Reaction kinetics – second order MCQs With Answer

Reaction kinetics – second order MCQs With Answer

Understanding reaction kinetics is essential for B.Pharm students involved in drug stability, formulation science and metabolic studies. These 30 MCQs focus on second-order reactions — bimolecular processes where the rate depends on the concentrations of two reactant molecules or on the square of one reactant. Topics include differential and integrated rate laws, half-life dependence on initial concentration, units of the rate constant (M⁻¹ s⁻¹), graphical analysis (1/[A] vs. t), pseudo-first-order conditions, and pharmaceutical applications such as degradation kinetics. Questions emphasize derivations, experiment interpretation and problem-solving to build laboratory and exam readiness. Now let’s test your knowledge with 30 MCQs on this topic.

Q1. Which integrated rate law corresponds to a second-order reaction of type 2A → products?

• ln[A] = -kt + ln[A]0
• 1/[A] – 1/[A]0 = kt
• [A] = [A]0 e^{-kt}
• [A] – [A]0 = -kt

Correct Answer: 1/[A] – 1/[A]0 = kt

Q2. What are the units of the rate constant k for a second-order reaction in concentration units of mol L⁻¹ and time in seconds?

• s⁻¹
• mol L⁻¹ s⁻¹
• L mol⁻¹ s⁻¹ (or M⁻¹ s⁻¹)
• mol² L⁻² s⁻¹

Correct Answer: L mol⁻¹ s⁻¹ (or M⁻¹ s⁻¹)

Q3. For a second-order reaction 2A → products, how does the half-life (t1/2) depend on the initial concentration [A]0?

• t1/2 is independent of [A]0
• t1/2 = ln 2 / (k [A]0)
• t1/2 = 1 / (k [A]0)
• t1/2 = [A]0 / k

Correct Answer: t1/2 = 1 / (k [A]0)

Q4. Which plot yields a straight line for a second-order reaction with respect to reactant A?

• ln[A] vs. t
• [A] vs. t
• 1/[A] vs. t
• ln(1/[A]) vs. t

Correct Answer: 1/[A] vs. t

Q5. If rate = k[A][B] and initially [A]0 = [B]0, what is the apparent rate law in terms of [A] only?

• rate = k[A]
• rate = k[A]²
• rate = k[A]^{1/2}
• rate = k

Correct Answer: rate = k[A]²

Q6. Which experimental observation supports that a reaction is second-order?

• Plot of ln[A] vs. t is linear
• Half-life is constant over different [A]0
• Plot of 1/[A] vs. t is linear with slope = k
• Rate is independent of concentration

Correct Answer: Plot of 1/[A] vs. t is linear with slope = k

Q7. In a bimolecular reaction A + B → products, under pseudo-first-order conditions when [B] >> [A], the observed rate constant kobs equals:

• k / [B]
• k [B]
• k [A]
• k

Correct Answer: k [B]

Q8. For a second-order reaction with rate = k[A]², what is the slope of the 1/[A] vs. t plot?

• -k
• k
• k/2
• -1/k

Correct Answer: k

Q9. Which statement best describes a molecularity of two in an elementary reaction step?

• Involves one molecule undergoing rearrangement
• Involves two reactant molecules colliding in a single step
• Occurs in two separate steps each unimolecular
• Always implies third-order kinetics experimentally

Correct Answer: Involves two reactant molecules colliding in a single step

Q10. How does temperature generally affect the second-order rate constant k?

• k decreases with increasing temperature
• k is independent of temperature
• k increases with temperature according to the Arrhenius equation
• k changes unpredictably and not described by Arrhenius

Correct Answer: k increases with temperature according to the Arrhenius equation

Q11. Given the second-order integrated rate law 1/[A] – 1/[A]0 = kt, what is [A] at time t?

• [A] = [A]0 e^{-kt}
• [A] = 1 / (1/[A]0 + kt)
• [A] = [A]0 – kt
• [A] = [A]0 / (1 + k t [A]0^{2})

Correct Answer: [A] = 1 / (1/[A]0 + kt)

Q12. For a reaction A + B → products with initial concentrations [A]0 ≠ [B]0, the integrated rate expression is:

• 1/[A] – 1/[A]0 = kt for all cases
• ln([B]/[A]) = k t ([B]0 – [A]0)
• More complex logarithmic form involving [A]0 and [B]0
• [A] = [A]0 e^{-kt}

Correct Answer: More complex logarithmic form involving [A]0 and [B]0

Q13. Which experimental technique is commonly used to follow concentration vs. time for kinetic analysis in pharmaceutical studies?

• Mass spectrometry without quantitation
• UV–visible spectroscopy monitoring absorbance changes
• X-ray crystallography
• Polarography only for solids

Correct Answer: UV–visible spectroscopy monitoring absorbance changes

Q14. When plotting kinetic data for a second-order reaction, the y-intercept of the 1/[A] vs. t plot equals:

• k
• 1/[A]0
• [A]0
• 0

Correct Answer: 1/[A]0

Q15. If two reactants follow rate = k[A][B] and initial rates doubling [A] while [B] constant quadruples the rate, what is the order with respect to A?

