Download Energy Changes

Energy Changes
Energy changes associated with
chemical reactions.
Notes (1)
All chemical reactions involve energy. If you
have ever sat near a campfire, you have
heat
experienced this energy as _____and
_______.
light
In addition to producing energy, chemical
use energy.
reactions also _____
photosynthesis
For example, plants perform ____________
which is a chemical reaction that uses energy
from sunlight.
Notes (2)
Energy is involved in chemical reactions in two
ways:
(1) At the start of a chemical reaction, energy is
break
used to ___________
bonds between atoms in
the reactants so that the atoms are available to
form new bonds; and
(2) Energy is ___________
released when new bonds form
as the atoms recombine into the new
compounds of the products.
We classify chemical reactions based on how the
energy used in (1) compares to the energy released
in (2).
Notes (3)
If forming new bonds releases more energy than
it takes to break the old bonds, the reaction is
exothermic Once started, exothermic
____________.
reactions tend to keep going because each
releases enough energy to start the
reaction _______
reaction in neighboring molecules.
Notes (4)
If forming new bonds in the products releases
less energy than it took to break the original
____
bonds in the reactants, the reaction is
endothermic Endothermic reactions absorb
___________.
energy. In fact, endothermic reactions need
more energy to keep going. An example of an
important endothermic reaction is photosynthesis
__________.
An interesting question
Exothermic reactions occur because the atoms
___________
products
arranged as compounds of the ____________
have lower energy and are more stable than they
are when arranged as compounds of the
reactants
___________.
Chemical reactions
________________—like
other systems—move
toward more stable circumstances. If this is true,
why don’t all the elements combine into the
molecules that have the lowest possible energy?
Notes (5)
activation energy a reaction
Without enough ________________,
will not happen even if it is __________.
exothermic That is
why a flammable material like gasoline does not
burn without a spark or flame. The spark
supplies the activation energy to start the
reaction.
Notes (6)
The activation energy must be supplied to break the
molecules of hydrogen and oxygen apart. Energy is then
released when the four free hydrogen and two free
oxygen atoms combine to form two water molecules.
The reaction is _____________
exothermic because the energy
released by forming water is greater than the activation
energy. Once the reaction starts, it supplies its own
activation energy and quickly grows.
Enthalpy
The symbol ______,
H is used to represent
internal energy of a
enthalpy, which is the _______________
substance. It is this change in enthalpy, or
ΔH that determines which way the energy
______,
flows.
negative (-)
If the energy flows out, the sign is __________,
exothermic
and the reaction is __________.
positive (+)
If the energy flows in, the sign is __________,
endothermic
and the reaction is ____________.
Notes (7)
You learned that photosynthesis is an
endothermic reaction. Through photosynthesis,
______________
plants capture energy from the Sun. The way that
humans and other life forms obtain this energy is by
respiration
a chemical reaction called _______________
which
is the reverse of photosynthesis.
Glucose is metabolically “burned” in the presence
of oxygen and carbon dioxide and water is released.
What kind of reaction is respiration—exothermic or
endothermic? How do you know?
Fossil Fuels
Energy is obtained from fossil fuels (like natural
gas, kerosene, and coal) by using
_______________________,
which are
Combustion Reactions
exothermic
____________.
In addition to producing energy, combustion
reactions yield ______________
carbon dioxide and _______
water as
products.
Notes (8)
Certainly, it’s useful for chemical reactions to
__________
produce more energy than they use. But, how
so we benefit from reactions that use more energy
than they produce?
It turns out that most of the reactions used in
industry to produce useful materials _________
require
more energy than they produce. This is one of the
reasons sources of energy are so important to
industry. In other words, exothermic reactions are
needed to cause endothermic reactions to run!
Notes (9)
ICE PACKS - An ice pack gets very cold because it
ionic bonds in
takes energy to dissolve the ____________
the ammonium nitrate. Besides being
endothermic this reaction is also a
____________,
dissolution reaction
__________________.