Rucete ✏ AP Chemistry In a Nutshell
10. Kinetics — Practice Questions 2
This chapter introduces the factors that influence the rate of chemical reactions, including rate laws, reaction mechanisms, and activation energy.
(Multiple Choice — Click to Reveal Answer)
1. Which of the following does NOT affect the rate of a chemical reaction?
(A) Activation energy
(B) Temperature
(C) Enthalpy change
(D) Catalyst
Answer
(C) — Enthalpy change relates to thermodynamics, not kinetics.
2. In a first-order reaction, the half-life is:
(A) Constant
(B) Dependent on concentration
(C) Dependent on pressure
(D) Inversely proportional to [A]
Answer
(A) — First-order reactions have constant half-life regardless of concentration.
3. What does the area under a Maxwell-Boltzmann distribution curve represent?
(A) Total energy
(B) Total number of molecules
(C) Rate constant
(D) Activation energy
Answer
(B) — The area under the curve represents all particles in the system.
4. What happens to the reaction rate if a catalyst is added to a reaction already at equilibrium?
(A) Increases product formation
(B) Decreases product formation
(C) Increases the rate of both forward and reverse reactions
(D) Shifts equilibrium to the right
Answer
(C) — A catalyst increases the rate at which equilibrium is reached but does not shift it.
5. In a two-step mechanism, the overall rate law is determined by:
(A) The fastest step
(B) The last step
(C) The slowest step
(D) The step involving the catalyst
Answer
(C) — The slowest elementary step is rate-determining.
6. What does a large rate constant (k) suggest about the speed of the reaction?
(A) The reaction is slow
(B) The reaction has high activation energy
(C) The reaction is fast
(D) The temperature is low
Answer
(C) — A large k indicates a fast reaction under given conditions.
7. If a reaction is second-order in A and zero-order in B, the rate law is:
(A) rate = k[A][B]
(B) rate = k[A]²
(C) rate = k[B]²
(D) rate = k[A][B]⁰
Answer
(B) — B has no effect; zero-order means independent of B's concentration.
8. What is the graphical method to identify a zero-order reaction?
(A) Plot of [A] vs. time is linear
(B) Plot of ln[A] vs. time is linear
(C) Plot of 1/[A] vs. time is linear
(D) Plot of rate vs. time is linear
Answer
(A) — A straight [A] vs. time plot indicates zero-order.
9. How do heterogeneous catalysts work?
(A) Increase enthalpy of activation
(B) Form temporary bonds with reactants
(C) Lower entropy of the system
(D) Increase temperature
Answer
(B) — They adsorb reactants onto their surface, weakening bonds.
10. The rate law for a reaction is rate = k[A]². If [A] is tripled, the rate increases by a factor of:
(A) 3
(B) 6
(C) 9
(D) 27
Answer
(C) — (3)² = 9
11. What will happen to the rate of a reaction if the temperature is decreased?
(A) It increases
(B) It remains unchanged
(C) It decreases
(D) It becomes zero
Answer
(C) — Lower temperature reduces the number of molecules with enough energy to overcome activation energy, so the rate decreases.
12. The rate law of a reaction is: rate = k[A][B]. If [A] is doubled and [B] is halved, the new rate will be:
(A) Unchanged
(B) Halved
(C) Doubled
(D) Quartered
Answer
(A) — 2 × 0.5 = 1, so the rate remains the same.
13. In the reaction A → B, which of the following graphs confirms first-order kinetics?
(A) [A] vs. time is linear
(B) ln[A] vs. time is linear
(C) 1/[A] vs. time is linear
(D) Rate vs. [A] is constant
Answer
(B) — ln[A] vs. time linearity confirms first-order behavior.
14. What is the unit of the rate constant for a zero-order reaction?
(A) mol⁻¹·L·s⁻¹
(B) mol·L⁻¹·s⁻¹
(C) mol·L⁻¹
(D) mol·s⁻¹
Answer
(D) — Zero-order rate constants have units of concentration/time (e.g., mol/s).
