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Changes in Matter
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Chapter Seventeen: Chemical
Reactions
• 17.1 Understanding Chemical Reactions
• 17.2 Energy and Chemical Reactions
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Investigation 17B
• How do scientists describe what happens
in a chemical reaction?
Conservation of Mass
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17.2 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.
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17.2 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?
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17.2 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?
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17.2 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.
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17.2 Activation energy
• This diagram shows how the energy flows in the reaction of hydrogen and oxygen.
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17.2 Addition reactions
• In an addition reaction, two or more substances combine to form a new compound.
• The process of creating large molecules from small ones is called polymerization.
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17.2 Decomposition reactions
• A chemical reaction in which a single compound is broken down to produce two or more smaller compounds is called a decomposition reaction.
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17.2 Reaction symbols
• The small symbols in
the parentheses (s, l,
g, aq) next to each
chemical formula
indicate the phase of
each substance in the
reaction.
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17.2 Displacement reactions
• In single-displacement reactions, one element replaces a similar element in a compound.
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17.2 Precipitation reactions
• A precipitate is a new
solid product that comes
out of solution in a
chemical reaction.
• The formation of a cloudy
precipitate is evidence
that a double-
displacement reaction
has occurred.
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17.2 Precipitation
• The limewater test for carbon dioxide is a precipitation reaction.
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17.2 Petroleum refining
• The refining process separates petroleum into molecules with different numbers of carbon atoms.
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17.2 Petroleum refining
• These are some of the molecules found in gasoline.
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17.2 Combustion reactions
• In a perfect reaction, all the hydrocarbon molecules are completely burned to into carbon dioxide and water.
• In an engine not all the fuel burns completely and pollutants such as carbon monoxide are also formed.
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17.2 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.
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Chemistry Connection
How do Hot and Cold Packs Work?
Have you ever used a hot
or cold pack?
Although it’s seem like
magic, it’s really a mini
chemistry lab inside that
plastic wrapper.
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Activity
• Most hot and cold
packs work by
breaking a membrane
that separates a solid
and water.
Explore Hot and Cold Packs