The document discusses balancing chemical equations by following the law of conservation of mass. It explains that a chemical equation uses symbols to represent a chemical reaction, with reactants on the left side of the yield sign and products on the right. For the equation to be balanced, the number of atoms of each element must be equal on both sides after coefficients are added. This ensures the mass is also conserved, as required by the law of conservation of mass. An example demonstrates balancing the equation for the combustion of propane by adding coefficients to match the number of carbon, hydrogen and oxygen atoms on each side.

1. TEKS 8.5D recognize that chemical formulas are used to identify substances
and determine the number of atoms of each element in chemical formulas
containing subscripts
TEKS 8.5F recognize whether a chemical equation containing coefficients is
balanced or not and how that relates to the law of conservation of mass
Balancing
Chemical
Equations

2. A chemical reaction
occurs when the
chemical bonds
between elements
are formed or
broken.

3. This requires
a gain or loss
of energy.

4. A chemical equation
is a short, simple
way to describe a
chemical reactions
using symbols and
formulas instead of
words.

5. Reactants
are the substances
you begin with in a
chemical reaction.

6. These are shown to the
left side of an arrow
called the yield sign.
Fe
+
2 HCl
reactants
FeCl2
yield
sign
+
products
H2

7. During a chemical reaction,
the atoms rearrange to form
new bonds between atoms.

8. Products
are the substances you
end with in a chemical
reaction.

9. Products are shown to
the right of the yield
sign arrow.
Fe
+
2 HCl
reactants
FeCl2
yield
sign
+
products
H2

10. The products formed in a
chemical reaction have
different properties from the
reactants.

11. A yield sign
is used to show the
direction of the
chemical reaction.
Fe + 2 HCl
FeCl 2 + H 2

12. A chemical equation is read
like this:
Fe +
1 atom
of iron
2 HCl
2 molecules
reacts
of
with hydrochloric
acid
FeCl2 +
to yield
1
molecule
of Iron
chloride
H2
1
and
molecule
of
hydrogen

13. During a chemical reaction, atoms
cannot be created or destroyed,
so the number of atoms at the
beginning of a reaction MUST
equal the number at the end of
the reaction!

14. This is called the Law of
Conservation of Mass.

15. This law also means that
the mass of the reactants
must equal the mass of the
products.

16. What if the number of
atoms in the reactants
does not match the
number in the products?

17. C3H8 +
O2
mass =76 grams
3-carbon
8-hydrogen
2-oxygen
CO2
+
H2O
mass = 62 grams
1-carbon
2-hydrogen
3-oxygen

18. The Law of Conservation of Mass
says that the number of each
atom must be equal on both
sides of the yield sign along
with the masses. Let’s balance
the equation using some basic
rules.

19. Basic Rules for
Balanced
Equations

20. Chemical
formulas
CANNOT be
altered!

21. When balancing an
equation, the subscript
cannot be changed.
Don’t
change me!
H 2O

22. Coefficients can be
added before an
element or
molecule.

23. C3H8 + 5 O2 => 3CO2 + 4 H2O
Add coefficients to
the equation to
balance the number of
atoms

24. C3H8 + 5O2 => 3CO2 + 4H2O
3-carbon
8-hydrogen
10-oxygen
3-carbon
8-hydrogen
10-oxygen
Atoms on
each side are
equal!!

25. Coefficients were used
to balance the number
of atoms on each side
of the chemical
equation.

26. C3H8 + 5O2 => 3CO2 + 4H2O
mass = 204 grams =>mass = 204 grams
If we mass the reactants
and products before and
after the reaction, the
masses are equal. We have
followed the law!