15. What is the overall order of a reaction with rate = k[A][B]²?
(A) First-order
(B) Second-order
(C) Third-order
(D) Zero-order
Answer
(C) — 1 (from A) + 2 (from B) = 3
16. Which of the following best describes a reaction mechanism?
(A) A balanced chemical equation
(B) A list of possible reactants
(C) A series of elementary steps
(D) A list of catalysts used
Answer
(C) — Reaction mechanisms break a reaction into a series of smaller steps.
17. If the initial rate of a reaction is directly proportional to [A] and [B], the rate law is:
(A) rate = k[A][B]
(B) rate = k[A]²
(C) rate = k[B]²
(D) rate = k
Answer
(A) — Proportional to both means first-order in each reactant.
18. Which of the following factors increases the frequency of effective collisions?
(A) Using a larger container
(B) Lowering temperature
(C) Increasing concentration
(D) Removing a catalyst
Answer
(C) — More particles in the same space increase collision frequency.
19. What happens when a reaction reaches its transition state?
(A) Bonds are completely broken
(B) New bonds are fully formed
(C) The system has maximum potential energy
(D) Reaction has stopped
Answer
(C) — Transition state = highest energy point (activated complex).
20. What is a key assumption in collision theory?
(A) Reactions occur randomly without energy
(B) All collisions lead to reactions
(C) Particles must collide with correct orientation and energy
(D) Temperature has no effect
Answer
(C) — Correct orientation and sufficient energy are both necessary.
21. Which energy profile corresponds to a catalyzed reaction?
(A) Higher peak
(B) Same peak
(C) Lower peak
(D) Flat line
Answer
(C) — Catalysts lower activation energy, reducing the energy peak.
22. What is the relationship between temperature and rate constant k in the Arrhenius equation?
(A) Inversely proportional
(B) No relation
(C) Directly proportional (exponentially)
(D) Square root relationship
Answer
(C) — k increases exponentially with temperature due to e^(-Ea/RT).
23. Which condition would favor the faster reaction rate?
(A) Larger activation energy
(B) Lower temperature
(C) Smaller surface area
(D) Higher pressure (for gases)
Answer
(D) — Higher pressure increases collision frequency for gas-phase reactions.
24. Why does a powdered solid react faster than a chunk of the same substance?
(A) It has higher temperature
(B) It dissolves better
(C) It has a greater surface area
(D) It forms intermediates more easily
Answer
(C) — More surface area allows more contact between reactant particles.
25. What is the term for a species that appears in the mechanism but not in the overall equation?
(A) Product
(B) Intermediate
(C) Catalyst
(D) Reactant
Answer
(B) — Intermediates are produced and consumed within the mechanism only.
26. A reaction has the rate law: rate = k[A][B]². Which change will cause the rate to increase by a factor of 8?
(A) Double [A], triple [B]
(B) Triple [A], double [B]
(C) Double both [A] and [B]
(D) Triple both [A] and [B]
Answer
(C) — Rate = k[A][B]² → (2)[2]² = 8
27. For a second-order reaction with respect to [A], what happens to the rate if [A] is reduced to 1/2?
(A) Halved
(B) Unchanged
(C) Doubled
(D) One-fourth
Answer
(D) — Rate ∝ [A]², so (½)² = ¼
28. What is the purpose of determining a reaction’s rate-determining step?
(A) To calculate ΔH
(B) To write the balanced equation
(C) To establish the rate law
(D) To predict products
Answer
(C) — The slowest step governs the overall rate law.
29. For a reaction mechanism, what must be true for the proposed mechanism to be valid?
(A) All steps must be exothermic
(B) The sum of elementary steps equals the overall reaction
(C) The rate law must include all intermediates
(D) Activation energy must be constant
Answer
(B) — The elementary steps must add to the overall balanced equation.
