Lesson 12.4 Collision Theory
Suggested Reading
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Zumdahl Chapter 12 Sections 12.7 & 12.8
Essential Question
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What is collision theory?
Learning Objectives:
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Describe the kinetic theory in terms of the movement of particles whose
average energy is proportional to temperature in Kelvins.
Describe the collision theory.
Define the term activation energy.
Describe transition state theory.
Predict and explain, using collision theory, the qualitative effects of
particle size, temperature, concentration and pressure on the rate of
reaction.
Sketch and explain qualitatively the Maxwell-Boltzmann energy
distribution curve for a fixed amount of gas at different temperatures and
its consequences for changes in reaction rate.
Describe the effect of a catalyst on a chemical reaction.
Sketch and explain Maxwell-Boltzmann curves for reactions with and
without catalysts.
Introduction
You now know that the rate of a reaction depends on temperature. This shows
up in the rate law through the rate constant, which varies with temperature. In
most cases, the rate increases with temperature. How do you explain the
dependence of reaction rate on temperature? To understand it, we need to
look at a simple theoretical explanation of reaction rates. This is collision
theory. There are many learning objectives in this area, lets take them one at
a time.
Describe the kinetic theory in terms of the movement
of particles whose average energy is proportional to
temperature in Kelvins.
This objective was previously covered during our study of gases, but the IB
also includes in as part of collision theory (IB 6.2.1), because it is relevant to
both KMT and collision theory. Thornely's video on the topic should provide
sufficent review.
Watch the following YouTube Video:
https://www.youtube.com/watch?v=WuElPE0Heaw
Describe the collision theory.
Define activation energy.
Why the rate constant depends on temperature can be explained by collision
theory. For a reaction to take place between two reactant particles, three
conditions are necessary:
1.
2.
3.
The particles (atoms, ions, or molecules) must come into physical
contact (collide) with one another.
They must collide in the correct orientation.
The reactant molecules myst collide with sufficient kinetic energy to
bring about the reaction.
These three conditions are collectively know as collision theory. The
minimum energy required for two molecules to react is called the
activation energy, Ea. Activation energy depends on the particular
reaction.
Watch the following YouTube Video:
https://www.youtube.com/watch?v=mBTSwJnZ6Sk
Predict and explain, using collision theory, the qualitative effects of
particle size, temperature, concentration and pressure on the rate of
reaction.
Describe the effect of a catalyst on a chemical reaction.
Watch the following YouTube Video:
https://www.youtube.com/watch?v=KQW1N-ERGBs
Factors that increase the rate of a reaction:
Concentration: Increasing concentration increases the frequency of the
collisions, which results in a greater number of successful collisions and
consequently, a faster rate.
Pressure: In order to increase pressure we must reduce volume.
This increases the frequency of the collisions and results in a faster rate as
stated above.
Temperature: The rate of a reaction depends on temperature because
collision frequency depends on temperature. As the temperature rises, the
molecules move faster and collide more frequently. Collision frequency is
proportional to the root-mean-square (rms) speed, which is in turn
proportional to temperature.
Particle size: Reducing the particle size increases the rate of reaction,
because it greatly increases the surface area. The greater surface area
allows form more collisions, because there are more places in which a
collision can occur.
Catalysis: A catalyst is a substance that increases the rate of reaction
without itself being consumed, or chemically changed during the reaction.
Catalysts work by bringing the reactant particles into close contact with
one another. The catalyst is said to provide an alternative pathway with a
lower activation energy. More particles will possess this lower activation
energy, and so the rate increases.
Sketch and explain qualitatively the Maxwell-Boltzmann energy
distribution curve for a fixed amount of gas at different temperatures
and its consequences for changes in reaction rate.
Watch the following YouTube Video:
https://www.youtube.com/watch?v=YnHIfqUZi48
A Maxwell-Boltmann distribution shows the kinetic energy amongst particles
for a fixed amount of gas. It has a characteristic shape and several features
you should be aware of although you will not be tested on them directly.
Features:
1.
2.
3.
The area under the curve is directly related to number of particles, and
so for a fixed amount of gas this area must remain constant.
For a given T, most particles possess a KE close to the average.
At the average value the area under the graph is equal on both sides,
because the same number of particles will have a lower kinetic energy
as have a higher kinetic energy.
Why do we care? These curves can help us to understand activation energy
as well as the effect of temperature on the rate of reaction.
More generally, we can see that at higher temperatures, more molecules will
posses the minimum threshold energy (activation energy) needed in order for
a reaction to occur.
Describe transition state theory.
Collision theory explains some important features of a reaction, but it is
limited in that it does not explain the role of activation energy. Transition
state theory attempts to describe the role of activation energy
by explaining the reaction resulting from the collision of two molecules in
terms of an activated complex. An activated complex (transition state) is
an unstable grouping of atoms that can break up to form products.
Consider the reaction between hydroxide and bromomethane above. When
the molecules come together with the proper orientation, a C-O bond
begins to form. At the same time, the kinetic energy of the collision is
absorbed by the activated complex as a vibrational motion of the atoms.
This energy becomes concentrated in the bonds denoted by the dashed
lines and can flow between them. If, at some moment, sufficient energy
becomes concentrated in one of the bonds of the activated complex, that
bond breaks and falls apart. Depending on whether the C-O or C-Br bond
breaks, the activated complex either reverts back to reactants or yields the
products.
Potential-Energy Diagrams for Reactions
A potential energy diagram can help you understand what is going on. The
curve shows the change in potential energy that occurs during the progress
of reaction. The potential energy curves starts at the left with the energy of
the reactants, OH- and CH3Br. Moving along the curve toward the right, the
potential energy increases to a maximum corresponding to the activated
complex. Farther to the right the energy decreases to that of the products.
Only if the reactant molecules have sufficient kinetic energy is it possible
for the reaction to occur. This kinetic energy must be equal to or greater
than the difference in potential energy between the reactants and the
activated complex. The energy difference is the activation energy for the
forward reaction. The difference in energy between the reactants and
products equals the heat of reaction, ∆H. More generally,
If the exothermic reaction above left was reversed, it would become an
endothermic reaction. This is why we change the sign for enthalpy when
we use Hess's law. Also, the activation energy would increase. It would
equal ∆H + Ea. Similarly, the endothermic reaction on the right could be
reversed. The reaction would then become exothermic and the activation
energy would equal Ea - ∆H.
Sketch and explain Maxwell-Boltzmann curves for reactions with and
without catalysts.
Watch the following YouTube Video:
https://www.youtube.com/watch?v=RUNhM4AtMZ0
The distribution shows that the catalyzed reaction has a lower activation
energy. More particles will now have the minimum energy required to react.
The potential energy diagram also shows that the activation energy is lower
for the catalyzed reaction. Not, however, that the change in energy for the
reaction, ∆H (denoted by ∆E in the diagram), does not change for the
catalyzed reaction.
HOMEWORK: Make sure that you have all diagrams and drawing for
these notes!!