Transcript Document

Section 2.7—Balancing
Equations
Objectives:
•Use the Law of Conservation of Mass to
explain the need for balanced equations
•Convert word equations to balanced
chemical equations
Review
1. How to write chemical formulas
a. For ionic compounds:
1) write the symbol and charge for the cation
2) write the symbol and charge for the anion
3) criss-cross the NUMBER only of the charge
to obtain a subscript
4) Reduce the subscripts, if necessary
5) Use parentheses when there is more than
one polyatomic ion needed.
Review
b. For binary covalent compounds:
1) Write the symbols of the first and
second elements
2) Translate any PREFIXES into a
subscript of that element
Example: dinitrogen pentoxide
N 2O 5
Review
c. For acids without oxygen (the acid will
be named with the prefix “hydro”):
1) The cation will be H+
2) The anion will be found in the root of
the acid’s name (following the
“hydro”) – determine its symbol &
charge
3) Criss-cross the numbers of the
charges to get the formula
Review
d. For acids with oxygen (acid name will
end in “ic” or “ous”):
1) The cation is H+
2) If the acid ends in “ic”, look for a
polyatomic anion that ends in “ate”
3) If the acid ends in “ous”, look for a
polyatomic anion that ends in “ite”
4) Criss-cross the numbers of the
charges to get the formula
REMEMBER: Bases are just ionic compounds
with the anion “hydroxide”. NH3 (ammonia) is an
exception.
Review
e. There are seven elements that are
found in nature as diatomic molecules.
They must be written that way in a
formula.
H2
N2
O2
F2
Cl2
Br2
I2
Review
2. How to write formula equations
a. Locate each name of a compound and
write its formula (using those rules
reviewed in previous slides.)
b. Separate all reactants and products
with a “+” sign
c. Separate reactants ffrom products with
an 
Practice
Hydrogen and oxygen react to form water.
Law of Conservation of Matter/Mass
Notice that, in this practice equation, the
number of atoms that we start with does not
equal the number of atoms we finish with.
2 H atoms + 2 O atoms  2 H atoms + 1 O atom
This violates a scientific law that says
matter cannot be destroyed (or created.)
Law of Conservation of Matter/Mass
Law of Conservation of Matter –
Matter cannot be created nor destroyed
during chemical or physical changes
Also called the Law of Conservation
of Mass (since all matter has mass)
How Does the Law Lead to Balancing?
Law of
Conservation
of Matter
Matter cannot
be created nor
destroyed
during a
chemical or
physical
change
Therefore…
So we must…
The matter on
the reactants
side and the
matter on the
products side
must be the
same
Ensure the
numbers of
each type of
atom are the
same on both
sides of the
equation…by
balancing!
How do we Balance Equations?
Number of
compounds in
the reaction
Coefficients
2 H2 + O2  2 H2O
Subscripts
# of atoms in a
compound
Subscripts balance charges within a compound.
Coefficients balance atoms in an equation
What do Coefficients Really Mean?
CH4 + 2 O2  CO2 + 2 H2O
H
H
C
H
O
O
O
H
O
O
C
O
H
O
O
H
Total:
1C
4H
4O
The equation is balanced.
H
Total:
1C
4H
4O
H
How to Balance Chemical
Equations
How to Balance By Inspection:
1
Make a table of elements
_____ CH4 + _____ O2  _____H2 O + _____ C O2
Reactants
C
H
O
Products
C
H
O
How to Balance By Inspection:
2
Count the number of each element on the reactants
and products side.
Don’t forget to add all the atoms of the same element together—
even if it appears in more than one compound!
_____ CH4 + _____ O2  _____H2 O + _____ C O2
Reactants
Products
1
C
1
C
4
H
2
H
2
O
3
O
How to Balance By Inspection:
3
ADD COEFFICIENTS TO BALANCE THE NUMBERS
Each time you add a coefficient, update your table with the new
quantities of each atom.
_____ CH4 + _____
2 O2 _____
2 H2 O + _____ C O2
Reactants
Products
1
C
1
C
4
H
2 4H
2 4O
3 4O
Let’s Practice #1
Example:
Balance the
following
equation
__ H2 + __ O2  __ H2O
Let’s Practice #2
Example:
Balance the
following
equation
2 H2 + __ O2  __
2 H 2O
__
Let’s Practice #2
Example:
Balance the
following
equation
__ HCl + __ Ca(OH)2  __ CaCl2 + __ H2O
Let’s Practice #1
Example:
Balance the
following
equation
Did you see the “OH” polyatomic ion &
change H2O to HOH?
2 HCl + __ Ca(OH)2  __ CaCl2 + __
2 H2O
__
Let’s Practice #3
Example:
Write and
balance the
following
equation
Iron and oxygen combine to produce iron (III)
oxide.
Let’s Practice #3
Example:
Write and
balance the
following
equation
4 Fe + __
3 O2  ___
2 Fe2O3
__