AP Chemistry : Reaction Mechanisms

Study concepts, example questions & explanations for AP Chemistry

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Example Questions

Example Question #1 : Reaction Mechanisms

Which of the following is true?

Possible Answers:

Since intermediate compounds can be formed, the chemical equations for the elementary reaction in a multistep mechanism do not always have to add to give the chemical equation of the overall process.

In a reaction mechanism an intermediate is identical to an activated complex

All of the above

The rate-determining step of a reaction is the rate of the slowest elementary step of its mechanism

If we know that a reaction is an elementary reaction, then we know its rate law.

Correct answer:

All of the above

Explanation:

All of the above describe elementary reactions and how they give an overall mechanism.

Example Question #2 : Reaction Mechanisms

A possible mechanism for the overall reaction Br2 (g) + 2 NO (g)  ->  2 NOBr(g) is 

02

The rate law for the formation of NOBr based on this mechanism is rate =           .

Possible Answers:

Correct answer:

Explanation:

Based on the slowest step the rate law would be:  Rate = k2 [NOBr2] [NO], but one cannot have a rate law in terms of an intermediate (NOBr2).

 

Because the first reaction is at equilibrium the rate in the forward direction is equal to that in the reverse, thus:

 

  and:  

 

Substitution yields:  

Example Question #3 : Reaction Mechanisms

For the reaction NO2 (g) + CO (g) -> NO (g) + CO2 (g), the reaction was experimentally determined to be Rate = k[NO2]2.  If the reaction has the following mechanism, what is the rate limiting step, and why?

 

Step 1:  2 NO2  -> NO3 + NO                            (slow)

Step 2:  NO3 + CO -> NO2 + CO2                     (fast)

Possible Answers:

Step 1 is limiting because the NO2 is a reactant.

Not enough information

Step 2 is limiting because the NO3 intermediate has to be formed before the reaction can occur.

Step 2 is limiting because the fast step determines how quickly the reaction can occur.

Step 1 is limiting because the reaction can not go faster than its slowest step.

Correct answer:

Step 1 is limiting because the reaction can not go faster than its slowest step.

Explanation:

The reaction can never go faster than its slowest step. 

Example Question #1 : Reaction Mechanisms

04

Based on the figure above, what arrows corresponds to the activation energy of the rate limiting step and the energy of reaction?  Is the reaction endo- or exothermic?

Possible Answers:

Exothermic

Endothermic

Correct answer:

Endothermic

Explanation:

Since the products are higher in energy than the reactions, the reaction is endothermic.

Example Question #2 : Reaction Mechanisms

Consider the following mechanism:  

A + B ->  R +  C              (slow)

A + R  ->  C                   (fast)

Possible Answers:

There are no intermediates

A

R

B

C

Correct answer:

R

Explanation:

R is the intermediate.  It is formed in Step 1 and consumed in Step 2.

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