The Study of Chemical Reactions
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Transcript The Study of Chemical Reactions
The Study of Chemical Reactions
Writing and balancing the overall
equation for the reaction barely
scrapes the surface…You must
understand the
mechanism,
thermodynamics, and
kinetics
of the reaction as well.
The Chlorination of Methane
We will see how to investigate
each of these areas by studying
the following gas-phase reaction:
∆ or hν
CH4 + Cl2
HCl + CH3Cl
(+ CH2Cl2 + CHCl3 + CCl4)
The Chlorination of Methane
Does not occur at room
temperature in the absence of
light.
The most effective light for the
reaction is blue and is absorbed
by the Cl2.
The light-initiated reaction has a
high quantum yield.
Free-Radical Chlorination of
Methane
Proceeds by a chain reaction.
Steps in any chain reaction:
Initiation
the generation of a reactive intermediate
Propagation
Products form. Reaction continues until
reactants or intermediates are depleted.
Termination
removes the reactive intermediates
Initiation: Generation of a Free Radical
With either heat (∆) or the appropriate
wavelength of light (hν), Cl2 undergoes
homolytic cleavage, one electron in the
bond going to each of the Cl atoms:
Propagation
Propagation refers to the steps in the reaction
that generate the products and regenerate the
reactive intermediates.
step 1
reactive intermediates
step 2
Termination
Propagation continues until
a reactant is used up, or
the reactive intermediates get
depleted by nonproductive reactions.
These are some of the termination reactions.
Thermodynamics of the FreeRadical Chlorination of Methane
Thermodynamics tell a lot about a
system at equilibrium.
∆G°(25°C) = -108.6 kJ
KP = e-∆G°/RT =
e108600/2477.7=e43.83=1.1x1019
CH4 + Cl2 HCl + CH3Cl
KP
PHCl PCH 3Cl
PCH 4 PCl 2
Thermodynamics of the FreeRadical Chlorination of Methane
The large value of K and the large
negative value of ∆G° say the
reaction goes to completion.
In general, a reaction goes to >99%
completion if its ∆G is < -12kJ.
Thermodynamics of the FreeRadical Chlorination of Methane
The free energy change depends on the enthalpy
change for the reaction, the temperature at which the
reaction takes place, and the entropy change:
∆G = ∆H - T∆S
Examining ∆H and ∆S will show what drives the
reaction and, consequently, how the reaction will
behave at different temperatures.
∆H°(25°C) = -105 kJ The reaction is very exothermic,
which favors products.
∆S°(25°C) = 12.16 J/K The entropy change is positive,
which also favors products.
At 25°C, the bigger influence on ∆G° is the fact that the
reaction is so exothermic.
Bond-Dissociation Enthalpies
A measure of the strength of a bond is
how much energy it takes to break it
homolytically:
CH3-H + 435* kJ/mol ▪CH3 + H▪
The weaker the bond, the less energy
is needed to break it.
Cl-Cl + 242 kJ/mol 2Cl▪
This explains why blue light initiates
the formation of Cl free radicals but
not methyl radicals.
*From Table 4-2
Bond-Dissociation Enthalpies
Since we know the mechanism of the
chlorination reaction, we can calculate ∆H based
on bond dissociation enthalpies (BDEs).
To break bonds
requires
435+242=677
kJ/mol.
431 kJ
435 kJ
351 kJ
242 kJ
To form bonds
releases
431+351=782
kJ/mol.
Bond-Dissociation Enthalpies
∆H° = BDE(bonds broken)-BDE(bonds
formed)
= 677-782 = -105 kJ/mol
This value tells us the reaction is very
exothermic, primarily because the Cl-Cl
bond is fairly weak.
Kinetics
Thermodynamics tell a lot about a
system at equilibrium.
Kinetics tell how fast a system
will reach equilibrium.
Reaction rates are determined
experimentally.
Kinetics
rate = k[reactant 1]a[reactant 2]b …
[reactant 1] is the molarity of
reactant 1.
a is the order of the reaction for
reactant 1.
a+b+… is the overall order of the
reaction.
k is the rate constant.
Kinetics
k is the rate constant.
k is given by the Arrhenius equation
k=Ae-Ea/RT
A is a constant that incorporates collision
frequency and orientation
Ea is the activation energy, the energy
needed to form the transition state.
R is the gas constant (8.3145 J/mol K)
Reaction-Energy Diagram for a
Single-Step Reaction
transition state
Ea, the activation energy
ΔE, the energy
change for the
reaction
Reaction-Energy Diagram for the Two
Propagation Steps of the Chlorination of
Methane
rate equation for step 1:
rate=k1[CH4][Cl▪]
Temperature Dependence of the
Rate Constant
k increases with T.
At a higher temperature, more reactant molecules
will have kinetic energies ≥ Ea.
Estimation: Rate doubles for every 10°C the
temperature increases.
Activation
energy
Ea
Chlorination of Other Alkanes
For ethane and the cycloalkanes, the
mechanism is very similar to that of
methane.
initiation
propagation step 1
Chlorination of Ethane
propagation step 2
termination
overall reaction
Chlorination of Other Alkanes
Can you write the complete mechanism
for the chlorination of cyclopentane?
initiation
propagation step 1
propagation step 2
termination
overall reaction