Thermodynamics

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Thermodynamics

• Tells if a reaction
• will occur

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Kinetics

• Tells how fast a reaction will occur

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Reaction Rate

• Speed of the reaction
• Found experimentally
• Measure change in concentration of reactant or
  product over time
• Rate ?Concentration
• ?time

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How do you measure rates?

• Measure concentration of 1 or more
  reactants/products over time
• Reactants disappear
• Products appear
• reaction rate change in concentration of
  reactants products in given amount time

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Concentration of Reactants, Products
Appearance of products
Disappearance of reactants
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How do reactions occur?

• Must have an effective collision between reacting
  particles
• Collision Theory
• Collision must be energetic
• Collision must occur at an effective angle

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Particle Diagram of Collision
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Reaction Rates depend on

• frequency of collisions
• how often occur
• And
• efficiency of collisions
• what percentage are effective

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Collision Theory

• Molecules must collide in order to react
• Effective collisions lead to formation of
  products
• Ineffective collisions do not lead to products

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Effective Collisions

• Energetic
• Favorable Orientation

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Effective vs. Ineffective Collision
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Most collisions are NOT effective!
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Why Do Collisions Have to be Energetic?
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Activation Energy Reaction
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Energy Diagram of a Reaction
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Activation Energy

• Energy needed to initiate reaction
• Energy needed to overcome reaction barrier
• difference between where reactants start top
  of hill
• Difference between reactants activated complex

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Activation Energy

• Using match to start a fire
• spark plug in car engine

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Potential Energy Curve Endothermic
Endothermic Reaction Products have more P.E.
than reactants Start low, end high
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Potential Energy Curve Exothermic
Exothermic Reaction Products have less P.E.
than reactants Start high, end low
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Have to label 6 energies on curve reactants
products
Both endo exo reactions
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6 Energies to Label
Label on both endo exo P.E. curves

1. PE reactants
2. PE products
3. PE activated complex
4. Ea forward reaction
5. Ea reverse reaction
6. ?H

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Ea for reverse rxn
Ea for forward rxn
P.E. of activated complex
P.E. of products
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?H of reaction
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P.E. of reactants
P.E. of activated complex
P.E. of products
Time What kind
of reaction is represented?
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?H of reaction
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Why does collision have to be energetic?

• kinetic energy(KE) of reactants is used to
  overcome reaction barrier
• KE is transformed into PE

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Factors that determine reaction rates

1. Nature of reactants (ions vs molecules)
2. Temperature
3. Concentration
4. Pressure (gases only)
5. Surface Area
6. presence of catalyst

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Nature of the reactantsIons or Molecules?

• Ions in solution react quickly
• Covalently bonded molecules react slowly
• takes time to break all those bonds!
• 2 gas phase reactants react more quickly than 2
  liquids or 2 solids

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Temperature

• measure of average KE of molecules in system
• faster molecules are moving, will collide more
  often
• faster molecules are moving, more energetic the
  collisions

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Increase in Temperature

• Increases frequency of collisions
• Increases percentage of collisions that lead to
  reaction

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Concentration

• Increase in concentration
• more particles per unit volume
• more collisions in given amount time

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Pressure

• systems involving gases changing pressure
  analogous to changing concentration
• ? Pressure, ? particles per unit volume
• ? Pressure, ? particles per unit volume

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Surface Area

• Higher surface area
• more particles exposed for reaction
• Higher surface area means smaller particle size
• (For heterogeneous reactions)

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Vocabulary Interlude

• Homogeneous Reaction
• all reactants in same phase
• Heterogeneous Reaction
• reactants in different phases

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Catalyst

• Substance that increases rate of reaction without
  itself being consumed
• does not participate in reaction
• Provides alternate reaction pathway with lower
  energy barrier

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Reaction Mechanism

• series of steps that leads from reactants to
  products
• describes how bonds break, atoms rearrange, and
  bonds form in reaction

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Elementary Step

• each individual step in reaction mechanism
• lowest elementary step called
• rate-determining step