Law of Conservation of
Matter:

Matter cannot be created or
destroyed.
AMOUNT OF REACTANTS =
AMOUNT OF PRODUCTS ! ! ! !
Guidelines for Balancing
Chemical Equations:

 1) Do NOT change the subscripts !
 2) Every element MUST have the same amount of
atoms on both sides of the equation.
 3) To balance, stoichiometric coefficients (numbers) are
added in front of the chemical formula until we have
the same atom amount on both sides of the equation.
Guidelines (cont.)

 4) Once a coefficient is added, go back and check
the rest of the equation.
 5) Balance every element EXCEPT Hs and Os. Then
balance Hs, Os last. Balance hydrogens with an
even number
 6) CHECK EQUATION TO BE SURE
EVERYTHING BALANCES/CANCELS OUT ON
BOTH SIDES ! ! !
Helpful Hints

 Elements found in only ONE compound on each side—balance
first
 Polyatomic ions usually stay together—balance as a unit.
 Elements in diatomic state? Balance compounds with these
elements using an EVEN number
 Do not be afraid to use FRACTIONS as coefficients !!
 Sometimes the easiest way to balance
 Use common multiplier and multiply all coefficients to get rid of the
fraction
Example 1:

KI (aq) +
Pb(NO3)2 
KNO3 +
PbI2
Example 2:

C2H6
+
O2 
CO2
+
H2O
Practice

1) Zn + CrCl3  CrCl2 + ZnCl2
2) Cu + H2SO4  CuSO4 + SO2 + H2O
3) FeI3 +
K2SO4 
KI +
Fe2(SO4)3
4) CaSO4 + Al(NO3)3  Ca(NO3)2 + Al2(SO4)3
Telling a story….

Writing Chemical
Equations

1) Identify reactants, write chemical formulas.
1) Identify products, write chemical formulas.
3) Write the complete chemical equation.
4) Balance the equation.
Example 1:

Copper (II) carbonate  copper (II)
oxide + carbon dioxide
Example 2:

Merry and Pippin find lead (II) chloride
and lithium sulfate in Gandalf’s chemical
supply cabinet. They combine these
chemical compounds and then run to
Gandalf so he may identify the produced
chemical compounds. Gandalf identifies
the compounds as lead (II) sulfate and
lithium chloride.
Diatomic elements

 Bromine (Br2)
 Chlorine (Cl2)
 Fluorine (F2)
 Hydrogen (H2)
 Iodine (I2)
 Nitrogen (N2)
 Oxygen (O2)
Example 3:

A chemist at a Wake Forest chemistry
lab prepares a potassium bromide
solution dissolved in water to react
with chlorine gas. This reaction
produces bromine in the liquid state
and potassium chloride in solution.
Example 4:

 Merry and Pippin have gotten in over their heads
AGAIN!! Without Gandalf, they have combined a
liquid phosphorus-chlorine compound which is
77.45% Cl by mass with water. Gandalf hears the
commotion and immediately returns to identify an
aqueous solution of phosphoric acid and
hydrochloric acid.
Practice

1) Ariel swam down to her treasure trove to collect
silver iodide and iron (III) carbonate, which she
transformed into iron (III) iodide and silver
carbonate for Cinderella’s birthday.
2) Cinderella decided to combine some leftover zinc
sulfate and lithium carbonate in her mop bucket. After
some bibbidi-bobbidi-boo; she discovers she has made
zinc carbonate and lithium sulfate.
Homework

 Read pp. 92-97
 Problems p. 118 #53-57