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Reaction Mechanisms

Reaction Mechanisms. eg:. 2 H 2(g) + O 2(g)  2 H 2 O (g). A “termolecular collision” (collision of 3 molecules) is an unlikely event. Most reactions occur as a series of bimolecular collisions:. step 1:. H 2 + O 2  H 2 O 2. (slow). step 2:. H 2 O 2 + H 2  2 H 2 O.

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Reaction Mechanisms

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  1. Reaction Mechanisms eg: 2 H2(g) + O2(g) 2 H2O(g) • A “termolecular collision” (collision of 3 molecules) is an unlikely event. • Most reactions occur as a series of bimolecular collisions: step 1: H2 + O2 H2O2 (slow) step 2: H2O2 + H2 2 H2O (fast) overall: 2 H2 + O2 2 H2O • This is a two step mechanism. • The first step is the “rate determining” step (slowest) • H2O2 is a “reaction intermediate”

  2. EA 2 H2+ O2 H 2 H2O • Highest peak represents the rate determining step. • The number of peaks shows the number of steps in the mechanism. • Reaction intermediate forms at the valley.

  3. eg: An exothermic reaction mechanism step 1: HBr + O2 HOOBr (slow) step 2: HOOBr + HBr  2 HOBr (fast) HOBr + HBr  H2O + Br2 step 3: (fast) HOBr + HBr  H2O + Br2 overall: 4 HBr + O2 2 H2O + 2 Br2 EA H • note that EA is always defined by the rate determining step.

  4. Practice: For the following 3 step reaction mechanism: (Exothermic). K + 2 NO  KN2O2 (fast) KN2O2 + H2 N2O + H2O + K (slow) N2O + H2 N2 + H2O (fast) Write the balanced net equation. Which step is rate determining? Name two reaction intermediates. Draw a PE diagram for this 3 step mechanism Identify the catalyst in this reaction.

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