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Chemical KineticsChemical KineticsChapter 15Chapter 15
H2O2 decomposition in an insect
H2O2 decomposition catalyzed by MnO2
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 KineticsChemical Kinetics
Energy Diagram
thermodynamics
KINETICS
4
the study of the study of REACTION RATESREACTION RATES and their relation to the way and their relation to the way the reaction proceeds, i.e., the reaction proceeds, i.e.,
its its MECHANISMMECHANISM..
•The reaction mechanism is our goal!
the study of the study of REACTION RATESREACTION RATES and their relation to the way and their relation to the way the reaction proceeds, i.e., the reaction proceeds, i.e.,
its its MECHANISMMECHANISM..
•The reaction mechanism is our goal!
KINETICS
5Reaction Reaction MechanismsMechanisms
The sequence of events at the molecular level that The sequence of events at the molecular level that control the speed and outcome of a reaction.control the speed and outcome of a reaction.
Br from biomass burning destroys stratospheric ozone. Br from biomass burning destroys stratospheric ozone. (See R.J. Cicerone, (See R.J. Cicerone, ScienceScience, volume 263, page 1243, 1994.), volume 263, page 1243, 1994.)
Step 1:Step 1:Br + OBr + O33 ---> BrO + O ---> BrO + O22
Step 2:Step 2:Cl + OCl + O33 ---> ClO + O ---> ClO + O22
Step 3:Step 3:BrO + ClO + light ---> Br + Cl + OBrO + ClO + light ---> Br + Cl + O22
NET: NET: 2 O2 O33 ---> 3 O ---> 3 O22
REACTION RATES
RR = [P ] = - [R ] t t
P =products R = reactants
7Determining a Reaction Determining a Reaction RateRate
Rate = the change Rate = the change in [dye] divided in [dye] divided by timeby time
The rate is The rate is determined from determined from the plot.the plot.
Rate = the change Rate = the change in [dye] divided in [dye] divided by timeby time
The rate is The rate is determined from determined from the plot.the plot.
Dy
e C
on
ce
ntr
atio
nD
ye
Co
nc
en
trat
ion
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
Collisions
EnergyCollisions
Energy
Orientation
NO
NO
YES
12
Concentrations and Concentrations and Rates Rates
To postulate a reaction To postulate a reaction mechanism, we studymechanism, we study
•• reaction ratereaction rate and and
•• its its concentration concentration dependencedependence
To postulate a reaction To postulate a reaction mechanism, we studymechanism, we study
•• reaction ratereaction rate and and
•• its its concentration concentration dependencedependence
13
Concentration and rateConcentration and rate
What is concentration of reactant as function of time?
The rate law is
Rate -[A]
time = k [A]
REACTION ORDERREACTION ORDERIn general, forIn general, for
a a AA + b + b BB --> x --> x X X
Rate = k [A]Rate = k [A]mm[B][B]nn
The exponents The exponents m,, and n m,, and n •• are the reaction order
• can be 0, 1, 2 or fractions
•• must be determined by must be determined by experiment!experiment!
Rate contantRate contant: : Arrhenius equation Arrhenius equation Rate contantRate contant: : Arrhenius equation Arrhenius equation
k Ae -Ea/RTk Ae -E
a/RTRate
constant
Temp (K)
8.31 x 10-3 kJ/K•molActivation energy
Frequency factor
Frequency factor = frequency of collisions with correct geometry.
Rate constant is dependent on only the activation energy and temperature
Simulation: RATE
17MECHANISMSA Microscopic View of Reactions
MECHANISMSA Microscopic View of Reactions
Mechanism: how reactants are converted to products at the molecular level.RATE LAW ----> MECHANISM
experiment ----> theory
18More on MechanismsMore on MechanismsMore on MechanismsMore 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
19Collision TheoryCollision TheoryCollision TheoryCollision Theory
Reactions require
(a) activation energy and
(b) correct geometry.
O3(g) + NO(g) ---> O2(g) + NO2(g)
2. Activation energy 2. Activation energy and geometryand geometry
1. Activation energy 1. Activation energy
20MechanismsMechanisms
O3 + NO reaction occurs in a single ELEMENTARY step. Most others involve a
sequence of elementary steps.
Adding elementary steps gives NET reaction.
212 I- + 2 H+ + H2O2---> I2 + 2 H2O
1. Rate law determined from experiment is:
Rate = k [IRate = k [I--] [H] [H22OO22]]
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.
22MechanismsMechanisms
Proposed MechanismProposed Mechanism
Step 1 — slow HOOH + I- --> HOI + OH-
Step 2 — fast HOI + I- --> I2 + OH-
Step 3 — fast 2 OH- + 2 H+ --> 2 H2O
Rate is controlled by slow step —
RATE DETERMINING SSTEP, RDS.
Rate can be no faster than RDS!
Proposed MechanismProposed Mechanism
Step 1 — slow HOOH + I- --> HOI + OH-
Step 2 — fast HOI + I- --> I2 + OH-
Step 3 — fast 2 OH- + 2 H+ --> 2 H2O
Rate is controlled by slow step —
RATE DETERMINING SSTEP, RDS.
Rate can be no faster than RDS!
2 I- + H2O2 + 2 H+ ---> I2 + 2 H2O Rate = k [I-] [H2O2]
23MechanismsMechanisms
Elementary Step 1 is bimolecular and involves I- and HOOH. Therefore, this predicts the rate
law should be
Rate [I-] [H2O2] — 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-] [H2O2] — as observed!!
The species HOI and OH- are reaction intermediates.
2 I- + H2O2 + 2 H+ ---> I2 + 2 H2O
Rate = k [I-] [H2O2]
Step 1 — slow HOOH + I- --> HOI + OH-
Step 2 — fast HOI + I- --> I2 + OH-
Step 3 — fast 2 OH- + 2 H+ --> 2 H2O
24
Simulation:” Mechanisms
25
Sovled problems: pg 144
26
NONO22 + CO NO + CO + CO NO + CO22
NO2 + CO reaction:
Rate = k[NO2]2
Single step
Two possible mechanisms
Two steps: step 1 Two steps: step 2
27
Ozone Decomposition Ozone Decomposition MechanismMechanism
Proposed mechanismProposed mechanism
Step 1: fast, equilibriumStep 1: fast, equilibrium
O3 (g) <--> O2 (g) + O (g)
Step 2: slowO3 (g) + O (g) ---> 2 O2 (g)
2 O2 O33 (g) ---> 3 O (g) ---> 3 O22 (g) (g)
Rate = k [O3]2
[O2]
28