Chemical Reactions
• Chemical Reactions form new
substances by breaking and
making chemical bonds
Chemical Reactions
• Chemical reactions change the
way the atoms are arranged
• Fireworks are a result of
chemical reactions
Difference between
Physical and Chemical
Changes
• Physical
• A change in the
state
• Solid to liquid
• Liquid to gas
• Gas to liquid
• Atoms/molecules
of the
substances do
not change
• Chemical
• Atoms/molecules
broken down or
combined to
form new
substances
• Properties of
new substances
are different
than properties
of the
substances that
make them up
Evidence of Chemical
Reactions
• Color Change
• Formation of Precipitate
(solid formed from two
liquids)
• Formation of gas
• Temperature change:
endothermic (temp down)
exothermic (temp up)
• Change in smell
Reactants and Products
• Reactants: present at the beginning
of the reaction
• Products: are the substances formed
by the chemical reaction
• Example burning natural gas
• CH4 + 2O2 > CO2 +2H2O
• Reactants > Products
Classification of
Chemical Reactions
• Synthesis: a
new compound
is formed by the
combination of
simpler
reactants
• Smog formed
when nitrogen
and oxygen
combine
• N2 +2O2 > 2 NO2
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Classification of Chemical
Reactions
• Decomposition: A
reactant breaks
down into simpler
products (reverse
of synthesis)
• Water can be
decomposed into
hydrogen and
oxygen
• 2 H2O > 2H2 +O2
Classification of
Chemical Reactions
• Combustion: One reactant is
always oxygen and another
reactant often contains carbon
and hydrogen
• Burning of methane
• CH4 + 2O2 > CO2 +2H2O
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The Rates of Chemical
Reactions can vary
• Concentration: measures the number of
particles present in a certain volume.
• A high concentration of reactants means
there is a large number of particles that
can collide and react
• Turning the valve on a gas stove increases
the methane molecules that can combine
with oxygen and results in a bigger flame
and faster combustion reaction
Temperature
• Rate of reaction can be increased
by an increase in temperature
which increases how fast the
particles are moving and increases
the number of collisions
• Remember the cold and hot water
distribution of food coloring
The Rates of Chemical
Reactions can vary
• Surface Area: the exposed surface of
a substance.
• To increase reaction break a large
piece of the material so that there is
more surface area of the material
• Increase surface area=Increase
reaction rate
Catalyst
• Catalyst: is a substance that
increases the rate of chemical
reaction but is not consumed in the
reaction
• Enzymes are catalysts that are used
by living things to cause a chemical
reaction. Resulting in a new product
being made and the catalyst not
being changed.
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The masses of reactants and
products are equal
Chemical Equations
• Reactants > Products
• Balancing Chemical
Equations
• Use conservation of
mass to balance: atoms
can not be created or
destroyed.
• Reactants = products
• Law of conservation
of mass: states that
in a chemical
reaction atoms are
neither created nor
destroyed. (Antoine
Lavoisier)
• All atoms present in
the reactants are
also present in the
products
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Chemical equations
•
•
•
•
•
CH4 + O2 > CO2 + H2O
Balanced? No
Reactants: C=1, H=4, O=2
Products: C=1, H=2, O=3
H and O not equal
•
•
•
•
Use coefficients to balance equations
CH4 + 2O2 > CO2 + 2H2O
C=1, H=4, O=4 > C=1, H= 4, O=4
Equation is now balanced!
Chemical Reactions
Involve Energy Changes
• It takes energy to break
chemical bonds. This
energy is called bond
energy.
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Exothermic Reactions
• Exothermic
reaction: energy
is released and
temp goes
up…more energy
is released
when products
are formed.
• Space shuttle on
take off: white
clouds of water
vapor are
formed
Exothermic Reactions
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• Exothermic reactions release
energy!
• All common combustion reactions
are exothermic
• Fireflies release energy in the form
of light as an exothermic reaction
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Endothermic reactions
Endothermic Reactions
• Photosynthesis
• Endothermic reactions: produce a
decrease in temperature because
the bond energies of the reactants
are greater than the bond energies
of the products
(general formula)
• 6CO2 +6H2O > C6H12O6 +6O2
• All endothermic reactions absorb
energy!
• Alka Seltzer and water…temperature
goes down.
• Energy is stored in the
glucose molecule to be used
when needed.
• plants absorb energy from
sunlight to turn carbon
dioxide and water into
oxygen and glucose (sugar).
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Life and Industry depend
on Chemical Reactions
• Living things require chemical reactions
• Respiration: living things get energy from
glucose by respiration. (combustion of glucose)
• Photosynthesis
• 6CO2 +6H2O + energy> C6H12O6 +6O2
• Respiration
• C6H12O6 +6O2 > 6CO2 +6H2O + energy
• Respiration is the opposite of
photosynthesis!
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