Notes – Chemical Equations
Chapter 8, Lesson 2
Conservation of Mass
• When the end of the last period bell rang,
everyone moved to a new room, and ended up
in different groups in different rooms. The
number of students IN the whole school did not
change, only their arrangement.
• The amount of matter in a chemical reaction
does not change, so the total mass at the start
MUST equal the total mass at the end.
Conservation of Mass
• Matter is ALWAYS conserved in chemical
reactions.
• Chemical equations show that in chemical
reactions, atoms rearrange, but no atoms
are gained or lost.
Conservation of Mass
• Antoine Lavoisier invented a balance to
make more precise measurements.
• He also determined that the mass before
and after a chemical reaction was the
same.
Mass of
elements
BEFORE
Mass of
elements
AFTER
Conservation of Mass
• The law of conservation of mass states
that the total mass before a chemical
reaction is equal to the total mass after the
reaction.
Chemical Equations
• Reactants are the starting materials in a
chemical reaction and are placed on the
LEFT side.
• Products are the ending materials in a
chemical reaction and are placed on the
RIGHT side.
Reactant + Reactant → Product
H2
+
O2
→ H2O2
Chemical Equations
• “Tin + oxygen → tin oxide” reads as “tin
plus oxygen produces tin oxide.”
• Word equations can be long and do not
show that mass is conserved.
Chemical Equations
• Instead of writing long word equations,
scientists use symbols and formulas.
– Symbols represent atoms.
– Formulas represent molecules.
– Molecules may be elements or compounds.
Chemical Equations
• A molecule is the combination of 2 or more
atoms.
• Some molecules have atoms that are alike,
called diatomic (O2).
• Most molecules are made of more than one
type of atom (H2O) – a compound.
Chemical Equations
Chemical Equations
• Chemical equations use the symbols and
formulas instead of words.
Balancing a Chemical Equation
• Atoms are neither gained nor lost in a
reaction, so both sides of the equation
must have the same number of atoms.
Balancing a Chemical Equation
• A subscript tells how many
atoms of an element are in one
molecule.
• A coefficient tells how many
atoms, molecules, or formula
units are in a reaction.
subscript
2H2O
coefficient
Balancing a Chemical Equation
• (i.e.) “Oxygen reacts with hydrogen to form water.”
H2 + O2 → H2O
Reactants
H
2
Products
O
2
H
2
O
1
• This is unbalanced, and we CANNOT change the
subscripts!
• We add coefficients, a number in FRONT of a chemical
formula that tells you how many molecules or atoms of each
reactant & product act in the reaction. We do not write “1.”
• We add a coefficient of “2” in front of the product:
H2 + O2 → 2 H2O
Reactants
H
O
2
2
Products
H
4
O
2
• The oxygen atoms are now balanced, but the hydrogens
are not anymore. Add a coefficient of “2” in front of the H2:
2 H2 + O2 → 2 H2O
Reactants
H
O
4
2
Products
H
4
O
2
Balancing a Chemical Equation
Writing Balanced Equations
Step 1
Step 2
Step 3
Step 4
Step 5
Determine the correct symbols and formulas
for reactants and products.
Write reactant symbols and formulas to the left
of an arrow and product symbols and formulas
to the right.
Count the number of each kind of atom on
both sides.
Use coefficients to make the number of
each kind of atom the same on both sides
of the arrow.
Check to see that each kind of atom balances.