AP®ChemistryCollege BoardExam QuestionsUnit 5: KineticsReaction Mechanisms

Reaction Mechanisms (College Board AP® Chemistry): Exam Questions

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Reaction Mechanisms

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11 mark

What is the significance of an intermediate species in a proposed reaction mechanism?

  • It appears in the overall balanced equation but not in the elementary steps.

  • It is produced in one elementary step and consumed in a subsequent step.

  • It catalyzes the reaction without being consumed.

  • It is always the limiting reagent in the overall reaction.

Choose your answer

21 mark

Which of the following best describes the rate-limiting step in a multi-step reaction mechanism?

  • The step with the lowest activation energy.

  • The step with the highest rate constant.

  • The step with the highest activation energy.

  • The step that produces the most stable intermediate.

Choose your answer

11 mark

The mechanism for the following reaction between iodine and propanone is shown. 

CH3COCH3 (aq) + I2 (aq) → CH3COCH2I (aq) + HI (aq)

Step 1: CH3COCH3 + H+ → CH3COH+CH3        

Step 2: CH3COH+CH3 → CH3COHCH2 + H+ 

Step 3: CH3COHCH2 + I2 → CH3COHCH2I + I-

Step 4: CH3COHCH2I → CH3COCH2I + H+ 

Which classifications of CH3COCH3, H+ and CH3COHCH2 are correct?

 

CH3COCH3

H+

CH3COHCH2

A

Intermediate

Intermediate

Catalyst

B

Reactant

Intermediate

Product

C

Reactant

Catalyst

Intermediate

D

Reactant

Product

Intermediate

    Choose your answer

    21 mark

    Consider the following mechanism for a reaction:

    Step 1: N2​O5​ (g) → NO2​ (g) + NO2+ (g) ​

    Step 2: N2​O5 (g) + NO2+ (g) ​→ NO2​ (g) + NO3− (g) ​

    Step 3: 2NO3− ​(g) → O2​ (g) + 2NO2 (g)

    Which of the following correctly identifies the reaction intermediates?​

    • NO2​ (g) and NO2+ (g) ​

    • N2​O5​ (g) and NO2​ (g)

    • N2​O5​ (g) and NO2+ (g) ​

    • NO2+ (g) and NO3− (g) ​

    Choose your answer

    31 mark

    Which of the following best describes the process of chemical adsorption in heterogeneous catalysis?

    • Dissolution of the catalyst in the reaction mixture.

    • Electrostatic repulsion between reactants and catalyst surface.

    • Formation of weak covalent bonds between reactants and catalyst surface.

    • Physical attraction between reactants and catalyst surface through van der Waals forces.

    Choose your answer

    41 mark

    To catalyze a biochemical reaction, an enzyme typically

    • drives the reaction to completion by consuming byproducts of the reaction.

    • binds temporarily to reactant molecules to lower the activation energy of the reaction.

    • dissociates into additional reactant molecules, thereby increasing the reaction rate.

    • decomposes and releases energy to increase the number of successful collisions between reactant molecules.

    Choose your answer

    51 mark

    2CH3CHO → CH3CH(OH)CH2CHO

    The proposed mechanism for the reaction of ethanal in dilute alkaline solution to form 3-hydroxybutanal is shown: 

    1: CH3CHO + :OH- → :CH2CHO + H2O (slow)

    2: CH3CHO + :CH2CHO → CH3CH(O:-)CH2CHO (fast)

    3: CH3CH(O:-)CH2CHO + H2O → CH3CH(OH)CH2CHO + :OH- (fast)

    Which of the following statements is not correct?

    • The rate expression is rate = [CH3CHO] [OH-].

    • Step 1 is the rate-determining step.

    • OH- is a catalyst.

    • Steps 2 and 3 have a lower activation energy than step 1.

    Choose your answer

    11 mark

    Consider the following reaction mechanism:

    A + B ⇌ C (fast)

    C + D → E (slow)

    If the experimental rate law is found to be Rate = k[A][B][D], what can be concluded about the relationship between k and the rate constants of the individual steps?

    • k = k1k2, where k1 is the forward rate constant of step 1 and k2 is the rate constant of step 2.

    • k = k1k2/k-1, where k1 and k-1 are the forward and reverse rate constants of step 1, and k2 is the rate constant of step 2.

    • k = k2, where k2 is the rate constant of step 2.

    • k = k1 + k2, where k1 is the forward rate constant of step 1 and k2 is the rate constant of step 2.

    Choose your answer

    21 mark

    The following fourth-order reaction was studied at a constant temperature:

    5 Br- (aq) + BrO3- (aq) + 6 H+ (aq) → 3 Br2 (aq) + 3 H2O (l)

    Data was collected for the rate of the reaction as follows:

    conc-v-rate-graph-
    time-v-conc-graph-bromide-ions

    Which rate expression is consistent with the data?

    • Rate = k [Br-] [BrO3-] [H+]

    • Rate = k [Br-]2 [H+]2

    • Rate = k [Br-] [BrO3-] [H+]2

    • Rate = k [Br-]2 [BrO3-] [H+]

    Choose your answer

    31 mark

    The reaction between ethyl bromide and hydroxide ions was studied at a constant temperature:

    C2H5Br (aq) + OH⁻ (aq) → C2H5OH (aq) + Br⁻ (aq)

    Experimental data for the reaction is provided:

    [C2H5Br] (mol dm-3)

    [OH-] (mol dm-3)

    Rate (mol dm-3 s-1)

    2.0 x 10-3

    1.0 x 10-2

    4.0 x 10-4 

    4.0 x 10-3

    1.0 x 10-2

    8.0 x 10-4 

    8.0 x 10-3

    2.0 x 10-2

    3.2 x 10-3 

    What is the correct equation to calculate the value of the rate constant, k?

    • fraction numerator rate over denominator left square bracket straight C subscript 2 straight H subscript 5 Br right square bracket space left square bracket OH to the power of minus right square bracket end fraction

    • fraction numerator left square bracket straight C subscript 2 straight H subscript 5 Br right square bracket space left square bracket OH to the power of minus right square bracket over denominator rate end fraction

    • fraction numerator rate over denominator left square bracket straight C subscript 2 straight H subscript 5 Br right square bracket space left square bracket OH to the power of minus right square bracket squared end fraction

    • begin mathsize 14px style fraction numerator rate over denominator left square bracket straight C subscript 2 straight H subscript 5 Br right square bracket squared space left square bracket OH to the power of minus right square bracket squared end fraction end style

    Choose your answer

    41 mark

    The following mechanism has been proposed for the reaction between nitrogen(II) oxide and carbon monoxide:

    NO2 (g) + CO (g) → NO (g) + CO2 (g)

    Mechanism:

    Step 1:   NO2 + NO2 ⇌ N2O4   (fast equilibrium)

    Step 2:   N2O4 + 2CO → 2NO + 2CO2   (slow step)

    Which rate expression is consistent with the mechanism?

    • Rate = k [NO2] [CO]2

    • Rate = k [N2O4] [CO]2

    • Rate = k [NO2]2 [CO]2

    • Rate = k [N2O4] [CO]

    Choose your answer