Chemical Kinetics — Advanced Notes & 50 MCQs

1. The rate of a chemical reaction is defined as:

1. Total heat released per second
2. Change in concentration of reactant or product per unit time ✅
3. Change in pressure only
4. Change in volume only

2. In rate law rate = k[A]^2[B], overall order of reaction is:

1. 1
2. 3 ✅
3. 2
4. 4

3. Molecularity refers to:

1. Order observed experimentally
2. Number of species colliding in an elementary step ✅
3. Overall order of multistep reaction
4. Temperature dependence only

4. A first-order reaction has a half-life that is:

1. Dependent on initial concentration
2. Constant and independent of initial concentration ✅
3. Proportional to square of initial concentration
4. Infinite

5. For a second-order reaction 2A → products, integrated rate law is:

1. [A]_t = [A]_0 - kt
2. 1/[A]_t = 1/[A]_0 + kt ✅
3. ln[A]_t = ln[A]_0 - kt
4. [A]_t = [A]_0 e^{-kt}

6. The Arrhenius equation relates rate constant k to:

1. Activation energy only
2. Temperature and activation energy ✅
3. Pressure only
4. Concentration only

7. Activation energy E_a is:

1. Energy released during reaction
2. Minimum energy required for reactant molecules to form products ✅
3. Always negative
4. Independent of temperature

8. If plot of ln k vs 1/T is a straight line, the slope equals:

1. A (pre-exponential factor)
2. -E_a/R ✅
3. +E_a/R
4. 0

9. For the reaction A → products, rate = k[A]^0.5. The order is:

1. Zero
2. Half-order ✅
3. First-order
4. Negative order

10. Collision theory states reaction rate depends on:

1. Only concentration of products
2. Number of effective collisions per unit time ✅
3. Only pressure
4. Color of reactants

11. A catalyst speeds up a reaction by:

1. Changing equilibrium constant
2. Lowering activation energy and providing alternate pathway ✅
3. Changing stoichiometry
4. Altering initial concentrations only

12. In a reaction where rate doubles when concentration of A doubles, the order with respect to A is:

1. 0
2. 1 ✅
3. 2
4. Half-order

13. Which experimental method is commonly used to determine rate law?

1. Bragg's diffraction
2. Method of initial rates ✅
3. Titration of final mixture only
4. Melting point analysis

14. For first-order reaction, a plot of ln[A]_t vs t is:

1. Parabolic
2. Linear with slope -k ✅
3. Exponential growth
4. Horizontal line

15. The pre-exponential factor A in Arrhenius equation represents:

1. Activation energy
2. Frequency of collisions with correct orientation ✅
3. Temperature dependence only
4. None of the above

16. If rate is independent of concentration of reactant, the reaction is:

1. First-order
2. Zero-order ✅
3. Second-order
4. Fractional-order

17. The units of rate constant k for a second-order reaction are:

1. s^-1
2. M^-1 s^-1 (or concentration^-1 time^-1) ✅
3. M s^-1
4. Unitless

18. Which theory explains reaction rates by considering activated complex and its equilibrium with reactants?

1. Collision theory only
2. Transition state (activated complex) theory ✅
3. Bohr theory
4. Rayleigh scattering

19. Temperature increase generally affects reaction rate by:

1. Decreasing k
2. Increasing k because more molecules exceed E_a ✅
3. No change
4. Changing order only

20. A chain reaction often involves which characteristic step?

1. Only unimolecular decomposition
2. Propagation steps where intermediates regenerate reactive species ✅
3. Only termination by catalysts
4. No intermediate formation

21. For reaction A + B → products, experimentally rate ∝ [A]^0[B]^1. This means rate depends on:

1. Only A concentration
2. Only B concentration ✅
3. Both A and B equally
4. Neither A nor B

22. In an elementary bimolecular reaction, molecularity is:

1. Zero
2. Two ✅
3. One
4. Three

23. A pseudo-first-order reaction occurs when:

1. All reactant concentrations change equally
2. One reactant is in large excess so its concentration is effectively constant ✅
3. Temperature is constant only
4. No reactant is in excess

24. The half-life of a zero-order reaction depends on:

1. Only temperature
2. Initial concentration ([A]_0) and rate constant (t_1/2 = [A]_0 / 2k) ✅
3. Only activation energy
4. Independent of [A]_0

25. Which experimental plot would you use to confirm a first-order reaction?

1. [A]_t vs t linear
2. ln[A]_t vs t linear ✅
3. 1/[A]_t vs t linear
4. t vs 1/[A]_t quadratic

