Factors Effecting Reaction Rate

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Factors Effecting Reaction Rate
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Activation Energy

• A certain amount of energy must be available to
  propel the molecules from one chemical state to
  another
• This energy barrier is called the Activation
  Energy (Ea)?
• Arrangement of atoms that has this maximum energy
  is called the activated complex

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Energy Profile
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Collision Theory

• In order to react molecules and atoms must touch
  each other.
• They must hit each other hard enough to react.
• Anything that increase these things will make the
  reaction faster.

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Reactants
Energy
Products
Reaction coordinate
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Activation Energy - Minimum energy to make the
reaction happen
Reactants
Energy
Products
Reaction coordinate
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Activated Complex or Transition State
Reactants
Energy
Products
Reaction coordinate
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Reactants
Energy
Overall energy change
Products
Reaction coordinate
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Things that Effect Rate

• Temperature
• - Higher temperature faster particles.
• - More and harder collisions.
• - Faster Reactions.
• Concentration
• - More concentrated closer together the
  molecules.
• - Collide more often.
• - Faster reaction.

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Things that Effect Rate

• Particle size
• - Molecules can only collide at the surface.
• - Smaller particles bigger surface area per
  volume.
• - Smaller particles faster reaction.

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Orientation

• Colliding alone is not enough to guarantee
  reaction
• Relative orientations of the molecules may
  determine if the energy gets to the right place
  or if they are suitably oriented to form the new
  bonds
• As temp increases, the number of collisions
  increases as does the fraction that are oriented
  correctly

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Things that Effect Rate

• Catalysts- substances that speed up a reaction
  without being used up.(enzyme).
• - Speeds up reaction by giving the reaction a
  new path.
• - The new path has a lower activation energy.
• - More molecules have this energy.
• - The reaction goes faster.
• Inhibitor- a substance that blocks a catalyst.

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Reactants
Energy
Products
Reaction coordinate
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Homogenous Catalyst

• Catalyst present in the same phase as the
  components of the reaction
• Example NO acts as a catalyst for the
  decomposition of ozone
• NO provides another mechanism for the reaction
  which has a lower Ea

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Heterogeneous Catalyst

• Exists in a phase different from reactants
• Catalyzed reactions of gases often occur at the
  surface of metals
• Places where reacting molecules are absorbed on
  the metal surface are called active sites

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Catalysts

• Hydrogen bonds to surface of metal.
• Break H-H bonds

Pt surface
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Catalysts
Pt surface
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Catalysts

• The double bond breaks and bonds to the catalyst.

Pt surface
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Catalysts

• The hydrogen atoms bond with the carbon

Pt surface
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Catalysts
Pt surface
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Example

• A reaction occurs in a two step mechanism as
  shown
• A C ? B D (slow)
• B A ? C F (fast)
• What substance acts as a catalyst?
• What substance is an intermediate?
• Write the overall reaction equation.
• Write the rate law for the overall reaction.