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Section 7-4
Energy and Chemical Reactions
March 29, 2011
Pages 197 and 199
Energy and chemical reactions
• All chemical reactions
involve energy.
– Burning is a chemical
reaction that gives off energy
in the form of heat and light.
– In plants, photosynthesis is a
reaction that uses energy
from sunlight.
8.1 Why chemical bonds form
• It takes energy to
separate atoms that
are bonded together.
• The same energy is
released when
chemical bonds form.
• Atoms form bonds to
reach a lower energy
state.
Types of energy in reactions
• If forming new bonds releases more energy than
it takes to break the old bonds, the reaction is
exothermic.
• Exothermic reactions tend to keep going
because each reaction releases enough energy
to start the reaction in neighboring molecules.
How can the energy from an exothermic reaction be useful?
Types of energy in reactions
• Endothermic reactions absorb energy.
• These reactions need energy to keep going.
Where does the energy needed for this reaction come from?
Activation energy
• Activation energy is the energy needed to start a
reaction and break chemical bonds in the
reactants.
– This 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.
Activation energy
So even if you end up at a lower energy
level, you have to start by adding energy
10.2 Nuclear reactions
• Nuclear reactions change the
nucleus of an atom.
• Because they affect the nucleus
itself, nuclear reactions can change
one element into a different element.
• This means that nuclear reactions
don’t balance!
• Also, in nuclear reactions, some
matter is converted into energy!
(This is what E=mc2 means!)
Fission
• There are two kinds of nuclear reaction
• In a fission reaction, a neutron is shot at a
big atom to break it into smaller pieces.
• Nuclear reactors and simple nuclear
bombs use fission.
BOOM!
Fusion
• Fusion reactions happen when small
atoms are combined to make bigger atoms
• Big nuclear bombs use fusion.
• The sun produces energy through fusion
4 Hydrogens
1 Helium