CHEMICAL REACTIONS
7th Grade Science
Bowling Green Junior High
WHAT ARE CHEMICAL
REACTIONS?
Chemical Reaction – a change that takes
place when two or more substances
(reactants) interact to form new substances
(products) with new properties.
COMPOUNDS
Matter
made of two or
more different elements
chemically bonded.
Cannot
be separated by
physical means
Has
properties that are
different from the
elements that make it up.
 MORE
COMMON THAN ELEMENTS DUE TO MANY
ELEMENTS BEING REACTIVE WITH EACH OTHER
 THE
ELEMENTS THAT COMBINE MAKE A NEW SUBSTANCE
WITH NEW PHYSICAL PROPERTIES
 FOR
A COMPOUND TO FORM OR BE BROKEN DOWN, A
CHEMICAL REACTION MUST TAKE PLACE
NaCl
+
=
TABLE
SALT
 Respiration
(breathing)
 Photosynthesis
 Grilling
food
 Starting
a vehicle
 Digestion
 Rusting
metal
EVERYDAY EXAMPLES OF CHEMICAL REACTIONS
HOW DO YOU KNOW WHEN A CHEMICAL
REACTION HAS TAKEN PLACE?
A new substance with
new properties is
formed
SIGNS OF A CHEMICAL REACTION
 Temperature
 FIZZES
Change (heat given off or required)
OR BUBBLES
 COLOR
CHANGE
 ODOR
 LIGHT
 NEW
GIVEN OFF
SUBSTANCE FORMED
Precipitate
(solid)
Precipitate
(gas bubbles)
TWO PARTS OF A CHEMICAL REACTION
Reactants – Substances that start a
chemical reaction (EX: chemicals on
match head)
Products – Substances produced in the
reaction (EX: black material on match)
CHEMICAL EQUATIONS
• Chemical equations are symbols used to describe
the details of a chemical reaction.
• Shows how the reactants changed into the
product.
• This involves indicating all the atoms involved in
the reaction.
Fe + O2
Reactants:
Iron and oxygen
Plus Sign:
Shows substances
combine
Reactants are ALWAYS to the
left of the arrow
FeO2
Arrow:
Means “yields”
takes the place
of an = sign
Product:
Ferrous oxide
(rust)
Products are ALWAYS to the
right of the arrow
TYPES OF CHEMICAL REACTIONS
 Combustion
 Synthesis
 Decomposition
 Single
replacement
 Double
replacement
 Neutralization
 Oxidation/Reduction
 Hydrolysis
 Endothermic/Exothermic
WHAT DO YOU HAVE TO HAVE TO
BURN SOMETHING?
COMBUSTION REACTIONS
When
oxygen (O2) combines with another
compound to form water and carbon
dioxide.
 Needs
 Takes
 Fast
a fuel source
place at high temperatures
process that results in an increase of
temperature and production of fire.
Chemical reactions can be
classified
Combustion Reaction – always involves oxygen (O )
2
as a reactant.
CH4 + 2O2
Methane
H
H
C
Oxygen
O O
H
H
+
O O
CO2 + 2H2O
Water
Carbon Dioxide
+
4 TYPES OF REACTIONS
SYNTHESIS REACTIONS
Two
or more substances react
to form a new substance(s)
A + B  AB
S
+ O2  SO2
CHEMICAL REACTIONS CAN BE
CLASSIFIED
Synthesis Reaction – combines two or more
simpler reactants to form new, more complex
products.
N2 + 2O2
Nitrogen
N N
Oxygen
+
2NO2
Nitrogen Dioxide
OO
OO
Simple to complex
DECOMPOSITION REACTION
One
substance breaks down into
two or more simpler substances
AB
A+B
CaCO3
 CaO + CO2
Chemical reactions can be
classified
Decomposition reaction
– breaks a reactant
into two or more simpler products
2H2O
Water
H
O
2H2 +
O2
Oxygen
Hydrogen
H
+
H
Complex
to
simple
O
H
SINGLE REPLACEMENT
One
element replaces another
element in a compound
AB + C  AC + B
Zn
+ 2HCl  H2 + ZnCl2
Chemical reactions can be
classified
Replacement Reaction – elements switch places
to form new compounds.
1) Single Replacement
Zn +
Zinc
Zn
+
2HCl
Hydrochloric Acid
Cl
H
ZnCl2 +
Zinc Chloride
Cl
H
+
Cl
H
Cl
H
H2
Hydrogen
DOUBLE REPLACEMENT
Elements
from two different
compounds switch places
AB + CD  AC + BD
HCl
+ NaOH  NaCl + H2O
Chemical reactions can be
classified
Replacement Reaction – elements switch places
to form new compounds.
Double Replacement
FeS + 2HCl
Iron Sulphide
Fe
S
Hydrochloric Acid
+
Cl
H
Cl
H
FeCl2 +
Iron Chloride
H2 S
Hydrogen Sulfide
+
 All
chemical reactions are going to release
(give off) energy or absorb (take in) energy.
 Some
will require energy to start the reaction
(activation energy)

