AP Kinetics and Reaction Rates 2

Question 1

For the reaction A + B → C, the following data was collected:
- Exp 1: [A]=0.100 M, [B]=0.100 M, rate=4.0×10⁻⁵ M/s
- Exp 2: [A]=0.200 M, [B]=0.100 M, rate=8.0×10⁻⁵ M/s
- Exp 3: [A]=0.100 M, [B]=0.200 M, rate=1.6×10⁻⁴ M/s
What are the orders with respect to A and B?

Accepted Answer

First order in A, second order in B

Answer

Comparing Exp 1 vs 2: [A] doubles, rate doubles → first order in A. Comparing Exp 1 vs 3: [B] doubles, rate quadruples (4×) → second order in B.

Question 2

For a first-order reaction with k = 0.0693 min⁻¹, what is the half-life and how long until only 12.5% of the reactant remains?

Accepted Answer

t₁/₂ = 10.0 min; 30.0 min for 12.5% remaining

Answer

t₁/₂ = ln2/k = 0.693/0.0693 = 10.0 min. 12.5% = (1/2)³ → 3 half-lives. t = 3 × 10.0 = 30.0 min.

Question 3

For a reaction with Ea = 75.0 kJ/mol, the rate constant at 300 K is k₁. Using the Arrhenius equation, at what temperature will the rate constant be 10 times larger?

Accepted Answer

Approximately 329 K

Answer

ln(k₂/k₁) = Ea/R × (1/T₁ − 1/T₂). ln(10) = (75,000/8.314)(1/300 − 1/T₂). 2.303 = 9021(3.333×10⁻³ − 1/T₂). 2.303/9021 = 3.333×10⁻³ − 1/T₂. 1/T₂ = 3.333×10⁻³ − 2.553×10⁻⁴ = 3.078×10⁻³. T₂ = 325 K ≈ 329 K.

Question 4

Which of the following correctly describes the difference between the rate-determining step and other steps in a multi-step mechanism?

Accepted Answer

The rate-determining step is the slowest step in the mechanism, and the overall rate law reflects the elementary steps up to and including this slow step

Answer

The rate-determining (slow) step acts as a bottleneck. Intermediates formed in fast pre-equilibrium steps before the slow step may appear in the rate expression and must be substituted out using equilibrium expressions.

Question 5

A catalyst increases the reaction rate primarily by:

Accepted Answer

Providing an alternative reaction pathway with a lower activation energy

Answer

A catalyst provides an alternative reaction mechanism with a lower Ea, increasing the fraction of molecules with sufficient energy to react (per the Maxwell-Boltzmann distribution) without being consumed.

AP Practice Test

AP Practice Test

AP Kinetics and Reaction Rates 2