Rucete ✏ AP Chemistry In a Nutshell
10. Kinetics — Practice Questions
This chapter introduces the factors that affect reaction rates, how to determine rate laws, and the role of activation energy, temperature, and catalysts in chemical kinetics.
(Multiple Choice — Click to Reveal Answer)
1. Why does an increase in temperature increase the rate of a reaction?
(A) The activation energy of the reaction increases.
(B) The temperature acts as a catalyst in the reaction.
(C) There are more collisions per second that will have the proper energy to exceed the activation energy.
(D) There are a greater proportion of collisions with the correct orientation to be effective.
Answer
(C) — Increasing temperature raises the kinetic energy, leading to more effective collisions that can overcome activation energy.
2. A change in temperature of 10°C was found to triple the rate of a chemical reaction. What does this imply about the reacting molecules?
(A) The average velocity of the molecules triples.
(B) The number of collisions per second triples.
(C) The number of molecules above the reaction energy threshold triples.
(D) The average energy of the molecules triples.
Answer
(C) — More molecules have energy exceeding the activation energy barrier at higher temperatures.
3. How do reaction intermediates differ from activated complexes?
(A) Intermediates have characteristics of both reactants and products.
(B) Intermediates are unstable and can never be isolated.
(C) Intermediates contain normal bonds and may occasionally be isolated.
(D) All reactions involve intermediates, but not all have activated complexes.
Answer
(C) — Intermediates are actual species that can sometimes be isolated, unlike the short-lived activated complex.
4. The rate of a chemical reaction is related to:
(A) Activation energy and potential energy curve
(B) Orientation
(C) Activation energy, orientation, and frequency
(D) Frequency
Answer
(C) — All three factors — activation energy, orientation, and frequency of collisions — influence the reaction rate.
5. What is the overall reaction for the mechanism involving Cl₂ and CHCl₃?
(A) Cl(g) + CHCl₃(g) → CCl₃(g) + HCl(g)
(B) Cl₂(g) + CHCl₃(g) → HCl(g) + CCl₄(g)
(C) CCl₃(g) + Cl(g) → CCl₄(g)
(D) Cl₂(g) + CHCl₃(g) + CCl₃(g) → HCl(g) + CCl₄(g)
Answer
(B) — Add all elementary steps and cancel intermediates to get the overall reaction.
6. Which rate law is consistent with the mechanism involving Cl₂ and CHCl₃?
(A) Rate = k[CHCl₃][Cl]
(B) Rate = k[CCl₃][Cl]
(C) Rate = k[Cl₂]
(D) Rate = k[Cl₂][CHCl₃]
Answer
(A) — The rate-determining step determines the rate law, which includes CHCl₃ and Cl.
7. In a reaction energy profile, what does point C represent?
(A) Potential energy of reactants
(B) Activation energy
(C) Activated complex
(D) Enthalpy change
Answer
(C) — Point C is the peak of the energy diagram and corresponds to the activated complex.
8. In the reaction profile, if the forward Ea is less than the reverse Ea and the reaction is exothermic, which is correct?
(A) Forward Ea > reverse Ea and ΔH is exothermic
(B) Reverse Ea > forward Ea and ΔH is endothermic
(C) Forward Ea < reverse Ea and ΔH is exothermic
(D) Reverse Ea < forward Ea and ΔH is endothermic
Answer
(C) — In exothermic reactions, products are lower in energy, and forward Ea is usually smaller than reverse Ea.
9. A catalyst affects which part of the energy diagram?
(A) Reactant energy
(B) Product energy
(C) Activation energy
(D) ΔH
Answer
(C) — Catalysts lower the activation energy, making it easier for the reaction to proceed.
10. If flask B has half the number of molecules as flask A, how does the rate compare?
(A) Rate of A is twice B
(B) Rate of B is same as A
(C) Rate of A is half of B
(D) Rate of B is half of A
Answer
(D) — Fewer molecules means fewer effective collisions, resulting in a slower reaction rate.
11. Which of the following would increase the rate of a reaction?
(A) Decreasing temperature
(B) Diluting the reactants
(C) Adding a catalyst
(D) Removing a product
Answer
(C) — Catalysts lower activation energy and speed up both forward and reverse reactions.
12. Which is true of a reaction mechanism?
(A) It is always a single step.
(B) It must match the overall stoichiometry.
