Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Chemical Kinetics Chapter 15 H2O2 decomposition in an insect H2O2 decomposition catalyzed by MnO2 REACTION RATES - RR = D [P ] = D [R ] Dt P =products Dt R = reactants Relative Rates Reactant 2A g - 4B + C D [A ] = D [B ] = D [C ] 2D t 4D t Dt Rate Calculations Collision Theory NO Collisions Collisions NO YES Energy Collisions Energy Orientation Factors Affecting RXN Rates • *Nature of Reactants • Temperature • Concentration • Surface Area/ Physical state • Catalysts Simulation: RATE MECHANISMS A Microscopic View of Reactions Sections 15.5 and 15.6 Mechanism: how reactants are converted to products at the molecular level. RATE LAW ----> MECHANISM experiment ----> theory 8 REACTION ORDER In general, for a A + b B --> x X with a catalyst “C” Rate = k m n p [A] [B] [C] The exponents m, n, and p • are the reaction order • can be 0, 1, 2 or fractions • must be determined by experiment! More on Mechanisms Reaction of cis-butene --> trans-butene is UNIMOLECULAR only one reactant is involved. A bimolecular reaction BIMOLECULAR — two different molecules must collide --> products Exo- or endothermic? 10 Collision Theory Reactions require (a) activation energy and (b) correct geometry. O3(g) + NO(g) ---> O2(g) + NO2(g) 1. Activation energy 2. Activation energy and geometry 11 Mechanisms O3 + NO reaction occurs in a single ELEMENTARY step. Most others involve a sequence of elementary steps. Adding elementary steps gives NET reaction. 12 Mechanisms Most rxns. involve a sequence of elementary steps. 2 I- + H2O2 + 2 H+ ---> I2 + 2 H2O Rate = k [I-] [H2O2] NOTE 1. Rate law comes from experiment 2. Order and stoichiometric coefficients not necessarily the same! 3. Rate law reflects all chemistry down to and including the slowest step in multistep reaction. 13 Mechanisms Most rxns. involve a sequence of elementary steps. 2 I- + H2O2 + 2 H+ ---> I2 + 2 H2O Rate = k [I-] [H2O2] Proposed Mechanism Step 1 — slow HOOH + I- --> HOI + OHStep 2 — fast Step 3 — fast HOI + I- --> I2 + OH2 OH- + 2 H+ --> 2 H2O Rate of the reaction controlled by slow step — RATE DETERMINING STEP, rds. Rate can be no faster than rds! 14 Mechanisms 15 2 I- + H2O2 + 2 H+ ---> I2 + 2 H2O Rate = k [I-] [H2O2] Step 1 — slow Step 2 — fast Step 3 — fast HOOH + I- --> HOI + OH- HOI + I- --> I2 + OH2 OH- + 2 H+ --> 2 H2O Elementary Step 1 is bimolecular and involves Iand HOOH. Therefore, this predicts the rate law should be Rate [I-] [H2O2] — as observed!! The species HOI and OH- are reaction intermediates. 16 Simulation:” Mechanisms Rate Laws and Mechanisms 17 NO2 + CO reaction: Rate = k[NO2]2 Two possible mechanisms Two steps: step 1 Single step Two steps: step 2 18 Ozone Decomposition Mechanism 2 O3 (g) ---> 3 O2 (g) [O3 ]2 Rate = k [O2 ] Proposed mechanism Step 1: fast, equilibrium O3 (g) <--> O2 (g) + O (g) Step 2: slow O3 (g) + O (g) ---> 2 O2 (g) 19 Sovled problems: pg 144 20