FSc Pre-Engineering Chemistry: Reaction Kinetics MCQs

Practice Reaction Kinetics MCQs for FSc Pre-Engineering Chemistry — topic-wise sets with solved answers.

FSc Pre-Engineering Chemistry: Reaction Kinetics MCQs — sample questions

  1. Question 1

    Q1. For a first-order reaction, the rate constant is 0.01 s^-1. What is the half-life?

    • A) 69.3 s
    • B) 100 s
    • C) 50 s
    • D) 200 s

    Answer: 69.3 s

    Explanation: For a first-order reaction, t1/2 = 0.693 / k = 0.693 / 0.01 = 69.3 s, using the formula t1/2 = ln(2) / k.

  2. Question 2

    Q2. The rate of reaction A + B -> products is given by rate = k[A]²[B]. The order of reaction is

    • A) 1
    • B) 2
    • C) 3
    • D) 0

    Answer: 3

    Explanation: Order is the sum of exponents in the rate law, here 2 + 1 = 3, so the order is 3.

  3. Question 3

    Q3. For the reaction 2NO + O2 -> 2NO2, the rate law is rate = k[NO]²[O2]. If [NO] is doubled and [O2] is halved, the rate

    • A) doubles
    • B) halves
    • C) remains same
    • D) becomes 2 times

    Answer: becomes 2 times

    Explanation: New rate = k(2[NO])²([O2]/2) = k(4[NO]²)([O2]/2) = 2k[NO]²[O2], so rate becomes 2 times the original.

  4. Question 4

    Q4. The unit of rate constant for a zero-order reaction is

    • A) s^-1
    • B) mol L^-1 s^-1
    • C) L mol^-1 s^-1
    • D) mol² L^-2 s^-1

    Answer: mol L^-1 s^-1

    Explanation: For zero-order, rate = k, so k has units of rate, which is concentration/time, i.e., mol L^-1 s^-1.

  5. Question 5

    Q5. The half-life of a radioactive substance is 20 years. What fraction remains after 1 / 2 40 years?

    • A) 1/4
    • B) 1/2
    • C) 1/8
    • D) 1/16

    Answer: 1/16

    Explanation: After 20 years, 1/2 remains; after 40 years, 1/4; after 60 years, 1/8; after 80 years (2 * 40 years), 1/16 remains.

  6. Question 6

    Q6. For a reaction with rate law rate = k[A][B]², if [A] is doubled and [B] is tripled, the rate increases by a factor of

    • A) 6
    • B) 12
    • C) 18
    • D) 24

    Answer: 18

    Explanation: New rate = k(2[A])(3[B])² = k(2[A])(9[B]²) = 18k[A][B]², so rate increases by 18 times.

  7. Question 7

    Q7. The rate constant of a reaction is 0.05 s^-1 at 25°C and 0.15 s^-1 at 35°C. The activation energy is

    • A) 50 kJ/mol
    • B) 60.3 kJ/mol
    • C) 70 kJ/mol
    • D) 80 kJ/mol

    Answer: 60.3 kJ/mol

    Explanation: Using Arrhenius equation, ln(k2/k1) = (Ea/R)(1/T1 - 1/T2), we get Ea = 60.3 kJ/mol, with R = 8.314 J/(mol K).

  8. Question 8

    Q8. The reaction A -> B is first-order with k = 0.02 min^-1. If [A]0 = 1 M, [A] at t = 50 min is

    • A) 0.368 M
    • B) 0.135 M
    • C) 0.368 M (2)
    • D) 0.5 M

    Answer: 0.368 M

    Explanation: For first-order, [A] = [A]0 * e^(-kt) = 1 * e^(-0.02*50) = e^(-1) = 0.368 M.

  9. Question 9

    Q9. For a second-order reaction, the half-life is

    • A) independent of initial concentration
    • B) directly proportional to initial concentration
    • C) inversely proportional to initial concentration
    • D) directly proportional to square of initial concentration

    Answer: inversely proportional to initial concentration

    Explanation: For second-order, t1/2 = 1/(k[A]0), so t1/2 is inversely proportional to [A]0.

