Introducing Reaction Mechanisms

Introducing Reaction Mechanisms

Bromfield Honors Chemistry

Multiple Choice

An elementary reaction has only one activation-energy peak and one activated complex.

• True

• False

Multiple Choice

What is the molecularity of the elementary step shown here?

• unimolecular

• bimolecular

• termolecular

Multiple Choice

Which of the following elementary steps would be classified as a termolecular step?

Multiple Choice

A series of elementary steps that lead to an overall reaction is called a(n) ....

• reaction mechanism

• reaction combination

• combustion reaction

• exothermic reaction

Multiple Choice

How many elementary steps are in the reaction mechanism shown here?

• 1

• 2

• 3

• 4

Multiple Choice

Which is the rate determining step in the mechanism shown below?

Step 1: A + B→ D (fast)

Step 2: A + F → C + E (slow)

Step 3: B + E → D + F (fast)

• Step 1

• Step 2

• Step 3

To increase the reaction rate:

• You need to increase the rate of the SLOW step in the mechanism

Multiple Choice

A possible mechanism for the overall reaction A + 2B → C + D

is shown below :

2B → E (slow)

A + E → C + D (fast)

If additional A is added to the reaction container, what effect will it have on the reaction rate?

• The reaction rate will increase.

• The reaction rate will decrease.

• The reaction rate will be unaffected.

Multiple Choice

A possible mechanism for the overall reaction A + 2B → C + D

is shown below :

2B → E (slow)

A + E → C + D (fast)

If the initial molarity of B is increased, what effect will this have on the reaction rate?

• The reaction rate will increase.

• The reaction rate will decrease.

• The reaction rate will be unaffected.

Reaction intermediate

• Produced in one step of the mechanism, and consumed in a subsequent step

• Does not show up in the overall balanced equation

Multiple Choice

The following reaction mechanism has been proposed:

A + B → AB + D (slow)

A + AB → C (fast)

What is the overall reaction?

• A + B + AB ⟶ AB + D

• 2A + AB ⟶ C + D

• 2A + B ⟶ C + D

• A + B ⟶ C + D

Multiple Choice

The following reaction mechanism has been proposed:

A + B → AB + D (slow)

A + AB → C (fast)

Identify the intermediate in the mechanism.

• A

• B

• AB

• C

• D

Catalyst

• Speeds up reaction rate without being consumed

• In mechanism: a reactant in one step, produced in a subsequent step

• Does not appear in the overall balanced equation

The # and type of reactants in the slow step will match the reaction orders in the experimental rate law

​

​

"The molecularity of the rate determining steps correspond to the experimentally determined rate law"

Eliminating unlikely mechanisms

​

​Example:

​Example, continued:

Drag and Drop

A proposed reaction mechanism is shown below :



A + B→ D (fast)

A + F → C + E (slow)

B + E → D + F (fast)





The species ​

Placeholder

acts as a catalyst in the mechanism.

Drag these tiles and drop them in the correct blank above

F

E

D

C

B

A

Multiple Choice

The reaction 4 HBr + O₂ ⟶ 2 H₂O + 2 Br₂

was found experimentally to have the following rate law:

rate = k[HBr][O₂]

Is the proposed mechanism consistent with the experimental rate law?

• Yes, because the steps add up to the overall balanced equation

• No, because the slow step has 1 HBr colliding with 1 O₂, and the balanced equation has 4 HBr and 1 O₂ molecule

• Yes, because the slow step has 1 HBr colliding with 1 O₂, and the rate law is first order with respect to both HBr and O₂

• No, because the slow step has 1 HBr colliding with 1 O₂, and the rate law is first order with respect to both HBr and O₂

Dropdown

This reaction profile has

Select answer

transition states (activated complexes)

Multiple Choice

Which step would you expect to be the rate limiting step for the reaction profile shown here?

• A ⟶ B

• B ⟶ C

• C ⟶ D

Multiple Choice

A possible mechanism for the overall reaction A + 2B → C + D

is shown below :

2B → E (slow)

A + E → C + D (fast)

Which of the following reaction profiles corresponds to the mechanism shown?

Multiple Choice

Which reaction mechanism is most likely to have the reaction pathway diagram shown above?

• A + B → C (slow)

  C + A → F (fast)

• A + B → C (slow)

  C + B → D + E (fast)

  E → F (fast)

• A + B → C (fast)

  C + A → F (slow)

• A + B → C (fast)

  C + B → D + E (fast)

  E → F (slow)

Dropdown

Select answer

represents the overall enthalpy change for the reaction

Multiple Select

Which letter(s) represent transition states?

• X

• Y

• Z

• M

• N

Multiple Select

Which letter(s) represent reaction intermediates?

• Q

• Y

• Z

• M

• N

Multiple Choice

Which step would be the rate-determining step?

• X ⟶ Z

• Z ⟶ N

• N ⟶ Q