BALANCING CHEMICAL EQUATIONS: HIGHLIGHT, AND THEN WRITE IN PENCIL!!!! http://web.jjay.cuny.edu/~acarpi/NSC/6-react.htm
Now that you know that chemicals bond to become more stable, you can look at some chemical reactions. Chemical reactions happen all around us: when we light a match, start a car, eat dinner or walk the dog. A chemical reaction is the way that two substances bond together. One example is the reaction of hydrogen with oxygen to form water. To write the chemical reaction you put the reactants (the substances reacting) on the left with an arrow pointing to the products
(the substances being formed) which are on the right. You might guess that it looks something like this:
H + O

H
2
O
However there are 2 problems with this chemical reaction. First, because atoms like to have full valence shells, single H or O atoms are rare (and unhappy) creatures. As you may have seen in a previous lesson, both hydrogen and oxygen react with themselves to form the molecules H
2
and
O
2
, respectively. These hydrogen and oxygen molecules are much more common. Given this correction, one might guess that the reaction looks like this:
H
2
+ O
2
 H
2
O
But we still have one problem. As written, this equation tells us that 1 hydrogen molecule (with
2 H atoms) reacts with 1 oxygen molecule (with 2 O atoms) to form 1 water molecule (with 2 H atoms and 1 O atom). In other words, we seem to have lost 1 O atom along the way!
To write a chemical reaction correctly, the number of atoms on the left side of a chemical equation has to be precisely balanced with the atoms on the right side of the equation. THEY MUST BE
EQUAL! How does this happen? In reality, the O atom that we 'lost' reacts with a 2nd molecule of hydrogen to form a second molecule of water. The reaction is therefore written:
2 H
2
+ O
2
 2 H
2
O
In the chemical reaction above, the number in front of the molecule (called a coefficient ) tells how many molecules are needed for a balanced reaction.

In order to write a correct chemical reaction, we must balance all of the atoms on the left side of the reaction with the atoms on the right side. Let's look at an example. Natural gas is primarily methane. Methane (CH
4
) is a molecule in which 4 hydrogen atoms are bonded to one carbon atom. If you have a gas stove, lighting the stove causes the methane to react with oxygen in the atmosphere to release heat (energy) and the atoms recombine to form carbon dioxide and water vapor. The unbalanced chemical reaction would be:
CH
4
+ O
2

CO
2
+ H
2
O
Look at the reaction atom by atom. On the left side we find 1 carbon atom, and 1 on the right.
There are 4 hydrogen atoms on the left, but only 2 on the right. Therefore, you know 2 water molecules must be formed. Adding this coefficient we get:
CH
4
+ O
2

CO
2
+ 2 H
2
O
Now we have to balance the oxygen atoms. On the left you find 2 atoms, on the right 4 (2 in the
CO
2
molecule and 1 in each of 2 H
2
O molecules). Therefore we need to start with 4 oxygen atoms, or 2 molecules. The balanced equation would then be:
CH
4
+ 2 O
2
 CO
2
+ 2 H
2
O
Try balancing the following equations on your own (note: while you don't need to write the coefficient 1 in a chemical reaction, these examples need you to write a 1 where needed):
1.) ______Na + ______ Cl
2

______NaCl
2.) ______N
2
+ ______H
2
 ______NH
3
3.) ______Zn + ______ HCl

______ZnCl
2
+ ______H
2
4.) ______Fe + ______O
2
 ______Fe
2
O
3