Elementary Reactions & Collisions (College Board AP® Chemistry): Exam Questions

The graph below shows the Maxwell-Boltzmann distribution curve for a gas at 300 K.

Sketch the distribution curve for the same gas at 350 K on the same graph. Ensure your curve shows the shift in the fraction of particles with energy ≥ E_a.

Explain how the shift in the fraction of particles with energy ≥ E_a​ affects the reaction rate at 350 K compared to 300 K.

The reaction between methane (CH₄​) and chlorine (C_l) is initiated by light:

CH₄ + Cl₂ → CH₃Cl + HCl

Explain the role of light in initiating the reaction.

Draw a reaction energy profile for the exothermic reaction described in part (a), labelling the activation energy and ΔH.

Explain what a transition state is in a reaction.

The decomposition of hydrogen peroxide (H₂O₂​) occurs via the following mechanism:

1. H₂O₂ → 2HO^. (slow)

2. HO^. + H₂O₂ → H₂O + HO₂^.​ (fast)

3. HO₂^. + HO^. → H₂O + O₂

Write the rate law for the overall reaction based on the rate-determining step.

Identify the intermediate(s) in this reaction mechanism and explain your reasoning.

Explain why increasing the temperature increases the rate of this reaction, referring to the Maxwell-Boltzmann distribution.

The decomposition of H₂O₂​ involves bond breaking in the first step. Explain how molecular collisions must have sufficient energy and correct orientation for this step to occur.

The decomposition of hydrogen peroxide (H₂O₂​) is commonly catalysed by manganese(IV) oxide (MnO_2​):

2H₂O₂ → 2H₂O + O₂

Explain how the steric factor (p) influences the rate constant (k) by affecting molecular collisions in the decomposition of H₂O₂​.

Use the collision model to explain why the decomposition of H₂O₂​ is slow without a catalyst.

Predict how the addition of MnO₂​ changes the activation energy and the reaction pathway, including its effect on intermediates.

The following data were collected for the rate constant (kkk) of a reaction at different temperatures:

The relationship between the rate constant and temperature is described by the Arrhenius equation:

k = Ae^−E_a/RT

Using the Maxwell-Boltzmann distribution, explain why the reaction rate increases as the temperature rises from 300 K to 320 K.

Predict whether the sensitivity of the rate constant (k) to temperature would increase, decrease, or remain the same if the activation energy (E_a) were higher. Justify your answer.

Explain how the data provided can be used to calculate E_a.

Consider the following reaction mechanism:

1. NO + O₃ → NO₂ + O₂ (fast)

2. NO + NO₂ → N₂O₃ (slow)

The overall reaction releases energy as products form. Write the overall balanced chemical equation for this reaction.

Draw the energy profile for this mechanism, clearly labelling the activation energy (E_a​) for each step, the intermediate, and ΔH.

Justify why the first step has a lower activation energy than the second step.

Explain how the addition of a catalyst changes the energy profile and the reaction rate.

Explain how increasing the concentration of NO₂​ would affect the overall rate of the reaction. Justify your answer.

A student proposes using temperature to speed up the reaction. Predict how increasing the temperature affects the fraction of collisions with energy ≥ E_a​ and explain the molecular basis of this change.