You Should Be Able To…
1. Define and explain the law of conservation of mass
2. Write and balance chemical equations from formula,
word equations, or descriptions of experiments
 Subscript
 Coefficient
 Law of Conservation of Mass
 Molecule
 Atom
 Skeleton Equation
 Balanced Equation
 Word Equation
• Chemical reactions result in chemical
changes.
– Chemical changes occur when new substances
are created.
– The original substance(s), called reactants,
change into new substance(s) called products.
Reactants
(c) McGraw Hill Ryerson 2007
Products
See pages 202 - 203
Reactants
(c) McGraw Hill Ryerson 2007
Products
See pages 202 - 203
• Chemical reactions can be written in different
ways.
– A word equation:
• Nitrogen monoxide + oxygen  nitrogen dioxide
– A symbolic equation:
• 2NO(g) + O2(g)  2NO2(g)
COEFFICIENTS
STATE OF MATTER
- Letters indicate the state of
each compound.
(aq) = aqueous/dissolved in
water
- Indicates how many of each
molecule there is.
(s) = solid
-Ie: there are 2 molecules of NO.
(g) = gas
(c) McGraw Hill Ryerson 2007
( ) = liquid
• When a chemical reaction occurs, new compounds are
created, BUT…
– No new matter is created or destroyed; atoms are
just rearranged as the atoms change partners to form
new compounds.
– If there are 3 atoms of oxygen in the reactants, there
MUST be 3 atoms of oxygen in the products.
– Number of each atom in reactants = number of each
atom in products.
• The law of conservation of mass:
– Mass of reactants = mass of products
If you could collect and measure all of the exhaust from
this car, you would find that mass of reactants (gas + O2) =
mass of products (exhaust).
(c) McGraw Hill Ryerson 2007
• The simplest form of chemical equation is a word
equation, a DESCRIPTION
– Potassium metal + oxygen gas  potassium oxide
• A skeleton equation shows the formulas of the
elements/compounds.
– A skeleton equation shows which atoms are involved, but
not how many molecules are involved.
• K + O2  K2O
• A balanced chemical equation shows all
atoms and the coefficients tells us how many
molecules (and atoms) there are.
– Balancing ensures that the number of each
atom is the same on both sides of the reaction
arrow.
4K
K K
K K
+
O2
O O

2K2O
K O K
K O K
• Using the law of conservation of mass, we
can count atoms to balance the number of
atoms in chemical equations.
– Word equation: methane + oxygen  water +
carbon dioxide
– Skeleton equation: CH4 + O2  H2O + CO2
• To balance the compounds, take note of how many
atoms of each element occur on each side of the
reaction arrow.
(c)
McGraw
Hill Ryerson 2007
See
Page 207
Skeleton equation:
The same number
of atoms must be on
each side.
Balanced equation:
(c) McGraw Hill Ryerson 2007
CH4 + O2  H2O + CO2
Carbon = 1
Hydrogen = 4
Oxygen = 2
Carbon = 1
Hydrogen = 2
Oxygen = 3
CH4 + 2O2  2H2O + CO2
Carbon = 1
Carbon = 1
Hydrogen = 4
Oxygen = 4
Hydrogen = 4
Oxygen = 4
See Page 207
Balancing Equations
hydrogen
H2
+ oxygen
+
O2


water
H2O
H
Reactants
2
Products
2
O
2
1
Balancing Equations
hydrogen
+ oxygen
H2 +
H
O
O2
Reactants
2
2

water
 2 H2O
Products
2
1
Balancing Equations
hydrogen
+ oxygen
H2 +
H
O
O2
Reactants
2
2

water
 2 H2O
Products
4
2
Balancing Equations
hydrogen
+ oxygen
2 H2(g) +
H
O

water
O2 (g)  2 H2O (l)
Reactants
4
2
Products
4
2
• Balance chemical equations by following these steps:
– Trial and error will work but can be very inefficient.
• USE A TABLE (write atoms underneath reactants and
products)
• Balance one compound at a time & rewrite the # of
atoms in the chart as things change.
• Only add coefficients; NEVER change subscripts!!!
• If H and O appear in more than one place, attempt to
balance them LAST.
– Balance everything that isn’t ‘H’ or ‘O’ 1st.
– Balance the ‘H’s 2nd to last.
– Balance the ‘O’s last.
– Always double-check after you think you are finished.
– CHECK YOUR ANSWERS!!!
See pages 209 - 211
(c) McGraw Hill Ryerson 2007
• Balance the following:
– Fe + Br2  FeBr3
– Sn(NO2)4 + K3PO4  KNO2 + Sn3 (PO4)4
– C2H6 + O2  CO2 + H2O