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2.Definition of Steady state approximation
3. In Chemical kinetics in steady state state approximation
4. Mechanism involving in steady state approximation
5. rate of formation, using steady state approximation plot
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1. What is the steady state approximation
2.Definition of Steady state approximation
3. In Chemical kinetics in steady state state approximation
4. Mechanism involving in steady state approximation
5. rate of formation, using steady state approximation plot
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So far I have uploaded RE1,2,3,4,5,6, and 7
Now is time to study catalysis and catalytic Reactor
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Chemical Kinetics: A common chapter covered in Gen Chem 2. Chemical kinetics, also known as reaction kinetics, is the branch of physical chemistry that is concerned with understanding the rates of chemical reactions
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2. Chemical
Kinetics
Kinetics
• Studies the rate at which a chemical
process occurs.
• Besides information about the speed at
which reactions occur, kinetics also
sheds light on the reaction mechanism
(exactly how the reaction occurs).
3. Chemical
Kinetics
Factors That Affect Reaction Rates
• Physical State of the Reactants
In order to react, molecules must come in
contact with each other.
The more homogeneous the mixture of
reactants, the faster the molecules can
react.
4. Chemical
Kinetics
Factors That Affect Reaction Rates
• Concentration of Reactants
As the concentration of reactants increases,
so does the likelihood that reactant
molecules will collide.
5. Chemical
Kinetics
Factors That Affect Reaction Rates
• Temperature
At higher temperatures, reactant
molecules have more kinetic energy,
move faster, and collide more often and
with greater energy.
6. Chemical
Kinetics
Factors That Affect Reaction Rates
• Presence of a Catalyst
Catalysts speed up reactions by
changing the mechanism of the
reaction.
Catalysts are not consumed during
the course of the reaction.
7. Chemical
Kinetics
Reaction Rates
Rates of reactions can be determined by
monitoring the change in concentration of
either reactants or products as a function of
time.
8. Chemical
Kinetics
Reaction Rates
In this reaction, the
concentration of
butyl chloride,
C4H9Cl, was
measured at various
times.
C4H9Cl(aq) + H2O(l) → C4H9OH(aq) + HCl(aq)
9. Chemical
Kinetics
Reaction Rates
The average rate of
the reaction over
each interval is the
change in
concentration divided
by the change in time:
Average rate =
∆[C4H9Cl]
∆t
C4H9Cl(aq) + H2O(l) → C4H9OH(aq) + HCl(aq)
10. Chemical
Kinetics
Reaction Rates
• Note that the average
rate decreases as the
reaction proceeds.
• This is because as the
reaction goes forward,
there are fewer
collisions between
reactant molecules.
C4H9Cl(aq) + H2O(l) → C4H9OH(aq) + HCl(aq)
11. Chemical
Kinetics
Reaction Rates
• A plot of concentration
vs. time for this reaction
yields a curve like this.
• The slope of a line
tangent to the curve at
any point is the
instantaneous rate at
that time.
C4H9Cl(aq) + H2O(l) → C4H9OH(aq) + HCl(aq)
12. Chemical
Kinetics
Reaction Rates
• All reactions slow down
over time.
• Therefore, the best
indicator of the rate of a
reaction is the
instantaneous rate near
the beginning.
C4H9Cl(aq) + H2O(l) → C4H9OH(aq) + HCl(aq)
13. Chemical
Kinetics
Reaction Rates and Stoichiometry
• In this reaction, the ratio
of C4H9Cl to C4H9OH is
1:1.
• Thus, the rate of
disappearance of C4H9Cl
is the same as the rate
of appearance of
C4H9OH.
C4H9Cl(aq) + H2O(l) → C4H9OH(aq) + HCl(aq)
Rate =
-∆[C4H9Cl]
∆t
=
∆[C4H9OH]
∆t
14. Chemical
Kinetics
Reaction Rates and Stoichiometry
• What if the ratio is not 1:1?
2 HI(g) → H2(g) + I2(g)
•Therefore,
Rate = − 1
2
∆[HI]
∆t
=
∆[I2]
∆t
15. Chemical
Kinetics
Reaction Rates and Stoichiometry
• To generalize, then, for the reaction
aA + bB cC + dD
Rate = −
1
a
∆[A]
∆t
= −
1
b
∆[B]
∆t
=
1
c
∆[C]
∆t
1
d
∆[D]
∆t
=
19. Chemical
Kinetics
Concentration and Rate
• This means
Rate ∝ [NH4
+
]
Rate ∝ [NO2
−
]
Rate ∝ [NH+
] [NO2
−
]
or
Rate = k [NH4
+
] [NO2
−
]
• This equation is called the rate law, and
k is the rate constant.
20. Chemical
Kinetics
Rate Laws
• A rate law shows the relationship
between the reaction rate and the
concentrations of reactants.
• The exponents tell the order of the
reaction with respect to each reactant.
• This reaction is
First-order in [NH4
+
]
First-order in [NO2
−
]
21. Chemical
Kinetics
Rate Laws
• The overall reaction order can be found
by adding the exponents on the
reactants in the rate law.
• This reaction is second-order overall.
22. Chemical
Kinetics
Integrated Rate Laws
Using calculus to integrate the rate law
for a first-order process gives us
ln
[A]t
[A]0
= −kt
Where
[A]0 is the initial concentration of A.
[A]t is the concentration of A at some time, t,
during the course of the reaction.
27. Chemical
Kinetics
First-Order Processes
• When ln P is plotted as a function of time, a
straight line results.
• Therefore,
The process is first-order.
k is the negative slope: 5.1 × 10-5
s−1
.
