Introduction: Determination of reaction order is a core concept in chemical kinetics and crucial for B.Pharm students studying drug stability, degradation kinetics, and formulation shelf-life. This topic covers experimental approaches — the initial rate method, integrated rate laws, graphical methods (plotting [A] vs time, ln[A] vs time, 1/[A] vs time), half-life behavior, and pseudo-order kinetics — to identify zero-order, first-order, and second-order reactions. Understanding units of the rate constant, relationship between molecularity and observed order, and implications for pharmacokinetics helps predict drug behavior under storage and physiological conditions. Mastery of these principles supports rational formulation and regulatory studies. ‘Now let’s test your knowledge with 30 MCQs on this topic.’
Q1. What is the primary experimental method used to determine the order of a reaction by comparing how initial rates change with concentration?
- Initial rate method
- Half-life method
- Continuous variation method
- Spectrophotometric endpoint method
Correct Answer: Initial rate method
Q2. For a first-order reaction, which plot gives a straight line?
- Concentration [A] versus time
- ln[A] versus time
- 1/[A] versus time
- Rate versus [A]^2
Correct Answer: ln[A] versus time
Q3. Which order of reaction has a half-life independent of initial concentration?
- Zero-order
- First-order
- Second-order
- Pseudo-first-order
Correct Answer: First-order
Q4. For a zero-order reaction, how does the concentration change with time?
- Exponential decay
- Linear decrease
- Inverse linear increase
- Logarithmic decrease
Correct Answer: Linear decrease
Q5. If a rate law is rate = k[A]^2, what is the overall order of the reaction?
- Zero-order
- First-order
- Second-order
- Third-order
Correct Answer: Second-order
Q6. What are the SI units of the rate constant k for a second-order reaction?
- s^-1
- M^-1 s^-1
- M s^-1
- M^-2 s^-1
Correct Answer: M^-1 s^-1
Q7. Which method isolates one reactant concentration in large excess to determine the order with respect to the limiting reagent?
- Initial rate method
- Isolation method
- Pseudo-order method
- Method of continuous variation
Correct Answer: Pseudo-order method
Q8. A plot of 1/[A] versus time is linear. What is the reaction order?
- Zero-order
- First-order
- Second-order
- Mixed-order
Correct Answer: Second-order
Q9. For a pseudo-first-order reaction, which statement is true?
- All concentration dependencies are second-order
- The rate behaves as first-order because one reactant is in large excess
- The rate constant becomes zero
- Half-life depends on initial concentration squared
Correct Answer: The rate behaves as first-order because one reactant is in large excess
Q10. Which kinetic parameter directly indicates how fast a reaction proceeds at a given concentration?
- Reaction order
- Rate constant k
- Molecularity
- Activation energy only
Correct Answer: Rate constant k
Q11. Molecularity differs from reaction order because molecularity is:
- Experimentally determined from rate laws
- A stoichiometric count of species in an elementary step
- Always equal to the reaction order
- Measured in units of concentration
Correct Answer: A stoichiometric count of species in an elementary step
Q12. Which integrated rate law corresponds to a zero-order reaction?
- [A] = [A]0 e^-kt
- 1/[A] = kt + 1/[A]0
- [A] = [A]0 – kt
- ln[A] = ln[A]0 – kt
Correct Answer: [A] = [A]0 – kt
Q13. If doubling [A] causes the initial rate to increase by a factor of four, the reaction is:
- Zero-order in A
- First-order in A
- Second-order in A
- Third-order in A
Correct Answer: Second-order in A
Q14. In pharmaceutical stability, why is determining reaction order important?
- It identifies toxic impurities directly
- It predicts shelf-life and concentration-time profiles
- It replaces the need for dissolution testing
- It measures only pH changes
Correct Answer: It predicts shelf-life and concentration-time profiles
Q15. The initial rate for rate = k[A][B] is measured while keeping [B] constant and varying [A]. Observed rate is directly proportional to [A]. What is the order with respect to A?
- Zero-order
- First-order
- Second-order
- Cannot be determined
Correct Answer: First-order
Q16. Which experimental technique is commonly used to monitor reactant concentration continuously for kinetic analysis in pharmaceutics?
- Thin-layer chromatography endpoint only
- Spectrophotometry or HPLC analysis over time
- Melting point determination
- Gravimetric titration after reaction completion
Correct Answer: Spectrophotometry or HPLC analysis over time
Q17. A reaction shows a half-life that increases as initial concentration decreases. This behavior suggests which order?
