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Chapter 9 Phenomenological chemical kinetics
(empirical/classical)
-- experimental aspects
§ 9.1 Introduction of chemical kinetics
Levine p. 528-529
17.1 reaction kinetics
rate of reaction, rate laws, reaction mechanism
17.2 measurement of reaction rates
17.3 integration of rate law
first-order reactions; second-order reactions; third-order
reactions
zero-order reaction;
n-th-order reaction
1.1 details about a chemical reaction
1) Stoichiometric: balance of the equation
2) Thermodynamic: Spontaneity and equilibrium
3) Kinetic: Rate and mechanism
The factor concerned by thermodynamics is the inherent tendency
or the possibility and the advancement of the reaction but not the
reality and the rate.
is spontaneous. Fortunately, this reaction is too slow to be detectable.
Thermodynamics and kinetics concern different aspects of
reaction, for example:
 2NH3
N2  3H2 
Cat .T , p
Thermodynamics:
reaction direction (spontaneity); maximum yield (equilibrium).
Kinetics:
rate (catalyst, temperature), mechanism
Chemical kinetics:
a branch of chemistry that concerns reaction rate
and mechanism.
Main concerns of chemical kinetics:
1) Reaction rate: factors that determines the rate of
reaction, rate law.
2) Reaction mechanism: the series of steps by which a
reaction takes place.
3) structure-dependence of reactivity: the correlation
between molecular structure and reaction potential.
1.2 reaction mechanism and elementary reaction
H2 + I2 = 2 HI
1) I2 = 2I
2) 2I + H2 = 2HI
H2 + Br2 = 2 HBr
1)
2)
3)
4)
5)
Br2 = 2 Br
Br + H2 = HBr + H
H + Br2 = HBr + Br
H + HBr = H2 + Br
2 Br = Br2
multistep reaction
the detailed way by which the reactants are converted into products,
or the series of steps by which a reaction takes place.
Reaction mechanism / reaction pathway
1)
Br2 = 2Br
2)
Br + H2 = HBr + H
3)
H + Br2 = HBr + Br
4)
H + HBr = H2 + Br
5)
2 Br = Br2
The reaction that completes in one act is called elementary
reaction. The sum of the elementary reactions and their series is
called the mechanism of the overall reaction.
The species such as H and Br formed in one step and consumed
in a subsequent step and never seen as a product is called an
intermediate.
1.3 Classification of reactions
1) elementary reactions
According to the number of molecules involved in the
reaction, elementary reactions can be divided into three kinds:
unimolecular reaction: (decomposition, isomerization) I2 = 2I
bimolecular reaction: (combination) Br + H2 = HBr + H
termolecular reaction: 2I + H2 = 2HI
The probability of three-molecule colliding simultaneously is very
small, termolecular reaction are quite rare. The elementary reaction
with four reactant molecules in gaseous phase has not been found.
2) overall reactions
According to the number of elementary reactions
involved, the overall reaction can be classified into:
simple reaction
Diels-Alder addition:
Overall
reaction
+
Ethylene butadiene cyclohexene
complex reaction
Find differences between mechanisms described in physical
chemistry and those shown in organic chemistry.
Hydrolysis of nitrile