MDCAT Chemistry: Reaction Kinetics MCQs

MDCAT Chemistry: Reaction Kinetics MCQs

1. Question 1

   Q1. A chemist observes a three-step reaction mechanism for the synthesis of a complex molecule. Which step dictates the overall rate of the reaction?

   • A) The fastest elementary step
   • B) The slowest elementary step
   • C) The step with the lowest activation energy
   • D) The step involving the most stable intermediate

   Answer: The slowest elementary step

   Explanation: The slowest step in a multi-step reaction is the rate-determining step. Option A is tempting but fast steps don't limit overall speed.

2. Question 2

   Q2. In an elementary reaction step involving the collision of two molecules, the term 'molecularity' refers specifically to which parameter?

   • A) The total number of molecules in the balanced equation
   • B) The sum of exponents in the rate law
   • C) The number of reacting species in an elementary step
   • D) The experimentally determined power of concentration

   Answer: The number of reacting species in an elementary step

   Explanation: Molecularity is the number of reactant molecules in an elementary step. Option D refers to the experimental order, which may differ.

3. Question 3

   Q3. During a chemical reaction, a species is produced in the first step and consumed in the second step. What is this species called?

   • A) Reaction intermediate
   • B) Homogeneous catalyst
   • C) Activated complex
   • D) Inhibitor

   Answer: Reaction intermediate

   Explanation: Intermediates are produced then consumed; they don't appear in the net equation. Option B describes a catalyst which is regenerated.

4. Question 4

   Q4. For the reaction NO2 + CO -> NO + CO2 at low temperature, the mechanism involves 2NO2 -> NO3 + NO (slow). What is the rate law?

   • A) Rate = k[NO2][CO]
   • B) Rate = k[NO2]
   • C) Rate = k[CO]
   • D) Rate = k[NO2]^2

   Answer: Rate = k[NO2]^2

   Explanation: The rate is determined by the slow step involving two NO2 molecules. Option B fails because CO is not in the rate-determining step.

5. Question 5

   Q5. In the context of collision theory, what represents the configuration of atoms at the peak of the activation energy barrier?

   • A) A stable product of a fast step
   • B) A temporary high-energy state during a collision
   • C) A substance that can be isolated and stored
   • D) The reactant with the highest kinetic energy

   Answer: A temporary high-energy state during a collision

   Explanation: Activated complexes are high-energy, unstable species at the peak of the energy barrier. Option C is wrong because they cannot be isolated.

6. Question 6

   Q6. A student adds a small amount of MnO2 to hydrogen peroxide to speed up its decomposition. How does the catalyst achieve this?

   • A) Decreasing the activation energy
   • B) Increasing the frequency of collisions
   • C) Increasing the heat of reaction
   • D) Decreasing the kinetic energy of reactants

   Answer: Decreasing the activation energy

   Explanation: Catalysts lower activation energy by providing an alternative pathway. Option C is incorrect as catalysts do not change the enthalpy (Delta H).

7. Question 7

   Q7. The mechanism for 2NO2 + F2 -> 2NO2F is: NO2 + F2 -> NO2F + F (slow). What is the order of reaction with respect to F2?

   • A) Second order with respect to NO2
   • B) Zero order with respect to F2
   • C) First order with respect to F2
   • D) Third order overall

   Answer: First order with respect to F2

   Explanation: The slow step involves one molecule of NO2 and one of F2. Option A is tempting but ignores the specific mechanism provided.

8. Question 8

   Q8. Which of these processes demonstrates a mechanism where the catalyst and the reactants exist in the same gaseous phase?

   • A) Pt catalyst in the contact process
   • B) Fe catalyst in Haber's process
   • C) Ni catalyst in hydrogenation of oils
   • D) Nitrogen oxides in the lead chamber process

   Answer: Nitrogen oxides in the lead chamber process

   Explanation: Homogeneous catalysis occurs when the catalyst and reactants are in the same phase. Option A describes heterogeneous catalysis (solid/gas).

9. Question 9

   Q9. In a complex reaction consisting of multiple elementary steps, what is the relationship between the molecularity and the experimentally determined order?

   • A) Molecularity is always equal to the overall order
   • B) Molecularity of the slow step equals the overall order
   • C) Order is a theoretical value while molecularity is experimental
   • D) Molecularity can be zero or a fraction

   Answer: Molecularity of the slow step equals the overall order

   Explanation: The slow step is rate-determining; its molecularity defines the order. Option D is wrong because molecularity applies to elementary steps only.

10. Question 10

    Q10. Which characteristic is unique to the mechanism of enzyme-catalyzed reactions compared to most inorganic catalysts?

    • A) High specificity for substrates
    • B) Inorganic nature and heat stability
    • C) Ability to shift the equilibrium position
    • D) Requirement in large stoichiometric amounts

    Answer: High specificity for substrates

    Explanation: Enzymes are protein catalysts that show extreme specificity. Option B is wrong because enzymes are organic/biological catalysts, not inorganic.

