Balancing Chemical Equations
Balancing chemical equations is an important skill used throughout chemistry. The Law
of Conservation of Matter explains why chemical equations are balanced. It states that in a
chemical reaction, matter (atoms) are not created or destroyed- the atoms are rearranged. A
similar idea is the law of conservation of mass. Since matter is not created or destroyed, the
total amount of mass at the beginning and the end of a reaction is the same.
Let’s review a little of the material we started our study of matter with. We said that
when new materials are produced, a chemical change (reaction) has taken place. A chemical
equation shows this and takes into consideration the amounts of material that is started with
and that are present at the end. Indications that a chemical change has occurred include
bubbling, an odor, temperature changes and formation of a precipitation.
A chemical equation uses symbols and formulas to show what happens during a
chemical change. The materials that enter into the reaction are called the reactants. The new
materials that form are called the products. Usually an arrow is between the 2 sides, pointing
towards the products. The arrow can be pointed either way- so be careful on determining
products and reactants. Sometimes the arrow is going both ways- showing that while new
products are being made, they are breaking back into the reactants at the same time. If one
arrow is bigger than the other, then that “direction” of the reaction is happening at a faster
rate.
There are several steps to follow to balance a chemical equation. Be careful in doing
each step. A mistake early on, makes doing the rest at times impossible! You will need to start
with a correctly written equation- that is all the symbols and formulas are written correctly. In
the first equation you will be doing, you will be given equations with correctly written symbols
and formulas. Later you will be writing your own. Second step is to count all the atoms on each
side of the equation. We call this taking an
inventory. I prefer the method shown. Other
websites show it a slightly different way. Listing
the elements down the center helps to separate
the products from the reactants. Notice in this
example, the polyatomic ion stays together in
the inventory. In this problem it is appropriate
to do that since the polyatomic ion is present
on both sides of the equation. If the polyatomic
ion is not present on both sides, then you cannot leave it together in the inventory. We will
work on recognizing when to do this later. The third step is to add coefficients to balance the
equation. A coefficient is a whole number written in front of a symbol or formula. It tells how
many of the material there is. For example 2NaCl says there are 2 salt molecules. In the 2,
there are 2 atoms of Na and 2 atoms of Cl. In 2Na2S, there are 4 Na’s and 2 S’s. Remember a
balanced equation has the same number of each kind of atom (or polyatomic ion) on each side
of the equation. Start with something that is not balanced. In the problem on the previous
page, maybe you would put a coefficient 2 in front of the Al on the reactant side. Then go to
the inventory and change the atom count. You will have 2 Al’s and 6 Br’s. Maybe next, maybe
you would put a coefficient of 6 in front of the KBr on the product side. That would give you
the 6 Br’s you need. Don’t forget to change the K’s in the inventory. You continue this process
until everything is the same on both sides. The final answer for this problem would be:
2AlBr3 + 3K2SO4
6KBr + Al2(SO4)3
Keep these things in mind:
*You can change a coefficient more than 1 time
*Be sure to change everything in the inventory affected by the addition of a coefficient
I would encourage you to watch the Khan video if want to view the process. He does these
without an inventory. I would encourage you not to do this- in some problems there can be a
lot to keep track off!
http://www.khanacademy.org/science/chemistry/chemical-reactionsstoichiometry/e/balancing_chemical_equations
Define chemical equation
Define reactant
Define product
Define coefficient
Define balanced chemical equation
What is the law of conservation of mass?
Why must chemical equations be balanced?
List and learn the seven diatomic elements.