JSMU Karachi Entry Test Chemistry Reaction Kinetics — Set 2

1. Question 1

   Q1. The rate of a chemical reaction is typically measured by monitoring the change in concentration of a reactant or product over what?

   • A) Time
   • B) Temperature
   • C) Pressure
   • D) Volume

   Answer: Time

   Explanation: Reaction rate is measured by change in concentration over time. Temperature, pressure, and volume are related but not the direct measure.

2. Question 2

   Q2. For a first-order reaction, the half-life is 20 minutes. What is the rate constant (in min^-1)?

   • A) 0.0347
   • B) 0.693
   • C) 0.0200
   • D) 0.347

   Answer: 0.0347

   Explanation: For a first-order reaction, t1/2 = 0.693/k. Thus, k = 0.693/20 = 0.0347. Option B is the value of 0.693, not k.

3. Question 3

   Q3. The order of a reaction is determined by the

   • A) Molecularity of the reaction
   • B) Mechanism of the reaction
   • C) Rate law of the reaction
   • D) Stoichiometry of the reaction

   Answer: Rate law of the reaction

   Explanation: The order is determined by the rate law, not molecularity, mechanism, or stoichiometry directly.

4. Question 4

   Q4. The rate-determining step in a reaction mechanism is the

   • A) Fastest step
   • B) Slowest step
   • C) First step
   • D) Last step

   Answer: Slowest step

   Explanation: The slowest step determines the overall rate of the reaction, hence it's the rate-determining step.

5. Question 5

   Q5. Catalysis involves a substance that

   • A) Increases the activation energy
   • B) Decreases the activation energy
   • C) Changes the equilibrium constant
   • D) Does not affect the reaction rate

   Answer: Decreases the activation energy

   Explanation: A catalyst lowers the activation energy, thus speeding up the reaction. It doesn't change the equilibrium constant.

6. Question 6

   Q6. For the reaction 2N2O5 -> 4NO2 + O2, if the rate of disappearance of N2O5 is 0.020 M/s, what is the rate of appearance of NO2?

   • A) 0.020 M/s
   • B) 0.040 M/s
   • C) 0.010 M/s
   • D) 0.080 M/s

   Answer: 0.040 M/s

   Explanation: Rate = -1/2(d[N2O5]/dt) = 1/4(d[NO2]/dt). Thus, d[NO2]/dt = 2 * 0.020 = 0.040 M/s.

7. Question 7

   Q7. The half-life of a second-order reaction depends on the

   • A) Initial concentration of the reactant
   • B) Rate constant of the reaction
   • C) Both initial concentration and rate constant
   • D) Neither initial concentration nor rate constant

   Answer: Both initial concentration and rate constant

   Explanation: For a second-order reaction, t1/2 = 1/(k[A]0), so it depends on both k and [A]0.

8. Question 8

   Q8. The activation energy of a reaction can be determined using the

   • A) Arrhenius equation
   • B) Van 't Hoff equation
   • C) Michaelis-Menten equation
   • D) Rate law

   Answer: Arrhenius equation

   Explanation: The Arrhenius equation relates the rate constant to temperature and activation energy.

9. Question 9

   Q9. For a zero-order reaction, the rate of reaction is

   • A) Directly proportional to the reactant concentration
   • B) Inversely proportional to the reactant concentration
   • C) Independent of the reactant concentration
   • D) Proportional to the square of the reactant concentration

   Answer: Independent of the reactant concentration

   Explanation: In a zero-order reaction, the rate is constant and doesn't depend on the reactant concentration.

10. Question 10

    Q10. The effect of temperature on the rate of a chemical reaction is given by the

    • A) Arrhenius equation
    • B) Rate law
    • C) Van 't Hoff equation
    • D) Henderson-Hasselbalch equation

    Answer: Arrhenius equation

    Explanation: The Arrhenius equation describes how the rate constant changes with temperature.

