You Should Be Able To…
1. Define and explain the law of conservation of mass
2. Represent chemical reactions and the conservation
of atoms, using molecular models
3. Write and balance (using the lowest whole number
coefficients) chemical equations from formulae, 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.
– 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 +

O2
hydrogen peroxide

H2O2
YOU CANNOT CHANGE THE SUBSCRIPTS
H
O
Reactants
2
2
Products
2
2
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)
• If they look the same on both sides of the equation, treat
polyatomic ions (such as SO42–) as a group & balance them as
such.
• If ‘OH’ and H2O are in the equation, write water as HOH.
• 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
http://www.authorstream.com/Presentation/kellymdeters-86103-section2-7-balancing-chemical-equations-education-ppt-powerpoint/
• Balance the following:
– Fe + Br2  FeBr3
– Sn(NO2)4 + K3PO4  KNO2 + Sn3 (PO4)4
– C2H6 + O2  CO2 + H2O
____Ba + ____H2O
____CO2 + ____H2O
____Fe2O3 + ____C
____Fe + ____H2O




____Ba(OH)2 + ____H2
____H2CO3
____Fe + ____CO
____H2 + ____Fe2O3
• If you don’t transform your word into a
skeleton equation properly, you won’t
be able to balance the equation
correctly.
– Change chemical names into chemical
formulas. 4 types:
• Simple ionic compounds
• Multivalent ionic compounds
• Ionic compounds with polyatomic ions
• Covalent compound
• Be careful of diatomic elements -remember the special seven!!
H2, N2, O2, F2, Cl2, Br2, I2
See page 208
(c) McGraw Hill Ryerson 2007
 Several common covalent molecules containing hydrogen have
common names that you should know and MEMORIZE!!
 methane = CH4
 glucose = C6H12O6
 ethane = C2H6
 ammonia = NH3
See page 208
(c) McGraw Hill Ryerson 2007
Example #1:
Word Equation: Solutions of lead nitrate react with potassium iodide to
produce solid lead iodide and a solution of potassium nitrate.
Skeleton Equation: Pb(NO3)2(aq) + KI(aq)  PbI2(s) + KNO3(aq)
Balanced Equation: Pb(NO3)2(aq) + 2KI(aq)  PbI2(s) + 2KNO3(aq)
Example #2:
Word Equation: Copper reacts with hydrogen nitrate to produce copper (II)
nitrate plus hydrogen.
Skeleton Equation: Cu + H(NO3)
Balanced Equation: Cu + 2H(NO3)

Cu(NO3)2 +
H2
 Cu(NO3)2 +
H2
See page 208
(c) McGraw Hill Ryerson 2007