(C) It cannot include intermediates.
(D) It cannot be observed experimentally.
Answer
(B) — The sum of all elementary steps in a mechanism must match the balanced overall equation.
13. What defines the rate-determining step in a reaction mechanism?
(A) The step with the lowest activation energy
(B) The step with the fastest rate
(C) The step with the highest activation energy
(D) The final step in the mechanism
Answer
(C) — The slowest step with the highest activation energy determines the rate of the reaction.
14. Which factor does NOT influence the rate of a chemical reaction?
(A) Temperature
(B) Pressure (for gases)
(C) Surface area (for solids)
(D) Color of the container
Answer
(D) — Container color has no impact on the speed of a chemical reaction.
15. What is the initial rate of a reaction?
(A) The speed when the reaction is complete
(B) The average speed over the reaction
(C) The rate at the very start before significant change in concentration
(D) The rate after equilibrium is reached
Answer
(C) — Initial rate is measured early on to reflect the most accurate rate unaffected by reverse reactions.
16. How does concentration affect reaction rate?
(A) It only affects equilibrium, not rate
(B) Higher concentrations result in more frequent collisions
(C) It lowers activation energy
(D) It has no effect on gaseous reactions
Answer
(B) — More particles mean more collisions, which increases the rate.
17. What is true about a first-order reaction?
(A) Rate is proportional to [A]²
(B) Rate is constant
(C) Rate is proportional to [A]
(D) Rate is independent of [A]
Answer
(C) — In a first-order reaction, rate = k[A]
18. If doubling [A] doubles the rate, the reaction is:
(A) First order with respect to A
(B) Second order with respect to A
(C) Zero order with respect to A
(D) Third order with respect to A
Answer
(A) — A linear relationship between rate and [A] indicates first-order behavior.
19. What are the units of the rate constant (k) for a first-order reaction?
(A) mol/L·s
(B) s⁻¹
(C) L/mol·s
(D) mol²/L²·s
Answer
(B) — First-order reactions have rate constants with units of s⁻¹.
20. A catalyst in a reaction provides:
(A) More energy to the molecules
(B) A new pathway with lower activation energy
(C) A decrease in the enthalpy of products
(D) A higher equilibrium constant
Answer
(B) — Catalysts offer an alternative pathway with lower activation energy.
21. The collision theory of reaction rates involves all EXCEPT:
(A) Proper orientation
(B) Sufficient energy
(C) Reactant phase change
(D) Particle collisions
Answer
(C) — Phase change isn’t required in collision theory; energy and orientation are.
22. For a reaction A → B, what does a straight-line plot of ln[A] vs. time suggest?
(A) Zero-order
(B) First-order
(C) Second-order
(D) Third-order
Answer
(B) — A linear ln[A] vs. time graph indicates a first-order reaction.
23. Which step in a mechanism is most affected by the presence of a catalyst?
(A) Fast steps
(B) Equilibrium step
(C) Rate-determining step
(D) Final step
Answer
(C) — Catalysts work to lower the activation energy of the slowest, rate-determining step.
24. What is an intermediate in a reaction mechanism?
(A) A reactant
(B) A product
(C) A catalyst
(D) A species formed and consumed during the reaction
Answer
(D) — Intermediates are transient species that appear in the mechanism but not in the net reaction.
25. The minimum amount of energy required for a reaction to proceed is:
(A) Heat of reaction
(B) Free energy
(C) Activation energy
(D) Potential energy
Answer
(C) — Activation energy is the energy barrier that must be overcome for a reaction to proceed.
26. The decomposition of hydrogen peroxide follows the rate law: rate = k[H₂O₂]. If the initial concentration of H₂O₂ is doubled, how is the rate affected?
(A) Doubled
(B) Tripled
(C) Quadrupled
(D) Unchanged
Answer
(A) — Since the reaction is first-order, doubling [H₂O₂] will double the rate.
27. A certain reaction has a rate law: rate = k[A]²[B]. What will be the effect on the rate if [A] is halved and [B] is doubled?
(A) Halved
(B) Doubled
(C) Reduced to one-fourth
(D) Remains unchanged
Answer
(C) — (½)² × 2 = ¼
28. Which graphical method would confirm a second-order reaction with respect to [A] in the reaction A → Products?
(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 [A]² vs. time is linear
Answer
(C) — A second-order reaction gives a straight line when 1/[A] is plotted against time.
