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17.3 - Mechanisms

What Really Goes on in Closed Flasks…. 17.3 - Mechanisms. Example:. Some reactions occur in one step. Ex: (CH 3 ) 3 CBr + H 2 O → (CH 3 ) 3 COH + HBr. But most do not. Step 1:. Step 2:. H. H. Step 3:. H. H. The steps of a reaction are known as its mechanism

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17.3 - Mechanisms

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  1. What Really Goes on in Closed Flasks… 17.3 - Mechanisms

  2. Example: Some reactions occur in one step

  3. Ex: (CH3)3CBr + H2O → (CH3)3COH + HBr But most do not Step 1: Step 2: H H Step 3: H H

  4. The steps of a reaction are known as its mechanism Each step of a mechanism is known as an elementary step The elementary steps add together to make the overall equation Mechanism

  5. Formed in one step, but used up in a later step • Do NOT appear in overall equation • Find the intermediate(s) below: Step 1: (CH3)3CBr → (CH3)3C+ + Br- Step 2: (CH3)3C+ + H2O → (CH3)3COH2+ Step 3: (CH3)3COH2+ → (CH3)3COH + H+ Intermediates

  6. Used in one step but remade in a later step (Opposite that of an intermediate) • Find the catalyst below! Step 1: NO2(g) + NO2(g)  NO3(g) + NO(g) Step 2: NO3(g) + CO(g)  NO2(g) + CO2(g) Find the Catalyst!

  7. Some steps are slower than others, and one is slowest of all. The slowest step determines how fast the entire reaction goes, and is called the rate-determining step The Rate-Determining Step (RDS)

  8. Because the RDS determines the rate of the reaction, you can write the reaction’s rate law directly from the RDS and its coefficients • Example 1: If RDS is (CH3)3CBr → (CH3)3C+ + Br-then Rate = k[(CH3)3CBr]1 • Example 2: If RDS is NO2(g) + NO2(g)  NO3(g) + NO(g) then Rate = k[NO2] [NO2]= k [NO2]2 A “Shortcut” to the Rate Law

  9. Energy Diagram of Single Step

  10. Energy Diagram of Multiple-Step Mechanisms EA1 EA2 ∆H

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