How to Balance Chemical Equations Step By Step

Balancing chemical equations can be one of the most confusing things for new chemistry students.  Everyone doesn't have the luxury of a good teacher who will take the time to explain it for those who just don't get it.  I'm not saying I'm the best, but maybe the following will help it click for someone who is just confused. 

When chemical reactions occur, most of the time the atoms in the resulting product molecules are not in the same amount as the individual reactants. 

Here is a skeleton view of a basic chemical reaction:

X + Y ---------->  XY


Although it looks like one X and one Y should also be the same on the other side, the product molecule may contain varying amounts of each atom.

For example:

H₂ + O₂ -------------> H₂O

On the reactant side are two Hydrogen atoms and two Oxygen atoms.  When reacted they yield H₂O molecules.

Although we start out with 2 Hydrogen atoms and 2 Oxygen atoms, the resulting H2O molecule only contains 1 Oxygen atom.   We must balance the equation so that there is the same number of  EACH atom on BOTH sides.

The balanced version of this equation would look like this:

2H₂ +O₂ --------------> 2H₂O

You can count and see that we have 4 H  and 2 O on the reactant side and we also have 4 H and 2 O on the product side.

(Note:  The coefficient 2 means that there are two molecules of Hydrogen and 2 molecules of H₂O.  Therefore, you would multiply the atoms of each molecule by the coefficient to calculate the number of atoms present.) 

I'm going to post a couple equations and just share my mental process in solving them.  As you learn to balance chemical reactions, keep the following in mind:

1.When you balance equations, the only numbers you ever change are the coefficients (the number in front of the molecule).   Never change the subscripts of the molecules.  This would make them different substances!

Note that 

H₄ = 4 atoms of Hydrogen
4H = 4 atoms of Hydrogen
2H₂ = 4 atoms of Hydrogen

If there is no coefficient or subscript, the atom is just counted once.

2. Work these out in pencil.  You will likely have to erase more than once before you correctly balance these reactions.

3. It may be helpful to write out how many atoms of each element are on each side as you go along. 

4. Remember that the end goal is to have the same amount of each element on both sides of the arrow.



1.   ____ H₂ +  ____ O₂  -------->  ______H₂O


When I begin my process of balancing an equation, my attention immediately goes to the atom in largest quantity. You know that you have to have to balance the equation to have at least that many atoms on the other side.
Since the Hydrogen atoms are already balanced on both sides (there are 2 Hydrogen atoms on each side), let's look at the O₂ on the reactant side.

There are 2 Oxygen atoms on the left side, right?   This means you must make the Oxygen atoms on the product side equal 2, also.  Do this by placing a 2 in front of the H₂O product.

_____ H₂ + _____O₂ ----------->  2 H₂O

Now we have 2 Oxygen atoms on the left and 2 Oxygen atoms on the right side.  But guess what happened when we did this.   When we added the 2 in front of the H₂O we increased the number of Oxygen atoms, but we also increased the number of Hydrogen atoms.

There are now 4 Hydrogen atoms on the right side.

What to do?   Well, we have to make it so that there are 4 Hydrogen atoms on the left side, too.   So, simply add a 2 in front of the Hydrogen molecule on the left to get this:

2 H₂ + ___ O2 ------> 2 H₂O

Now if you count everything, you see that there are 4 Hydrogen atoms on both sides and 2 Oxygen atoms on both sides.  Therefore this chemical equation is balanced.  We don't even have to change or add a coefficient to the O₂ on the left side.  Our work is done.

2 H₂ + O₂ -------> 2 H₂O

Here are a few more equations for you to try.  Find the answers and my personal process in solving them in my next post Balancing Equations For Chemistry.

___ N₂ + ____H₂ ------>  ___NH₃


___S₈ + ___O2 ------->  ____SO₃


___N₂ + ___O2 -------> ____ N₂O


___HgO ------> ____Hg + O₂


____CO₂ + ____H₂O ----->  ___C₆H₁₂O₆ +____ O₂


___Zn + __HCl ------->  ____ZnCl₂  + ___ H₂