29. A catalyzed and uncatalyzed reaction have the same ΔH. Why?
(A) Catalysts lower ΔH
(B) Catalysts alter products
(C) Catalysts provide different activation energy, but not enthalpy change
(D) Catalysts change the equilibrium constant
Answer
(C) — Catalysts change the pathway (Ea), not the thermodynamics (ΔH).
30. What happens to the rate constant k when temperature increases, assuming Arrhenius behavior?
(A) k remains unchanged
(B) k increases
(C) k decreases
(D) k becomes zero
Answer
(B) — According to the Arrhenius equation, increasing temperature increases the value of k.
31. What does a large activation energy imply about a reaction?
(A) The reaction is spontaneous
(B) The reaction is fast
(C) The reaction is slow
(D) The rate constant is large
Answer
(C) — High activation energy means few molecules have enough energy to react, resulting in a slower reaction.
32. The initial rate of a reaction is measured for various concentrations of reactant A. Doubling [A] results in no change in rate. What is the order with respect to A?
(A) Zero
(B) First
(C) Second
(D) Third
Answer
(A) — If the rate doesn’t change when [A] changes, the reaction is zero-order in A.
33. If a catalyst speeds up a reaction by a factor of 100, what happens to the half-life of a first-order reaction?
(A) It doubles
(B) It’s unaffected
(C) It decreases
(D) It increases
Answer
(C) — For a first-order reaction, half-life depends on k. Increasing rate increases k, so t½ decreases.
34. In a multistep reaction, which step determines the rate law?
(A) First step
(B) Last step
(C) Slowest elementary step
(D) Fastest step
Answer
(C) — The slowest elementary step determines the overall rate and the rate law.
35. For a reaction with rate = k[A][B], doubling both [A] and [B] will increase the rate by a factor of:
(A) 2
(B) 4
(C) 8
(D) 16
Answer
(B) — 2 × 2 = 4, so the rate increases by a factor of 4.
36. Define activation energy and explain its role in chemical reactions.
Answer
Activation energy is the minimum amount of energy required for reactants to collide and form products. It determines the rate at which a reaction occurs.
37. Explain why increasing temperature generally increases the rate of a chemical reaction.
Answer
Higher temperatures increase the kinetic energy of molecules, leading to more frequent and energetic collisions that can overcome activation energy.
38. Describe the difference between a catalyst and an intermediate in a reaction mechanism.
Answer
A catalyst is not consumed in the reaction and appears in both the beginning and end steps, while an intermediate is formed and consumed during the reaction.
39. What information can be obtained from the slope of a first-order reaction plot of ln[A] vs. time?
Answer
The slope equals -k, where k is the rate constant for the reaction.
40. Why does increasing surface area increase the reaction rate for a solid reactant?
Answer
It exposes more particles for collisions, leading to more frequent interactions with other reactants.
41. What does it mean if a reaction is zero-order with respect to a reactant?
Answer
The rate of the reaction is independent of the concentration of that reactant.
42. Write the integrated rate law for a first-order reaction.
Answer
ln[A] = -kt + ln[A]₀
43. What is the half-life formula for a first-order reaction?
Answer
t½ = 0.693 / k
44. How does a catalyst lower the activation energy of a reaction?
Answer
It provides an alternative reaction pathway with a lower energy barrier for the reaction to proceed.
45. What is the rate law expression for a reaction that is first-order in A and second-order in B?
Answer
Rate = k[A][B]²
46. A reaction proceeds via two steps. The first is fast and the second is slow. Which step determines the rate law and why?
Answer
The second (slow) step is the rate-determining step and governs the rate law because it limits the overall speed of the reaction.
47. What are typical units of the rate constant for a second-order reaction?
Answer
L·mol⁻¹·s⁻¹
48. What is the significance of the peak in a potential energy diagram?
Answer
It represents the activated complex or transition state — the highest energy point between reactants and products.
49. Describe one method for determining the order of a reaction experimentally.
Answer
Measure initial reaction rates at various concentrations of a reactant and analyze how rate changes with concentration.
50. How does the frequency factor (A) in the Arrhenius equation affect the rate constant?
Answer
It represents the number of collisions with proper orientation per unit time, directly affecting the value of the rate constant k.