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GOALS
1. Explain the role of activation energy and degree of randomness in
chemical reactions
3. Investigate the effects of a catalyst on chemical reactions and apply it
to everyday examples.
4. Experimentally determine indicators of a chemical reaction specifically
precipitation, gas evolution, water production, and changes in energy to
the system.
5. Demonstrate the effects of changing concentration,
temperature, and pressure on chemical reactions.
GOAL: Explain the role of activation energy and
degree of randomness in chemical reactions
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Reaction Directions
• Physical and Chemical systems attain the
lowest possible energy.
• Law of disorder: the natural tendency is for
systems to move in the direction of maximum
disorder (or randomness)- 2nd law of
thermodynamics
– Entropy is a measure of the disorder of a
system.
• An increase in entropy favors spontaneous
chemical reactions; decrease favors the
nonspontaneous reaction.
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Entropy
is a
measure
of the
disorder
of a
system.
• Rxns are
favorable
when they
result in a
decrease in
energy and an
increase in
entropy
(disorder)
• Rxn can
proceed if
products have
more order IF
energy is
supplied. 5
2nd Law of thermodynamics:
The total Entropy of the universe is
constantly increasing. The state of maximum
entroy is the most stable state.
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Chemical Reactions and Energy
• All chemical reactions release or absorb
energy.
– Heat, light, sound
• Chemical reactions are the making and
breaking or bonds.
Enthalpy (ΔH):
Heat Energy of
the System
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1. Exergonic
• Chemical
reactions that
releases energy
are called
exergonic.
– Glow sticks
• If heat is
released, it is
called
exothermic
– Combustion
– Decease in Enthalpy
(ΔH) of the system
Ch 17
2. Endergonic
• Chemical reactions
that require
energy are called
endergonic.
– Ex: Cold Packs
• If heat is
absorbed, it is
called endothermic
• Increase in
Enthalpy (ΔH) of
the system
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Ch 17
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Intro
Clip
Goals: Investigate the effects of a catalyst on chemical reactions and
apply it to everyday examples. Demonstrate the effects of changing
concentration, temperature, and pressure on chemical reactions.
Rates of Chemical Reactions
Collision theory
- For a chemical reaction to occur,
the reactant particles must collide.
But collisions with too little energy
do not produce a reaction.
- The particles must have enough
energy for the collision to be
successful in producing a reaction.
- The rate of reaction depends on
the rate of successful collisions
between reactant particles.
The more successful collisions
there are, the faster the rate of
reaction
Collision Theory (Youtube)
Clip
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Measuring the Rates of Chemical Reactions
• Rate: Expressed as the amount of reactant
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changing per unit of time.
• What are some ways that you might be able to
measure the rate of a reaction?
– Amount of a product produced over time
– Amount of reactant used up over time.
Factors Affecting Reaction Rates
1. Temperature
2. Concentration
3. Particle Size (surface area)
4. Pressure
5. Catalysts (“the match maker”)
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1-Temperature
• Particles can
only react when
they collide. If
you heat a
substance, the
particles move
faster and so
collide more
frequently. That
will speed up
the rate of
reaction.
 increase the temp, more
molecules are able to move
faster, so more of them
will have the minimum
energy for the reaction to
take place.
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2- Concentration
• Increasing the
concentration,
increases the
probability of a
collision between
reactant particles
because there are more
of them in the same
volume and so increases
the chance of a
successful collision
forming products.
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3-Particle Size
Smaller in size
means larger in
surface area and
hence a faster
rate of reaction.
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No so fun fact: On Feb. 7,
2008 a huge explosion and fire
occurred at the Imperial Sugar
Refinery in Georgia, USA
causing 14 deaths and seriously
injuring 38 others. The
explosion was caused by
accumulated sugar dust in the
packaging facility
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4-Pressure
• Increasing the
pressure on a
reaction
involving
reacting gases
increases the
rate of reaction.
• Changing the
pressure on a
reaction which
involves only
solids or liquids
has no effect on
the rate.
5-Catalysts
• A catalyst is a
substance which
speeds up a
reaction, but is
chemically
unchanged at the
end of the
reaction.
 Increases the frequency of collisions
 Changes orientation of molecules
 Can reduce intramolecular forces within
reactants
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Activation Energy
• To understand what catalysts
do, we need to go back and talk
about reactions and energy…..
• Collisions only result in a
reaction if the particles collide
with enough energy to get the
reaction started.
– This minimum energy required is
called the activation energy for
the reaction.
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Activation Energy
• The minimum
energy that
colliding particles
must have in order
to react is called
activation energy.
– energy can be
used to stretch,
bend, and
ultimately break
bonds, leading to
chemical reactions
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Catalysts
A catalyst provides
an alternative
route for the
reaction by
lowering its
activation energy
so more particles
will have enough
energy to react.
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Catalysts
Biological Catalyst:
Enzymes
Inorganic catalyst…
Metals…
.
– Catalysts
are not used
up in the
reaction.
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Catalysts and Inhibitors
Some reactions proceed too fast.
• They can be slowed down by inhibitors.
– EX: Preservatives in food
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“Everyday examples of Rates of
Reactions
1. Enzymes
2. Catalytic converters
Catalytic converters change poisonous molecules like carbon monoxide and various
nitrogen oxides in car exhausts into more harmless molecules like carbon dioxide and
nitrogen. They use expensive metals like platinum, palladium and rhodium as the
heterogeneous catalyst.
The metals are deposited as thin layers onto a ceramic honeycomb. This maximises
the surface area and keeps the amount of metal used to a minimum.
Taking the reaction between carbon monoxide and nitrogen monoxide as typical:
Reversibility of Reactions
• Some reactions are
reversible
• Chemical Equilibrium
– When the rates of
the forward rxn and
the reverse rxn are
equal
– Dynamic state
• Rxn still continues to
happen
• Funny Review Clip