AP Chemistry : Reaction Mechanisms
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Example Questions
Example Question #41 : Kinetics And Energy
Which of the following is true?
All of the above
If we know that a reaction is an elementary reaction, then we know its rate law.
The rate-determining step of a reaction is the rate of the slowest elementary step of its mechanism
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
All of the above describe elementary reactions and how they give an overall mechanism.
Example Question #42 : Kinetics And Energy
A possible mechanism for the overall reaction Br2 (g) + 2 NO (g) -> 2 NOBr(g) is
The rate law for the formation of NOBr based on this mechanism is rate = .
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:
Substitution yields:
Example Question #1 : 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)
Step 2 is limiting because the fast step determines how quickly the reaction can occur.
Step 1 is limiting because the NO2 is a reactant.
Step 1 is limiting because the reaction can not go faster than its slowest step.
Step 2 is limiting because the NO3 intermediate has to be formed before the reaction can occur.
Not enough information
Step 1 is limiting because the reaction can not go faster than its slowest step.
The reaction can never go faster than its slowest step.
Example Question #1 : Reaction Mechanisms
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?
Exothermic
Endothermic
Endothermic
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)
B
There are no intermediates
R
A
C
R
R is the intermediate. It is formed in Step 1 and consumed in Step 2.
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