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Title Lesson 2 Collision Theory and
Maxwell-Boltzmann Distribution

• Learning Objectives
• Understand the four main influences on the rate
  of a reaction
• Use a simulation to explore the Maxwell-Boltzmann
  Distribution

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Practice

• What is the best definition of rate of reaction?
• The time it takes to use up all the reactants
• The rate at which all the reactants are used up
• The time it takes for one of the reactants to be
  used up
• The increase in concentration of a product per
  unit time

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Collision Theory

• Reactions occur as the result of two particles
  colliding with enough activation energy.
• Anything that increases the number of collisions,
  or the proportion of collisions that have enough
  energy will increase the rate of reaction

How to speed up chemical reactions - and get a
date! - TedEd Video
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Kinetic Energy and Temperature

• Particles move randomly because of the kinetic
  energy they possess.
• Random nature of movement and collisions ? not
  all particles will have the same values of
  kinetic energy at any one time ? they will have a
  range.
• Average of values can be taken ? directly related
  to its absolute temperature.
• Increasing temperature ? increases average
  kinetic energy of the particles

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A Glass of Water

• DISCUSS
• What is the relationship between the temperature
  of this water and the movement of its molecules?
• In a glass of room temperature water, are all the
  molecules moving at the same speed?
• Bonus Where is there more heat energy, a glass
  of room temperature water or a red-hot needle?

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Distribution of particle speeds
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Energy distribution curves
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The Maxwell-Boltzmann Distribution

• Describes how energy is distributed in a
  collection of particles

• x-axis Energy of particles
• y-axis Number of particles with a given energy

• Emp most-populated energy level
• E the average energy, i.e the temperature

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Simulating the Maxwell-Boltzmann Distribution

• As with much physical chemistry, computer-based
  simulations are one of the best ways to get to
  grips with this.
• Follow the instructions here (Maxwell-Boltzmann
  Distribution Simulation)
• This will require you to have installed Java

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The effect of changing temperature
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Small temperature changes
The MaxwellBoltzmann distribution shows that for
a small increase in temperature, there is a
relatively large increase in the number of
particles with at least the activation energy.
A small increase in temperature therefore leads
to a large increase in rate.
The increase in collision frequency is also a
factor, but its effect is small compared to the
increase in energy.
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Effect of temperature summary
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The Boltzmann Graphs
Red 100K Green 300K Blue 500K
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Review

• The particles in a body possess a range of
  energies.
• This range is described by the Maxwell-Boltzmann
  Distribution

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What is collision theory?
Collision theory states that for a reaction to
occur

• particles must collide

• particles must have sufficient energy

• particles must collide with the correct
  orientation.

Most collisions do not result in reaction because
they do not meet the second and third criteria.
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Energy of collision
Ea has a different value for different reactions!

• In order for a reaction to happen, the particles
  must have a certain minimum value for their
  kinetic energy ? activation energy (Ea).
• Ea is the energy needed to overcome repulsions
  between molecules and for breaking bonds.
• Ea allows reactants to achieve the transition
  state from which products can form.

Only particles with at least this amount of
kinetic energy will be able to successfully
collide (think about the Maxwell-Boltzmann
distribution curve)!
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Activation energy
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Geometry of collision

• Collisions are random ? orientation of collisions
  are random.
• Some orientations result in effective collisions
  (correct collision geometry) Some dont!

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Solutions
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What are kinetics?
Ch 1.1
A2
Temperature Pressure Concentration Particle
size Catalyst
How fast a chemical reaction goes is affected by
many factors. List four of these.
What are chemical kinetics and what affects them?
Discuss in your groups how each factor affects
the reaction rate
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Chemical reaction rates
Ch 1.1
A2
Temperature Food spoils more quickly at room
temperature than in a refrigerator bacteria grow
faster at RT than at lower temperatures
How do factors affect reaction rate?
At higher temperatures, reactant molecules have
more kinetic energy and move faster.
Consequently, they collide more often and with
greater energy.
simulation
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Effect of temperature on particles
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Effect of temperature on rate graph
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Factors affecting reaction rates
Ch 1.1
A2
There are fewer red particles inc the same volume
so there is less chance of a collision
Concentration As the concentration of reactants
increases, so does the likelihood that reactant
molecules will collide, the rate of reaction
generally increase.
How do factors affect reaction rates?
There are more red particles in the same volume
so there is more chance of a collision so the
reaction goes faster
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Effect of concentration on rate
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Effect of concentration on rate graph
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Factors affecting reaction rates
Ch 1.1
A2
Surface area As surface area increases the rate
of reaction generally increases More contact
between exposed surfaces of reactants
How do factors affect reaction rates?
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Factors affecting reaction rates
Ch 1.1
A2