  10. Question 10

    Q10. The rate of reaction 2A + B -> products is given by rate = k[A][B]. If [A] = [B] = 0.1 M, the rate is 0.01 M/s. The rate constant k is

    • A) 1 M^-1 s^-1
    • B) 0.1 M^-1 s^-1
    • C) 10 M^-1 s^-1
    • D) 0.01 M^-1 s^-1

    Answer: 1 M^-1 s^-1

    Explanation: k = rate / ([A][B]) = 0.01 / (0.1 * 0.1) = 1 M^-1 s^-1.

  11. Question 11

    Q11. The activation energy for a reaction is 80 kJ/mol. At what temperature will the rate be double at 300 K?

    • A) 310 K
    • B) 320 K
    • C) 330 K
    • D) 340 K

    Answer: 310 K

    Explanation: Using Arrhenius equation and given that k2 = 2k1, we can find T2 = 310 K, with Ea = 80 kJ/mol and T1 = 300 K.

  12. Question 12

    Q12. For the reaction A + 2B -> C, the rate law is rate = k[A]². The order with respect to B is

    • A) 0
    • B) 1
    • C) 2
    • D) 3

    Answer: 0

    Explanation: The rate law does not contain [B], so the order with respect to B is 0.

  13. Question 13

    Q13. The decomposition of N2O5 is a first-order reaction with k = 0.001 s^-1 at 25°C. The half-life is

    • A) 693 s
    • B) 1000 s
    • C) 500 s
    • D) 2000 s

    Answer: 693 s

    Explanation: For first-order, t1/2 = 0.693 / k = 0.693 / 0.001 = 693 s.

  14. Question 14

    Q14. For a reaction with rate law rate = k[A]²[B], if [A] and [B] are both doubled, the rate increases by a factor of

    • A) 4
    • B) 6
    • C) 8
    • D) 12

    Answer: 8

    Explanation: New rate = k(2[A])²(2[B]) = k(4[A]²)(2[B]) = 8k[A]²[B], so rate increases by 8 times.

  15. Question 15

    Q15. The unit of rate constant for a third-order reaction is

    • A) L² mol^-2 s^-1
    • B) L mol^-1 s^-1
    • C) mol L^-1 s^-1
    • D) s^-1

    Answer: L² mol^-2 s^-1

    Explanation: For third-order, rate = k[A]³, so k has units of L² mol^-2 s^-1, derived from rate / [A]³.

  16. Question 16

    Q16. For the reaction 2NO2 -> 2NO + O2, the rate law is rate = k[NO2]². The order is

    • A) 1
    • B) 2
    • C) 3
    • D) 0

    Answer: 2

    Explanation: The order is the exponent in the rate law, here 2, so the order is 2.

  17. Question 17

    Q17. The rate constant of a reaction at 25°C is 0.1 s^-1. The activation energy is 50 kJ/mol. The rate constant at 35°C is

    • A) 0.15 s^-1
    • B) 0.2 s^-1
    • C) 0.3 s^-1
    • D) 0.4 s^-1

    Answer: 0.2 s^-1

    Explanation: Using Arrhenius equation, we can find k2 = 0.2 s^-1, with Ea = 50 kJ/mol, T1 = 298 K, and T2 = 308 K.

  18. Question 18

    Q18. The half-life of a first-order reaction is 100 s. The rate constant k is

    • A) 0.00693 s^-1
    • B) 0.0693 s^-1
    • C) 0.693 s^-1
    • D) 6.93 s^-1

    Answer: 0.00693 s^-1

    Explanation: For first-order, k = 0.693 / t1/2 = 0.693 / 100 = 0.00693 s^-1.

  19. Question 19

    Q19. For a reaction A -> B, the rate law is rate = k[A]. If [A]0 = 1 M and k = 0.1 min^-1, [A] after 10 min is

    • A) 0.368 M
    • B) 0.135 M
    • C) 0.5 M
    • D) 0.1 M

    Answer: 0.368 M

    Explanation: For first-order, [A] = [A]0 * e^(-kt) = 1 * e^(-0.1*10) = e^(-1) = 0.368 M.

  20. Question 20

    Q20. The rate of reaction A -> B is given by rate = k[A]^n. If the rate is doubled when [A] is doubled, the order n is

    • A) 1
    • B) 2
    • C) 0
    • D) 3

    Answer: 1

    Explanation: For rate = k[A]^n, if doubling [A] doubles the rate, then n = 1, since 2 = 2^n implies n = 1.

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