18.3. Reaction Mechanisms http://www.ck12.org
- Reaction mechanisms must be experimentally determined, and the rate law for the rate-determining step of
the mechanism must agree with the rate law for the overall reaction. - Mechanisms show the presence of intermediates and catalysts, neither of which are present in the balanced
equation for the overall reaction. - A potential energy diagram can show the mechanism of a reaction by a series of peaks and valleys correspond-
ing to each elementary step.
Lesson Review Questions
Reviewing Concepts
- How is an elementary step different from an overall chemical reaction?
- Why can a rate law be written based on the equation for an elementary step?
- The overall rate for a reaction depends most on which step of the mechanism? Explain.
- Classify the following elementary reactions as unimolecular, bimolecular, or termolecular.
a. SO+O 2 →SO 2 +O
b. CH 3 NC→CH 3 CN
c. H 2 +2I→2HI - The equation for the production of ammonia from nitrogen and hydrogen gases is: Why is it unlikely that this
reaction proceeds in a single elementary step?
Problems
- Given the reaction and experimentally determined rate law below:
a. Why must the mechanism of the reaction consist of more than one step?
b. The mechanism below has been proposed for the reaction.
If this mechanism is correct, which step is the rate-determining step? Explain. - Consider the following two-step reaction mechanism. Write the overall reaction. Which substance is an
intermediate in the reaction? Which substance is a catalyst? - Given the overall reaction below: By experiment it is found that doubling the concentration of A 2 doubles the
rate. Doubling the concentration of B also doubles the rate. Doubling the concentration of C has no effect on
the rate.
a. Write the rate law for the reaction.
b. How can changing the concentration of C have no effect on the rate?
c. Propose a reaction mechanism that is consistent with the rate law. Indicate the relative speed (fast or
slow) of each step.
Further Reading / Supplemental Links
- ReactionMechanism–elementaryprocess, http://www.science.uwaterloo.ca/~cchieh/cact/c123/elmntary.html
- ReactionMechanisms, http://www.chem.arizona.edu/~salzmanr/480a/480ants/reacmech/reacmech.html