The Study of Chemical Reactions

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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

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