8th Grade
Unit 3: Interactions of
Matter and Energy
Lesson 1:
Chemical Reactions
Vocabulary of Instruction:
1. Chemical Change
•
The atoms of a
substance are
rearranged into new
substances with
different chemical
composition.
• Evidence of a chemical
change include:
– color change.
– odor change.
– Bubbling.
– fizzing (production of
gas).
– production of light or
heat.
– formation of a solid
(precipitate).
2. Chemical Equation
• A chemical equation consists of the chemical formulas of
the substances that are mixed (called the reactants), and
the chemical formula of the substances that result from
the chemical reaction (called the products). The two are
separated by an → — usually read as "yields“ — and
each chemical formula is separated from others by a plus
sign. Sometimes a triangle is drawn over the arrow
symbol to denote energy must be added to the
substances for the reaction to begin.
• As an example, a reaction where hydrogen combines with
oxygen to produce water can be denoted as follows:
H2 + O → H2O
• This can be read as "H two plus O, yields H two O".
2-A. Chemical Equation
• A short, simple way to describe a chemical
reaction using symbols and formulas instead of
words. The substances to the left of the
are
called reactants and the substances to the right
are called products.
Example Equation for Photosynthesis:
6CO2 + 6H2O + Sunlight  C6H12O6 + 6O2 + Heat
Carbon dioxide + water + energy from the sun
yields
Glucose + Oxygen + Heat.
3. Chemical Formula
• Combination of
symbols and
numbers that
represent the
elements in a
compound. It
shows the ratio
of elements in a
compound.
Structural Formula
C16H2N2Cl2
Chemical Formula
4. Chemical Reaction
• Describes the
interaction or
combination
between two or
more substances
that produces
new substances,
which
characteristics are
different from the
original
substances.
5. Chemical Symbol
• A chemical symbol is an abbreviation or
short representation of the name of a
chemical element. Natural elements all have
symbols of one or two letters. The first letter
will always be Capitalized. For example:
•
•
•
•
•
•
•
H = Hydrogen
Cl = Chlorine
C = Carbon
Mg = Magnesium
Ag = Silver
Au = Gold
P = Phosphorus
5-A. Chemical Symbols Chart
6. Coefficient
• A coefficient is the number of molecules of
reactants or products in a chemical equation.
– For example:
• 2 H2 + O2 > 2 H2O
means that two molecules of hydrogen react with
one molecule of oxygen to produce two
molecules of H2O (water).
6-A. Coefficient
• A chemical formula may be preceded by a
coefficient indicating the proportion of that
substance necessary to produce the chemical
reaction in a formula. For instance, the formula
for the burning of methane
» CH4 + 2O2 → CO2 + 2H2O
indicates that twice as much O2 as CH4 is
needed, and when they react will produce twice
as much H2O as CO2. This is because during the
reaction, each atom of carbon needs exactly two
atoms of oxygen to combine with and to produce
the CO2, and every two atoms of hydrogen need
an atom of oxygen to combine with and to
produce the H2O.
•
•
6-B. Coefficient
Using Coefficients with Formulas
You have learned that the subscript numbers in a chemical formula
represent the number of atoms in one molecule or in one formula unit of an
ionic compound. Now you will learn about the other numbers, called
coefficients, that you often see to the left of a chemical formula. While a
subscript number acts as a multiplier for a single element (unless there are
parenthesis), a coefficient number acts as a multiplier for all of the atoms in
the entire compound. As with subscripts, when no number is present then "1"
is understood.
• Look at the example below:
•
• CO2
Here we have one molecule of carbon dioxide. The subscript 2 in the
formula above only pertains to the oxygen in the compound. The total
number of atoms in the compound is 3.
•
– Now let us put a coefficient in front of the molecule and see how that
changes things.
• 5 CO2
The coefficient 5 refers to the entire molecule. It shows that there are 5
molecules of carbon dioxide. Since each molecule is made up of 3 atoms, the
total number of atoms is now 15.
– There are 5 carbon atoms and 10 oxygen atoms.
