AP Chemistry Rules for Balancing Equations Name: __________________

1. Balance elements that appear only once on each side of the equation first. 2. Usually balance elements in this order:

Metals

Polyatomic Ions

Nonmetals

H and O

3. Save for last those elements that appear more than twice in the equation. 4. In some cases where diatomic elements (H2, O2, N2, F2, Cl2, Br2, and I2) are

found, whole number coefficients cannot be used, so multiply the entire equation

by 2.

5. Check your final answer to see if you have the same number of each type of atom on both sides of the equation.

6. Make sure that the coefficients in the equation form the smallest whole number ratio.

Example 1:

___ C7H7OH + ___ O2 ___ CO2 + ___ H2O

1. First balance elements that appear only once on each side of the equation.

___ C7H7OH + ___ O2 7 CO2 + 4 H2O

2. Next balance elements that appear more than twice in the equation.

3. In this equation, there is an even number of O on the product side, but only a possible odd number on the reactant side. In this case, multiply everything by 2.

This is common for combustion reactions.

2 C7H7OH + ___ O2 14 CO2 + 8 H2O

4. Then balance the O remembering that it appears in 4 places.

2 C7H7OH + 17 O2 14 CO2 + 8 H2O

5. Always check your answer to see that there are the same number of each type of

atom on both sides of the equation.

Example 2:

___ I4O9 ___ I2O6 + ___ I2 + ___ O2

1. In this case, both I and O appear in more than two places. The I is already balanced. In general balance metals before nonmetals and both before O and H.

2. There is an odd number of O on the reactant side while only a possible even amount on the reactant side. Therefore multiply everything by 2.

2 I4O9 2 I2O6 + 2 I2 + ___ O2

3. Now balance the O.

2 I4O9 2 I2O6 + 2 I2 + 3 O2

4. Always check your answer to see that there are the same number of each type of

atom on both sides of the equation.

Example 3:

___ Ba(OH)2.8H2O + ___ NH4SCN ___ Ba(SCN)2 + ___ H2O + ___ NH3

1. Sometimes it is useful when balancing to keep track of entire polyatomic ions. First balance the barium because it is a metal and it appears only 2 times in the

equation.

Ba(OH)2.8H2O + ___ NH4SCN Ba(SCN)2 + ___ H2O + ___ NH3

2. Since the ammonium ion is not on both sides, we will next balance the thiocyanate ion.

Ba(OH)2.8H2O + 2 NH4SCN Ba(SCN)2 + ___ H2O + ___ NH3

3. Now balance the N.

Ba(OH)2.8H2O + 2 NH4SCN Ba(SCN)2 + ___ H2O + 2 NH3

4. Now balance the waters because water is on both sides of the equation. But

remember that there are 2 hydroxide ions on the reactant side and that there are 2

left over H from converting ammonium ions into ammonia. If hydroxide ions do

not remain intact during a reaction, they are almost always converted into water.

Ba(OH)2.8H2O + 2 NH4SCN Ba(SCN)2 + 10 H2O + 2 NH3

5. Always check your answer to see that there are the same number of each type of

atom on both sides of the equation.