Chemical Kinetics - Cabrillo College - Breakthroughs …dscoggin/chem1b/lecturenotes/Ch16... · PPT...
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Chemical Kinetics Chapter 16 Chapter 16
Transcript of Chemical Kinetics - Cabrillo College - Breakthroughs …dscoggin/chem1b/lecturenotes/Ch16... · PPT...
Chemical Kinetics
Chapter 16Chapter 16
Kinetics
Reaction RatesReaction RatesFactors affecting rateFactors affecting rateQuantitative rate expressionsQuantitative rate expressions
DeterminationDeterminationFactorsFactors
Models for RatesModels for RatesReaction MechanismsReaction MechanismsEffects of catalystsEffects of catalysts
Rates
Change in concentration of a reactant or product Change in concentration of a reactant or product per unit timeper unit time
tA
t- tA - A
t time,in ChangeA conc, in Change
0t
0t
B A
12_292
.20
.40
.60
.80
1.00
400 800 1200 1600 2000
Rate = 5.4 x 10-4 mol/L.s
Rate = 2.7 x 10-4 mol/L.s
[N2O
5] (m
ol/L
)
Time (s)
Factors affecting rates
Nature of the reactantsNature of the reactantsState of subdivision/surface areaState of subdivision/surface areaConcentrationConcentrationTemperatureTemperatureCatalystsCatalysts
Reactants
ComplexityComplexityBond strengthsBond strengthsEtc.Etc.
242342 O OHC O HC
242342 O OHC O HC
242342 O OHC O HC
Concentrations as functions of time
22 O 2NO 2NO
Time(s) [NO2] [NO] [O2]0 0.0100 0.0000 0.000050 0.0079 0.0021 0.0011
100 0.0065 0.0035 0.0018150 0.0055 0.0045 0.0023200 0.0048 0.0052 0.0026250 0.0043 0.0057 0.0029300 0.0038 0.0062 0.0031350 0.0034 0.0066 0.0033400 0.0031 0.0069 0.0035
Graph: Concentration vs. time
22 O 2NO 2NO
M10725.1 0 - 400
0.0100 - 0.0031 t- t
NO - NO
tNO 5
0400
0240022
Average Rate
-[NO2]/t time period(s)–4.20E-05 0 - 50 –2.80E-05 50 - 100 –2.00E-05 100 - 150 –1.40E-05 150 - 200 –1.00E-05 200 - 250 –1.00E-05 250 - 300 –8.00E-06 300 - 350 –6.00E-06 350 - 400 –1.75E-05 0 - 400
Change of concentration in a time intervalChange of concentration in a time interval
Average Rate
0
0.002
0.004
0.006
0.008
0.01
0.012
0 50 100 150 200 250 300 350 400 450
Time, sec
Con
c.,m
ol/L
[NO2]
[NO]
[O2]
sM10725.1
0 - 4000.0100 - 0.0031
t- tNO - NO
t
NO 5
0400
0240022
Slope of line between two points on the graphSlope of line between two points on the graph
Instantaneous rate
xy line tangent of slope
t
NO rate 2
s 375M0.009
tNO s 100 @ rate 2
sM10.42 s 100 @ rate 5-
Slope of tangent line at a point on the graphSlope of tangent line at a point on the graph
Instantaneous Rate
0.009 M
375 s
12_291
0.000370s
O2
0.0025
0.005
0.0075
0.0100
0.0006
70s
0.0026
110 s
NO2
NO
50 100 150 200 250 300 350 400
Con
cent
ratio
ns (
mol
/L)
Time (s)
[NO2 ]
t
Initial Rate (t = 0)
Initial rate
xy line tangent of slope
t
NO rate 2
s 225M0.010
tNO s 0 @ rate 2
sM10.44 s 0 @ rate 5-
Slope of tangent line at time 0 (y intercept)Slope of tangent line at time 0 (y intercept)
Rate Laws
k k == rate constantrate constantm, n m, n == orderorder
nm BAk rate
22 O 2NO 2NO
rate rate == k[NOk[NO22]]nn
Introduction to Rate Laws
Reversible chemical reactionsReversible chemical reactions
Forward:Forward:
Backward:Backward:
Equilibrium:Equilibrium:
22 O 2NO 2NO
2NO O 2NO 22
22 O 2NO 2NO
Introduction
Dominant Reaction:Dominant Reaction:
Rate Law:Rate Law:
k, k’:k, k’: specific rate constantspecific rate constant n :n : order of reactantorder of reactant
can be zero, fractional, or negativecan be zero, fractional, or negative
22 O 2NO 2NO
n22 NOk
tNO rate
-
n22 NOk
tO erat
Method of Initial Rates
Unknown:Unknown: k, m, nk, m, n
Initial rate:Initial rate: instantaneous rate just after instantaneous rate just after reaction is initiatedreaction is initiated
nm BAk rate
Initial Rates, NO2 decomposition
22 O 2NO 2NO
Experiment Initial Conc.