• Catalyst
• a substance that increases the rate of a reaction
  without being consumed in the reaction
• Enzymes
• biological catalysts
• proteins that increase the rate of biochemical
  reactions

How do factors affect reaction rates?
Surface area (Particle size) as surface area
increases the rate of reaction generally
increases
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Effect of surface area on rate graph
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Factors affecting reaction rates
Ch 1.1
A2
Pressure Higher pressure compresses the gas,
effectively increasing its concentration. This
will increase frequency of collision.
How do factors affect reaction rates?
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Effect of gas pressure on rate
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Effect of pressure on rate graph
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Factors affecting reaction rates
Ch 1.1
A2
Catalysts A substance that increases the rate of
a reaction without itself undergoing permanent
change. Most catalysts provide an alternative
route for the reaction that has a lower
activation energy. Catalysts are often changed
during the course of a reaction, but will always
be re-formed by the end of it.
How do factors affect reaction rates?
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Factors affecting reaction rates
Ch 1.1
A2
Catalysts This means without increasing the
temperature, a larger number of particles will
now have values of kinetic energy greater than
the activation energy ? more successful
collisions
Catalysts bring about an equal reduction in the
activation energy of both the forward and the
reverse reaction ? no change in the position of
the equilibrium or yield
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Effect of catalysts on rate graph
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What do catalysts do?
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How do catalysts work?
Catalysts increase the rate of reactions without
being used up during the reaction.
One way in which this occurs is for the catalyst
to be changed during the reaction, then changed
back in a second reaction with one of the
reactants or products. This is an alternative
reaction pathway.
An example is the oxidation of sulfur dioxide
This is catalyzed by vanadium(V) oxide
The catalyst is re-formed by reacting with oxygen
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Catalysts and energy distribution curves
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Heterogeneous catalysts
There are two types of catalysts heterogeneous
and homogeneous.
Heterogeneous catalysts are in a different phase
to the reactants. The catalyst is usually a solid
and the reactants are liquids or gases (e.g.
solid catalysts for gas reactions in catalytic
converters).
Industrial examples of heterogeneous catalysis
include the iron catalyst used in ammonia
production and the ZieglerNatta catalyst used in
poly(e)thene production.
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Homogeneous catalysts
Homogeneous catalysts are in the same phase as
the reactants. The catalyst and the reactants are
usually liquids, such as the hardener added to
fibreglass resin.
Another example of homogeneous catalysis is the
destruction of atmospheric ozone catalyzed by
chlorine free radicals. In this reaction the
catalyst and reactants are in the gas phase.
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Advantages of catalysts
Using a catalyst means that a reaction can take
place at the same rate as the uncatalyzed
reaction, but at a lower temperature and/or
pressure. This has the following advantages,
which are particularly important in industry

• lower energy demands

• therefore less CO2 produced

• therefore less environmental impact

• and lower production costs.

A non-industrial example is enzyme catalysis in
biological washing powders, allowing efficient
washing at a lower temperature.
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Catalysts true or false?
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Catalysts in Practice

• Complete the experiment here to investigate the
  effect of a Co2 catalyst on the oxidation of
  tartrate ions by hydrogen peroxide.
• C4H4O62-(aq) 5H2O2(aq) ? 2OH-(aq) 4CO2(g)
  6H2O(l)
• You will need to adapt the given method to give
  you a quantitative measure of the rate.
• Review lesson 1 if you need to

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Collision theory summary
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Rate of reaction summary
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Simulation

• This should be downloaded and installed before
  the lesson!
• https//phet.colorado.edu/en/simulation/reactions-
  and-rates
• Complete the activity here.
• Once you have finished, you should attempt to
  develop an analogy to describe your findings.

Factor Effect Chemical Explanation Analogy Explanation
Concentration (pressure in gases)
Temperature
Particle size
Catalyst
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Solutions
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Practice Questions

• Attempt the practice questions here
• Complete them for homework

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Review

• Reaction rates increase if you increase
• The number of collisions
• The energy of those collisions
• Increasing temperature, concentration and surface
  area will increase the number of collisions
• Increasing temperature increases the energy of
  collisions.
• Using a catalysts decreases the energy needed.