Chap14-Part6

Chap14-Part6 - REVIEW
SESSION:
Wednesday
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Unformatted text preview: REVIEW
SESSION:
Wednesday
 Chemical
Kine8cs
 EXAM‐2:
March
4th,
Friday
 Chapters
13
and
14
 50:50‐
Descrip8ve
Q’s
and
concepts
 Rate
Determining
Step
 Many
chemical
reac8ons
have
mul8ple
steps.
 Each
step
has
its
own
ac8va8on
energy
and
rate
constant.
 If
one
step
is
much
slower
than
the
others,
the
overall
reac8on
 cannot
proceed
faster
than
this
slow
step.
 In
this
case,
the
slow
step
is
called
the
rate‐determining
step,
or
 the
rate‐limi8ng
step.
 The
rate‐limi8ng
step
determines
the
overall
reac8on
rate.
 Rate‐Limi0ng
Step
 The
rate
of
traffic
flow
from
place

 1
to
place
3
is
determined
by
the
 rate
of
movement
through
plaza
A.
 The
rate
of
traffic
flow
from
place

 1
to
place
3
is
determined
by
the
 rate
of
movement
through
plaza
B.
 Two‐Step
Mechanisms
with
a
Slow
First
Step
 Example: 
NO2(g)
+
CO(g)
→
NO(g)
+
CO2(g)
 1st
Step: 
 
NO2
(g)
+
NO2(g)
→
NO3(g)
+
NO(g) 
(k1,
slow)
 2nd
Step: 
 
NO3(g)
+
CO(g)
→
NO2(g)
+
CO2(g) 
(k2,
fast)
 The
overall
reac8on
rate
is
controlled
by
the
first
step.

Thus,
 Rate = k1 [ NO2 ] 2 Two
step
mechanism
with
a
Fast
Ini0al
Step
 •  A
proposed
mechanism
is
 Step
1:

NO
+
Br2
 NOBr2




(fast)
 Step
2:

NOBr2
+
NO
→
2
NOBr




(slow)
 Step
1
includes
the
forward
and
reverse
reac8ons.
 Two
step
mechanism
with
a
Fast
Ini0al
Step
 •  The
rate
of
the
overall
reac8on
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]?
 Two
step
mechanism
with
a
Fast
Ini0al
Step
 •  NOBr2
can
react
two
ways:
 –  With
NO
to
form
NOBr
 –  By
decomposi8on
to
reform
NO
and
Br2
 •  The
reactants
and
products
of
the
first
step
 are
in
equilibrium
with
each
other.
 •  Therefore,
 Ratef
=
Rater
 Two
step
mechanism
with
a
Fast
Ini0al
Step
 •  Because
Ratef
=
Rater
,
 k1
[NO]
[Br2]
=
k−1
[NOBr2]
 •  Solving
for
[NOBr2]
gives
us
 k1
 [NO]
[Br2]
=
[NOBr2]
 k−1
 Two
step
mechanism
with
a
Fast
Ini0al
Step
 
Subs8tu8ng
this
expression
for
[NOBr2]
in
 the
rate
law
for
the
rate‐determining
step
 gives
 Rate
=
 k2k1
 k−1
 [NO]
[Br2]
[NO]
 =
k
[NO]2
[Br2]
 Consider
the
diagram
to
the
 right,
represen8ng
two
steps
 in
an
overall
reac8on.
 red
=
O,
blue
=
N,
green
=

F
 (a)  Write
the
equa8on
for
the
overall
reac8on.
 (c)
Find
the
rate
if
the
first
 step
is
slow.
 (b)
Iden8fy
the
intermediate. 

 Consider
the
diagram
to
the
 right,
represen8ng
two
steps
 in
an
overall
reac8on.
 red
=
O,
blue
=
N,
green
=

F
 (a)  Write
the
equa8on
for
the
overall
reac8on.
 NO2
+
F2

NO2F+
F
 NO2+F

NO2F
 ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐
 2NO2+F2+
F

2NO2F
+F
 (b)
Iden8fy
the
intermediate. 

 (c)
Find
the
rate
if
the
first
 step
is
slow.
 Consider
the
diagram
to
the
 right,
represen8ng
two
steps
 in
an
overall
reac8on.
 red
=
O,
blue
=
N,
green
=

F
 (a)  Write
the
equa8on
for
the
overall
reac8on.
 NO2
+
F2

NO2F+
F
 NO2+F

NO2F
 ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐
 2NO2+F2+
F

2NO2F
+F
 (b)
Iden8fy
the
intermediate. 
F
is
the
intermediate
 (c)
Find
the
rate
if
the
first
 step
is
slow.
 Consider
the
diagram
to
the
 right,
represen8ng
two
steps
 in
an
overall
reac8on.
 red
=
O,
blue
=
N,
green
=

F
 (a)  Write
the
equa8on
for
the
overall
reac8on.
 NO2
+
F2

NO2F+
F
 NO2+F

NO2F
 ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐
 2NO2+F2+
F

2NO2F
+F
 (b)
Iden8fy
the
intermediate. 
F
is
the
intermediate
 (c)
Find
the
rate
if
the
first
 step
is
slow.
 Rate
=
k[NO2][F]
 Catalyst:
  A
catalyst
is
a
substance
that
changes
the
rate
of
a
chemical
reac8on


 


without
undergoing
a
chemical
change
itself.
  A
catalyst
lower
the
ac8va8on
barrier
for
a

