Rucete ✏ AP Chemistry In a Nutshell
10. Kinetics — Practice Questions 3
This chapter introduces the concepts of reaction rates, rate laws, reaction mechanisms, the Arrhenius equation, and the effect of temperature and catalysts on reaction rate.
(Multiple Choice — Click to Reveal Answer)
1. Which condition would most likely lead to an increase in the rate constant (k) for a reaction?
(A) Decreasing the temperature
(B) Using a less polar solvent
(C) Increasing temperature
(D) Removing a catalyst
Answer
(C) — Increasing the temperature increases the fraction of particles with energy greater than Ea, raising the rate constant.
2. A second-order reaction has units for the rate constant of:
(A) mol·L⁻¹·s⁻¹
(B) L²·mol⁻²·s⁻¹
(C) L·mol⁻¹·s⁻¹
(D) s⁻¹
Answer
(C) — For a second-order reaction, the unit of k is L·mol⁻¹·s⁻¹.
3. What information is necessary to determine the rate constant from a first-order graph?
(A) Concentration and temperature
(B) ln[reactant] vs. time slope
(C) Initial rate and final concentration
(D) Molecular structure
Answer
(B) — For first-order kinetics, the slope of ln[reactant] vs. time gives –k.
4. Which of the following changes would decrease the frequency of effective collisions?
(A) Increasing temperature
(B) Increasing concentration
(C) Diluting the reactants
(D) Adding a catalyst
Answer
(C) — Diluting the reactants lowers particle concentration and reduces collision frequency.
5. What is the molecularity of a reaction step involving two reactants forming one product in a single step?
(A) Unimolecular
(B) Bimolecular
(C) Termolecular
(D) Complex
Answer
(B) — Two reactant molecules colliding in a single step indicates bimolecular molecularity.
6. A graph of 1/[A] versus time produces 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
(C) — A linear 1/[A] vs. time graph indicates second-order kinetics.
7. What is the effect of a catalyst on the enthalpy change (ΔH) of a reaction?
(A) Increases ΔH
(B) Decreases ΔH
(C) No change
(D) Reverses sign of ΔH
Answer
(C) — Catalysts do not affect thermodynamic properties like ΔH; only activation energy is changed.
8. What does a reaction rate law describe?
(A) The equilibrium constant
(B) The number of steps in the mechanism
(C) The rate of the reaction as a function of concentration
(D) The temperature dependence of product formation
Answer
(C) — Rate laws show how reaction rate depends on reactant concentrations.
9. In the Arrhenius equation, which term represents the fraction of molecules that collide with enough energy to react?
(A) A
(B) T
(C) e^(-Ea/RT)
(D) k
Answer
(C) — The exponential factor represents the fraction of collisions with sufficient energy to overcome the activation barrier.
10. The initial rate of a reaction doubles when [A] is doubled. What is the order with respect to A?
(A) Zero
(B) First
(C) Second
(D) Third
Answer
(B) — A linear relationship between concentration and rate indicates first-order dependence.
11. Which type of catalyst exists in a different phase than the reactants?
(A) Homogeneous
(B) Endothermic
(C) Heterogeneous
(D) Biological
Answer
(C) — Heterogeneous catalysts are in a different phase (e.g., solid catalyst with gas reactants).
12. Which of the following is a valid reason for using the method of initial rates?
(A) It determines final concentrations
(B) It reveals the slowest step in the mechanism
(C) It eliminates complications from reverse reactions
(D) It measures ΔH directly
Answer
(C) — Initial rate data avoids effects from reverse reactions or equilibrium.
13. What is the primary effect of increasing pressure on a gas-phase reaction?
(A) Lowers activation energy
(B) Increases particle size
(C) Increases collision frequency
(D) Changes ΔH
Answer
(C) — Higher pressure increases the number of collisions per unit time.
14. Which of the following graphs is linear for a zero-order reaction?
(A) ln[A] vs. time
(B) [A] vs. time
(C) 1/[A] vs. time
(D) ln(rate) vs. ln[A]
Answer
(B) — For zero-order, [A] vs. time gives a straight line with a constant slope.
