Reaction Rates

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Reaction Rates

• Ch 18 Notes

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Collision Theory

• Reacting substances (atoms, ions, molecules,
  etc.) must collide in order to react
• Collisions have to happen with the correct
  orientation
• Reacting substances must have enough energy to
  form the activation complex
• this amount of energy is the activation
  energy

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sToP tHinK

• Space shuttles use liquid H2 and O2 in a chemical
  reaction to get enough energy for propulsion.
• Why do they use these substances in liquid phase
  and not gaseous phase?

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1. Reacting substances (atoms, ions, molecules,
etc.) must collide in order to react
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2. Collisions have to happen with the correct
orientation
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3. Reacting substances must have enough energy
to form the activation complex
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Collision Theory - Pictoral Summary
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Activation Energy
difference between starting energy and activated
complex.
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Reactions happen in steps

• Most reactions are complex, meaning they happen
  in two or more elementary steps
• example

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Different Energy diagrams
lt- single step
complex reaction -gt
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sToP tHinK

• Assume that this reaction is reversible, meaning
  that products can react to reform reactants.
  Which direction of the reactants would have a
  larger activation energy?
• reactants to products OR products to reactants

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Factors that affect Reaction Rate

1. Nature of Reactants
2. Concentration
3. Surface Area
4. Temperature
5. Catalysis

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1. Nature of Reactants

• Some elements/substances are more reactive than
  others
• Ionization energy
• Electronegativity
• Solid, Liquid, or Gas
• Single, Double, or Triple Bonds

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2. Concentration

• Higher reactant concentrations means
• more particle collisions and, therefore, a faster
  reaction rate because more collisions increases
  the chance of having collisions with enough
  energy and correct orientation.

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3. Surface Area

• Greater surface area creates a larger interface
  for particle collisions, so increases reaction
  rate.

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4. Temperature

• Higher temperature means higher kinetic energy,
  more particle collisions, and faster reaction
  rate.

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5. Catalysts

• Very different from the other factors that affect
  reaction rate
• Instead of increasing/decreasing collisions,
  catalysts lower the activation energy needed for
  the reaction to occur
• activation energy - amount of energy needed for a
  reaction to take place
• inhibitor - slows down or even blocks a reaction