in solution kinetics are:

• mol L^-1
• L mol^-1 s^-1
• s^-1
• mol L^-1 s^-1

Correct Answer: L mol^-1 s^-1

Q15. If concentration is expressed in mg L^-1 rather than mol L^-1, the units of a second‑order rate constant become:

• L mg^-1 s^-1
• M^-1 s^-1
• mg L^-1 s^-1
• s^-1

Correct Answer: L mg^-1 s^-1

Q16. For a second‑order reaction with true k (L mol^-1 s^-1) and excess [B]0 (mol L^-1), the pseudo‑first‑order kobs = k[B]0. What are the units of kobs?

• L mol^-1 s^-1
• mol L^-1 s^-1
• s^-1
• L^2 mol^-2 s^-1

Correct Answer: s^-1

Q17. On a plot of ln[A] versus time for a first‑order reaction, the slope equals:

• +k with units L mol^-1 s^-1
• -k with units s^-1
• -k with units L mol^-1 s^-1
• +k with units s^-1

Correct Answer: -k with units s^-1

Q18. For a second‑order reaction 2A → products, which plot yields a straight line and what is the slope unit?

• [A] vs time, slope mol L^-1 s^-1
• ln[A] vs time, slope s^-1
• 1/[A] vs time, slope L mol^-1 s^-1
• [A]^2 vs time, slope mol^2 L^-2 s^-1

Correct Answer: 1/[A] vs time, slope L mol^-1 s^-1

Q19. For a zero‑order reaction, which statement is correct about the plot and units?

• ln[A] vs time is linear; slope s^-1
• [A] vs time is linear; slope mol L^-1 s^-1
• 1/[A] vs time is linear; slope L mol^-1 s^-1
• [A] vs time is exponential; slope s^-1

Correct Answer: [A] vs time is linear; slope mol L^-1 s^-1

Q20. Clearance (CL) relates to elimination rate constant by ke = CL/Vd. If ke is in h^-1 and Vd in L, what are the units of CL?

• L h^-1
• h L^-1
• L^-1 h
• h^-1

Correct Answer: L h^-1

Q21. Which expression correctly gives the units of k for overall reaction order n?

• time^-1 × concentration^(n-1)
• time^-1 × concentration^(1-n)
• time^(n-1) × concentration^1
• time × concentration^(1-n)

Correct Answer: time^-1 × concentration^(1-n)

Q22. If a second‑order rate constant is reported as 0.02 M^-1 min^-1, what are its units?

• M min^-1
• M^-1 min^-1
• min^-1
• min M^-1

Correct Answer: M^-1 min^-1

Q23. For an overall third‑order reaction, how do units of k scale with concentration and time?

• concentration^2 time^-1
• concentration^-2 time^-1
• concentration^-1 time^-1
• time^-3 concentration^2

Correct Answer: concentration^-2 time^-1

Q24. Enzyme kinetics under conditions of substrate much less than Km can often show apparent first‑order behavior. What are the units of the observed first‑order rate constant?

• M^-1 s^-1
• s^-1
• L mol^-1 s^-1
• mol L^-1 s^-1

Correct Answer: s^-1

Q25. A drug has k = 0.1386 day^-1 following first‑order kinetics. What is its half‑life (t1/2)?

• 0.693 days
• 5 days
• 0.1386 days
• 10 days

Correct Answer: 5 days

Q26. For a second‑order reaction with k = 2 M^-1 s^-1 and initial [A] = 0.10 M (2A → products), what is the half‑life?

• 0.5 s
• 5 s
• 0.05 s
• 10 s

Correct Answer: 5 s

Q27. True or False: The numerical value and units of a rate constant depend on the concentration units chosen (e.g., M vs mg L^-1).

• True
• False
• Only depends on time unit
• Only depends on reaction order

Correct Answer: True

Q28. For a reaction of fractional order 0.5, the units of k (with concentration in M and time in s) are:

• M^-0.5 s^-1
• M^0.5 s^-1
• s^-1
• L mol^-1 s^-1

Correct Answer: M^0.5 s^-1

Q29. Which plot is best to determine k for a first‑order degradation experimentally?

• [A] vs time
• ln[A] vs time
• 1/[A] vs time
• [A]^2 vs time

Correct Answer: ln[A] vs time

Q30. Which statement about units of rate constants is false?

• Units depend on overall reaction order
• Changing concentration units changes k units
• All rate constants always have units s^-1 regardless of order
• Pseudo‑first‑order kobs has units of s^-1 if time is in seconds

Correct Answer: All rate constants always have units s^-1 regardless of order