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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?
A. The time it takes to use up all the reactantsB. The rate at which all the reactants are used upC. The time it takes for one of the reactants to be
used upD. 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 WaterDISCUSS: 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 Ē: 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 Maxwell–Boltzmann 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.
no
. o
f p
arti
cles
energy Ea
no. particles with Ea almost
doubled
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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 with the correct orientation.
particles must have sufficient energy
particles must collide
Most collisions do not result in reaction because they do not meet the second and third criteria.
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Energy of collision 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)!
Ea has a different value for different reactions!
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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 don’t!
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Solutions
What are “kinetics”?Ch 1.1 A2
Wha
t ar
e ch
emic
al k
ineti
cs a
nd w
hat
affec
ts th
em?
How fast a chemical reaction goes is affected by many factors. List four of these.
TemperaturePressureConcentrationParticle sizeCatalyst
Discuss in your groups how each factor affects the reaction rate
Chemical reaction ratesCh 1.1 A2
How
do
fact
ors
affec
t rea
ction
rate
?Temperature:Food spoils more quickly at room temperature than in a refrigerator
bacteria grow faster at RT than at lower temperatures
simulation
At higher temperatures, reactant molecules have more kinetic energy and move faster. Consequently, they collide more often and with greater energy.
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Effect of temperature on particles
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Effect of temperature on rate: graph
Factors affecting reaction ratesCh 1.1 A2
How
do
fact
ors
affec
t rea
ction
rate
s?Concentration:
As the concentration of reactants increases, so does the likelihood that reactant molecules will collide, the rate of reaction generally increase.
There are fewer red particles inc the same volume so there is less chance of a collision
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
Factors affecting reaction ratesCh 1.1 A2
How
do
fact
ors
affec
t rea
ction
rate
s?Surface area
As surface area increases the rate of reaction generally increases…
More contact between exposed surfaces of reactants
The particles on the surface can react
When cut into smaller pieces the particles on the inside can react
Factors affecting reaction rates
Catalysta substance that increases the rate of a reaction
without being consumed in the reactionEnzymes
– biological catalysts– proteins that increase the rate of biochemical
reactions
Ch 1.1 A2
How
do
fact
ors
affec
t rea
ction
rate
s?
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
Factors affecting reaction ratesCh 1.1 A2
How
do
fact
ors
affec
t rea
ction
rate
s?Pressure
Higher pressure compresses the gas, effectively increasing its concentration.
This will increase frequency of collision.
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Effect of gas pressure on rate
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Effect of pressure on rate: graph
Factors affecting reaction ratesCh 1.1 A2
How
do
fact
ors
affec
t rea
ction
rate
s?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.
Factors affecting reaction ratesCh 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?
An example is the oxidation of sulfur dioxide:
SO2(g) + V2O5(s) ® SO3(g) + V2O4(s)
The catalyst is re-formed by reacting with oxygen:
V2O4(s) + ½O2(g) ® V2O5(s)
SO2(g) + ½O2(g) ® SO3(g)
This is catalyzed by vanadium(V) oxide:
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.
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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 Ziegler–Natta 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:
A non-industrial example is enzyme catalysis in biological washing powders, allowing efficient washing at a lower temperature.
lower energy demands…
…therefore less CO2 produced…
…therefore less environmental impact…
…and lower production costs.
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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.
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