Collision Theory,
Elementary Steps,
Intermediates, Rate
Determining Steps
Page 387 # 1
390 # 2
391 # 1 - 4
Prepare Iodine Clock Lab
RATE DETERMINING STEP
Many reactions consist of a series of separate stages.
Each step has its own rate and rate constant.
The overall rate of a multi-step process is governed by the slowest
step (like a production line where overall output can be held up by
a slow worker).
This step is known as the RATE DETERMINING STEP .
If there is more than one step, the rate equation may not contai n
all the reactants in its format.
RATE DETERMINING STEP
THE REACTION BETWEEN PROPANONE AND IODINE
Iodine and propanone
react in the presence of acid
CH 3 COCH 3 + I 2
The rate equation is...
r = k [CH 3 COCH 3 ] [H + ]
Why do H + ions appear in
the rate equation?
Why does I 2 not appear
in the rate equation?
CH 3 COCH 2 I + HI
RATE DETERMINING STEP
THE REACTION BETWEEN PROPANONE AND IODINE
Iodine and propanone
react in the presence of acid
CH 3 COCH 3 + I 2
CH 3 COCH 2 I + HI
The rate equation is...
r = k [CH 3 COCH 3 ] [H + ]
Why do H + ions appear in
the rate equation?
The reaction is catalysed by acid
[H + ] affects the rate but is unchanged overall
Why does I 2 not appear
in the rate equation?
The rate determining step doesn't involve I 2
RATE DETERMINING STEP
THE REACTION BETWEEN PROPANONE AND IODINE
Iodine and propanone
react in the presence of acid
CH 3 COCH 3 + I 2
CH 3 COCH 2 I + HI
The rate equation is...
r = k [CH 3 COCH 3 ] [H + ]
Why do H + ions appear in
the rate equation?
The reaction is catalysed by acid
[H + ] affects the rate but is unchanged overall
Why does I 2 not appear
in the rate equation?
The rate determining step doesn't involve I 2
The slowest step of any multi-step reaction is known as the rate determining step and it
is the species involved in this step that are found in the overa ll rate equation.
Catalysts appear in the rate equation because they affect the ra te but they do not
appear in the stoichiometric equation because they remain chemically unchanged
RATE DETERMINING STEP
HYDROLYSIS OF HALOALKANES
OH -
Haloalkanes (general formula RX) are
hydrolysed by hydroxide ion to give alcohols.
RX +
With many haloalkanes the rate equation is...
This is because both the RX and OH - must
collide for a reaction to take place in ONE STEP
r = k [RX][OH -]
ROH +
X-
SECOND ORDER
RATE DETERMINING STEP
HYDROLYSIS OF HALOALKANES
OH -
RX +
With many haloalkanes the rate equation is...
This is because both the RX and OH - must
collide for a reaction to take place in ONE STEP
r = k [RX][OH -]
SECOND ORDER
but with others it only depends on [RX]...
r = k [RX]
FIRST ORDER
The reaction has taken place in TWO STEPS...
- the first involves breaking an R-X bond
- the second step involves the two ions joining
i)
ii)
ROH +
X-
Haloalkanes (general formula RX) are
hydrolysed by hydroxide ion to give alcohols.
RX
R+ + XR + + OH ROH
Slow
Fast
The first step is slower as it involves bond breaking and energy has to be put in.
The first order mechanism is favoured by tertiary haloalkanes because the hydroxide ion is hindered
in its approach by alkyl groups if the mechanism involves the hy droxide ion and haloalkane colliding.
