Kinetics Part IV: Activation Energy Chapter 13 Sec 5

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KineticsPart IV Activation EnergyChapter 13
Sec 5 6of Brady Senese 5th Edition

• Dr. C. Yau
• Fall 2009

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The Collision Theory

• The rate of a reaction is proportional to the
  number of effective collisions per second among
  the reactant molecules.
• Only EFFECTIVE collisions lead to products.
• We already know concentration plays an important
  part.
• Only a small fraction of collisions lead to
  products. Now we consider two other factors 1)
  Activation Energy
• 2) Molecular Orientation

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Kinetic Energy Distribution
Ea Activation energy minimum energy needed
for collision to be effective
REMEMBER This is the graph for Kinetic Energy.
Fig 13.12 p.545
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Collision Theory Of Reactions

• For a reaction to occur, three conditions must be
  met
• Reactant particles must collide.
• Collision energy must be enough to break
  bonds/initiate.
• Particles must be oriented so that the new bonds
  can form.
• e.g. NO2Cl Cl NO2 Cl2

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Eqn Summarizing 3 Factors in Collision Theory

• Particulate Level
• Rxn Rate (molecules L-1 s-1)
• N x forientation x fKE
• N collisions per second per liter of mixture
• forientation fraction of collisions with
  effective orientation
• fKE fraction of collisions with sufficient
  kinetic energy for effective collision (area
  under the curve with KE ? Ea

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• Mathematically, fKE
• has been found to be related to Ea and T in this
  equation

Still remember what fKE stands for?
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Eqn Summarizing 3 Factors in Collision Theory

• Macroscopic Level
• Equation has to be in terms of moles instead of
  molecules.
• Conversion factor is
• So we divide the previous equation by Avogadros
  number to get
• reaction rate in units of mol L-1 s-1

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Temperature Effects

• Changes in temperature affect the rate constant,
  k, according to the
• Arrhenius equation
• p is the steric factor
• Z is the frequency of collisions.
• Ea is the activation energy
• R is the Ideal Gas Constant (8.314 J/mol K)
• T is the temperature (K)
• We substitute A (the frequency factor) for (pZ)

This is an important equation to remember!
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Graphical Determination of Ea
You are expected to be able to derive this
yourself.
How exactly do we determine Ea? What do we plot
on the x-axis? on the y-axis? How do we find Ea
on the graph?
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Example 13.11 p.549 Determine Ea in kJ/mol
What do we do with this data?
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(No Transcript)
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Then what?... How do we find Ea?
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Determination of Ea from k at 2 temperatures
Ratio form Can be used when A isnt known.
For those who like math, turn in a careful
derivation for bonus points. You are not allowed
to get help from anyone.
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Example

• Given that k at 25C is 4.6110-1 M/s and that at
  50C it is 4.6410-1 M/s, what is the activation
  energy for the reaction?

Ea 208 J/mol
Can you think of a reason why the graphical
method would give a more accurate value for Ea?
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Working With The Arrhenius Equation

• Given the following data, predict k at 75C using
  the graphical approach

slope -0.0278 K and y-intercept -0.1917
k8.2510-1
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• In the reaction 2N2O5(g) ?4 NO2(g) O2(g) the
  following temperature and rate constant
  information is obtained. What is the activation
  energy of the reaction?
• 102 kJ
• -102 kJ
• 1004 kJ
• -1004 kJ
• none of these

Practice with Example 13.12 p.550, Exer.24, 25
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Potential Energy Diagrams

• demonstrate the energy needs and products as a
  reaction proceeds
• tell us whether a reaction is exothermic or
  endothermic
• tell us if a reaction occurs in one step or
  several steps
• show us which step is the slowest
• Do not confuse PE diagram with KE diagram! Learn
  the terminology!
• So, remember which is the KE diagram?

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Potential Energy Diagrams
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Potential Energy Diagram

• What would the potential energy diagram look like
  for an endothermic reaction?
• Make a sketch of a PE diagram for an endothermic
  reaction.
• Where do we look to find the activation energy?
• Where do we look to find the heat absorbed during
  the reaction?

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Catalysts

• speed a reaction, but are not consumed by the
  reaction
• may appear in the rate law
• lower the Ea for the reaction
• may be heterogeneous or homogeneous

Ea of uncatalyzed rxn
Ea of catalyzed rxn
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Catalytic Actions

• may serve to weaken bonds through induction
• may serve to change polarity through
  amphipathic/surfactant effects
• may reduce geometric orientation effects
• Heterogeneous catalyst reactant and product
  exist in different states.
• Homogeneous catalyst reactants and catalyst
  exist in the same physical state

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Example of a heterogeneous catalyst
Well-known The Haber Process.
3H2 (g) N2 (g) 2NH3 (g)
Note Fe is never consumed.