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Reaction Mechanisms

Reaction Mechanisms. -- the processes by which reactions occur. --. i.e., what happens in getting from R to P. -- sometimes, mechanisms are dependent on temp. CH 3 CN CH 3 NC. NO + O 3 NO 2 + O 2. elementary steps (or elementary reactions) :.

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Reaction Mechanisms

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  1. Reaction Mechanisms -- the processes by which reactions occur -- i.e., what happens in getting from R to P -- sometimes, mechanisms are dependent on temp.

  2. CH3CN CH3NC NO + O3 NO2 + O2 elementary steps (or elementary reactions): R turn to P in a single step A reaction’s molecularity is defined by the number of particles that participate as reactants in an elementary step. unimolecular bimolecular termolecular very rare

  3. NO2 + CO NO + CO2 NO2 + NO2 NO + NO3 Elem. Step 1: NO3 + CO NO2 + CO2 NO2 + CO NO + CO2 Elem. Step 2: these must add to give the overall reaction For multistep mechanisms, sequences of elementary steps are needed to go from R to P. e.g., For the reaction Above, NO3 is an intermediate. All multistep mechanisms have them.

  4. Molecularity Elementary Step Rate Law for that Elementary Step uni- A P bi- A + A P bi- A + B P ter- A + A + A P ter- A + A + B P ter- A + B + C P These rate laws are for elementary steps, not necessarily for the R P reaction. The rate law and relative speed of each elementary step (no matter how many there are) determine the overall rate law for the reaction; that is, the mechanism gives us the overall rate law. Rate laws for the elementary steps are found as follows... rate = k [A] rate = k [A]2 rate = k [A] [B] rate = k [A]3 rate = k [A]2 [B] rate = k [A] [B] [C]

  5. In producing vegetables from a garden, the rate-determining step is the growing of the food itself. Most reactions have multiple elementary steps. The slowest of these is the... rate-determining step. The correct rate law for a reaction must: 1) be based on… the r-d step 2) have only… REACTANTS in it (NOT intermediates).

  6. (not a REACTANT) CHCl3(g) + Cl2(g) CCl4(g) + HCl(g) has the following elementary steps: E. S. 1: Cl2 2 Cl (fast, eq) E. S. 2: CHCl3 + Cl CCl3 + HCl (slow) E. S. 3: CCl3 + Cl CCl4 (fast) State the rate law and the units on the rate constant. rate = k2 [CHCl3][Cl] For fast eq. steps, you can write a rate law in either dir. e.g., eq rate = k1f[Cl2] = k1r[Cl]2  [Cl] = (k1f/k1r[Cl2])1/2 So rate = k [CHCl3][Cl2]1/2 sub. this in for [Cl] above units on k = M–1/2 s–1

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