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 a reactant or
  a product over time.
• Rate ?Conc
• ?time

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How do you measure rates?

• Measure the concentration of 1 or more reactants
  or products over time.
• Reactants disappear
• Products appear
• The reaction rate is the change in concentration
  of reactants products in a given amount of 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 for reaction to occur. Collision
  Theory
• Collision must be energetic.
• Collision must occur at an effective angle.

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Particle Diagram of Collision
Activated complex or transition state.
Reactants
Products
NO O3 ? NO2 O2
Activated Complex is NOT in equation!
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Reaction Rates depend on

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

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Collision Theory

• Molecules must collide in order to react.
• Effective collisions lead to the 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
Activated Complex
Reactants
Enthalpy or Potential Energy
Products
Reaction Pathway
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Activation Energy

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

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Activation Energy

• Using a match to start a fire.
• The spark plug in a 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.
1)Ea Activation Energy 2)?H Hproducts
Hreactants
Potential Energy
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6 Energies to Label
Label on both endo exo P.E. curves.

1. P.E. of reactants
2. P.E. of products
3. P.E. of activated complex
4. Ea for the forward reaction
5. Ea for the reverse reaction
6. ?H

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They mix the arrows up!

• You cant memorize them by location they move
  them around.
• Have to memorize them by where they start and
  where they stop.
• The 3 arrows for Potential Energy of start at
  the baseline.
• Eas start where you are end at the top of
  the hill.

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

• The kinetic energy of the reactants is used to
  overcome the reaction barrier.
• The kinetic energy is transformed into potential
  energy.

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Factors that determine reaction rates

• Nature of the reactants (ions vs. molecules)
• Temperature
• Concentration
• Pressure (for gases)
• Surface Area
• The presence of a catalyst

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Nature of the reactantsIons or Molecules?

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

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Temperature

• Rule of thumb
• Increasing the temperature 10oC doubles the
  reaction rate.

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Temperature

• A measure of the average kinetic energy of the
  molecules in a system.
• The faster they are moving, the more often they
  will collide.
• The faster they are moving, the more energetic
  the collisions.

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Maxwell-Boltzmann Distribution
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Increase in Temperature

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

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Concentration

• Increase in concentration means more particles
  per unit volume so more collisions in a given
  amount of time.

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Pressure

• For systems involving gases.
• Analogous to increasing concentration.
• ? Pressure, ? number of 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 are in the
  same phase.
• Heterogeneous Reaction reactants are in
  different phases.

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Catalyst

• Substance that increases the rate of reaction
  without itself being consumed.
• Provides an alternate reaction pathway with a
  lower energy barrier.

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Enzymes are catalysts!
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Catalytic Converter in Engines
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Hydrogenation Surface Catalysis
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Surface Science
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Reaction Mechanism

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

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Elementary Step

• Each individual step in a reaction mechanism.
• The slowest elementary step is called the
  rate-determining step.

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P.E. Curve for Multi-Step Rxn