11. Question 11

    Q11. In an energy profile diagram, what does the vertical distance between the energy of reactants and the energy of products represent?

    • A) Activation energy of the forward reaction
    • B) Activation energy of the reverse reaction
    • C) Enthalpy of the reaction
    • D) Potential energy of the activated complex

    Answer: Enthalpy of the reaction

    Explanation: Enthalpy change is the difference between product and reactant energy. Option A is activation energy, which is distinct from net heat.

12. Question 12

    Q12. According to the collision theory, which factor primarily determines the rate of a chemical reaction at a constant temperature?

    • A) The total number of molecules in the container
    • B) The frequency of effective collisions
    • C) The average velocity of all molecules
    • D) The total volume of the reaction mixture

    Answer: The frequency of effective collisions

    Explanation: Reaction rate depends on the number of effective collisions per unit time. Option D is incorrect because not all collisions lead to reaction.

13. Question 13

    Q13. In the contact process, arsenic impurities must be removed to prevent 'catalytic poisoning'. This mechanism involves which of the following?

    • A) Preferential adsorption of impurities on the catalyst surface
    • B) Increasing the surface area of the catalyst
    • C) Changing the physical state of the catalyst
    • D) The reaction of the catalyst with the products

    Answer: Preferential adsorption of impurities on the catalyst surface

    Explanation: Poisoning occurs when the catalyst surface is blocked by impurities. Option B refers to activation, which is the opposite of poisoning.

14. Question 14

    Q14. During the titration of KMnO4 with oxalic acid, the purple color disappears slowly at first but rapidly later. What explains this?

    • A) KMnO4 acts as the catalyst
    • B) Oxalic acid acts as the catalyst
    • C) The reaction is inhibited by acidic conditions
    • D) Mn2+ ions act as an autocatalyst

    Answer: Mn2+ ions act as an autocatalyst

    Explanation: Mn2+ acts as an autocatalyst, speeding up the reaction as it proceeds. Option A is wrong because KMnO4 is the reactant.

15. Question 15

    Q15. When the temperature of a reaction is increased by 10 K, the rate usually doubles. Mechanistically, what is the primary cause?

    • A) The activation energy decreases
    • B) The value of the rate constant (k) increases
    • C) The molecularity of the reaction changes
    • D) The order of the reaction decreases

    Answer: The value of the rate constant (k) increases

    Explanation: The rate constant k increases exponentially with temperature. Option A is wrong because Ea is generally independent of temperature.

16. Question 16

    Q16. If a reaction mechanism involves a step with a very high activation energy (Ea), what can be inferred about that reaction?

    • A) The reaction will be extremely fast
    • B) The reaction will be exothermic
    • C) The reaction will be slow at room temperature
    • D) The reaction will have a high molecularity

    Answer: The reaction will be slow at room temperature

    Explanation: Large Ea means a steep energy barrier and slower rate. Option D is wrong because Ea is the barrier, not the total energy.

17. Question 17

    Q17. Which of the following reaction mechanisms involves heterogeneous catalysis?

    • A) Hydrogenation of unsaturated oils using nickel
    • B) Hydrolysis of sugar in the presence of H2SO4
    • C) Lead chamber process for H2SO4
    • D) Decomposition of ozone in the presence of NO

    Answer: Hydrogenation of unsaturated oils using nickel

    Explanation: Heterogeneous catalysis involves different phases, usually a solid catalyst and gaseous/liquid reactants. Option B is homogeneous.

18. Question 18

    Q18. A reaction is found to have an order of 1.5 with respect to a reactant. What does this suggest about its mechanism?

    • A) The reaction is an elementary step
    • B) The reaction follows a complex multi-step mechanism
    • C) The reaction is zero order with respect to all reactants
    • D) The reaction is independent of temperature

    Answer: The reaction follows a complex multi-step mechanism

    Explanation: Fractional orders indicate complex mechanisms with multiple steps. Option A is wrong as elementary steps always have integer molecularity.

19. Question 19

    Q19. A catalyst is added to a reversible reaction. What is the mechanistic effect on the chemical equilibrium?

    • A) Increased yield of products
    • B) Change in the equilibrium constant value
    • C) Shift of equilibrium to the right
    • D) Faster attainment of the equilibrium state

    Answer: Faster attainment of the equilibrium state

    Explanation: Catalysts do not change the equilibrium constant (Kc), only the rate to reach it. Option B is a common misconception.

20. Question 20

    Q20. In the acid-catalyzed hydrolysis of ethyl acetate, the concentration of water remains virtually constant. What is the order of this reaction?

    • A) Second order
    • B) Zero order
    • C) First order
    • D) Third order

    Answer: First order

    Explanation: Hydrolysis of ethyl acetate by an acid is a first-order reaction. Option A is wrong because water is in excess (pseudo-first order).