The Collision Theory and
Activation Energy
Explaining how and why factors
affect reaction rates
The Maxwell-Boltzmann apparatus
• Maxwell and Boltzmann performed an
experiment to determine the kinetic energy
distribution of atoms
• Because all atoms of an element have roughly
the same mass, the kinetic energy of identical
atoms is determined by velocity (KE= ½mv2)
The Maxwell-Boltzmann apparatus
• Maxwell and Boltzmann performed an
experiment to determine the kinetic energy
distribution of atoms
• Because all atoms of an element have roughly
the same mass, the kinetic energy of identical
atoms is determined by velocity (KE= ½mv2)
The Maxwell-Boltzmann apparatus
• Maxwell and Boltzmann performed an
experiment to determine the kinetic energy
distribution of atoms
• Because all atoms of an element have roughly
the same mass, the kinetic energy of identical
atoms is determined by velocity (KE= ½mv2)
The Maxwell-Boltzmann distribution
• The resulting disk looks like this:
Basically, if we plot the
intensity of the dots on a
graph we get a graph of
fraction of atoms/molecules
vs. kinetic energy:
Fraction of
molecules
Molecules
hit disk last
Molecules
hit disk first
Kinetic energy 
Why is the graph skewed?
• This curve is characteristic of all molecules
• The curve is elongated due to how atoms
collide, and to the units of the graph
• Recall all particles are in motion. An average
speed will be reached.
• The graph is skewed because 0 is the lower
limit, but theoretically there is no upper limit
• More than that the graph is skewed because
the x-axis has units of energy not velocity
Same data, different
axes. E.g. v=1, KE=1
v=2, KE=4
v=3, KE=9
KE
velocity
Temperature and reaction rate
• By understanding the Maxwell-Boltzmann
distribution, we can begin to understand the
two reasons why an increase in temperature
causes an increase in reaction rate
• Read pg. 754 - 755 (starting at 18.7), answer
questions 18.61 - 18.64 on pg. 773 and …
Q- Look back at the five factors that affect
reaction rates. Three of these factors can be
(at least in part) explained by the collision
theory. Identify the 3 factors and explain how
the affect of each can be explained with
reference to the collision theory
Temperature and reaction rate
18.61 Rates are typically 2-3 times greater
18.62 Rate is proportional to the number of
collisions per second among reactants
18.63 reactant orientation and activation energy
18.64 This happens because a larger fraction of
the reactant molecules possess the minimum
energy necessary to surpass Ea.
A- Ability to meet (molecules that are well mixed
will have a greater chance of colliding)
Concentration of reactants (more molecules
means more collisions)
Temperature (faster moving molecules means
more collisions per unit of time). Plus…
Temperature and reaction rate
• Demonstrations: Mg + O2, H2 + O2
• By increasing the temperature, a small number
of molecules reach Ea. The reaction is
exothermic, further increasing temperature and
causing more molecules to reach Ea, etc.
• Draw the M-B distribution for H2 + O2 before
heat was applied. Show how heat affects the
diagram.
Shift due to higher
temperature
Fraction of
molecules
Kinetic energy 
Ea
Transition state theory
• Read remainder of 18.7 (pg. 756 – 757)
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