Chemistry Reference
In-Depth Information
that the elements combine. We have created a new compound that may
not even exist! Remember that when we went over oxidation numbers in
Chapter 5, we pointed out that each formula for a neutral compound must
show a net charge of zero. Magnesium has an oxidation number of +2 and
oxygen has an oxidation number of -2. They combine in a 1:1 ratio to form
magnesium oxide, because +2 + (-2) = 0. By changing the subscript to
satisfy the atomic tally, we violated a rule that we learned several chapters
ago! Thus, the rule was born.
What can we do to balance a chemical equation? All we are allowed to
do is add coefficients in front of the chemical formulas to achieve the proper
balance. The correct way to balance the equation from Example 2 is:
2Mg + O
2
2MgO
How can we be sure that this is the correct balanced equation? We can
do another atomic tally and make sure that the same number of atoms of
each element appear on both sides of the equation.
2Mg + O
2
2MgO
Elements
{
# of atoms
{
Mg
O
Mg
O
2222
Balanced!
Let's try another example. Do an atomic tally for the following equa-
tion. If the equation is not balanced, figure out what coefficient(s) you need
to add in order to achieve balance. Finally, do another atomic tally to make
sure that the new equation is balanced.
Example 3
Do an atomic tally for the following equation and determine if it is
balanced. If not, balance the equation using coefficients, and repeat
the tally.
CH
4
+ O
2
H
2
O + CO
2
I will go through the solution slowly, and if I lose you, go back to the
beginning and try to follow me again. It is very important that you learn the
process of balancing equations correctly.
First, we do an atomic tally to see if our equation is already balanced.
CH
4
+ O
2
H
2
O + CO
2
Elements
{
# of atoms
{
CHOCHO
142123
Not balanced!