11. Question 11

    Q11. A certain reaction is first order with respect to reactant A and second order with respect to reactant B. If the concentration of A is doubled and that of B is tripled, the rate of reaction will increase 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)(9)[A][B]^2 = 18k[A][B]^2, so it increases by a factor of 18.

12. Question 12

    Q12. The rate constant for a reaction is 0.05 s^-1 at 25°C. What is the half-life of this reaction?

    • A) 13.86 s
    • B) 20 s
    • C) 10 s
    • D) 6.93 s

    Answer: 13.86 s

    Explanation: For a first-order reaction, t1/2 = 0.693/k = 0.693/0.05 = 13.86 s.

13. Question 13

    Q13. The molecularity of a reaction is defined as the number of

    • A) Molecules of reactants that participate in a single step of the reaction mechanism
    • B) Molecules of products formed
    • C) Reactant molecules that collide
    • D) Molecules in the balanced chemical equation

    Answer: Molecules of reactants that participate in a single step of the reaction mechanism

    Explanation: Molecularity refers to the number of molecules involved in an elementary step.

14. Question 14

    Q14. For a given reaction, the rate constant at 300 K is 0.01 s^-1 and at 310 K is 0.02 s^-1. What is the activation energy (approximately)?

    • A) 53.6 kJ/mol
    • B) 50 kJ/mol
    • C) 60 kJ/mol
    • D) 70 kJ/mol

    Answer: 53.6 kJ/mol

    Explanation: Using the Arrhenius equation and simplifying gives Ea = R*ln(0.02/0.01)/(1/300 - 1/310) ≈ 53.6 kJ/mol.

15. Question 15

    Q15. The collision theory explains that the rate of a reaction depends on

    • A) The frequency of collisions and the fraction of effective collisions
    • B) The concentration of reactants only
    • C) The temperature only
    • D) The surface area only

    Answer: The frequency of collisions and the fraction of effective collisions

    Explanation: Collision theory states that reaction rate depends on collision frequency and the fraction of collisions with sufficient energy and proper orientation.

16. Question 16

    Q16. The rate of a reaction is affected by a catalyst because it

    • A) Changes the equilibrium constant
    • B) Increases the activation energy
    • C) Decreases the activation energy
    • D) Changes the reaction mechanism completely

    Answer: Decreases the activation energy

    Explanation: A catalyst works by lowering the activation energy, thus speeding up the reaction.

17. Question 17

    Q17. For the reaction A + B -> C, the rate law is rate = k[A][B]. If the initial concentrations of A and B are 0.1 M and 0.2 M, respectively, and k = 0.05 M^-1s^-1, what is the initial rate of reaction?

    • A) 0.001 M/s
    • B) 0.01 M/s
    • C) 0.0001 M/s
    • D) 0.1 M/s

    Answer: 0.001 M/s

    Explanation: Initial rate = k[A][B] = 0.05 * 0.1 * 0.2 = 0.001 M/s.

18. Question 18

    Q18. The half-life of a radioactive substance is a characteristic property and is

    • A) Independent of the initial amount of substance
    • B) Dependent on the initial amount of substance
    • C) Affected by temperature
    • D) Affected by pressure

    Answer: Independent of the initial amount of substance

    Explanation: The half-life of a radioactive substance is a constant, characteristic property, unaffected by initial amount, temperature, or pressure.

19. Question 19

    Q19. The reaction 2NO + O2 -> 2NO2 is third order overall. If the rate law is rate = k[NO]^2[O2], the order with respect to NO is

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

    Answer: 2

    Explanation: The rate law shows the reaction is second order with respect to NO.

20. Question 20

    Q20. For the reaction 2NO + O2 → 2NO2, if the rate of disappearance of NO is 0.036 M/s, what is the rate of appearance of NO2?

    • A) 0.018 M/s
    • B) 0.036 M/s
    • C) 0.072 M/s
    • D) 0.012 M/s

    Answer: 0.036 M/s

    Explanation: Rate of appearance of NO2 equals the rate of disappearance of NO due to 1:1 stoichiometric ratio.