Lesson 6.05
Rate and Collision
Theory
Introduction
• Have you ever driven a bumper car?
• The point of driving a bumper car, at least for
most people, is to collide with as many other cars
as possible.
• Imagine that you and your friends are driving the
bumper cars at night, and there are no lights on
the ride.
• This means that all of the drivers will be driving
around randomly, and it will be much harder to
aim your car at specific victims.
• Do you think that the number of bumper
cars being driven will affect the number of
collisions that you and the other drivers
experience?
• What about speed? How do you think the
number of collisions you and the other
drivers experience would change if you
increased the speed of all the cars?
Keep the relationship between amount, speed,
and collisions in mind as you continue into the
lesson to discuss the rates of chemical
reactions.
Objectives
After completing this lesson, you will be able
to:
• Explain how various factors affect the rate
of a chemical reaction.
• Describe the collision theory as a model of
reaction rate.
Is it worth it?
When chemists plan a reaction, they ask
questions like:
• Will it produce useful amounts of the
desired product?
• Will the reaction proceed rapidly enough to
be useful?
Kinetics
• The area of chemistry that studies reaction
rates is called kinetics.
• The measure of the rate of a reaction
involves measuring the change in
something over a given amount of time.
How is it measured?
The rate of a reaction is usually measured in
one of two ways:
• The decrease in concentration or amount
of reactant over time
• The increase in concentration or amount
of product over time
Collision Theory
• The idea that in order for compounds, atoms, or
ions to react successfully, the particles must
collide together.
Successful Collisions
A successful collision is one that results in making
the product(s).
Two factors must be true for a reaction to be
“successful”:
• Sufficient energy
• Correct orientation
Sufficient Energy
• The collision between reactants must have
enough energy for the reactants’ valence
energy levels to penetrate each other.
• This interaction between valence energy
levels allows the electrons to rearrange to
form new bonds.
• If a collision between two reactants does
not have enough energy, the collision will
not be able to produce the products.
Energy Distribution
• We can use a graph to represent the distribution
of energy within a sample of particles at a given
temperature.
• This general curve, called a Maxwell-Boltzmann
distribution, shows us that the majority of the
particles have moderate amounts of energy,
near the average kinetic energy of the sample,
while some particles have energy higher or lower
than that average value.
Energy Distribution
• Remember that for a reaction to happen
particles must collide with energy equal to or
greater than the activation energy for the
reaction.
• That activation energy requirement can be
marked on the diagram to show us the amount
of a given sample that has the possibility of
meeting this energy requirement in a collision.
A Maxwell-Boltzmann distribution graph
• The y-axis of the graph is labeled “number of
particles,” increasing as you go up the axis, and
the x-axis is labeled (energy), increasing left to
right.
• The shape of the graph is like that of a bell or hill,
showing that the majority of the particles have a
kinetic energy near the average kinetic energy of
the sample, while some particles have more
energy (far right of the graph) and some particles
have less energy (far left of the graph).
A Maxwell-Boltzmann distribution graph
A Maxwell-Boltzmann distribution graph
Correct Orientation
• When reactants come in contact with each other
during their random movement, the orientation
of their collision will determine if the collision is
successful
The hydrogen chloride molecule is correctly
oriented to the double bond in the ethene molecule.
Incorrect Orientation
These are all incorrect orientations
These will not be successful in producing the new
product; the two reactants will just “bounce” off of each
other.
Factors Affecting Reaction Rate- Concentration
Increasing Concentration
For many reactions involving liquids or gases, increasing the
concentration of the reactants increases the rate of reaction
Be careful: doubling the concentration of one of the
reactants doesn’t mean you will double the rate of the
reaction.
The amount by which an increase in concentration increases
the rate of a reaction depends on many different properties
of the reaction.
Factors Affecting Reaction Rate~ Temperature
• temperature is a measure of average
kinetic energy
• we know that the temperature of a system
will affect the number of collisions that
have sufficient energy to react
Factors Affecting Reaction Rate~ Temperature
This direct relationship between temperature and rate is
due to two reasons:
1. Increasing the temperature of a system increases the
average kinetic energy of the particles. This means more
collisions with enough energy to react successfully.
2. Increasing the temperature of a system increases the
kinetic energy ,meaning that the particles are moving
faster. There are more collisions per minute at a higher
temperature then at a lower temperature. More
collisions in a given amount of time means there are
more opportunities for collisions to meet both
requirements for success.
Factors Affecting Reaction Rate ~Catalysts
• Sometimes it is not possible to raise the
temperature to help the reaction move faster
• When a catalyst is used, it provides an alternate
pathway for the reaction that requires less
activation energy.
EXAMPLE: Enzymes. In your body, living cells can only
survive within a narrow temperature range.
Raising the temperature within the cells to speed up
the necessary chemical reactions would result in
damage to the very cells that the reactions support.
Catalysts
Maxwell-Boltzmann distribution curve
Did You Know?
• Some substances, called inhibitors, actually
decrease the rate of catalyzed chemical
reactions.
• They do this by interfering with the catalysts’
ability to participate in the reaction.
• Many biological poisons are inhibitors that
interfere with the enzymes in your body.
Increasing the Rate of a Reaction
• Increasing the temperature of a system allows
more particle collisions to meet the energy
requirement for a successful collision.
• Lowering the activation energy requirement of
a reaction allows more collisions to be
successful without changing any properties of
the collisions themselves. Adding a catalyst!
Increasing the Rate
Some other ways to increase the rate of a
reaction that you have already learned about
include:
• Agitation (stirring or mixing)
• Increasing the surface area (making the
particles smaller)
Congratulations!
You should now be able to:
• Explain how various factors affect the rate of a
chemical reaction.
• Describe the collision theory as a model of
reaction rate.
Please review this lesson again if you need to