 


transforma8on,
by
introducing
a
new
reac8on




 


pathway
  Catalyst
change
the
mechanism
of
the
reac8on
  A
catalyst
is
not
consumed
in
a
reac8on
  Important
proper8es
of
a
Catalyst
 


a)
Ac8vity
:
Enhancement
of
rate
 


b)
Selec8vity:
minimal
byproducts
 


c)
Life
8me:
reusability
 Importance
of
catalysis:
 ‐ Reduce
the
cost
of
produc8on
 ‐ Lead
to
beeer
selec8vity
and
less
waste
 Importance
of
catalysis:
 Journal
Source:
Inorg
Chem
Acta
1981
 Homogeneous
catalysis
Vs
Heterogeneous
Catalysis
 A
homogeneous
catalyst
is
a
substance
that
is
in
the
same
phase
as
the
 reactant
and
product
 –
For
example,
Wilkinson’s
catalyst
[RhCl(PPh3)]for
hydrogena8on
 A
heterogeneous
catalyst
is
material
that
is
in
a
different
phase
from
the
 reactant
and
product
 –
For
example,
Pd/C/
H2
for
hydrogena8on
 Catalysis
 Example
(slow): 
2
H2O2(aq)
→
2
H2O(l)
+
O2(g)
 With
bromine
as
a
catalyst
(fast):

 
step
1: 
2
Br‐(aq)
+
H2O2(aq)
+
2
H+(aq)
→
Br2(aq)
+
2H2O(l)
 
step
2: 
Br2(aq)
+
H2O2(aq)
→
2Br‐(aq)
+
2H+(aq)
+
O2(g)
 
sum: 

2
H2O2(aq)
→
2
H2O(l)
+
O2(g)
 A
catalyst
usually
speeds
up
the
reac8on
rate
by
providing
a
different
 pathway
for
the
reac8on
having
a
lower
ac8va8on
barrier.

The
energies
 of
the
reactants
and
products
are
not
changed,
only
the
speed
of
the
 reac8on.
 The
diagram
above
represents
an
imaginary
two‐step
mechanism.
The
orange,

 green
and
blue
spheres
represent
elements
A,
B
&
C,
respec8vely.

 (a)  Write
the
equa8on
for
the
net
reac8on
that
is
occurring.
 (b)
Iden8fy
the
intermediate. 

 (c)
Iden8fy
the
catalyst. 
 

 The
diagram
above
represents
an
imaginary
two‐step
mechanism.
The
orange,

 green
and
blue
spheres
represent
elements
A,
B
&
C,
respec8vely.

 (a)  Write
the
equa8on
for
the
net
reac8on
that
is
occurring.
 A2+
AB+
AC
BA2
+
A
+
AC
 BA2+
A
+
AC

A2+
BA2+
C
 Net:
AB+AC
BA2+
C
 (b)
Iden8fy
the
intermediate. 
A
 (c)
Iden8fy
the
catalyst.
A2
is
the
catalyst 
 

 Heterogenous
Catalysis
 A
heterogeneous
catalyst
exists
in
a
different
phase
than
the
 reactants
and
products.
 Usually
the
catalyst
is
a
solid
and
the
reactants
and
products
 are
in
a
gas
or
liquid
phase.
 The
ini8al
step
is
usually
the
adsorp8on
of
the
reactants
to
the
 surface
of
the
catalyst.
 Example
‐‐

the
reac8on
of
hydrogen
gas
with
ethylene
gas
to
 form
ethane
gas
 
 
C2H4(g)
+
H2(g)
→
C2H6(g)
 (a)  (b)  (c)  (d)  The
hydrogen
and
ethylene
adsorb
to
a
metal
surface.
 The
H‐H
bond
is
broken
to
give
hydrogen
atoms.
 The
hydrogen
atoms
move
to
the
adsorbed
ethylene
and
react.
 Fully
formed
ethane
molecules
desorb
and
return
to
the
gas
phase.
 Enzymes
 The
rates
of
most
biochemical
reac8ons
are
highly
regulated
 by
catalysts
called
enzymes.
 Most
enzymes
are
proteins
(polymers
of
amino
acids).
 These
proteins
have
large
molar
masses
(e.g.,
100,000
amu)
 and
very
well‐defined
three‐dimensional
structures.
 Enzymes
are
generally
highly
specific
for
the
reac8on
they
catalyze.
 The
ac8vi8es
of
enzymes
themselves
are
highly
regulated.
 An
enzyme
from
the
liver
called
catalase
causes
hydrogen
 peroxide
to
rapidly
decompose
into
water
and
oxygen.

 The
reac8on
catalyzed
by
an
enzyme
occurs
at
a
small
por8on
 of
the
enzyme
called
the
ac8ve
site.
 The
reactants
are
called
substrates.
 The
specificity
is
explained
in
part
by
the
lock‐and‐key
model.
 An
enzyme
called
lysozyme.
 Lysozyme
with
its
bound
substrate.
 Hermann
Emil
Fischer
(1852‐1919)
 ‐ 
German
Chemist
 ‐ 
Worked
on
heterocycles
and
Sugars
 ‐ 
Fischer
Esterifica8on
:
NOBLE
in
1902
 ‐ 
LOCK
&
KEY
model
to
visualize
substrate‐enzyme
interac8on
 ‐ 
Fischer
Indole
synthesis
 ‐ 
Fischer
Projec8on:
2D
representa8on
of
a
3D
organic
molecules
 ‐ Fischer
pep8de
synthesis
 ‐ Fischer
oxazole
synthesis
 ...
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