Reaction Rate

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Reaction Rate

• How Fast Does the Reaction Go

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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
4
Activation Energy - Minimum energy to make the
reaction happen
Reactants
Energy
Products
Reaction coordinate
5
Activated Complex or Transition State
Reactants
Energy
Products
Reaction coordinate
6
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.
• Smaller particles faster reaction.
• Smallest possible is molecules or ions.
• Dissolving speeds up reactions.
• Getting two solids to react with each other is
  slow.

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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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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
15
Catalysts
Pt surface
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Reaction Mechanism

• Elementary reaction- a reaction that happens in a
  single step.
• Reaction mechanism is a description of how the
  reaction really happens.
• It is a series of elementary reactions.
• The product of an elementary reaction is an
  intermediate.
• An intermediate is a product that immediately
  gets used in the next reaction.

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• This reaction takes place in three steps

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Ea

• First step is fast
• Low activation energy

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Ea
Second step is slow High activation energy
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Ea
Third step is fast Low activation energy
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Second step is rate determining
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Intermediates are present
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Activated Complexes or Transition States
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Mechanisms and rates

• There is an activation energy for each elementary
  step.
• Slowest step (rate determining) must have the
  highest activation energy.