Reaction Rate

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Reaction Rate

• How Fast Does the Reaction Go
• 5-1

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Collision Theory

• Chemists believe that all chemical change
  (rearrangement of matter) occurs due to the
  collision of the atoms or molecules that are
  reacting.

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• Collisions must have enough energy to produce the
  reaction (must equal or exceed the activation
  energy).
• Orientation of reactants must allow formation of
  new bonds

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For example 2BrNO(l) ? 2NO (g)
Br2 (g)
Molecules not correctly orientated, so no reaction
Molecules are correctly orientated, so reaction
occurs
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Activation Energy - Minimum energy to make the
reaction happen Ea
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 ?H
Products
Reaction coordinate
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Endothermic Reactions
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Exothermic Reactions
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Things that affect Rate

• 1. Temperature
• -Higher temperature faster particles.
• -More and harder collisions.
• -Faster Reactions.

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Temperature Reaction Rate
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Things that affect rate

• 2. Concentration
• -More concentrated closer together the
  molecules.
• -Collide more often.
• -Faster reaction.

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Things that Effect Rate

• 3. Nature of reactants
• a. Reactions are rapid at room temperature if
  no bond rearrangement are required
• Ionic reactions
• Pb2(aq) SO42-(aq) ? PbSO4(s)

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• b. Reactions are slow at room temperature when
  several bonds in the reacting substances must be
  broken
• Molecular reactions
• C2H5OH 3 O2 ? 2CO2 3H2O

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• 4. Surface area
• -Molecules can only collide at the surface.
• Smaller particles bigger surface area.
• Smaller particles faster reaction.

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• 5. a)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.
• b) Inhibitor- a substance that slows down a
  reaction (raises the activation energy).

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Endothermic Reaction witha Catalyst
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Exothermic Reaction with a Catalyst
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Catalysts Increase the Number of Effective
Collisions
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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.

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A chemical example of a Reaction Mechanism

• H2 (g)I2 (g) ? 2HI (g)
• This reaction is very slow at room temperature
  but if the system is exposed to sunlight ( source
  of UV light ) the reaction proceeds rapidly.

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• Step 1
• I2 (g) ?2I (g)
• Step 2
• H2 (g)2I (g) ? 2HI (g)
• Overall H2 (g)I2 (g) ? 2HI (g)

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Mechanism with catalyst

• Step rate
• Cl O3 ? O2 ClO Medium
• O3 ? O2 O Fast
• ClO O ? Cl O2 Fast
• Overall 2O3 ? 3O2
• Intermediates are ClO and O catalyst is Cl