Jeff Notes 2

Reaction Mechanisms Thursday, October 23, 2008 8:50 AM

Reactions H2O2(l) -> H2O(l) + 1/2O2(g) Only one particle involved with collision of peroxide molecules C(s) + O2(g) -> CO2(g) Two particles colliding 2K(s) + I2(g) -> 2KI(s) Three particles colliding. Reactions involving three or less particles Chances of all three particles having the proper energy and orientation is high The reaction will proceed in a single step Reactions that require more than 4 particles to collide simultaneously rarely occur in one step Odds that all the particles will have the required energy and proper orientation decreases rapidly with 4 or more reactant particles Reactions that have 4 or more reactant species occur in a series of steps called a REACTION MECHANISM

Reaction Mechanism A series of steps that will add up to give the overall reaction (Like Hess's Law) 4 A + 3 B -> A4B3 Target (very unlikely to occur in a single step) 1 A + 1B -> AB (The product is the reactant in the next step) AB + 1A -> A2B (The species that are not found in the original equation are called reactive intermediates and are crossed off, ie AB) A2B + 1B -> A2B2 A2B2 + 1A -> A3B2 A3B2 + 1A -> A4B2 A4B2 + 1B -> A4B3 4A + 3B -> A4B3

Rate Determining Step The overall rate will equal the rate of the rate determining step The rate expression CANNOT include reactive intermediates even if they are part of the rate determining step. Example: 3A + 2B -> A3B2 Target A + 2B -> AB2 Fast AB2 + 2A -> A3B2 Slow -> Rate determining step The rate expression is r=k[A]2 (because you were left with 2A after eliminating reactive intermediates)

Each individual step in a reaction mechanism is called an elementary step An elementary step will proceed as written (It cannot be broken down into a simpler step) The reactive intermediates are produced in one elementary step and are immediately consumed in the next elementary step Do all elementary steps occur at the same rate? -> NO!!! Rates of all the individual steps can be fast or slow. The slowest elementary step is called the RATE DETERMINING STEP The overall rate will equal the rate of the rate determining step.

Homework: Practice 1-6 pg 377 7-8 pg 381 Section 1-4 pg 381

Reaction mechanisms are only best guesses - they must conform to the following conditions 1. Each step MUST be an elementary step involving no more than 3 reactant particles 2. The rate determining step must be consistent with the rate equation 3. The elementary steps must add up to the overall equation More than one reaction mechanism maybe possible

Chemistry Page 1