- Zero-order
- First-order
- Second-order
- Fractional-order
Correct Answer: Second-order
Q18. Which statement about the rate constant k is correct?
- k is dimensionless for all orders
- k depends on temperature and activation energy
- k equals the reaction order numerically
- k is independent of catalysts
Correct Answer: k depends on temperature and activation energy
Q19. In a reaction A → products, the slope of ln[A] vs time is -0.023 min^-1. What does this slope represent?
- The half-life
- Positive rate constant k
- Negative rate constant -k for a first-order reaction
- The initial concentration
Correct Answer: Negative rate constant -k for a first-order reaction
Q20. When determining reaction order by the method of initial rates, why must measurements be taken at low conversion?
- To ensure pseudo-steady state of intermediates
- To approximate initial concentrations as constant and avoid reverse reaction effects
- To maximize detection limits of instruments
- To keep temperature fluctuations negligible
Correct Answer: To approximate initial concentrations as constant and avoid reverse reaction effects
Q21. For a reaction following rate = k[A]^0, which description fits?
- Rate independent of [A], zero-order
- Rate proportional to [A], first-order
- Rate proportional to [A]^2, second-order
- Rate depends on catalyst only
Correct Answer: Rate independent of [A], zero-order
Q22. What is the half-life expression for a first-order reaction?
- t1/2 = [A]0 / 2k
- t1/2 = 1 / (k[A]0)
- t1/2 = ln 2 / k
- t1/2 = k / ln 2
Correct Answer: t1/2 = ln 2 / k
Q23. In a complex mechanism, the experimentally observed order may differ from molecularity because:
- Experimental error always changes order
- Elementary steps and steady-state or pre-equilibrium approximations alter observed kinetics
- Molecularity is an average over time
- Order is defined by stoichiometry only
Correct Answer: Elementary steps and steady-state or pre-equilibrium approximations alter observed kinetics
Q24. A degradation reaction in solution shows linear decrease of drug concentration with time. This suggests what practical stability implication?
- Shelf-life independent of initial concentration (first-order)
- Predictable absolute loss per unit time (zero-order)
- Instability only at high concentration
- Reaction controlled by diffusion only
Correct Answer: Predictable absolute loss per unit time (zero-order)
Q25. Which graph would you use to verify zero-order kinetics experimentally?
- Plot of ln[A] vs time
- Plot of [A] vs time
- Plot of 1/[A] vs time
- Plot of rate vs ln[A]
Correct Answer: Plot of [A] vs time
Q26. If rate = k[A]^m[B]^n and doubling both [A] and [B] increases rate by 8, what is the overall order (m+n)?
- 1
- 2
- 3
- 4
Correct Answer: 3
Q27. The method of continuous variation (Job’s method) is mainly used to:
- Determine activation energy
- Find stoichiometry of a complex in binding studies
- Measure rate constants at different temperatures
- Isolate intermediates kinetically
Correct Answer: Find stoichiometry of a complex in binding studies
Q28. In a second-order reaction with one reactant A (A → products), how does half-life depend on initial concentration?
- t1/2 = ln2 / k (independent)
- t1/2 = 1 / (k[A]0) (inversely proportional)
- t1/2 = [A]0 / 2k (directly proportional)
- t1/2 = k[A]0^2
Correct Answer: t1/2 = 1 / (k[A]0) (inversely proportional)
Q29. Which kinetic model would you apply when a heterogeneous solid-catalyzed reaction shows concentration-independent surface saturation?
- First-order homogeneous model
- Langmuir-Hinshelwood or zero-order surface-limited kinetics
- Second-order homogeneous model
- Michaelis-Menten gas-phase kinetics only
Correct Answer: Langmuir-Hinshelwood or zero-order surface-limited kinetics
Q30. Which of the following changes can convert a second-order kinetic experiment into a pseudo-first-order one for practical analysis?
- Lowering temperature drastically
- Keeping one reactant in large excess
- Removing catalyst from the reaction mixture
- Decreasing ionic strength only
Correct Answer: Keeping one reactant in large excess
I am a Registered Pharmacist under the Pharmacy Act, 1948, and the founder of PharmacyFreak.com. I hold a Bachelor of Pharmacy degree from Rungta College of Pharmaceutical Science and Research. With a strong academic foundation and practical knowledge, I am committed to providing accurate, easy-to-understand content to support pharmacy students and professionals. My aim is to make complex pharmaceutical concepts accessible and useful for real-world application.
Mail- Sachin@pharmacyfreak.com