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Reaction Mechanisms
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Reaction Mechanisms Reactions may occur all at once or through several discrete steps. Each of these processes is known as an elementary reaction or elementary process.
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Multistep Mechanisms In a multistep process, one of the steps will be slower than all others. The overall reaction cannot occur faster than this slowest, rate-determining step.
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Reaction Mechanisms NO2 + CO NO + CO2 Rate = k[NO2]2
The sequence of events that describes the actual process by which reactants become products is called the reaction mechanism. NO2 + CO NO + CO2 Rate = k[NO2]2 Proposed Mechanism: Step 1: NO2 + NO2 NO3 + NO Step 2: NO3 + CO NO2 + CO2
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Slow Initial Step NO2 (g) + CO (g) NO (g) + CO2 (g)
The rate law for this reaction is found experimentally to be Rate = k [NO2]2 CO is necessary for this reaction to occur, but the rate of the reaction does not depend on its concentration. This suggests the reaction occurs in two steps.
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Reaction Mechanisms must satisfy 2 requirements:
The sum of the elementary steps must give the overall balanced equation. The mechanism must agree with the experimentally determined rate law.
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Reaction Mechanisms The molecularity of a process tells how many molecules are involved in the process. Termolecular elementary reactions seldom occur.
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Catalysts A catalyst speeds up a reaction without being consumed.
Identify the catalyst in the reaction mechanism below: Cl(g) + O3(g) ClO(g) + O2(g) O(g) + ClO(g) Cl(g) + O2(g) How is it different than an intermediate?
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