[NO2]Rate [O2] Formation
1 0.01 7.1 x 10-5
2 0.02 2.8 x 10-4
n22 NOk
tNO rate
-
Order of Reaction
General:General:
Substituting:Substituting:
Solution:Solution:
n21
n22
NOk-NOk-
1 rate2 rate
n1
n2
5-
-4
0.010k-0.020k-
10 7.110 2.8
2 ln n 4ln2 n(2) 4 n
so
Rate constant
Rate 1Rate 17.1 x 107.1 x 10-5 -5 M sM s-1-1 == -k[0.01 M]-k[0.01 M]22
kk == 0.71 M0.71 M-1-1 s s-1-1
Rate 2Rate 22.8 x 102.8 x 10-4 -4 M sM s-1-1 == -k[0.02 M]-k[0.02 M]22
kk == 0.70 M0.70 M-1-1 s s-1-1
n22 NOk
tNO rate
-
222 NO70.0
tNO law rate
You try
Experiment Initial Conc.
[H2]Initial Conc.
[I2] Rate
1 0.0113 0.0011 1.9 x 10-23
2 0.0220 0.0033 1.1 x 10-22
3 0.0550 0.0011 9.3 x 10-23
4 0.0220 0.0056 1.9 x 10-22
2HI I H 22
O2 + 2 NO 2NO2
Overall Order
Sum:Sum: 11 ++ 22 ++ 33== 66
Overall order of reaction:Overall order of reaction: 66
3232 IHSeOHk rate
Types
Differential:Differential:RateRate dependence on dependence on concentrationconcentration
Integrated:Integrated:ConcentrationConcentration dependence on dependence on timetime
n22 NOk
tNO rate
-
n22 NOk
tO erat
First Order Reactions
For aA For aA products products
Differential:Differential:
Integrated:Integrated:
AktA rate
-
0Alnkt - Aln t
kt AA
ln 0 t
Half-life, first order reactions
Integrated law:Integrated law:
Half-life:Half-life:
Half of initial reactedHalf of initial reacted[A][A]tt = = ½[A]½[A]00
Independent of Independent of [A][A]00
kt AA
ln 0 t
k0.693 t
kln2 t
21
21
Second Order Reactions
For aA For aA products products
Differential:Differential:
Integrated:Integrated:
2AktA rate
-
kt A1
A1
0
t
0A1 kt
A1
t
Half-life, second order reactions
Integrated law:Integrated law:
Half-life:Half-life:
Half of initial reactedHalf of initial reacted[A][A]tt = = ½[A]½[A]00
Inversely proportional to Inversely proportional to [A][A]00
kt A1
A1
0
t
0Ak1 t
21
Zero Order Reactions
For aA For aA products products
Differential:Differential:
Integrated:Integrated:
kAktA rate 0
-
kt- AA 0 t
0Akt - A t
Graphical Method
First orderFirst order
Second orderSecond order
Zero orderZero order
Straight lineStraight line
0Alnkt - Aln t
0A1 kt
A1
t
0Akt - A t
bmx y
First order
Plot:Plot:ln[A] vs. timeln[A] vs. time
ln[A]
time
ln[A]0
slope = -k 0Alnkt - Aln t
bmx y
Second order
Plot:Plot: 1 1 vs. time vs. time [A][A]
time
1 [A]o
slope = k
bmx y
0A1 kt
A1
t
1 [A]
Zero order
Plot:Plot:[A] vs. time[A] vs. time
[A]
time
[A]0
slope = -kbmx y
0Akt - A t
SummaryConditions set so dominant forward reactionConditions set so dominant forward reaction
Differential Rate LawsDifferential Rate Lawsrate as a function of concentrationrate as a function of concentrationmethod of initial ratesmethod of initial rates
Integrated Rate LawsIntegrated Rate Lawsconcentration as a function of timeconcentration as a function of timegraphical method graphical method
Experimental data collectionExperimental data collectionRate law types can be interconvertedRate law types can be interconverted
Reaction Mechanism