15. Why are multi-step mechanisms necessary in some reactions?
(A) All reactions happen in one step
(B) Intermediates need to be isolated
(C) Some reactions are too complex to occur in a single step
(D) Rate laws are constant for all reactions
Answer
(C) — Many reactions occur through simpler steps due to complex bond rearrangements.
16. The slope of the straight-line plot of ln[A] versus time for a first-order reaction equals:
(A) -k
(B) k
(C) 1/k
(D) -1/k
Answer
(A) — For first-order reactions, slope = -k
17. What is the main reason reaction rates generally increase with temperature?
(A) More reactants are formed
(B) Activation energy decreases
(C) Molecules move faster and collide more energetically
(D) Catalysts become more effective
Answer
(C) — Temperature increases molecular motion and the fraction of collisions with energy ≥ Ea.
18. What does a catalyst provide that speeds up a chemical reaction?
(A) Lower temperature
(B) Higher energy transition state
(C) Alternate reaction pathway with lower Ea
(D) Increased equilibrium constant
Answer
(C) — Catalysts offer a new pathway with reduced activation energy.
19. What characteristic distinguishes an intermediate from a catalyst?
(A) Intermediates appear in the overall reaction
(B) Catalysts are used up during reaction
(C) Intermediates are produced and consumed during the reaction
(D) Catalysts are present in the rate law
Answer
(C) — Intermediates are created in one step and consumed in another.
20. The minimum energy needed for a chemical reaction to occur is called:
(A) Collision energy
(B) Bond energy
(C) Activation energy
(D) Reaction enthalpy
Answer
(C) — Activation energy (Ea) is the minimum energy required for reactants to convert to products.
21. Which factor affects the rate constant k the most?
(A) Reactant concentration
(B) Temperature
(C) Volume of container
(D) Product stability
Answer
(B) — The rate constant is exponentially sensitive to temperature (via Arrhenius equation).
22. For a given reaction, the rate law is rate = k[A]². What is the effect of doubling [A]?
(A) Rate doubles
(B) Rate quadruples
(C) Rate triples
(D) Rate is unchanged
Answer
(B) — 2² = 4; doubling [A] quadruples the rate in second-order kinetics.
23. What does the term “reaction order” refer to?
(A) The number of steps in a mechanism
(B) The exponent of the reactant concentration in the rate law
(C) The number of products
(D) The position of reactants in the equation
Answer
(B) — Reaction order is the power to which the concentration is raised in the rate law.
24. What is the role of orientation in successful molecular collisions?
(A) Ensures minimum ΔH
(B) Guarantees equal bond energies
(C) Determines whether bonds break/form properly
(D) Prevents reverse reactions
Answer
(C) — Proper orientation allows for the correct breaking and forming of bonds.
25. In a proposed mechanism, which species must be omitted from the overall equation?
(A) Catalyst
(B) Reactant
(C) Product
(D) Solvent
Answer
(A) — Catalysts appear in mechanisms but not in the overall balanced equation.
26. A reaction has the rate law rate = k[A][B]². If the concentration of A is tripled and B is doubled, how much does the rate increase?
(A) 6×
(B) 12×
(C) 18×
(D) 24×
Answer
(C) — Rate ∝ [A][B]² → 3×(2)² = 3×4 = 12×
27. The decomposition of N₂O₅ follows first-order kinetics. If its half-life is 50 seconds, how much remains after 150 seconds?
(A) 1/8
(B) 1/2
(C) 1/4
(D) 1/16
Answer
(A) — Three half-lives pass: (1/2)³ = 1/8 remains.
28. Which of the following best explains why some reactions occur in multiple steps?
(A) Products are unstable
(B) Reactions must obey Hess’s law
(C) Activation energy for a one-step path is too high
(D) The rate law is unknown
Answer
(C) — Breaking a reaction into steps lowers the activation energy at each stage.
29. In the mechanism below, identify the intermediate:
Step 1: A + B → C
Step 2: C + D → E
Step 3: E → F
(A) A
(B) C
(C) E
(D) F
Answer
(B) — C is produced in one step and consumed in the next; it’s an intermediate.
