Post on 06-Aug-2015
HOMOGENEOUS REACTIONS
in which all reactants and products are in one phase
KINETICS Chemical kinetics, also known as reaction kinetics, is the study of rates of
chemical processes
KINETICS
Reaction RatesFactors affecting rate
Quantitative rate expressions
Determination
Factors
Models for Rates
Reaction Mechanisms
Effects of catalysts
RATES
Change in concentration of a reactant or product per unit rate
. per unit volume of the fluid for fluid-phase reactions, per unit area (or unit mass) of the
t
A
t- t
A - A
t time,in Change
A conc, in Change
0t
0t
B A
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
22 O 2NO 2NO
Graph: Concentration vs. time
Concentration vs Time
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
Co
nc.
,mo
l/L
[NO2]
[NO]
[O2]
22 O 2NO 2NO
M10725.1
0 - 400
0.0100 - 0.0031
t- t
NO - NO
t
NO 5
0400
0240022
-[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
Concentration vs Time
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
Co
nc.,
mo
l/L
[NO2]
[NO]
[O2]
Instantaneous rate
x
y line tangent of slope
t
NO rate 2
s 375
M0.009
t
NO s 100 @ rate 2
s
M10.42 s 100 @ rate 5-
Slope of tangent line at a point on the graph
INSTANTANEOUS RATE
Concentration vs Time
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
Co
nc.,
mo
l/L
[NO2]
[NO]
[O2]
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 (m
ol/L
)
Time (s)
[NO2 ]
t
Concentration vs Time
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]
t
NO rate 2
x
y line tangent of slope
s 225
M0.010
t
NO s 0 @ rate 2
s
M10.44 s 0 @ rate 5-
Slope of tangent line at time 0 (y intercept)
Initial Rate (t = 0)
RATE LAWS nm BAk rate k = rate constantm, n = order
22 O 2NO 2NO
rate = k[NO2]n
Order of Reaction In chemical kinetics, the order of reaction with respect to a certain reactant, is defined
as the power to which its concentration term.
Introduction to Rate Laws
Reversible chemical reactions
Forward:
Backward:
Equilibrium
:
22 O 2NO 2NO
2NO O 2NO 22
22 O 2NO 2NO
Introduction
Dominant Reaction:
Rate Law:
k, k’: specific rate constant
n : order of reactant
can be zero, fractional, or negative
22 O 2NO 2NO
n22 NOk
t
NO rate
-
n22 NOk
t
O erat
Method of Initial Rates
Unknown: k, m, n
Initial rate: instantaneous rate just after reaction 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
t
NO rate
-
General:
Substituting:
Solution:
n21
n22
NOk-
NOk-
1 rate
2 rate
n1
n2
5-
-4
0.010k-
0.020k-
10 7.1
10 2.8
2 ln n 4ln
2 n(2) 4 n
so
Rate constant
Rate 17.1 x 10-5 M s-1 = -k[0.01 M]2
k = 0.71 M-1 s-1
Rate 22.8 x 10-4 M s-1 = -k[0.02 M]2
k = 0.70 M-1 s-1
n22 NOk
t
NO rate
-
222 NO70.0
t
NO law rate
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
Types
Differential:
Rate dependence on concentration
Integrated:
Concentration dependence on time
n22 NOk
t
NO rate
-
n22 NOk
t
O erat
First Order Reactions
For aA products
Differential:
Integrated: first order reaction (order = 1) has a rate proportional to the concentration of one of the reactants. A common example of a first-order reaction is the phenomenon of radioactive decay. The rate law is:
rate = k[A] (or B instead of A), with k having the units of sec-1
Akt
A rate
-
0Alnkt - Aln t
kt A
Aln 0
t
The rate of reaction is proportional to the concentration of A
Half-life, first order reactions
Integrated law:
Half-life:
Half of initial reacted
[A]t = ½[A]0
Independent of [A]0
kt A
Aln 0
t
k
0.693 t
k
ln2 t
21
21
SECOND-ORDER REACTION
A second-order reaction (order = 2) has a rate proportional to the concentration of the square of a single reactant or the product of the concentration of two reactants:
rate = k[A]2 (or substitute B for A or k multiplied by the concentration of A times the concentration of B), with the units of the rate constant M-1sec-1
SECOND ORDER REACTIONS
For aA products
Differential:
Integrated:
2Akt
A rate
-
kt A
1
A
1
0
t
0A
1 kt
A
1
t
HALF-LIFE, SECOND ORDER REACTIONS
Integrated law:
Half-life:
Half of initial reacted
[A]t = ½[A]0
Inversely proportional to [A]0
kt A
1
A
1
0
t
0Ak
1 t
21
Zero-order reactions (order = 0) have a constant rate. This rate is independent of the concentration of the reactants. The rate law is:
rate = k, with k having the units of M/sec.
Zero Order Reactions
For aA products
Differential:
Integrated:
kAkt
A rate 0
-
kt- AA 0 t
0Akt - A t
Graphical Method
First order
Second order
Zero order
Straight line
0Alnkt - Aln t
0A
1 kt
A
1
t
0Akt - A t
bmx y
SummaryConditions set so dominant forward reaction
Differential Rate Lawsrate as a function of concentrationmethod of initial rates
Integrated Rate Lawsconcentration as a function of timegraphical method
Experimental data collectionRate law types can be interconverted
Reaction Mechanism
Chemical equation: Summary
Mechanism: Series of elementary steps
Elementary Steps: Reactions with rate laws
from molecularity
Molecularity: Number of species that must
collide to produce reaction
Reaction Mechanism
Proposed elementary steps must satisfy conditions:
— reasonable reactions
— sum of steps = overall balanced reaction
— mechanism rate law = experimental rate law
Rate-determining step
In a multi-step process:
SLOWEST step
Determines overall reaction rate
“Bottleneck”
Model for Kinetics
Collision Theory
rate determined by particle collisions
collision frequency and energy
Cl + NOCl → Cl2 + NO
Transition State Theory
how reactants convert to products
p fZ rate a
Z: no. of bimolecular collisions per secondfa: fraction with Ea
P: fraction with correct orientationEa: activation energy
Collision Theory (Bimolecular Collsions)
Arrhenius Equation
k: rate constant
Ea: activation energy (minimum required)T:absolute temperatureR: universal gas constantA: orientation factor
Energy & orientation requirements for reaction
RT
Ea
Ae k
Transition State Theory
Ea and internal energy:
Bonds breaking and forming
Atoms rearranging
“Transition State”
Unstable intermediate
At point of highest energy
Catalysts
• Speed reaction• Are not consumed
• Alternative pathway for reaction with lower Ea
Types
Homogeneous
Heterogeneous
Enzymes are biological catalysts
12_304
Ea (uncatalyzed )
Effectivecollisions(uncatalyzed)
Effectivecollisions(catalyzed)
Ea (catalyzed )
(a) (b)
Nu
mb
er
of
co
llis
ion
s
with
a g
ive
n e
ne
rgy
Nu
mb
er
of
co
llis
ion
s
with
a g
ive
n e
ne
rgy
Energy Energy