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Balancing Chemical Equations
Important Definitions:
Chemical Symbol
* 1 or 2 letters representing an element -- one per square in the Periodic Table of Elements
* always starts with a capital letter -- pay attention to upper & lower case letters
Chemical Formula
* chemical symbols showing how atoms combine to make compounds.
* read chemical formulas by saying the letters and numbers exactly as they appear
* Subscripts tell how many atoms of the element just in front of them are in each molecule
* Example:
H20 is the chemical formula for water.
* H and O are chemical symbols for hydrogen and oxygen.
* The "2" after the H means that there are two atoms of hydrogen in one water molecule.
* If the subscript is missing, that means there is just one atom of that element in the molecule, so
water has only 1 atom of oxygen in each molecule.
Chemical Equation
* uses chemical formulas to show what happens during a chemical reaction
* the left side is the reactants; these are the substances that are reacting
* the right side is the products; these are the substances that are produced (made)
* Chemical equations use arrows instead of equal signs; read the arrow as "yields"
* Matter is neither created nor destroyed in a chemical reaction, so all the atoms present on the
reactant side must be present on the product side of a chemical equation (Conservation of Mass).
* Coefficients are numbers placed in front of chemical formulas telling how many copies of that
molecule are used in the reaction. If the coefficient is missing, only 1 copy of the molecule is
used in the reaction.
* Example: 2 H202 -> 2 H20 + 02
* This is the equation for the decomposition of hydrogen peroxide (H202) to water (H2O) and
oxygen gas (O2)
* 2 molecules of hydrogen peroxide yield 2 molecules of water and 1 molecule of oxygen gas.
Remember: Coefficients go in front of chemical formulas, and tell the number of molecules used
in a chemical reaction;
subscripts come after a chemical symbol in a chemical formula, and tell the number of atoms in
the molecule.
Balancing Chemical Equations
1. Find the number of atoms for each element.
1.1 Identify the elements on the reactant side. They will be the same on the product side.
* Look for capital letters to find the elements.
* List each element one time on each side.
Na
+
MgF2
Na =
Mg =
F =
->
NaF +
Na =
Mg =
F =
Mg
1.2 Count the number of atoms for each element on each side of the equation.
* The number of atoms = the coefficient x the subscript
* Draw the molecules if you are having trouble figuring out how many atoms there are
Na
+
MgF2
Na = 1
Mg = 1
F = 2
->
NaF + Mg
Na = 1
Mg = 1
F = 1
Coefficient on MgF2 is 1; the subscript on F is 2, so 1x2 = 2 atoms of F
2. Choose an element that is not balanced.
2.1 Elements are not balanced if they have a different number of atoms on the reactant side than
they do on the product side.
2.2. In choosing which element to balance first, choose an element that is in a compound. It is
easy to balance elements that are not combined with others.
Na
+
MgF2
Na = 1
Mg = 1
F = 2
->
NaF + Mg
Na = 1
Mg = 1
F = 1
F is not balanced; there are 2 atoms on the reactant side, but only 1 on the product
side.
3. Add a coefficient in front of the formula with that element and adjust your
counts.
3.1 Find the side of the equation which has fewer atoms of the element you have chosen to
balance. You cannot take any atoms away, so you must add more to the side that needs it.
Na
+
MgF2
Na = 1
Mg = 1
F = 2
->
NaF + Mg
Na = 1
Mg = 1
F = 1
There are fewer F atoms on the product side.
3.2 Find the chemical formula on that side of the equation which has the element you want to
balance.
Na
+
MgF2
Na = 1
Mg = 1
F = 2
->
NaF + Mg
Na = 1
Mg = 1
F = 1
The chemical formula on the product side containing F is NaF.
3.3 Figure out how many copies of that molecule you need to balance that element. You
CANNOT add or change subscripts, you can only add or change coefficients. Change the
coefficient so you have enough copies of that molecule to balance the element you picked in step
2.
Na
+
MgF2
Na = 1
Mg = 1
F = 2
->
2 NaF +
Na = 1
Mg = 1
F = 1
Mg
There are 2 F atoms on the reactant side but only one on the product side. We must
add F to the product side by adding a coefficient in front of NaF. 2 copies of the
NaF molecule will give us the 2 F atoms we need.
3.4 Adjust the atom counts of the elements for the chemical formula you just added the
coefficient in front of.
Na
+
MgF2
Na = 1
Mg = 1
F = 2
->
2 NaF + Mg
Na = 1 2
Mg = 1
F = 1 2
Since we put a coefficient of 2 in front of NaF, we have to change the atom counts for
both Na and F. They now have 2 atoms each, the coefficient (2) times the subscript
(1) for each.
4. Repeat from Step 2 until all the elements are balanced.
2 Na
+
Na =
Mg =
F =
MgF2
1 2
1
2
->
2 NaF +
Na = 1 2
Mg = 1
F = 1 2
Mg
Na is now unbalanced, since there is 1 on the reactant side and now there are 2 on the
product side. By putting a coefficient of 2 in front of the Na on the reactant side,
we now have 2 Na atoms on both sides. The equation is balanced.