Chemical Kinetics Chapter 15 H 2 O 2 decomposition in an insect H 2 O 2 decomposition catalyzed by MnO 2

2 We can use thermodynamics to tell if a reaction is product or reactant favored. But this gives us no info on HOW FAST reaction goes from reactants to products. KINETICS — the study of REACTION RATES and their relation to the way the reaction proceeds, i.e., its MECHANISM. We can use thermodynamics to tell if a reaction is product or reactant favored. But this gives us no info on HOW FAST reaction goes from reactants to products. KINETICS — the study of REACTION RATES and their relation to the way the reaction proceeds, i.e., its MECHANISM. Chemical Kinetics

Energy Diagram thermodynamics KINETICS

4 the study of REACTION RATES and their relation to the way the reaction proceeds, i.e., its MECHANISM. The reaction mechanism is our goal! the study of REACTION RATES and their relation to the way the reaction proceeds, i.e., its MECHANISM. The reaction mechanism is our goal! KINETICS

5 Reaction Mechanisms The sequence of events at the molecular level that control the speed and outcome of a reaction. Br from biomass burning destroys stratospheric ozone. (See R.J. Cicerone, Science, volume 263, page 1243, 1994.) Step 1:Br + O 3 ---> BrO + O 2 Step 2:Cl + O 3 ---> ClO + O 2 Step 3:BrO + ClO + light ---> Br + Cl + O 2 NET: 2 O 3 ---> 3 O 2

REACTION RATES RR =  [P ] = -  [R ]  t  t P = products R = reactants

7 Determining a Reaction Rate Rate = the change in [dye] divided by time The rate is determined from the plot. The rate is determined from the plot. Rate = the change in [dye] divided by time The rate is determined from the plot. The rate is determined from the plot. Dye Concentration Time

Relative Rates Reactant 2A  4B + C -  [A ] =  [ B ] =  [ C ] 2  t 4  t  t

Rate Calculations

Factors Affecting RXN Rates *Nature of Reactants Temperature Concentration Surface Area/ Physical state Catalysts

Collision Theory Collisions Energy Collisions Energy Orientation NO YES

12 Concentrations and Rates To postulate a reaction mechanism, we study reaction rate andreaction rate and its concentration dependenceits concentration dependence To postulate a reaction mechanism, we study reaction rate andreaction rate and its concentration dependenceits concentration dependence

13 Concentration and rate What is concentration of reactant as function of time? The rate law is

REACTION ORDER In general, for a A + b B --> x X Rate = k [A] m [B] n The exponents m,, and n are the reaction order can be 0, 1, 2 or fractions must be determined by experiment! must be determined by experiment!

Rate contant: Arrhenius equation Rate constant Temp (K) 8.31 x kJ/Kmol Activation energy Frequency factor Frequency factor = frequency of collisions with correct geometry. Rate constant is dependent on only the activation energy and temperature

SimulationSimulation: RATE

17 MECHANISMS A Microscopic View of Reactions Mechanism: how reactants are converted to products at the molecular level. RATE LAW ----> MECHANISM experiment ---->theory

18 More on Mechanisms Reaction is UNIMOLECULAR if only one reactant is involved. BIMOLECULAR if two different molecules must collide to form a products A bimolecular reaction

19 Collision Theory Reactions require (a) activation energy and (b) correct geometry. O 3 (g) + NO(g) ---> O 2 (g) + NO 2 (g) 2. Activation energy and geometry 1. Activation energy

20 Mechanisms O 3 + NO reaction occurs in a single ELEMENTARY step. Most others involve a sequence of elementary steps. Adding elementary steps gives NET reaction.

21 2 I H + + H 2 O 2 ---> I H 2 O 1.Rate law determined from experiment is: Rate = k [I - ] [H 2 O 2 ] Most rxns. have sequence of elementary steps. NOTE 2. Order and stoichiometric coefficients NOT the same! 3.Rate law reflects all chemistry down to and including the slowest step in multistep reaction.

22 Mechanisms Proposed Mechanism Step 1 — slow HOOH + I - --> HOI + OH - Step 2 — fastHOI + I - --> I 2 + OH - Step 3 — fast2 OH H + --> 2 H 2 O Rate is controlled by slow step — S RATE DETERMINING STEP, RDS. Rate can be no faster than RDS! Proposed Mechanism Step 1 — slow HOOH + I - --> HOI + OH - Step 2 — fastHOI + I - --> I 2 + OH - Step 3 — fast2 OH H + --> 2 H 2 O Rate is controlled by slow step — S RATE DETERMINING STEP, RDS. Rate can be no faster than RDS! 2 I - + H 2 O H + ---> I H 2 O Rate = k [I - ] [H 2 O 2 ]

23 Mechanisms Elementary Step 1 is bimolecular and involves I - and HOOH. Therefore, this predicts the rate law should be Rate  [I - ] [H 2 O 2 ] — as observed!! The species HOI and OH - are reaction intermediates. Elementary Step 1 is bimolecular and involves I - and HOOH. Therefore, this predicts the rate law should be Rate  [I - ] [H 2 O 2 ] — as observed!! The species HOI and OH - are reaction intermediates. 2 I - + H 2 O H + ---> I H 2 O Rate = k [I - ] [H 2 O 2 ] Step 1 — slowHOOH + I - --> HOI + OH - Step 2 — fastHOI + I - --> I 2 + OH - Step 3 — fast2 OH H + --> 2 H 2 O

24 SimulationSimulation:” Mechanisms

25 Sovled problems: pg 144

26 NO 2 + CO NO + CO 2 NO 2 + CO reaction: Rate = k[NO 2 ] 2 Single step Two possible mechanisms Two steps: step 1Two steps: step 2

27 Ozone Decomposition Mechanism Proposed mechanism Step 1: fast, equilibrium O 3 (g) O 2 (g) + O (g) Step 2: slowO 3 (g) + O (g) ---> 2 O 2 (g) 2 O 3 (g) ---> 3 O 2 (g)

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