• Zero order in A
• First order in A
• Second order in A
• Fractional order in A

Correct Answer: First order in A

Q16. In enzymatic degradation approximated as second-order between drug and nucleophile, which approximation can simplify kinetics when nucleophile is in excess?

• Steady-state approximation
• Pseudo-first-order approximation
• Equilibrium approximation only
• Negligible rate approximation

Correct Answer: Pseudo-first-order approximation

Q17. Which quantity is directly obtained from the slope of 1/[A] vs. t for a 2A → products reaction?

• Activation energy
• Rate constant k
• Half-life t1/2
• Order of reaction only

Correct Answer: Rate constant k

Q18. For a second-order reaction, doubling the initial concentration [A]0 will change the half-life t1/2 how?

• t1/2 doubles
• t1/2 halves
• t1/2 remains unchanged
• t1/2 increases fourfold

Correct Answer: t1/2 halves

Q19. Which kinetic observation can distinguish between an elementary bimolecular step and a chain mechanism giving apparent second-order kinetics?

• Only rate law measurement at one concentration
• Temperature dependence and mechanistic probes (isotope effects, intermediate detection)
• Measuring pH alone
• Observing color change

Correct Answer: Temperature dependence and mechanistic probes (isotope effects, intermediate detection)

Q20. In a second-order degradation study of a drug, why is it important to report the units of k?

• Units are irrelevant for comparing rates
• Units indicate experimental error
• Units identify reaction order and allow comparison between studies
• Units determine the stoichiometry of products

Correct Answer: Units identify reaction order and allow comparison between studies

Q21. Which expression gives the time required for concentration to fall to one-quarter for a second-order reaction 2A → products?

• t = (3)/(k [A]0)
• t = (1)/(k [A]0)
• t = ln4 / (k [A]0)
• t = (1/4)/(k [A]0)

Correct Answer: t = (3)/(k [A]0)

Q22. If an experimental plot of 1/[A] vs. t gives a slope of 0.02 L mol⁻¹ s⁻¹, what is the rate constant k?

• 0.02 s⁻¹
• 0.02 L mol⁻¹ s⁻¹
• 50 L mol⁻¹ s⁻¹
• 0.02 mol L⁻¹

Correct Answer: 0.02 L mol⁻¹ s⁻¹

Q23. Which scenario would make a bimolecular drug degradation appear first order experimentally?

• Both reactants at identical low concentrations
• Reaction in gas phase only
• One reactant maintained in large excess (pseudo-first-order)
• Using a catalyst

Correct Answer: One reactant maintained in large excess (pseudo-first-order)

Q24. How would you experimentally confirm that a reaction is truly second-order rather than a mixture of orders?

• Measure only one time point
• Fit multiple kinetic models and test linearity of 1/[A] vs. t across concentrations
• Assume second-order from stoichiometry alone
• Use color change as sole evidence

Correct Answer: Fit multiple kinetic models and test linearity of 1/[A] vs. t across concentrations

Q25. Which of the following is a common application of second-order kinetics in pharmaceutical science?

• Zero-order release from matrix tablets only
• Enzyme-catalyzed one-substrate reactions exclusively
• Bimolecular drug–nucleophile degradation and second-order hydrolysis
• Diffusion-limited processes that are always zeroth order

Correct Answer: Bimolecular drug–nucleophile degradation and second-order hydrolysis

Q26. During kinetics analysis, experimental noise causes some scatter. Which method reduces scatter impact for second-order plotting?

• Plot raw concentration vs. t without transformation
• Average replicate measurements and use 1/[A] vs. t linear regression
• Normalize data to arbitrary units only
• Use only the last data point

Correct Answer: Average replicate measurements and use 1/[A] vs. t linear regression

Q27. If rate = k[A][B] and initial rates show a zero-order dependence on B, which conclusion is likely?

• B is in large excess making rate independent of B
• B is absent from the mechanism
• The reaction is third order

Correct Answer: B is in large excess making rate independent of B

Q28. Which replacement of concentration with absorbance is valid for kinetic plotting when Beer–Lambert law holds?

• Use absorbance directly in 1/[A] vs. t without calibration
• Convert absorbance to concentration using A = εlc, then plot 1/[A] vs. t
• Plot ln(absorbance) vs. absorbance
• Absorbance cannot be used for kinetics

Correct Answer: Convert absorbance to concentration using A = εlc, then plot 1/[A] vs. t

Q29. Which of the following kinetic parameters is most useful for comparing degradation rates of two drugs undergoing second-order hydrolysis?

• Initial concentration only
• Rate constant k with proper units
• Color of solution
• Half-life without reporting [A]0

Correct Answer: Rate constant k with proper units

Q30. In a batch experiment you find that doubling both [A]0 and [B]0 increases the initial rate by a factor of four. Which overall order does this indicate?

• Zero order
• First order
• Second order
• Third order

Correct Answer: Second order