Podcast 12.4 Mechanisms.mp4
X
AP Chemistry Podcast 12.4
Reaction ____________________:
How Do We Know the Mechanism is Correct?
Example One: nitrogen dioxide and carbon monoxide react by the balanced chemical equation:
Intermediates
Molecularity: the number of pieces that must come together.
Example Two
Molecularity and Rate Law
Example Two
Molecularity and Rate Law
RATE-DETERMINING STEP:
Getting Rid of Intermediates in the Rate Law
Example 3: An Intermediate Formed in Reversible Reactions
Example Four: Intermediates Formed in Fast Reactions
Example 3: An Intermediate Formed in Reversible Reactions
Example Five: The reaction between NO and H2 is believed to occur in the following three-step process.
Example Four: Intermediates Formed in Fast Reactions
Example Five: The reaction between NO and H2 is believed to occur in the following three-step process.
00:00
/
00:00
CC
AP
ChemistryPodcast
12.4 Mechanisms
of
Reactions 1
Reaction
____________________: A
method
that
is
used
to
show
the
intermediate
processes
that
occur
during
a
reactionLists
the
proposed
changes
that
take
place
to
the
reactants
as
the
product(s)
is
being
formed.
Two
or
three
chemical
reactions,
referred
to
as
_______________________
reactions
or
elementary
steps,
shown
one
on
top
of
the
other.
2
How
Do
We
Know
the
Mechanism
is
Correct? Must
follow
the
_________
________
to
be
considered
validMust
also
follow
the
stoichiometry
of
the
overall
reaction 3
Example
One:
nitrogen
dioxide
and
carbon
monoxide
react
by
the
balanced
chemical
equation:
NO2
(g)
+
CO(g)
→
NO(g)
+
CO2
(g)
Analysis
of
the
reaction
has
detected
the
presence
of
the
substance
NO3
,
which
is
neither
a
reactant
nor
a
product
of
the
reaction.
One
explanation
for
this
finding
is
that
the
reaction
proceeds
in
two
steps:
Step
1
)
NO2
+
NO2
→
NO3
+
NO
Step
2
)
NO3
+
CO
→
NO2
+
CO2
5
Intermediates Reaction
___________________
:
Any
substance
that
is
neither
a
reactant
nor
a
product
but
that
exists
during
the
reactionIntermediates
are
produced
during
the
reaction,
but
are
__________________
in
a
later
step
6
Molecularity:
the
number
of
pieces
that
must
come
together. In
each
elementary
step,
the
number
of
molecules
that
take
part
in
the
reaction
determines
the
_______________________
of
that
step.Unimolecular:
_________
molecule
is
involved
Bimolecular:
____________
molecules
reactingTermolecular:
reactions
involve
___________
molecules
(reacting)
but
are
quite
rare
because
they
require
the
simultaneous
collisions
of
three
molecules.
7
Example
Two Using
the
two
elementary
steps
shown
below,
construct
the
balanced
equation,
and
determine
the
molecularity
of
each
step.
NO2
+
F2
→
NOF2
+
O
NO2
+
O
→
NO3
8
Molecularity
and
Rate
Law A
products
Rate
=
k[A]A+A
productsRate=
k[A]22A
productsRate=
k[A]2A+B
productsRate=
k[A][B]A+A+B
products
Rate=
k[A]2[B]2A+B
products
Rate=
k[A]2[B]A+B+C
products
Rate=
k[A][B][C] 9
Example
Two Using
the
two
elementary
steps
shown
below,
construct
the
balanced
equation,
and
determine
the
molecularity
of
each
step.
NO2
+
F2
→
NOF2
+
O
NO2
+
O
→
NO3
8
Molecularity
and
Rate
Law A
products
Rate
=
k[A]A+A
productsRate=
k[A]22A
productsRate=
k[A]2A+B
productsRate=
k[A][B]A+A+B
products
Rate=
k[A]2[B]2A+B
products
Rate=
k[A]2[B]A+B+C
products
Rate=
k[A][B][C] 9
RATE-DETERMINING
STEP:
In
reaction
mechanisms,
the
different
elementary
steps
tend
to
occur
at
different
_____________.
The
____________________
step
is
known
as
the
rate-determining
step
for
the
reaction
and
will
determine
the
overall
rate
at
which
the
reaction
will
proceed.
10
Getting
Rid
of
Intermediates
in
the
Rate
Law Use
the
reactions
that
form
themIf
the
reactions
are
fast
and
irreversible
-
the
concentration
of
the
intermediate
is
based
on
____________________.If
it
is
formed
by
a
reversible
reaction
set
the
rates
____________
to
each
other. 11
Example
3:
An
Intermediate
Formed
in
Reversible
Reactions 2
NO
+
O2
2
NO2Mechanism2
NO
N2O2
(fast)N2O2
+
O2
2
NO2
(slow) 12
Example
Four:
Intermediates
Formed
in
Fast
Reactions 2
IBr
I2+
Br2
MechanismIBrI
+
Br
(fast)IBr
+
Br
I
+
Br2(slow)I
+
II2
(fast) 13
Example
3:
An
Intermediate
Formed
in
Reversible
Reactions 2
NO
+
O2
2
NO2Mechanism2
NO
N2O2
(fast)N2O2
+
O2
2
NO2
(slow) 12
Example
Five:
The
reaction
between
NO
and
H2
is
believed
to
occur
in
the
following
three-step
process.
NO
+
NO
→
N2O2
(fast)
N2O2+
H2
→
N2O
+
H2O
(slow)
N2O
+
H2
→
N2
+
H2O
(fast)
(a)
Write
a
balanced
equation
for
the
overall
reaction.
(b)
Identify
the
intermediates
in
the
reaction.
Justify
your
answer.
(c)
From
the
mechanism
represented
above,
a
student
correctly
deduces
that
the
rate
law
for
the
reaction
is
rate
=
k
[NO]2
[H2].
The
student
then
concludes
that
(1)
the
reaction
is
third-order
and
(2)
the
mechanism
involves
the
simultaneous
collision
of
two
NO
molecules
and
an
H2
molecule.
Are
conclusions
(1)
and
(2)
correct?
Explain.
14
Example
Four:
Intermediates
Formed
in
Fast
Reactions 2
IBr
I2+
Br2
MechanismIBrI
+
Br
(fast)IBr
+
Br
I
+
Br2(slow)I
+
II2
(fast) 13
Example
Five:
The
reaction
between
NO
and
H2
is
believed
to
occur
in
the
following
three-step
process.
NO
+
NO
→
N2O2
(fast)
N2O2+
H2
→
N2O
+
H2O
(slow)
N2O
+
H2
→
N2
+
H2O
(fast)
(a)
Write
a
balanced
equation
for
the
overall
reaction.
(b)
Identify
the
intermediates
in
the
reaction.
Justify
your
answer.
(c)
From
the
mechanism
represented
above,
a
student
correctly
deduces
that
the
rate
law
for
the
reaction
is
rate
=
k
[NO]2
[H2].
The
student
then
concludes
that
(1)
the
reaction
is
third-order
and
(2)
the
mechanism
involves
the
simultaneous
collision
of
two
NO
molecules
and
an
H2
molecule.
Are
conclusions
(1)
and
(2)
correct?
Explain.
14