Activation energies
Or, why things don’t fall apart
Outline
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Activation energies
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As we saw, rearranging chemical bonds
can yield a lot of energy. Why don’t
“spontaneous” reactions happen
spontaneously?
Activation energies are what keeps
high-energy compounds from falling
apart!
Rearranging chemical bonds can yield
a lot of energy
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Energy released:
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778 kJ/mole
48 kJ/gram
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Why does methane
exist in the presence
of oxygen?
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Shouldn’t it
immediately change
into CO2 and water?
Activation energy
You have to put extra energy in to get the
reaction going… which means that a
reaction that would normally be selfsustaining may not start without a “boost”!
Activation energies are barriers
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Extra energy (a spark, for instance) has
to be put in to get the reaction started.
This extra energy is the “activation
energy” that has to be supplied before the
reacton can occur.
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Exothermic reactions supply their own
activation energy, once started!
Catalysts lower activation energies
Without catalyst
With catalyst
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Mixed hydrogen & oxygen need a spark
(high-energy) to begin combining
But… a bit of metallic platinum will reduce
the activation energy to almost zero
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Puncturing a hydrogen balloon with a platinum
pin will make a big boom!
Stoichiometry and the Mole
Thinking about chemical reactions
without using microscopes and
tweezers
Outline
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Stoichiometry and the mole
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We can’t pick out individual atoms and
molecules very conveniently
But materials combine in Definite Proportions
according to the number of atoms/molecules
required
{ Balanced chemical reaction equations
If we define a quantity as “the molecular
weight in grams” we can directly measure
macroscopic quantities that correspond to the
quantities in chemical reaction equations
Equal masses of materials may have
very different numbers of particles
ping-pong balls
golf balls
But…
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Materials combine in definite
proportions atom-by-atom
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compounds have specific ratios of
atoms
CH4 + 2 O2 → CO2 + 2 H2O
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We don’t have a small enough
tweezers for this…!
Suppose that we define a unit based on
unit weight…
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Suppose we call
this a “blass” for
“ball-mass”
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A “blass” of
ping-pong balls
weighs 10 grams
A “blass” of golf
balls weighs
200 grams
Both quantities
have the same
number of balls
Suppose that we define a unit based on
unit weight…
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We call this a
“mole” for
“molecular mass”
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A “mole” of
carbon atoms
weighs 12 grams
A “mole” of
oxygen molecules
(O2) weighs
32 grams
Both quantities
have the same
number of atoms!
6.02 × 1023 is NOT a useful number
because we don’t use tiny tweezers!
1 mole = 23 g
1 mole = 4 g
1 mole = 207 g
Problem set
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Posted to Jenzabar
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Working with moles:
Given a chemical reaction equation…
{ How many grams of A are required to
completely react with x grams of B?
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