30. What is the main factor determining whether a collision leads to a reaction?
(A) Temperature only
(B) Volume of system
(C) Both proper orientation and sufficient energy
(D) Time duration of the collision
Answer
(C) — Effective collisions require both proper orientation and enough energy (≥Ea).
31. Which of the following is NOT an assumption in collision theory?
(A) Molecules must collide to react
(B) All collisions result in reaction
(C) Collisions must have sufficient energy
(D) Proper orientation is required
Answer
(B) — Not all collisions are effective; only some result in reaction.
32. A student plots [A] vs. time and gets a curve. ln[A] vs. time is also curved, but 1/[A] vs. time gives a straight line. What is the reaction order?
(A) First
(B) Second
(C) Zero
(D) Third
Answer
(B) — Linear 1/[A] vs. time implies second-order kinetics.
33. The energy profile for a reaction shows two humps. What does this indicate?
(A) Two reactants
(B) Two transition states and one intermediate
(C) Two products
(D) Equilibrium process
Answer
(B) — Each “hump” is a transition state; a valley between them is an intermediate.
34. Which expression correctly defines the rate of disappearance of B in the reaction A + 2B → C?
(A) rate = –Δ[B]/Δt
(B) rate = –(1/2)Δ[B]/Δt
(C) rate = –2Δ[B]/Δt
(D) rate = Δ[C]/Δt
Answer
(B) — Since B has a coefficient of 2, divide its rate of change by 2.
35. If a proposed mechanism gives a rate law that does NOT match the experimental rate law, then:
(A) The mechanism is valid
(B) The mechanism must be modified
(C) The overall reaction is wrong
(D) The rate law must be ignored
Answer
(B) — A valid mechanism must yield a rate law consistent with experimental data.
36. What does the term "transition state" refer to in a chemical reaction?
Answer
The transition state is a high-energy, unstable arrangement of atoms that exists momentarily as reactants are converted into products.
37. Explain why a catalyst does not appear in the overall chemical equation.
Answer
A catalyst is not consumed in the reaction; it is used and regenerated, so it does not appear in the net equation.
38. How can you determine the rate law of a reaction using experimental data?
Answer
By varying the concentration of reactants and observing changes in the initial rate, you can determine the order with respect to each reactant.
39. A reaction has a half-life that doubles as the concentration decreases. What is the likely order of the reaction?
Answer
It is likely a second-order reaction, since half-life increases with decreasing concentration in second-order kinetics.
40. Define the activation energy of a chemical reaction.
Answer
Activation energy is the minimum amount of energy required for a reaction to proceed.
41. Why does a reaction proceed faster at higher temperatures?
Answer
Higher temperature increases the kinetic energy of particles, leading to more frequent and more energetic collisions.
42. What does the slope of a ln[k] vs. 1/T graph represent in the Arrhenius equation?
Answer
The slope is equal to –Ea/R, which allows you to calculate the activation energy.
43. What is the primary purpose of studying reaction mechanisms?
Answer
To understand the sequence of steps and identify the rate-determining step to predict or control the reaction rate.
44. How does increasing surface area affect the rate of a heterogeneous reaction?
Answer
It increases the rate by providing more area for reactants to collide with the surface, leading to more frequent interactions.
45. What role does the rate-determining step play in a multistep reaction?
Answer
The rate-determining step is the slowest step and governs the overall reaction rate.
46. How can you experimentally verify if a proposed intermediate exists in a mechanism?
Answer
You can look for spectroscopic evidence or isolate it under specific conditions during the reaction.
47. Describe how the use of a catalyst affects the activation energy diagram of a reaction.
Answer
The diagram shows a lower peak for the catalyzed pathway, indicating a reduced activation energy.
48. What does it mean if the rate of a reaction is independent of concentration?
Answer
It is a zero-order reaction, meaning the rate does not change as reactant concentration changes.
49. Why are termolecular reactions rare in a single elementary step?
Answer
Because the probability of three particles colliding simultaneously with correct orientation and sufficient energy is extremely low.
50. What are the units of the rate constant k for a zero-order reaction?
Answer
The units are mol·L⁻¹·s⁻¹ (or M/s).