RATE DETERMINING STEP
The reaction
H 2 O 2 + 2H 3 O + 2 I+
Step 1
H 2 O 2 +I
Step 2
IO
Step 3
HIO + H 3 O + I+
+H 3 O +
I2 + 4H 2 O
IO
HIO
takes place in 3 steps
+H 2 O
SLOW
+
FAST
H 2O
I2 + 2H 2 O
The rate determining step is STEP 1 as it is the slowest
FAST
RATE DETERMINING STEP
The reaction
H 2 O 2 + 2H 3 O + 2 I+
I2 + 4H 2 O
Step 1
H 2 O 2 +I
IO
Step 2
IO
Step 3
HIO + H 3 O + I+
+H 3 O +
HIO
takes place in 3 steps
+H 2 O
SLOW
+
FAST
H 2O
I2 + 2H 2 O
FAST
The rate determining step is STEP 1 as it is the slowest
The reaction
2N 2 O 5
4NO 2 + O 2
Step 1
N2O 5
Step 2
NO 2 + NO 3
Step 3
NO +
takes place in 3 steps
NO 2 + NO 3
NO 3
SLOW
NO + NO 2 + O 2
2NO 2
The rate determining step is STEP 1
from another Step 1
FAST
FAST
rate = k [N2O5]
OTHER TOPICS
Autocatalysis
A small number of reactions appear to speed up, rather than slow down, for a time.
This is because one of the products is acting as a catalyst and as more product is
formed the reaction gets faster. One of the best known examples is the catalytic
properties of Mn 2+ (aq) on the decomposition of MnO 4
(aq). You will notice it in a
titration of KMnO 4 with either hydrogen peroxide or ethanedioic (oxalic) acid.
Molecularity
e.g.
The number of individual particles of the reacting species takin g
part in the rate determining step of a reaction
A + 2B
A
C+D
2B
molecularity is 3 - one A and two B's need to collide
however has a molecularity of 1 - only one A is involved
Rate Laws and Elementary Steps
Unimolecular reaction
A
products
rate = k [A]
Bimolecular reaction
A+B
products
rate = k [A][B]
Bimolecular reaction
A+A
products
rate = k [A]2
Reaction Mechanisms
The overall progress of a chemical reaction can be represented
at the molecular level by a series of simple elementary steps
or elementary reactions.
The sequence of elementary steps that leads to product
formation is the reaction mechanism.
2NO (g) + O2 (g)
2NO2 (g)
N2O2 is detected during the reaction!
Elementary step:
NO + NO
N2O2
+ Elementary step:
N2O2 + O2
2NO2
Overall reaction:
2NO + O2
2NO2
Reaction Intermediates
Intermediates are species that appear in a reaction
mechanism but not in the overall balanced equation.
An intermediate is always formed in an early elementary step
and consumed in a later elementary step.
Elementary step:
NO + NO
N2O2
+ Elementary step:
N2O2 + O2
2NO2
Overall reaction:
2NO + O2
2NO2
Rate Laws and Rate Determining Steps
Writing plausible reaction mechanisms:
•
The sum of the elementary steps must give the overall
balanced equation for the reaction.
•
The rate-determining step should predict the same rate
law that is determined experimentally.
The experimental rate law for the reaction between NO 2
and CO to produce NO and CO2 is rate = k[NO2]2. The
reaction is believed to occur via two steps:
Step 1:
NO 2 + NO2
NO + NO3
Step 2:
NO3 + CO
NO2 + CO2
What is the equation for the overall reaction?
NO2+ CO
NO + CO2
What is the intermediate? Catalyst?
NO3
NO2
What can you say about the relative rates of steps 1 and 2?
rate = k[NO2]2 is the rate law for step 1 so
step 1 must be slower than step 2
Write the rate law for this reaction.
Rate = k [HBr] [O2]
List all intermediates in this reaction.
HOOBr, HOBr
List all catalysts in this reaction.
None
Ostwald Process
4NH3 (g) + 5O2 (g)
2NO (g) + O2
2NO2 (g) + H2O (l)
Pt catalyst
4NO (g) + 6H2O (g)
2NO2 (g)
(g)
HNO2 (aq) + HNO3 (aq)
Pt-Rh catalysts used
in Ostwald process
Hot Pt wire
over NH3 solution
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