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 of 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 as products appear
• reaction rate change in concentration of
  reactants products over 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?

• Collision Theory
• must have effective collisions between reacting
  particles
• 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
• percentage that 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 must

• be energetic
• have 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 (hill)
• difference between where reactants start top of
  hill

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Examples of Activation Energy

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

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Potential Energy Curve Endothermic
endothermic rxn products have more PE than
reactants start low, end high
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Potential Energy Curve Exothermic
exothermic rxn products have less PE than
reactants start high, end low
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PE diagram must be able to labellabel on both
endo exo PE 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
PE of activated complex
PE of products
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?H of reaction
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PE of reactants
PE of activated complex
PE 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?

• KE of colliding reactant particles 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 vs Molecules?

• Type of particles
• ions in solution react quickly
• covalently bonded molecules react slowly
• takes time to break all those bonds!
• Phase of particles
• 2 gas phase reactants react more quickly than 2
  liquid reactants or 2 solid reactants

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Temperature

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

• only pertains to systems involving gases
• ? pressure analogous to ? 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 only

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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 rxn pathway with lower energy
  barrier

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Reaction Mechanism

• series of steps that leads from reactants to
  products
• also referred to as transition state
• process of rxn during which
• bonds break
• atoms rearrange
• new bonds form

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Elementary Steps

• each individual step in reaction mechanism
• lowest elementary step called
• rate-determining step
• step that must get enough energy to occur or the
  whole reaction cant occur