30. In an experiment, a reaction was found to be first-order with a rate constant k = 0.693 s⁻¹. What is the half-life?
(A) 0.693 s
(B) 1 s
(C) 2 s
(D) 0.347 s
Answer
(A) — For first-order: t½ = 0.693 / k = 1 s
31. Which of the following could indicate a reaction is multi-step?
(A) It occurs slowly
(B) The rate law includes an intermediate
(C) The rate law includes reactants not in the overall equation
(D) The activation energy is low
Answer
(C) — Presence of species in rate law not in overall equation suggests a multi-step mechanism.
32. In the Arrhenius equation, what does the “A” term represent?
(A) Activation energy
(B) Temperature
(C) Orientation factor and frequency of collisions
(D) Enthalpy change
Answer
(C) — “A” is the frequency factor, accounting for proper collisions.
33. If a catalyst is used in a reaction, which of the following is true?
(A) The equilibrium shifts toward products
(B) ΔH increases
(C) Ea decreases
(D) The reaction rate remains unchanged
Answer
(C) — Catalysts lower the activation energy, speeding up the reaction.
34. A student plots ln[A] versus time and gets a straight line. What is the order of the reaction with respect to A?
(A) Zero-order
(B) First-order
(C) Second-order
(D) Third-order
Answer
(B) — ln[A] vs. time = first-order reaction.
35. Which of the following changes will NOT affect the value of the rate constant, k?
(A) Temperature
(B) Catalyst
(C) Reactant concentration
(D) Activation energy
Answer
(C) — k is constant for a given reaction at a fixed temperature; concentration affects rate, not k.
36. Define the term "reaction mechanism" in the context of chemical kinetics.
Answer
A reaction mechanism is a series of elementary steps that represent the pathway from reactants to products in a chemical reaction.
37. What is meant by the term “rate-determining step”?
Answer
The rate-determining step is the slowest step in a reaction mechanism, which limits the overall reaction rate.
38. Describe how a catalyst affects the activation energy and rate of a chemical reaction.
Answer
A catalyst lowers the activation energy by providing an alternative pathway, thereby increasing the reaction rate.
39. Why is the orientation of colliding molecules important in a chemical reaction?
Answer
Only collisions with correct orientation allow bonds to break and new ones to form, leading to a successful reaction.
40. How can you experimentally determine the rate law of a reaction?
Answer
By measuring the initial reaction rates at different concentrations and analyzing how rate changes with each reactant's concentration.
41. What is the significance of the Arrhenius equation in kinetics?
Answer
It relates the rate constant (k) to temperature and activation energy, helping to understand how reaction rates change with temperature.
42. How does surface area affect the rate of a heterogeneous reaction?
Answer
Increasing surface area increases the number of available sites for reaction, leading to more collisions and a higher rate.
43. Explain why the half-life of a first-order reaction is constant.
Answer
In a first-order reaction, the rate depends only on the current concentration, so the time to halve the concentration remains constant.
44. What units does the rate constant (k) have in a second-order reaction?
Answer
L·mol⁻¹·s⁻¹
45. How does temperature influence the Maxwell-Boltzmann distribution?
Answer
Higher temperatures broaden the distribution and shift it toward higher energies, increasing the number of particles with sufficient energy to react.
46. Give one example of an experimental method to measure the rate of reaction.
Answer
Measuring the change in gas volume over time using a gas syringe.
47. What happens to the rate of a zero-order reaction as the concentration decreases?
Answer
The rate remains constant because zero-order reactions are independent of reactant concentration.
48. What is an intermediate in a reaction mechanism?
Answer
An intermediate is a species formed in one step of a mechanism and consumed in a later step; it does not appear in the overall equation.
49. Explain the difference between instantaneous rate and average rate.
Answer
Instantaneous rate is the rate at a specific moment, while average rate is calculated over a time interval.
50. Describe how a rate-concentration graph can help determine reaction order.
Answer
The shape of the graph (linear, curved, etc.) reveals the order: zero-order (flat line), first-order (linear), or second-order (curve).