30. Chemical
Kinetics
Second-Order Processes
The decomposition of NO2 at 300°C is described by
the equation
NO2 (g) NO (g) + 1/2 O2 (g)
and yields data comparable to this:
Time (s) [NO2], M
0.0 0.01000
50.0 0.00787
100.0 0.00649
200.0 0.00481
300.0 0.00380
31. Chemical
Kinetics
Second-Order Processes
• Graphing ln [NO2] vs. t
yields:
Time (s) [NO2], M ln [NO2]
0.0 0.01000 −4.610
50.0 0.00787 −4.845
100.0 0.00649 −5.038
200.0 0.00481 −5.337
300.0 0.00380 −5.573
• The plot is not a straight
line, so the process is not
first-order in [A].
32. Chemical
Kinetics
Second-Order Processes
• Graphing ln 1/
[NO2] vs. t,
however, gives this
plot.
Time (s) [NO2], M 1/[NO2]
0.0 0.01000 100
50.0 0.00787 127
100.0 0.00649 154
200.0 0.00481 208
300.0 0.00380 263
• Because this is a
straight line, the
process is second-
order in [A].
33. Chemical
Kinetics
Half-Life
• Half-life is defined
as the time required
for one-half of a
reactant to react.
• Because [A] at t1/2 is
one-half of the
original [A],
[A]t = 0.5 [A]0.
34. Chemical
Kinetics
Half-Life
For a first-order process, this becomes
0.5 [A]0
[A]0
ln = −kt1/2
ln 0.5 = −kt1/2
−0.693 = −kt1/2
= t1/2
0.693
kNOTE: For a first-order
process, the half-life does
not depend on [A]0.
37. Chemical
Kinetics
The Collision Model
• In a chemical reaction, bonds are
broken and new bonds are formed.
• Molecules can only react if they collide
with each other.
39. Chemical
Kinetics
Activation Energy
• In other words, there is a minimum amount of energy
required for reaction: the activation energy, Ea.
• Just as a ball cannot get over a hill if it does not roll
up the hill with enough energy, a reaction cannot
occur unless the molecules possess sufficient energy
to get over the activation energy barrier.
40. Chemical
Kinetics
Reaction Coordinate Diagrams
It is helpful to
visualize energy
changes
throughout a
process on a
reaction coordinate
diagram like this
one for the
rearrangement of
methyl isonitrile.
41. Chemical
Kinetics
Reaction Coordinate Diagrams
• It shows the energy of
the reactants and
products (and,
therefore, ∆E).
• The high point on the
diagram is the transition
state.
• The species present at the transition state is
called the activated complex.
• The energy gap between the reactants and the
activated complex is the activation energy
barrier.
44. Chemical
Kinetics
Maxwell–Boltzmann Distributions
• If the dotted line represents the activation
energy, as the temperature increases, so does
the fraction of molecules that can overcome
the activation energy barrier.
• As a result, the
reaction rate
increases.
46. Chemical
Kinetics
Arrhenius Equation
Svante Arrhenius developed a mathematical
relationship between k and Ea:
k = A e−Ea/RT
where A is the frequency factor, a number that
represents the likelihood that collisions would
occur with the proper orientation for reaction.
47. Chemical
Kinetics
Arrhenius Equation
Taking the natural
logarithm of both
sides, the equation
becomes
ln k = -Ea ( ) + ln A
1
RT
y = mx + b
Therefore, if k is determined experimentally at
several temperatures, Ea can be calculated
from the slope of a plot of ln k vs. 1/T.
51. Chemical
Kinetics
Multistep Mechanisms
• In a multistep process, one of the steps will
be slower than all others.
• The overall reaction cannot occur faster than
this slowest, rate-determining step.
52. Chemical
Kinetics
Slow Initial Step
• The rate law for this reaction is found
experimentally to be
Rate = k [NO2]2
• CO is necessary for this reaction to occur, but the
rate of the reaction does not depend on its
concentration.
• This suggests the reaction occurs in two steps.
NO2 (g) + CO (g) → NO (g) + CO2 (g)
53. Chemical
Kinetics
Slow Initial Step
• A proposed mechanism for this reaction is
Step 1: NO2 + NO2 → NO3 + NO (slow)
Step 2: NO3 + CO → NO2 + CO2 (fast)
• The NO3 intermediate is consumed in the second step.
• As CO is not involved in the slow, rate-determining
step, it does not appear in the rate law.
54. Chemical
Kinetics
Fast Initial Step
• The rate law for this reaction is found to
be
Rate = k [NO]2
[Br2]
• Because termolecular processes are
rare, this rate law suggests a two-step
mechanism.
2 NO (g) + Br2 (g) → 2 NOBr (g)
55. Chemical
Kinetics
Fast Initial Step
• A proposed mechanism is
Step 2: NOBr2 + NO → 2 NOBr (slow)
Step 1 includes the forward and reverse reactions.
Step 1: NO + Br2 NOBr2 (fast)
56. Chemical
Kinetics
Fast Initial Step
• The rate of the overall reaction depends
upon the rate of the slow step.
• The rate law for that step would be
Rate = k2 [NOBr2] [NO]
• But how can we find [NOBr2]?
57. Chemical
Kinetics
Fast Initial Step
• NOBr2 can react two ways:
With NO to form NOBr
By decomposition to reform NO and Br2
• The reactants and products of the first
step are in equilibrium with each other.
• Therefore,
Ratef = Rater
58. Chemical
Kinetics
Fast Initial Step
• Because Ratef = Rater ,
k1 [NO] [Br2] = k−1 [NOBr2]
• Solving for [NOBr2] gives us
k1
k−1
[NO] [Br2] = [NOBr2]