reaction rate how fast does the reaction go collision theory chemists believe that all chemical...
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Reaction RateHow Fast Does the Reaction Go
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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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Molecules not correctly orientated, so no reaction
Molecules are correctly orientated, so reaction occurs
For example:2BrNO(l) 2NO (g) + Br2 (g)
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Ene
rgy
Reaction coordinate
Reactants
Products
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Ene
rgy
Reaction coordinate
Reactants
Products
Activation Energy - Minimum energy to make the reaction happen Ea
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Ene
rgy
Reaction coordinate
Reactants
Products
Activated Complex or Transition State
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Ene
rgy
Reaction coordinate
Reactants
Products
Overall energy change ΔH
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Endothermic Reactions
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Exothermic Reactions
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Bimolecular Reaction
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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 bonds are required
Ionic reactions:
Pb2+ + SO42- → PbSO4
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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 blocks a catalyst (raises the activation energy).
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Ene
rgy
Reaction coordinate
Reactants
Products
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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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Second step is slowHigh activation energy
Ea
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Ea
Third step is fastLow 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
1. Cl + O3 → O2 + ClO Medium
2. O3 → O2 + O Fast
3. ClO + O → Cl + O2 Fast
Overall 2O3 → 3O2
Intermediates are ClO and O; catalyst is Cl