Transcript 1.4 The rate-determining step

The rate-determining step
L.O.:
 What is the rate-determining step of a
reaction?
 What is the connection between the rate
equation for a reaction and the mechanism?
Discuss in pairs any of these two situations.
Think what are the steps involved and which
one is the slowest step.
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 contain all the reactants in its format.
RATE DETERMINING STEP
THE REACTION BETWEEN PROPANONE AND IODINE
Iodine and propanone
react in the presence of acid
CH3COCH3 + I2
The rate equation is...
r = k [CH3COCH3] [H+]
Why do H+ ions appear in
the rate equation?
Why does I2 not appear
in the rate equation?
CH3COCH2I + HI
RATE DETERMINING STEP
THE REACTION BETWEEN PROPANONE AND IODINE
Iodine and propanone
react in the presence of acid
CH3COCH3 + I2
CH3COCH2I + HI
The rate equation is...
r = k [CH3COCH3] [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 I2 not appear
in the rate equation?
The rate determining step doesn’t involve I2
RATE DETERMINING STEP
THE REACTION BETWEEN PROPANONE AND IODINE
Iodine and propanone
react in the presence of acid
CH3COCH3 + I2
CH3COCH2I + HI
The rate equation is...
r = k [CH3COCH3] [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 I2 not appear
in the rate equation?
The rate determining step doesn’t involve I2
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 overall rate equation.
Catalysts appear in the rate equation because they affect the rate but they do not appear in
the stoichiometric equation because they remain chemically unchanged
In pairs: remember the two mechanisms for
the hydrolysis of haloalkanes. Discuss how
can you make the reaction faster in each
case.
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+ + OHROH
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 hydroxide ion and haloalkane colliding.
Worksheet
The rate-determining step
L.O.:
 What is the rate-determining step of a
reaction?
 What is the connection between the rate
equation for a reaction and the mechanism?
RATE DETERMINING STEP
The reaction
H2O2 + 2H3O+ + 2I¯
Step 1
H2O2 + I¯
Step 2
IO¯ + H3O+
Step 3
HIO + H3O+ + I¯
I2 + 4H2O
IO¯ + H2O
HIO
+
H2O
I2 + 2H2O
The rate determining step is STEP 1 as it is the slowest
takes place in 3 steps
SLOW
FAST
FAST
RATE DETERMINING STEP
The reaction
H2O2 + 2H3O+ + 2I¯
Step 1
H2O2 + I¯
Step 2
IO¯ + H3O+
Step 3
HIO + H3O+ + I¯
I2 + 4H2O
IO¯ + H2O
HIO
+
takes place in 3 steps
SLOW
H2O
FAST
I2 + 2H2O
FAST
The rate determining step is STEP 1 as it is the slowest
The reaction
2N2O5
4NO2 + O2
takes place in 3 steps
Step 1
N2O5
NO2 + NO3
Step 2
NO2 + NO3
NO + NO2 + O2
FAST
Step 3
NO +
2NO2
FAST
NO3
The rate determining step is STEP 1
SLOW
from another Step 1
rate = k [N2O5]