Balancing Chemical Equations
You may remember that the law of conservation of mass
says that matter is neither created nor destroyed during a chemical
reaction. This means that all chemical reactions must be balanced—the
number of atoms, moles, and ultimately the total mass must be conserved during
a chemical process. Here are the rules to follow when balancing
Determine the correct formulas for all the reactants
and products in the reaction.
balancing with the most complicated-looking group. A polyatomic ion
that appears unchanged on both sides of the equation can be counted
as a single unit.
the elemental (single elements) reactant and products for last, especially if
it is hydrogen or oxygen. Keep your eye out for diatomic molecules
such as oxygen, hydrogen, and the halogens.
you get stuck, double the most complicated-looking group and try
make sure that all coefficients are in the lowest-possible ratio.
when to quit! None of the reactions you will encounter will be that difficult.
If the coefficients are getting wild, double-check what you’ve done since
you may have a simple mistake.
When balancing reactions, keep your hands off the subscripts!
Use only coefficients to balance chemical equations. Now let’s try
an example. When you solve it yourself, make sure to follow the
Write the balanced equation for the reaction between chlorine
and sodium bromide, which produces bromine and sodium chloride.
First write the chemical formulas—be on the lookout for
the diatomic elements (such as Cl2):
Cl2 + NaBrBr2 + NaCl
Next, find the reagent with the scariest subscripts. In
this case, start with Cl2. You need a coefficient
of 2 in front of NaCl, which then requires a coefficient of 2 in
front of NaBr. The balanced equation becomes
Cl2 + 2NaBrBr2 + 2NaCl
Finally, count up everything to make sure you balanced
the equation correctly. You have two chlorine atoms, two sodium
atoms, and two bromines on the reactants side and two bromines,
two sodiums, and two chlorines on the products side. You’re done.
Write the balanced equation for the reaction between aluminum
sulfate and calcium chloride, which produces aluminum chloride and
Write the chemical formulas on their correct sides:
In this reaction, the aluminum sulfate looks the most
complicated, so start there. Look at what happens with sulfate—since
it remains sulfate on the right side of the reaction, treat it as
a unit. You have three on the left side and only one on the right
side, so place a coefficient of 3 in front of calcium sulfate. Now
deal with the aluminum. You have three on the left and one on the
right, so place a coefficient of 2 in front of aluminum chloride.
Last, you must place a coefficient of 3 in front of calcium chloride.
Count the atoms on both sides of the reaction and you’ll
see that you’re done.