1. Two types of equations are written by chemists:
  • word equations: describe the substances that react in a chemical reaction (termed reactants ), and the products that are formed, along with their states
  • formula equations are a shorthand method used to describe the same reactions. These are of two types:
    • Skeleton equations: (unbalanced) which lists the correct formula of each reacting substance and product substances, and their states.
    • Balanced equations: which list the correct formulas , states and balances the equation for the number of atoms present. That is it takes into account the Law of Conservation of Mass, and makes sure there is the same number and type of atom in the reactant and product. In Chemistry 30 we should only use balanced equations.
2. We balance equations by changing the coefficients or numbers in front of the substance.
WE NEVER CHANGE THE FORMULAS OF SUBSTANCES IN ORDER TO BALANCE.
3. Counting the atoms correctly is therefore critical. The balance (coefficient) we use is always multiplied by the subscripts used in each formula, to indicate how many atoms are represented. If atoms are in two different reactant or product compounds , they are added together to determine how many are present in total.
Reactants
number of each atom
2 SO2 + O2
2 Sulfur
4 + 2 = 6 Oxygen
4 CO2 + 6 H2O
4 Carbon
8 + 6 = 14 Oxygen
12 Hydrogen


Assignment #1: Counting Atoms
Determine the # of atoms of each type present in the following reactants.
Reactants
# of each atom
PbS + 2 PbO
 Pb =
S =
O =
Ca(NO3)2 + 2 KOH
 Ca =
N =
O =
K =
H =
2 NH4NO3 + H2S
 N =
H =
O =
S =
Fe(NO3)3 + 3 LiOH
 Fe =
N =
O =
Li =
H =
Ca3(PO4)2 + 3 H2SO4
 Ca =
P =
O =
H =
S =
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