AP - Advanced Placement Kinetics and Reaction Rates 1

Question 1

A chemist plots [A] vs. t, ln[A] vs. t, and 1/[A] vs. t for a decomposition reaction.
Only the plot of 1/[A] vs. t produces a straight line with a positive slope.
Which integrated rate law and reaction order apply?

Accepted Answer

Second order; 1/[A] = 1/[A]₀ + kt

Answer

First order; ln[A] = ln[A]₀ − kt

Answer

Zero order; [A] = [A]₀ − kt

Answer

Second order; ln[A] = ln[A]₀ − kt

Question 2

For a second-order reaction A → products, the half-life is 40.0 s when [A]₀ = 0.500 M. What is the rate constant k, and what will the half-life be when [A]₀ is halved to 0.250 M?

Accepted Answer

k = 0.0500 M⁻¹s⁻¹; t₁/₂ = 80.0 s

Answer

k = 0.0500 M⁻¹s⁻¹; t₁/₂ = 40.0 s

Answer

k = 0.0173 M⁻¹s⁻¹; t₁/₂ = 20.0 s

Answer

k = 0.0500 M⁻¹s⁻¹; t₁/₂ = 20.0 s

Question 3

A reaction follows the rate law: rate = k[A][B]². What are the correct units of the rate constant k if concentration is expressed in M and time in seconds?

Accepted Answer

M⁻²s⁻¹

Answer

M⁻¹s⁻¹

Answer

M·s⁻¹

Answer

s⁻¹

Question 4

A catalyst is added to a reaction at equilibrium. Which statement best describes its effect on the reaction's thermodynamics and kinetics?

Accepted Answer

It lowers Ea for both the forward and reverse reactions without altering ΔG° or the equilibrium constant

Answer

It increases ΔH° of the reaction and lowers Ea for the forward step only

Answer

It shifts the equilibrium position toward products by selectively stabilizing them

Answer

It raises the temperature of the system, increasing the fraction of molecules with sufficient energy

Question 5

Consider the two-step mechanism below:
  Step 1 (fast equilibrium): A ⇌ B    (equilibrium constant K₁)
  Step 2 (slow):            B + C → D  (rate constant k₂)
What is the rate law for the overall reaction in terms of the original reactants A and C?

Accepted Answer

rate = K₁k₂[A][C]

Answer

rate = k₂[B][C]

Answer

rate = k₁k₂[A][C]

Answer

rate = k₂[A][C]

Question 6

A zero-order reaction A → products has k = 2.50 × 10⁻³ M·s⁻¹ and [A]₀ = 0.400 M. How many seconds does it take for [A] to fall from 0.400 M to 0.100 M?

Accepted Answer

120 s

Answer

40.0 s

Answer

80.0 s

Answer

160 s