Objectives:Describe how
chemical reactions produce
new substances with different
properties. Identify four signs
that a chemical reaction might
be taking place. Explain what
happens to chemical bonds
during a chemical reaction.
• Chemical Reaction
process of a substance
completely changing to a
different substance with
different properties.
• Clues of a chemical
reaction
• 1. color change
• 2. gas formation
• 3. solid formation called a
precipitate
• 4. energy given off as light,
thermal, or electrical energy.
• Chemical reactions happen
when molecules bump into
each other causing the
starting bonds to break
apart, the atoms rearrange,
and new bonds are formed
•Example:
CH4 + O2 → CO2 + H2O
• Objectives:
• Interpret and write simple
chemical formulas.
• Write and balance simple
chemical equations.
• Explain how a balanced
equation shows the law of
conservation of mass
•Chemical Symbol
Shorter way to write
an element’s name.
• Chemical Formula
combination of chemical
symbols and numbers to
represent a substance. It
an easier way to write a
molecules and
compounds.
• A chemical formula
shows the number of
atoms of each element in
a molecule
• Examples:
H2O
SO4
Ca(OH)2
• Chemical formulas use
subscripts and
coefficients to show the
number of atoms and
molecules.
•Subscript
the small # to the
right of the element
that tells the # of
atoms:
•Examples:
H2O
SO4
Ca(OH)2
•Coefficients
large # to the left of
the element.
Applies to the whole
molecule!
EXAMPLES
•2H2O
3Ca(OH)2
4SO4
Writing Formulas for Covalent
Compounds
You can use the name of
a covalent compound
to write its chemical
formula. The names of
covalent compounds
use prefixes. Each
prefix represents a
number, as shown
below.
mono- 1 hexadi2 hepta
tri3 octatetra- 4 nonapenta- 5 deca-
6
7
8
9
10
How to Write Formulas for
Covalent Compounds
1. name the first element
2. add a prefix if it has a subscript
3. name the second element
4. Add a prefix
5. Change ending of second element to “
ide ”
Let’s try some practice
1.
2.
3.
4.
5.
6.
7.
8.
9.
CO2
CO
SO2
SO3
N2O
N2O3
NO
CCl4
SiO2
1.
2.
3.
4.
5.
6.
7.
8.
9.
Carbon dioxide
Carbon monoxide
Sulfur dioxide
Sulfur trioxide
Dinitrogen monoxide
Dinitrogen trioxide
Nitrogen monoxide
Carbon tetrachloride
Silicon dioxide
Exceptions
• Some important exceptions to this naming scheme
occur because the compounds were originally named
before the methodical naming scheme above became
widespread. Now a days, these names are so common
that they're officially recognized:
• H2O is "water"
PO4 is “phosphate”
• NH3 is "ammonia"
CLO3 is “chlorate”
• CH4 is "methane“
NO3 is “nitrate”
• SO4 is “Sulfate”
• CO3 is “carbonate”
• OH is “hydroxide”
Ionic Compounds
• To write the formula for an ionic compound,
make sure the compound’s charge is 0.
• No prefixes…ends in ide
•Chemical Equation
an easier and shorter
way to write a chemical
reaction using chemical
symbols and formulas as
a shortcut to describe a
chemical reaction
•Example:
CH4 + O2 → CO2 + H2O
• Reactants
what you start with
Products
what you end with
• CH4 + O2 → CO2 + H2O
Reactants
Products
•Law of conservation
of Mass
states that mass can
not be created or
destroyed
•An equation is balanced
when the # of atoms in
the reactants are = to
the # of atoms in the
products
•You balance an
equation by
changing the
coefficients, not the
subscript!
• Steps to balancing a chemical
equation:
1. Write the unbalanced
equation on your paper.
2. Draw a small line in front
of each molecule.
3. Draw boxes around all the
chemical formulas.
• 4. Make an element inventory.
5. Write numbers in front of
each of the boxes until the
inventory for each element is
the same both before and
after the reaction
Example
• __ Fe + __S → __FeS
Element
Before
After
Fe = ____ _____
S = ____ _____
• __H2 + __O2 → __H2O
Element Before
After
H = ____ _____
O=
____ _____
• __Mg + __O2 → __MgO
Element
Mg =
O=
Before
After
____ _____
____ _____
• _Ca + _H2O → _Ca(OH)2 + _H2
Element Before
After
H = ____ _____
O=
____ _____
Ca =
____ _____