Reaction kinetics – first order MCQs With Answer

Reaction kinetics – first order is a core topic for B.Pharm students studying drug stability, degradation and pharmacokinetics. First-order kinetics describe processes where the rate depends on the concentration of a single reactant and follows exponential decay, governed by the rate constant (k) and the integrated rate law ([A] = [A]0 e^-kt). Key concepts include half-life (t_1/2 = ln2/k), units of k (s^-1 or h^-1), pseudo-first order conditions, Arrhenius temperature dependence and analytical plots such as ln[A] vs t. Mastery of these ideas helps predict shelf life, drug degradation and concentration–time profiles. Now let’s test your knowledge with 30 MCQs on this topic.

Q1. What defines a first-order reaction in reaction kinetics?

• The rate is proportional to the square of the concentration of the reactant
• The rate is independent of reactant concentration
• The rate is proportional to the concentration of a single reactant
• The rate depends on the concentrations of two reactants

Correct Answer: The rate is proportional to the concentration of a single reactant

Q2. Which expression represents the integrated first-order rate law?

• [A] = [A]0 – kt
• [A] = [A]0 e^-kt
• ln([A]0/[A]) = kt²
• 1/[A] = 1/[A]0 + kt

Correct Answer: [A] = [A]0 e^-kt

Q3. For a first-order reaction, which plot yields a straight line?

• Plot of [A] vs t
• Plot of 1/[A] vs t
• Plot of ln[A] vs t
• Plot of [A]² vs t

Correct Answer: Plot of ln[A] vs t

Q4. What is the slope of the ln[A] vs t plot for a first-order reaction?

• +k
• -k
• k/2
• -ln k

Correct Answer: -k

Q5. What are the SI units of the first-order rate constant k?

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

Correct Answer: s^-1

Q6. Which formula gives the half-life (t_1/2) for a first-order process?

• t_1/2 = 1/(k[A]0)
• t_1/2 = ln2 / k
• t_1/2 = 2.303/k
• t_1/2 = k / ln2

Correct Answer: t_1/2 = ln2 / k

Q7. How does the half-life of a first-order reaction depend on initial concentration?

• It increases with increasing initial concentration
• It decreases with increasing initial concentration
• It is independent of initial concentration
• It depends on the square of initial concentration

Correct Answer: It is independent of initial concentration

Q8. If a drug follows first-order kinetics and t_1/2 = 4 h, what fraction remains after 8 h?

• 50%
• 25%
• 12.5%
• 0%

Correct Answer: 25%

Q9. Which condition describes pseudo-first-order kinetics?

• The reaction is second order but both reactants are at equal low concentrations
• One reactant is in large excess so the rate depends effectively on the other reactant only
• The rate is independent of all reactant concentrations
• The reaction follows a zero-order rate law

Correct Answer: One reactant is in large excess so the rate depends effectively on the other reactant only

Q10. Which expression gives the concentration [A] at time t for a first-order reaction?

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

Correct Answer: [A] = [A]0 e^-kt

Q11. For first-order degradation, the shelf-life (t₉₀) corresponding to 10% loss is given by:

• t₉₀ = ln(0.1)/k
• t₉₀ = ln(0.9)/-k
• t₉₀ = 0.1/k
• t₉₀ = k/ln(0.9)

Correct Answer: t₉₀ = ln(0.9)/-k

Q12. Which method is commonly used experimentally to confirm first-order kinetics?

• Plot of [A] vs 1/t
• Plot of 1/[A] vs t giving a straight line
• Plot of ln[A] vs t giving a straight line
• Plot of [A]² vs t giving a straight line

Correct Answer: Plot of ln[A] vs t giving a straight line

Q13. If the slope of ln[A] vs t is -0.2 h^-1, what is the half-life?

• 0.2 h
• 3.47 h
• ln2 × 0.2 h
• 5 h

Correct Answer: 3.47 h

Q14. A reaction showing exponential decay of concentration with time is most likely:

• Zero order
• First order
• Second order
• Mixed order

Correct Answer: First order

Q15. The initial rate of a first-order reaction is given by:

• k
• k [A]₀
• [A]₀/k
• k / [A]₀

Correct Answer: k [A]₀

Q16. If the rate constant k for a first-order process doubles, what happens to t_1/2?