Chemical equation:Chemical equation: SummarySummary
Mechanism:Mechanism: Series of elementary stepsSeries of elementary steps
Elementary Steps:Elementary Steps: Reactions with rate lawsReactions with rate lawsfrom molecularityfrom molecularity
Molecularity:Molecularity: Number of species that must Number of species that must collide to produce reactioncollide to produce reaction
Reaction Mechanism
Proposed elementary steps must satisfy conditions:Proposed elementary steps must satisfy conditions:
— reasonable reactionsreasonable reactions
— sum of steps = overall balanced reactionsum of steps = overall balanced reaction
— mechanism rate law = experimental rate mechanism rate law = experimental rate lawlaw
Intermediates
—appear in stepsappear in steps
—produced in one stepproduced in one step
—used in subsequentused in subsequent
—not in overall equationnot in overall equation
Rate-determining step
In a multi-step process:In a multi-step process:
SLOWEST stepSLOWEST step
Determines overall reaction rateDetermines overall reaction rate
““Bottleneck”Bottleneck”
Model for Kinetics
Collision TheoryCollision Theoryrate determined by particle collisionsrate determined by particle collisionscollision frequency and energycollision frequency and energy
Transition State TheoryTransition State Theoryhow reactants convert to productshow reactants convert to products
Collision Theory (Bimolecular Collsions)
Z:Z: no. of bimolecular collisions per no. of bimolecular collisions per secondsecond
ffaa:: fraction with Efraction with Eaa
P:P: fraction with correct orientationfraction with correct orientation
EEaa:: activation energyactivation energy
p fZ rate a
Arrhenius Equation
k:k: rate constantrate constantEEaa:: activation energy (minimum required)activation energy (minimum required)T:T: absolute temperatureabsolute temperatureR:R: universal gas constantuniversal gas constantA:A: orientation factororientation factor
Energy & orientation requirements for reactionEnergy & orientation requirements for reaction
RTEa
Ae k
Hydrolysis of an ester
Transition State Theory
EEaa and internal energy: and internal energy:
Bonds breaking and formingBonds breaking and formingAtoms rearrangingAtoms rearranging
““Transition State”Transition State”Unstable intermediateUnstable intermediateAt point of highest energyAt point of highest energy
forward reaction reverse reaction
exothermic reaction
I- + CH3Cl Cl- + CH3I
Catalysts
Speed reactionSpeed reactionAre not consumedAre not consumed
Alternative pathway for reaction with lower EAlternative pathway for reaction with lower Eaa
TypesTypesHomogeneousHomogeneousHeterogeneousHeterogeneous
Enzymes are biological catalystsEnzymes are biological catalysts
12_304
Ea (uncatalyzed )
Effectivecollisions(uncatalyzed)
Effectivecollisions(catalyzed)
Ea (catalyzed )
(a) (b)
Num
ber o
f col
lisio
nsw
ith a
giv
en e
nerg
y
Num
ber o
f col
lisio
nsw
ith a
giv
en e
nerg
y
Energy Energy
Adsorption, activation, reaction, desorption