EX: before you use a new cell phone, what’s got to
happen?
 Activation
energy=energy required to start a
chemical reaction.
ENDOTHERMIC VS. EXOTHERMIC
PROCESSES
EXOTHERMIC REACTIONS OR
PROCESSES
 Exothermic
reactions are exactly the opposite. While they
take some energy to get going, called the activation energy
of reaction, these reactions give off heat during the reaction
 Good
examples of exothermic reactions are explosions
like fireworks or combustion in engines.

Forming a chemical bond releases energy and is
exothermic
 Usually
feel hot because it is giving heat to you
ENDOTHERMIC REACTIONS OR
PROCESSES
 Endothermic
reactions are those which absorb heat during the
reaction. They take in more energy than they give off, which leaves
the surroundings cooler than the starting point

Evaporation of water by sunlight is a great example. The sun
and the liquid water combine and the water absorbs energy
and eventually becomes as gas.

Breaking a chemical bond requires energy and is endothermic
 Usually
feel cold because it is taking heat away from you
CATALYST
Substance
which speeds up a chemical
reaction but is chemically unchanged at
the end of the reaction.
•The catalytic converter in a car contains platinum, which serves
as a catalyst to change carbon monoxide, which is toxic, into
carbon dioxide.
•If you light a match in a room with hydrogen gas and oxygen
gas, there will be an explosion and most of the hydrogen and
oxygen will combine to create water molecules.
A
way of writing
which type of
atoms and how
many of each
there are in a
compound.
CHEMICAL FORMULAS
Written as: C4H10
Butane
Written as: CH4
Methane
= how many total
molecules
Subscripts= how many
atoms
COUNTING ATOMS
FeO2
H2O
CO2
MgBr2
C6H12O6
3OH
2H2O
COUNTING ATOMS IN CHEMICAL EQUATIONS
2Na + MgF2  2NaF + Mg
2K + Cl2  2KCl
COUNTING ATOMS IN CHEMICAL EQUATIONS
2Na2O  4Na + O2
COUNTING ATOMS IN CHEMICAL EQUATIONS
Law of Conservation of Matter
Matter cannot be created or destroyed,
it just changes forms.
*The total mass of the reactants MUST
EQUAL the total mass of the product.
Law of Conservation of Mass
Alka-Seltzer and Water
http://www.skyweb.net/science/balancing_chemical_equations_examples.htm
BALANCING EQUATIONS
•
The number of atoms of the
reactants must equal the number of
atoms in the product. (Law of
Conservation of Matter)
Ex: 2Na + Cl2 -> 2NaCl
4P + 5O2 -> P4O10
BALANCING EQUATIONS
•
Rules
–
Make sure that all atoms are equal on both
sides.
–
You can only add coefficients.
•
Changing the subscripts will change the identity
of the compound.
– H2O
& H2O2
EX: 2Na + Cl2 -> 2NaCl
H2 + O2 -> 2H2O
(Not balanced… So…)
2H2 + O2 -> 2H2O
Balancing Chemical Equations
Hg + O2
HgO
H2 + Cl
HCl
Mg + O2
MgO
O2 + H 2
H 2O
CH4 + O2
CO2 + H2O
Fe + Cl2
FeCl3
Hg +
O2
HgO
H2 + Cl
HCl
Mg + O2
MgO
O2 +
H2
H2O
Fe + Cl2
FeCl3
CH4 +
O2
CO2 +
H2O