2. ReviewReview
A chemical equation describes aA chemical equation describes a
chemical change (rxn).chemical change (rxn).
2NO(g) + O2(g) 2NO2(g)
BEFOREBEFORE
Reaction!Reaction!Reaction!Reaction!
AfterAfter
3. Reaction RatesReaction Rates
Reaction rate = “speed” of reaction.Reaction rate = “speed” of reaction.
– EX: Burning of hydrogenEX: Burning of hydrogen fastfast
– EX: Rusting of ironEX: Rusting of iron slowslow
– EX: Conversion of graphite to diamondEX: Conversion of graphite to diamond
extremely slowextremely slow
Collision theory: reactions happen whenCollision theory: reactions happen when
molecules collide.molecules collide.
– Reaction rate depends on two thingsReaction rate depends on two things (that(that
we’re going to discuss today)we’re going to discuss today)::
How frequently molecules collide.How frequently molecules collide.
How fast they are moving when they do collide.How fast they are moving when they do collide.
4. Activation EnergyActivation Energy
All reactions have an energy profile:All reactions have an energy profile:
Reaction Coordinate Reaction Coordinate
Energy
Energy
Reactants
Products
+∆H
Activation
Energy
-∆H
Reactants Products
Activation
Energy
5. Collision TheoryCollision Theory
When reactant molecules collide,When reactant molecules collide,
they must collide withthey must collide with at leastat least asas
much total energy as the activationmuch total energy as the activation
energy.energy.
– Any molecules that collide withAny molecules that collide with less thanless than
the activation energy will simply bouncethe activation energy will simply bounce
apart w/o reacting.apart w/o reacting.
The more molecules there areThe more molecules there are
colliding with sufficient energy tocolliding with sufficient energy to
react, the faster the reaction runs.react, the faster the reaction runs.
6. TemperatureTemperature
In general, rxns run faster at highIn general, rxns run faster at high
temperatures.temperatures.
– Rule of thumb: A rxn’s rate approx.Rule of thumb: A rxn’s rate approx.
doubles when the temp. rises by 10ºC.doubles when the temp. rises by 10ºC.
– NotNot alwaysalways true, but generally accurate.true, but generally accurate.
Rise in temp. means molecules moveRise in temp. means molecules move
faster.faster.
– More collisions per second.More collisions per second.
– Collisions w/ greater energy.Collisions w/ greater energy.
7. ConcentrationConcentration
As solute concentration increases, rxnAs solute concentration increases, rxn
rate usually increases also.rate usually increases also.
– More molecules colliding per unit of time.More molecules colliding per unit of time.
– EX: Zn(s) + 2HCl(aq)EX: Zn(s) + 2HCl(aq) ZnClZnCl22(aq) + H(aq) + H22(g)(g)
– 6.00 M HCl soln. will “eat” Zn much faster6.00 M HCl soln. will “eat” Zn much faster
than 1.00 M HCl soln.than 1.00 M HCl soln.
Explains why we usually mix reactantsExplains why we usually mix reactants
with one reactant in great excess.with one reactant in great excess.
– Keeps rxn rate high for entire rxn.Keeps rxn rate high for entire rxn.
8. ConcentrationConcentration
Mg + H2SO4 MgSO4 + H2
Set-Up 1Set-Up 1
Add 1 mole of MgAdd 1 mole of Mg
to 1 mole of Hto 1 mole of H22SOSO44..
ResultResult
Mg and HMg and H22SOSO44 are bothare both
completely used up, butcompletely used up, but
the reaction getsthe reaction gets veryvery slowslow
close to the end.close to the end.
Set-Up 2Set-Up 2
Add 1 mole of Mg to 5Add 1 mole of Mg to 5
moles of Hmoles of H22SOSO44..
ResultResult
4 moles of H4 moles of H22SOSO44 remain,remain,
but the reaction stays fastbut the reaction stays fast
all the way through.all the way through.
9. ConcentrationConcentration
Which bumper car ride will result inWhich bumper car ride will result in
more collisions per minute for themore collisions per minute for the
red car?red car?
10. Particle SizeParticle Size
The smaller the reactant particles,The smaller the reactant particles,
the faster the rxn.the faster the rxn.
– Grind up solids.Grind up solids.
– Dissolve solids.Dissolve solids.
Breaking up reactant particlesBreaking up reactant particles
increases:increases:
– surface area.surface area.
11. Particle SizeParticle Size
Aluminum powder bears a warning that itAluminum powder bears a warning that it
is an explosion hazard. Aluminum foilis an explosion hazard. Aluminum foil
bears no such warning. Why is Al powderbears no such warning. Why is Al powder
so dangerous when Al foil is not?so dangerous when Al foil is not?
– Powdered Al has aPowdered Al has a muchmuch greater surface areagreater surface area
exposed to oxygen.exposed to oxygen.
– The oxidation of aluminum is highlyThe oxidation of aluminum is highly
exothermic.exothermic.
– With powdered aluminum, a highly exothermicWith powdered aluminum, a highly exothermic
rxn can occur at a much greater rate.rxn can occur at a much greater rate.
12. Particle SizeParticle Size
These two blocks
have the same total
volume, but the one
on the right has been
broken into 1000
equal pieces. This
increases its surface
area by 60 times.
1meter
10 cm
14. CatalystCatalyst
CatalystCatalyst – a substance that speeds up a– a substance that speeds up a
reaction.reaction.
– Not consumed in rxn.Not consumed in rxn.
– Lowers activation energy.Lowers activation energy.
– Written above the reaction arrow.Written above the reaction arrow.
Example: Platinum metal is often used asExample: Platinum metal is often used as
a catalyst to convert methanol vapors intoa catalyst to convert methanol vapors into
formaldehyde.formaldehyde.
CH4O →Pt
CH2O + H2
15. Catalysts and Activation EnergyCatalysts and Activation Energy
Reaction Coordinate
PotentialEnergy
Uncatalyzed
Catalyzed
16. CatalystsCatalysts
Enzymes are proteins that act asEnzymes are proteins that act as
biological catalysts.biological catalysts.
– They speed up many biochemicalThey speed up many biochemical
reactions that would take far too long toreactions that would take far too long to
happen naturally.happen naturally.
– Enzymes are essential in just aboutEnzymes are essential in just about
every aspect of cell functioning.every aspect of cell functioning.
– Enzymes function best in specificEnzymes function best in specific
temperature ranges.temperature ranges.
Human enzymes work best around 37ºC.Human enzymes work best around 37ºC.
