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Ch. 9 Notes
8th Grade Physical Science
By Lisa Anglim
9.1 Forming New Substances
Chemical ReactionsThe process by which 1
or more substances
undergo change to
produce 1 or more
different substances
Reactions occur when
chemical bonds are
broken. The atoms
rearranged and form
new bonds
Clues that a reaction has taken place
Formation of a gas
Formation of a solid
Color Change
Energy change – heat, cold, light or sound produced
Conservation of Energy
Chemical reactions involve energy
It takes a certain amount of energy to start the
reaction (activation energy)
Some reactions release more energy (exothermic)
Other reactions absorb energy (endothermic)
Energy is conserved just like mass. The
energy is always somewhere, in exothermic
reactions it is stored in the reactants and then
released. In endothermic reactions it is
stored in the products.
Exothermic Reactions:
The energy of the reactants is greater than the
products. Energy is released during the reaction in
the form of light, heat or electricity.
Exothermic Reaction
-
Takes very little activation
energy to start the reaction
-
Reactants have more energy
stored in their bonds than
the products
-
The extra energy will be
released in the form of heat
and/or light
Activation
Energy
Reactants
Energy given
off
Products
-
Examples
-
Fire, glow stick, food
Endothermic Reactions:
The energy of the reactants is less than the
products. Energy is absorbed (the reaction requires
energy to take place)
Photosynthesis
Endothermic Reaction
Takes lots of activation
energy to start the reaction
(hard to start)
Activation
Energy
Products
Products have more energy
stored in their bonds than
the reactants
The extra energy is stored in
the bonds of the molecules
(like a battery)
Energy
stored
(absorbed)
Reactants
Examples
Photosynthesis (energy
absorbed from the sun),
photo paper, instant ice pack
9.2 Chemical Formula’s and
Equations
Chemical Formula
A shorthand
notation for a
compound or a
diatomic element
using chemical
symbols and
numbers
Chemical Formula’s
The formula gives you the names of the
elements and tells you how many atoms of each
element are in that substance.
The small numbers that are written below and
to the right of the chemical symbols are called
Subscripts, and notes the number of atoms of
that element.
A number in front of a chemical formula is a
coefficient and it is multiplied by the subscript
of all the atoms that are in the formula
Chemical Formula
the shorthand notation for a
compound using chemical
symbols and numbers
Symbols
5 CO2
Subscript goes with the
preceding symbol
Coefficient
Diatomic molecules
O2
Has 2 oxygen atoms
N2
Has 2 nitrogen atoms
Compounds
H2O
Has 2 hydrogen atoms
and 1 oxygen
C6H12O6
Has 6 carbon, 12 hydrogen
and 6 oxygen atoms
Counting Atoms
Mg (HCO3)2
means 1 Magnesium, 2 Hydrogen, 2 Carbon atoms and
6 Oxygen atoms
AgNO3
means 1 silver atom, 1 Nitrogen atom and 3 oxygen
atoms
2 MgCl2
means 2 magnesium atoms and 4 chlorine atoms
Writing Formulas
Ionic compounds
For example
Remember there must be a
metal and a nonmetal
Metal comes first
Add “ide” to the end of
the nonmetal
Ignore the subscripts
MgCl2
Magnesium chloride
chlorine
AlBr3
Aluminum bromide
Examples of ionic compounds (the charges of the
ions must be balanced)
Lithium fluoride
metal
Lithium oxide
nonmetal
+
Li
F
LiF
Charges are
balanced
Calcium oxide
metal
+2
Ca
nonmetal
-2
O
O
CaO
Charges are
balanced
metal
+
Li
nonmetal
-2
O
LiLiO
2O
Charges are
not balanced
balanced
Calcium
metalfluoride
nonmetal
+2
Ca
F
CaF
CaF2
Charges are
not balanced
balanced
Covalent compounds
Only nonmetals are found
Use the prefixes to show the
subscripts
“Mono” is not used for the
first elements in the
compound
Add “ide” to the last nonmetal
in the formula
For example
CO2
Carbon dioxide
CH4
Carbon tetrahydride
H2O
Prefixes
Dihydrogen monoxide
Mono-
1
Hexa-
6
Di-
2
Hepta-
7
Tri-
3
Octa-
8
Tetra-
4
Nona-
9
Penta-
5
Deca-
10
Caution: This is a lethal substance
(kills thousands of people each
year), can be used for rocket fuel,
causes billions of dollars in damage,
found everywhere
Chemical Equations
A shorthand description of a chemical reactions using
chemical symbols and formulas.
C + O2
CO2
(Yields)
Reactants
Products
Balancing Equations
Law of Conservation of
Mass
Matter is not created or
destroyed in ordinary
chemical and physical
changes
This means that chemical
equations must balance.
The number and types of
atoms that goes into a
reaction is the same as
what comes out.
Example
2
Hydrogen and oxygen
mix to form water.
1
H2 +
+
O2
2
H2O
Equations can only be
balanced by changing the
coefficient, you can not
change a subscript.
Balancing Chemical Equations
Practice
Follow along on your handout as we go through the program.
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