14-3-2011
1
Reaction Mechanism and Rate
Laws
An elementary reaction is a simple reaction that
proceeds in one step.
Most reactions are complex and require more than
one step.
A reaction mechanism is the sequence of
elementary steps that leads to product
formation.
An intermediate is a species that appears in
the mechanism of the reaction, but not in the
overall balanced reaction.
2
Most chemical reactions occur by
mechanisms that involve more than
one step. As a result, the rate law can
not be directly deduced from the
stoichiometry of a balanced chemical
equation. Only elementary reactions
(those which occur in one step) can
give the rate law. The rate law of an
elementary process does follow from
the coefficients of its balanced
equation.
3
Among the steps in a multi-step reaction,
there is always one that acts like a
bottleneck. There is usually one step
that is slowest compared to the rest.
This slowest step limits the overall
reaction rate and is called the ratedetermining step.
The rate-determining step determines the
rate law for the overall reaction.
4
5
6
Here is another example:
7
8
It is not always easy
9
The result is a sort of useless since it contains
a term that does not correspond to either any
of the reactants or the product.
A rate law should only consist of
concentrations of reactants and/or products,
no intermediates.
Since the intermediate, dinitrogen dioxide,
reacts slowly with oxygen, the reverse
reaction of Step 1 is possible.
10
To solve this problem we assume that the first
production of N2O2 from the first step is
consumed and there is an equilibrium
conditions where:
2NO D N2O2
Kequil = [N2O2]/[NO]2
Substitution in the rate equation gives
Rate = K*Kequil [NO]2[O2]
Rate = Kobs [NO]2[O2]
11
12
By doing rate of reaction experiments, you
find this rate equation:
The reaction is first order with respect to the
organic compound, and zero order with
respect to the hydroxide ions. The
concentration of the hydroxide ions isn't
affecting the overall rate of the reaction.
13
Increasing the concentration of the
hydroxide ions will speed up the fast step,
but that won't have a noticeable effect on
the overall rate of the reaction. That is
governed by the speed of the slow step.
14
Rate law in presence of a catalyst
Example: decomposition of H2O2 in
presence of IRate = k[H2O2][I-]
Step1 (slow): H2O2 + I- g H2O + IOStep 2 (fast): H2O2 + IO- g H2O + O2 + IOverall: 2H2O2 g 2H2O + O2
Therefore, rate depends on the
concentration of the catalyst although
catalyst does not show up in the overall
equation.
15
Catalysts
A catalyst is characterized by the ability
to do each of the following:
1. Catalysts increase the rate of reaction.
2. Catalysts are not consumed by the
reaction.
3. A small quantity of catalyst should be
able to affect the rate of reaction for a
large amount of reactant.
4. Catalysts do not change the equilibrium
constant for the reaction.
16
Example: Decomposition of potassium
chlorate
2KClO3 g 2 KCl + 3O2
This reaction is very slow except when a
catalyst like MnO2 where the reactions
proceeds very fast.
A catalyst speeds the reaction by providing a
set of elementary steps with more favorable
kinetics. In most cases Ea is decreased
17
18
Heterogeneous catalyst- the reactants
and the catalyst are in different phases.
catalyst = solid reactants = liquid/gas
Example: Ostwald process for
production of nitric acid. The role of the
catalyst (Pt-Rh solid catalyst) is to
speed up the first step of the reaction
of ammonia with oxygen:
19
4NH3(g) + 5O2(g) g 4NO(g) + 6H2O (g)
(Catalysed step)
2NO(g) + O2(g) g 2NO2(g)
2NO2(g) + H2O(l) g HNO2(l) + HNO3(l)
Heating nitrous acid will produce nitric
acid
3HNO2(l) g HNO3(l) + 2NO(g) + H2O(l)
20
Another example: Catalytic
converters
In the engine, nitrogen and oxygen from air will
form NO
N2(g) + O2(g) g 2NO(g)
After emission to atmosphere, NO2 , which is a
known pollutant, is formed:
2NO(g) + O2 g 2NO2
Therefore, before emission to atmosphere, a
catalytic converter containing Cr2O3 or CuO
is used to convert NO back to N2 and O2.
2NO g N2(g) + O2(g) (Catalytic step)
21
Catalytic Converters
catalytic
CO + Unburned Hydrocarbons + O2 converter CO2 + H2O
22
Homogeneous catalyst- catalyst and
reactants are in the same phase,
usually liquid.
Example: Hydrolysis of an acetic acid
ester is very slow in absence of a
catalyst:
Ester + water g acetic acid + alcohol
Rate = K[ester]
However, in presence of an acid catalyst,
the reaction is fast and the rate law is:
Rate = kobs[ester][H+]
23
Enzyme Catalysts
24
Enzyme Catalysts
25
Enzyme Catalysis
26
13.6
• Selected problems:
2, 5-7, 9, 10, 13, 15, 17-25, 27-29, 31, 3337, 40, 44, 46, 47, 49, 51-53.
27