6-C. Coefficient
•
•
•
•
•
Using Coefficients with Formulas
– Now, for an example with parenthesis:
• Ba(NO3)2
Here we have one formula unit of the ionic compound, barium nitrate. We
say "formula unit" instead of "molecule" because ionic compounds don't form
molecules. The subscript 3 pertains to the oxygen, showing 3 oxygen atoms
for each polyatomic ion of nitrate. The subscript 2 is a multiplier for
everything in the parenthesis, because it is showing that there are two nitrate
ions for every barium ion. The total number of atoms for each formula unit of
barium nitrate is 9.
There are: 1 barium atom, 2 nitrogen atoms, and 6 oxygen atoms.
– Now let's put a coefficient in front of the formula unit and see how it
changes the tally:
• 3 Ba(NO3)2
Now we have 3 formula units of barium nitrate. The 3 coefficient acts as a
multiplier for the entire compound. If there are 9 atoms in one formula unit of
barium nitrate, then there must be 27 atoms in three formula units.
There are: 3 barium atoms, 6 nitrogen atoms and 18 oxygen atoms.
7. Compound
• A substance made of
two or more elements
that are chemically
bound together in a set
ratio.
Examples:
– Carbon dioxide
• (CO2)
– Water
• (H2O)
8. Endothermic Reaction
• Endothermic Chemical Reaction describes a
chemical reaction that takes in heat energy.
• Example:
 Photosynthesis - process in which plants use or
take in the energy from the sun to convert carbon
dioxide and water into glucose and oxygen.
• The Chemical Reaction for Photosynthesis is:
Sunlight + 6CO2 + H2O = C6H12O6 + 6O2
9. Exothermic Reaction
• Exothermic Chemical Reaction describes a
chemical reaction that gives off heat energy.
 Example: mixture of sodium and chlorine to yield
table salt. This reaction produces energy for each
molecule of table salt that is produced.
• Chemical Reaction for Sodium and Chlorine is:
Na + Cl2 = NaCl + heat
10. Physical Change
• The
transformation
of the external
appearance of
a substance that
does not
convert it into
a new
substance.
11. Precipitate
• Precipitates can form when two soluble salts react
in solution to form one or more insoluble products.
The insoluble product separates from the liquid
and is called a precipitate.
• Precipitates can also form when the temperature of
a solution is lowered. The low temperature reduces
the solubility of a salt, resulting in its precipitation as
a solid.
11-A. Precipitate
• A precipitate is a solid that forms out of solution. A common
example is that of the mixing of two clear solutions: (1) silver
nitrate (AgNO3) and (2) sodium chloride (NaCl): The
chemical reaction is:
• The precipitate forms because the solid (AgCl) is insoluble in
water. That is true for all precipitates - the solids are
insoluble in aqueous solutions.
11-B. Precipitate
• Precipitation reactions occur all around us.
For example, sometimes the pipes in our
homes get clogged because precipitates of
magnesium and calcium oxides have
deposited themselves within the pipes. This can
happen with "hard" water. Another example is a
kidney stone. A kidney stone is nothing more
than a precipitate - often of calcium ions (from
cheese) and oxalates. It is often suggested
that a good way to avoid kidney stones is to
drink a lot of water. This helps because the
solubility of the precipitate increases with the
amount of water - thus avoiding the formation of
the kidney stone to begin with.
12. Reactants
• The substances or compounds that you start
with in a chemical equation (left side).
• Substances initially present in a chemical
reaction that are consumed during the reaction to
make products.
• 6CO2 + 6H2O + Sunlight  C6H12O6 + 6O2 + Heat
Reactants
Products
12. Reactants
• The substances or compounds that you start
with in a chemical equation (left side).
REACTANT
13. Products
• A product is a substance that forms as a result of
a chemical reaction. The end product of some
chemical reactions may be the result of a relatively
rapid reaction, nanoseconds to seconds.
• The substances or compounds that you have at
the end of a chemical equation (right side).
• 6CO2 + 6H2O + Sunlight  C6H12O6 + 6O2 + Heat
Reactants
Products
13. Products
• The substances or compounds that you have at
the end of a chemical equation (right side).
PRODUCT
14. Subscript
• Subscripts are placed to the lower right of the elements
symbol to show the number of atoms of the element in a
molecule.
• Formulas for molecules use chemical symbols with
subscript numbers to show the number of atoms of each
element:
• Examples:
• O2 for molecular oxygen
• O3 for ozone
• CH4 for methane
• C6H6 for benzene.
• Parentheses may enclose atoms that act as a group.