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AP®ChemistryCollege BoardExam QuestionsUnit 5: KineticsReaction Mechanisms

Reaction Mechanisms (College Board AP® Chemistry)

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Reaction Mechanisms

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Reaction Mechanisms

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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

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    21 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.

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    31 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.

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    41 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) ​

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    51 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.

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    61 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.

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    11 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.

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    21 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.

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    31 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+]

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    41 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

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    51 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]

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