Temperature dependence of reaction rates
Typically rates of reactions double for every 10oC rise in
temperature,
k  Ae(Ea /RT)
Arrhenius equation
Ea: activation energy
A: frequency factor
Ea
lnk  ln A 
RT
An Arrhenius plot of ln k
against 1/T is used to
determine Ea and A
The higher the Ea the stronger
the temperature dependence of
the rate constant
Collision Theory
Collisions between two (or more) atoms/molecules required
for a reaction.
However, every time two reactants collide they may not react
As temperature increases:
atoms/molecules collide more frequently
kinetic energy of atoms/molecules increases
Collision theory: reaction occurs only if the reactants collide
with a kinetic energy of at least the activation energy, and
they do so in the correct orientation.
Kinetic energy is important
Orientation is important
Cl
N
O
2 AB -> A2 + B2
2 NOCl  2 NO + Cl2
Animation 1
Animation 2
Animation 3
k  Ae(Ea /RT)
Ea
lnk  ln A 
RT
The factor e-Ea/RT: fraction of molecules that have at least the
minimum energy required for reaction.
For an Ea = 40 kJ/mol
Temperature (K)
e-Ea/RT
298
9.7 x 10-8
400
5.9 x 10-6
600
3.3 x 10-4
A: reflects orientation effect or steric effect
Measuring k as a function of T Ea to be determined
k2
Ea 1 1
ln   (  )
k1
R T2 T1
Reaction coordinate diagram
Activated complex or
transition state - highest
energy along reaction
coordinate
Reactants must collide with
sufficient energy to reach this
point and collide in a preferred
orientation to form the
activated complex
DE = (Ea)forward - (Ea)reverse
Higher temperatures favor products for an endothermic
reaction and reactants for an exothermic reaction
Endothermic reaction: Ea(forward) > Ea(reverse)
Exothermic reaction: Ea(forward) < Ea(reverse)
CH3OH(aq) + H+(aq)  CH3OH2+(aq)
CH3OH2+(aq) + Br- (aq)  CH3Br + H2O(aq)
Catalysis
Catalyst: a compound which speeds up the rate of a reaction,
but does not itself undergo a chemical change.
Simple mechanism
A + catalyst  intermediates
intermediates  B + catalyst
Overall:
AB
Concentration of catalyst is included in k; hence k varies with
concentration of catalyst
Presence of a catalyst provides an
alternate path with a lower Ea
2H2O2(aq)  2H2O(aq) + O2(g)
In the absence of a catalyst,
Ea = 76 kJ/mol
In the presence of a catalyst (I-);
Ea = 57 kJ/mol;
rate constant increases by a
factor of 2000
H 3C
CH3
C
H
H
(g)
C
H
cis-2-butene
CH3
C
H 3C
C
(g)
H
trans-2-butene
Catalyzed by I2
Pt
C2H4(g) + H2(g)  C2H6 (g)
Example of heterogenous catalysis
A catalyst does not effect the thermodynamics of the reaction
DG is not affected by catalyst; neither is K
Equilibrium concentrations are the same with and without
catalyst; just the rate at which equilibrium is reached
increases in the presence of a catalyst
K = k1/k-1; catalyst speeds up both the forward and reverse
reaction
Enzymes
Practically all living reactions are catalyzed by enzymes;
each enzyme specific for a reaction.
Enzymes typically speed up rates by 107 - 1014 times rate of
uncatalyzed reactions
Ea for acid hydrolysis of sucrose: 107 kJ/mol
Ea for catalyzed acid hydrolysis of sucrose: 36 kJ/mol
Rate increase of 1012 at body temperature
E + S  ES
ES  P + E
“Poisoning” a catalyst
Arsenic poisoning: Ingestion of As(V) as AsO43- results in
reduction to As(III) which binds to enzymes, inhibiting their
action
Nerve gases - block enzyme-controlled reactions that allow
nerve impulses to travel through the nerves.
Catalytic Converters
Incomplete combustion of gasoline produces CO,
hydrocarbon fragments (CmHn)
High temperature in the engine causes oxidation of N2 to NO
and NO2
Conversion of these pollutants to less harmful compounds is
speeded up in the presence of catalysts.
2 NO(g)
catalyst
CO, CmHn, O2
N2(g) + O2(g)
catalyst
CO2, H2O
Catalyst: pellets of Pt, Pd, Rh
animation