• It doubles
• It halves
• It remains unchanged
• It becomes zero

Correct Answer: It halves

Q17. Radioactive decay follows which kinetic order?

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

Correct Answer: First order

Q18. Which statement about pseudo-first-order reactions is true?

• The observed rate constant equals the true k only when concentrations are equal
• The observed rate constant is independent of excess reagent concentration
• The observed rate constant equals k multiplied by the constant concentration of the excess reagent
• Pseudo-first-order conditions only occur in gas-phase reactions

Correct Answer: The observed rate constant equals k multiplied by the constant concentration of the excess reagent

Q19. For a first-order reaction with k = 0.693 day^-1, what is t_1/2?

• 0.693 day
• 1 day
• 2 days
• ln(2) days

Correct Answer: 1 day

Q20. Calculate k (in h^-1) if t_1/2 = 10 h.

• 0.0693 h^-1
• 0.693 h^-1
• 10 h^-1
• 1.0 h^-1

Correct Answer: 0.0693 h^-1

Q21. A drug concentration decreases from 100 µg/mL to 50 µg/mL in 6 hours. Assuming first-order kinetics, what is k (h^-1)?

• 0.1155 h^-1
• 0.693 h^-1
• 6 h^-1
• 0.5 h^-1

Correct Answer: 0.1155 h^-1

Q22. For first-order kinetics, how long will it take to reduce the concentration to 10% of initial if k = 0.23 h^-1?

• ln(0.1)/-0.23 ≈ 10 h
• ln(0.9)/-0.23 ≈ 0.46 h
• ln(10)/0.23 ≈ 10 h
• ln(0.1)/0.23 ≈ -10 h

Correct Answer: ln(0.1)/-0.23 ≈ 10 h

Q23. Which kinetic parameter often used in stability studies is independent of initial concentration for first-order decay?

• Initial rate
• Half-life
• Initial concentration
• Activation energy

Correct Answer: Half-life

Q24. The Arrhenius equation relates the rate constant k to:

• Pressure only
• Temperature and activation energy
• Concentration and volume
• Catalyst concentration only

Correct Answer: Temperature and activation energy

Q25. In a first-order irreversible consecutive reaction A → B → C, the formation rate of B initially is approximately:

• k₂[B]
• k₁[A]
• k₁[B]
• k₂[A]

Correct Answer: k₁[A]

Q26. Which experimental observation would suggest first-order kinetics for drug degradation?

• Plot of [drug] vs time is linear
• Plot of 1/[drug] vs time is linear
• Plot of ln[drug] vs time is linear
• Rate of degradation is constant over time

Correct Answer: Plot of ln[drug] vs time is linear

Q27. If a first-order reaction has k = 0.3465 h^-1, what is the time to reduce concentration to 25%?

• ln(0.25)/-0.3465 ≈ 4 h
• ln(0.75)/-0.3465 ≈ 0.8 h
• ln(4)/0.3465 ≈ 4 h
• ln(0.25)/0.3465 ≈ -4 h

Correct Answer: ln(0.25)/-0.3465 ≈ 4 h

Q28. Which of the following best describes the concentration–time profile for a first-order elimination process in pharmacokinetics?

• Linear decline with time
• Exponential decline with time
• Parabolic decline with time
• Instantaneous drop to zero

Correct Answer: Exponential decline with time

Q29. To determine k from experimental data you should:

• Fit [A] vs t to a straight line
• Fit ln[A] vs t to a straight line and take negative slope
• Fit 1/[A] vs t to a straight line and take intercept
• Measure pH changes over time

Correct Answer: Fit ln[A] vs t to a straight line and take negative slope

Q30. Which factor does NOT directly affect the first-order rate constant k for a given reaction?

• Temperature
• Presence of catalyst
• Initial concentration of reactant
• Activation energy

Correct Answer: Initial concentration of reactant