Reaction Rates

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Reaction Rates
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What does rate mean?

• measure of the speed of any change that occurs
  within an interval of time
• time can range from microseconds (explosions) to
  centuries (plants to coal)
• in chemistryreaction rate is usually expressed
  as the amount of the reactant changes per unit
  time
• 3 moles/year
• 5 grams/second

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Collision Theory
http//www.mhhe.com/physsci/chemistry/essentialche
mistry/flash/collis11.swf

• only a small fraction of collisions produce a
  chemical reation
• effective collisions have enough kinetic energy
  and the collide at the correct angle
• need enough KE to
• assemble new compounds
• break the bonds of existing compounds

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Activation Energy
http//www.youtube.com/watch?vVbIaK6PLrRM

• minimum energy that colliding particles must have
  in order to react
• an activated complex is an unstable compound that
  forms momentarily at the peak of the barrier
• typically about 10-13 seconds
• both reactants reform and new products form
• also referred to as the transition state

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Activation Energy
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Factors Affecting Reaction Rate
The rate of a chemical reaction depends on

• temperature
• increasing temp. increases the frequency of
  collisions AND the KE necessary to overcome the
  activation energy barrier

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http//www.absorblearning.com/media/attachment.act
ion?quickvyatt2290

• concentration
• increasing the number of particles in a given
  volume increases the rate a which reactions occur

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• particle size
• surface area affects the reaction rate
• smaller particles have a greater ratio of surface
  area (x2) to volume (x3)
• this increases the amount of reactant exposed for
  reactions

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An iron bar is held in a flame and nothing
happens. Iron powder is then blown into the flame
and ignites
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• presence of a catalyst
• a substance that increases the rate of a reaction
  without being used up during the reaction

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• inibitors interfere with the action of the
  catalyst