26. The effect of pressure on reaction rate is most significant for reactions involving:

1. Only liquids
2. Gases (changing concentration by changing pressure) ✅
3. Only solids
4. All phases equally

27. Which statement is TRUE about multi-step mechanisms?

1. Rate law always follows stoichiometry of overall reaction
2. Rate law is determined by rate-determining (slow) step ✅
3. Mechanism cannot be inferred from rate law
4. Activation energy is same for all steps

28. Which of the following can act as homogeneous catalyst?

1. Solid platinum surface
2. An acid in solution that reacts with reactants and is regenerated ✅
3. Inert gas only
4. All solids only

29. If doubling temperature roughly doubles the rate of reaction, activation energy is likely:

1. Very high (>200 kJ/mol)
2. Moderate (depends, but small Ea gives less sensitivity) ✅
3. Negative always
4. Zero

30. Which method helps measure initial rate experimentally?

1. Monitor concentration change after long time only
2. Measure slope of concentration vs time at t≈0 ✅
3. Measure final product only
4. Use melting point depression

31. In an elementary termolecular step, molecularity is:

1. 1
2. 3 ✅
3. 2
4. Cannot be determined

32. Which parameter does not appear in rate law?

1. Concentrations of reactants
2. Stoichiometric coefficients (not necessarily) ✅
3. Rate constant k
4. Order with respect to each reactant

33. Which is a heterogeneous catalyst example?

1. Acid in homogeneous solution
2. Solid Pt surface catalyzing H2 + O2 reaction ✅
3. Enzyme in solution only
4. None of the above

34. The temperature coefficient (Q10) represents:

1. The change in rate for 1°C increase only
2. Factor by which rate increases for 10°C rise ✅
3. Change in activation energy per 10°C
4. Rate decrease per 10°C

35. Which plot would help determine order and rate constant for second-order reaction?

1. ln[A]_t vs t
2. 1/[A]_t vs t linear ✅
3. [A]_t vs t linear
4. 1/ln[A]_t vs t

36. In enzyme catalysis, the substrate binds to:

1. Solvent only
2. Active site of enzyme ✅
3. Product site only
4. None of the above

37. Which factor can change the value of rate constant k?

1. Concentration of reactants
2. Temperature ✅
3. Time only
4. Stoichiometric coefficients

38. The rate-determining step in a mechanism is:

1. Usually the fastest step
2. The slowest step that controls overall rate ✅
3. Always the first step
4. Never an elementary step

39. Which statement about elementary reactions is TRUE?

1. They always involve more than three molecules
2. They occur in a single step with molecularity equal to number of colliding species ✅
3. They cannot be bimolecular
4. Order cannot be determined from molecularity

40. A negative order reaction indicates:

1. Reaction rate increases with concentration
2. Rate decreases with increase in concentration of that reactant ✅
3. No reaction occurs
4. Impossible physically

41. Which experimental evidence supports a multi-step mechanism?

1. Rate law matches overall stoichiometry
2. Observed intermediates and rate law inconsistent with one-step mechanism ✅
3. Only equilibrium constant value
4. Only color change at end

42. The term 'steady state approximation' assumes:

1. Intermediate concentration remains constant during reaction ✅
2. Reactant concentration is zero
3. Products are immediately removed
4. Temperature is constant only

43. Which is true for a reaction with very large activation energy?

1. Reaction rate is very insensitive to temperature
2. Reaction rate is highly sensitive to temperature changes ✅
3. Rate constant k is very large at all temperatures
4. Reaction is instantaneous

44. In catalyst poisoning, catalyst activity is reduced because:

1. Catalyst dissolves completely
2. Impurities bind to active sites preventing reactant access ✅
3. Temperature drops drastically
4. Mass of catalyst increases only

45. The term 'chain branching' refers to:

1. Termination steps only
2. Steps where reactive intermediates produce more than one radical leading to explosion-like rates ✅
3. No propagation steps present
4. Only catalytic steps

46. If rate constant k doubles when temperature increases by 10°C, the Q10 value is approximately:

1. 1
2. 2 ✅
3. 10
4. 0.1

47. Which of the following increases the frequency factor A in Arrhenius equation?

1. Lower collision frequency
2. Better orientation and more frequent collisions ✅
3. Higher activation energy only
4. Lower temperature

48. Which order reaction has rate constant units of s^-1?

1. Zero-order
2. First-order ✅
3. Second-order
4. Third-order

49. The integrated rate law for zero-order reaction is:

1. ln[A]_t = ln[A]_0 - kt
2. [A]_t = [A]_0 - kt ✅
3. 1/[A]_t = 1/[A]_0 + kt
4. [A]_t = [A]_0 e^{-kt}