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Lecture

14webct Chemical Kinetics.pdf

20 Pages
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Department
Chemistry (Agric&EnvirSci)
Course Code
AECH 111
Professor
Alice Cherestes

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1/14/2012
1
Chapter 14
Chemical Kinetics
1
Kinetics
In kinetics we study the rate at which a
In
kinetics
we
study
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
Chemical
Kinetics
(exactly how the reaction occurs).
2
Factors That Affect Reaction Rates
Ph i l t t f th t t
Ph
ys
i
ca
l
s
t
a
t
e o
f
th
e reac
t
an
t
s.
In order to react, molecules must come
in contact with each other.
The more homogeneous the mixture
of reactants, the faster the molecules
Chemical
Kinetics
can react.
3
Factors That Affect Reaction
Rates
Concentration of
reactants.
As the
concentration of
reactants increases,
so does the
Chemical
Kinetics
likelihood that
reactant molecules
will collide.
4
1/14/2012
2
Factors That Affect Reaction Rates
Temperature
Temperature
At higher temperatures, reactant
molecules have more kinetic energy,
move faster, and collide more often
and with
g
reater ener
gy
.
Chemical
Kinetics
ggy
5
Factors That Affect Reaction Rates
Presence of a catalyst
Presence
of
a
catalyst
.
Catalysts speed up reactions by
changing the mechanism of the
reaction.
Catalysts are not consumed during
Chemical
Kinetics
Catalysts
are
not
consumed
during
the course of the reaction.
6
Reaction Rates
Chemical
Kinetics
Rates of reactions can be determined by
monitoring the change in concentration
of either reactants or products as a function
of time. 7
Reaction Rates
C4H9Cl(aq) + H2O(l) C4H9OH(aq) + HCl(aq)
In this reaction,
the concentration
of butyl chloride,
C4H9Cl, was
measured at
Chemical
Kinetics
.
8
1/14/2012
3
Reaction Rates
The average rate of
C4H9Cl(aq) + H2O(l) C4H9OH(aq) + HCl(aq)
The
average
rate
of
the reaction over each
interval is the change
in concentration
divided by the change
in time:
Chemical
Kinetics
in
time:
Average rate = [C4H9Cl]
t
9
Reaction Rates
Note that the
C4H9Cl(aq) + H2O(l) C4H9OH(aq) + HCl(aq)
average rate
decreases as the
reaction proceeds.
This is because as
the reaction goes
Chemical
Kinetics
forward, there are
fewer collisions
between reactant
molecules.
10
Reaction Rates
Aplotof[C
4
H
9
Cl]
C4H9Cl(aq) + H2O(l) C4H9OH(aq) + HCl(aq)
A
plot
of
[C
4
H
9
Cl]
versus time for this
reaction yields a curve
like this.
The slope of a line
tangent to the curve at
any point is the
Chemical
Kinetics
any
point
is
the
instantaneous rate at
that time.
11
Reaction Rates
All reactions slow down
C4H9Cl(aq) + H2O(l) C4H9OH(aq) + HCl(aq)
All
reactions
slow
down
over time.
Therefore, the best
indicator of the rate of a
reaction is the
instantaneous rate near
the
beginning
of the
Chemical
Kinetics
the
beginning
of
the
reaction.
12

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