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Balancing Chemical Equations
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
occurs when the chemical bonds between elements are formed or broken.
A chemical reaction
This requires a gain or loss
of energy.
is a short, simple way to describe a chemical reactions using symbols and formulas instead of words.
A chemical equation
are the substances you begin with in a chemical reaction.
Reactants
These are shown to the left side of an arrow called the yield sign.
Fe + 2 HCl FeCl2 + H2
reactants products yield sign
During a chemical reaction, the atoms rearrange to form
new bonds between atoms.
Products are the substances you end with in a chemical reaction.
Products are shown to the right of the yield
sign arrow.
Fe + 2 HCl FeCl2 + H2
reactants products yield sign
The products formed in a chemical reaction have
different properties from the reactants.
A yield sign
Fe + 2 HCl FeCl 2 + H 2
is used to show the direction of the
chemical reaction.
A chemical equation is read like this:
1 atom of iron
reacts with
2 molecules of
hydrochloric acid
to yield
1 molecule of Iron chloride
and 1
molecule of
hydrogen
Fe + 2 HCl FeCl2 + H2
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!
This is called the Law of Conservation of Mass.
This law also means that the mass of the reactants must equal the mass of the products.
What if the number of atoms in the reactants does not match the number in the products?
C3H8 + O2 CO2 + H2O
mass =76 grams mass = 62 grams 3-carbon 1-carbon 8-hydrogen 2-hydrogen 2-oxygen 3-oxygen
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.
Basic Rules for Balanced Equations
Chemical formulas
CANNOT be altered!
When balancing an equation, the subscript
cannot be changed.
H2O
Don’t change me!
Coefficients can be added before an
element or molecule.
C3H8 + O2 => CO2 + H2O 5 3 4
Add coefficients to the equation to
balance the number of atoms
C3H8 + 5O2 => 3CO2 + 4H2O 3-carbon 3-carbon 8-hydrogen 8-hydrogen 10-oxygen 10-oxygen
Atoms on each side are
equal!!
Coefficients were used to balance the number of atoms on each side
of the chemical equation.
If we mass the reactants and products before and after the reaction, the
masses are equal. We have followed the law!
mass = 204 grams =>mass = 204 grams
C3H8 + 5O2 => 3CO2 + 4H2O
