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스터디 가이드 - 스마트 노트
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Reaction Mechanisms
Introduction to Reaction Mechanisms
Reaction mechanisms describe the step-by-step sequence of elementary reactions by which an overall chemical change occurs. Understanding mechanisms helps explain how reactants are converted into products and how reaction rates are determined.
Overall Reaction: The net chemical equation showing the transformation of reactants to products.
Elementary Steps: Simpler reactions that make up the overall mechanism; each represents a single molecular event.
Intermediates: Species produced in one elementary step and consumed in another; they do not appear in the overall reaction.
Example: For the reaction NO + CO → NO + CO2, the mechanism can be broken down into:
NO2 + NO2 → NO3 + NO (slow)
NO3 + CO → NO2 + CO2 (fast)
Elementary Steps and Rate Laws
Each elementary step has its own rate law, which is determined by the molecularity (number of reactant molecules involved in the step).
Unimolecular: A → products; Rate = k[A]
Bimolecular: A + B → products; Rate = k[A][B]
Termolecular: 2A + B → products; Rate = k[A]2[B] (rare)
Relationship: The rate law for an elementary step directly reflects the stoichiometry of the reactants in that step.
Elementary Step | Rate Law |
|---|---|
A → something | Rate = k[A] |
2A → something | Rate = k[A]2 |
A + B → something | Rate = k[A][B] |
2A + B → something | Rate = k[A]2[B] |
Rate-Determining Step
The slowest elementary step in a reaction mechanism is called the rate-determining step. The overall rate law for the reaction is governed by this step.
Key Point: The rate law for the overall reaction matches the rate law for the slow (rate-determining) step.
Example: For the mechanism:
NO2 + NO2 → NO3 + NO (slow)
NO3 + CO → NO2 + CO2 (fast)
The rate law is .
Criteria for a Plausible Reaction Mechanism
A proposed reaction mechanism must satisfy several conditions to be considered plausible:
Elementary steps must add up to the overall balanced equation.
No elementary step should involve more than three molecules (termolecular steps are rare).
The rate law suggested by the mechanism must match the experimentally determined rate law.
There must be supporting experimental evidence.
Examples and Practice
Consider the overall reaction: SO2 + SO3 → S2O + 2 O2
Proposed mechanism:
SO2 + SO2 → S2O + O3 (slow)
SO3 + O3 → 2 O2 + SO2 (fast)
To evaluate plausibility:
Check if steps add up to the overall reaction.
Verify that the rate law matches experimental data.
Ensure no step involves more than three molecules.
Intermediates: O3 is an intermediate, as it is produced in the first step and consumed in the second.
Additional info:
These notes are foundational for understanding chemical kinetics, a topic relevant to both general biology and chemistry, as reaction rates and mechanisms are crucial in biological systems (e.g., enzyme catalysis).
