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Collision Theory and Activation Energy

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Collision Theory and Activation Energy Unit 3: Chemical Kinetics and Equilibrium – PowerPoint PPT presentation

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Title: Collision Theory and Activation Energy


1
Collision Theory and Activation Energy
  • Unit 3 Chemical Kinetics and Equilibrium

2
Review Chemical Reaction
  • Generic Format of reaction

A B C D
Reactants
Products
5 different types of chemical reactions
1) Synthesis
4) Double Displacement
2) Decomposition
5) Combustion
3) Single Displacement
3
Review Kinetic Molecular Theory
  • Used to explain many observations and chemical
    events!
  • All matter is made up of microscopic-sized
    particles (atoms, ions, molecules)
  • These particles are in constant motion (possess
    kinetic energy)
  • There is space between the particles (speed and
    spacing determine the physical state of matter)
  • Adding energy increases the speed of the moving
    particles (thus inc kinetic energy)

4
How do reactions occur?
Collision Theory
  • In order for a chemical reaction to take place,
    the reactants must come in contact and
    collide!!!!!
  • The collision transfers kinetic energy needed
    to break the necessary bonds so that new bonds
    can be formed.

5
Turns out
  • By calculating how many collisions are taking
    place per second and how quickly product is being
    produced chemists learned that most collisions
    are not successful (no product formed)
  • To think that reactant particles collide and
    products are automatically produced is over
    simplified.
  • There must be other requirements for a collision
    to be successful!

6
The Collision Theory
  • is an explanation of what is necessary for a
    chemical reaction to occur.
  • When a chemical reaction takes place, the
    reactant particles must meet two conditions (or
    requirements) during collision for the collisions
    to be successful
  • Proper orientation
  • Particles must collide with a certain minimum
    amount of energy, called activation energy.

7
1. Orientation
  • Particles must collide with the proper geometry
    or orientation for atoms to come in direct
    contact and form the chemical bonds of the
    products.

8
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9
2. Activation Energy
  • Particles must collide with a certain minimum
    amount of energy, called activation energy (Ea).
  • This energy is required to break chemical bonds
    in the reactants.
  • Note The energy of each particle is not
    important, it is the energy of the collision.

10
2. Activation Energy
11
Effect of Temperature
12
Potential Energy Diagram
  • We can represent the increase in potential energy
    during a chemical reaction using a potential
    energy diagram
  • The kinetic energy of reactants is transferred to
    potential energy as the reactants collide

13
The hill
  • The hill represents the activation energy that
    must be overcome for the reaction to occur
  • high hill ? slow rate of reaction
  • low hill ? fast rate of reaction
  • Top of hill is called change over point
  • There is a chemical species that exists here is
    referred to as activated complex
  • Activated complex
  • Neither reactant or product
  • Partial bonds, highly unstable

14
  • Potential Energy Diagram Handout

15
  • If both of these conditions are not met,
    particles will merely collide and bounce off one
    another without forming products.

16
Some collisions are successful.
  • Although, the percentage of successful collisions
    is extremely small, chemical reactions still take
    place at a reasonable rate because there are so
    many collisions per second between reactant
    particles!

17
Reaction Mechanism
  • Converting reactants to products often involves
    more then one stepas you know!
  • Each step is called an elementary reaction
  • Molecules formed during elementary reactions are
    called reaction intermediates (neither reactants
    nor products)
  • Example 2NO O2 ? 2NO2
  • Step 1 NO O2 ?NO3
  • Step 2 NO3 NO ?2NO2

18
The Rate Determining Step
  • In multi-step reactions there can be 2 or more
    elementary reactions.
  • There is always one reaction that is slowest
    this determines the overall rate of reaction
  • Thus, the slowest elementary reactions is called
    the rate determining reaction.

19
Catalysts
  • Works by lowering the activation energy of a
    reaction so that a larger fraction of reactants
    have sufficient energy to react
  • They do this by providing an alternative reaction
    mechanism

20
For example
  • A B ? AB (no catalyst)
  • With catalyst
  • Step 1 A catalyst ? A-cat
  • Step 2 A-cat B ? AB catalyst
  • Overall A B ? AB
  • Both steps are faster then the original,
    uncatalyzed reaction
  • Catalyst remains unchanged in the end

21
Catalysts contd
22
Real life catalyst example
  • Manganese dioxide (a black powder) will catalyze
    the breakdown of hydrogen peroxide.
  • 2H2O2 (aq) ? 2H2O(l) O2 (g) (uncatalyzed)
  • With catalyst
  • Car exhaust pipes use catalytic converters to get
    rid of some of the nasty gases from the engines.

23
Practice
  • Read Chapter 12!!!!!
  • Pg 484 9, 12a,b,d
  • Pg 486 1-4, 12
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