Today Announcements
is the last day of the second clicker
period. Clicker points for this period will be
calculated and posted over the weekend.
Suppose a polymerization reaction has a heat of reaction,
H =
and a S of
. At what temperature
would d epolymerization become possible?
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What is the
the Chapman Cycle?
of
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Chapman Cycle Reactions
(1) O2 + photon ---> 2 O
(2) O2 + O ----> O3
(3) O3 + photon ---> O2 + O (Shield)
(4) O3 + O ----> 2 O2
Ozone the transient
The average ozone molecule exists in
the atmosphere for 100-200 seconds!
BUT, it is constantly replaced by new
ozone molecules.
If you instantly swept away ALL of the
ozone, it would be back in roughly an
hour
What then is the ozone hole?
It is not a
hole.
It is a
depletion
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How does it happen?
Steady state production and destruction
of ozone is altered.
Production rate stays the same, and
destruction rate increases.
Process for increasing the rate is called
catalysis.
The destruction of ozone is catalyzed.
Defining Rate
In general a rate is defined as a change
per unit time.
What changes in a chemical reaction?
Often its the concentration of either a
reactant or product.
Mathematical Details
We need a single value for any rate.
"[ product ]
rate =
n product "t
!
rate =
"[ reactant ]
n reactant "t
!
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Why does the rate have a negative sign
when defined for a reactant?
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2.
3.
Because the initial
concentration is larger than
the final concentration.
Because the reaction is
slower as it continues.
Because the change in
concentration of the
reactant is negative, and
rate must be a positive
number.
Instantaneous vs. average
Consider
Figure Note 11.5
that
the rate of
the
reaction
changes
over time!
Ozone formation?
What does rate do for the formation of
ozone in the stratosphere?
Key idea - it cant happen very
efficiently at other levels of the
atmosphere because the concentration
of the various species must be right.
Suggests we need to think about
concentration.
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Rate Law
Mathematical expression that describes
the dependence of the rate on
concentration.
Definitions
rate = k [A]m[B]n
Rate constant: k
Is a function of temperature
m and n are often, but not always, integers.
Order of the reaction:
m (or n) = 0 zeroth order
m (or n) = 1 first order
m (or n) = 2 second order
For more than one reactant, the overall order is the
sum of the order for each reactant.
Key Idea
m (or n) can only be determined
experimentally.
They are not generally related to
stoichiometric coefficients
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Which equation is for a first
order rate law?
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Units and rate law
The units on rate are always
concentration per unit time.
molL-1s-1
In order to have this the overall unit, the
unit on the rate constant, k, varies
depending on the order of the reaction.
Units on rate constant
First order: s-1
Second order: Lmol-1s-1
If you see a question that provides the
units on k it is also telling you the order
of the reaction!
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Looking at the rate constant
Rate constant essentially tells us how
fast the reaction is.
If k is small, the rate is likely to be slow
(unless concentrations are very large)
If k is large, the rate is likely to be fast.
Alternatively, the unit of time on k is
often an indicator.
Using initial rates to determine
rate law
Strategy is to change concentration of
one reactant and measure initial rate.
Then change another, etc. to see how
the concentration affects the rate.
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