To balance equations:

1. Need to obtain the same number of each element on each side of the --->

2. May only change coefficients, never subscripts.

3. Check for even-odd relationships

4. Balance opposite side to compensate for the most complicated substance first, then finish balancing.

5. Subscripts outside a parenthesis apply to all elements inside the parenthesis.

6. If H₂O yields H₂ then write the H₂O as HOH.

Balancing Examples

Example 2:
__ N₂ + __ Cl₂ --> __ NCl₃
Even-odd relation in N's and Cl's so multiply NCl₃ by 2 and then finish balancing (see item 3 above).

N₂ + 3 Cl₂ --> 2 NCl₃

Example 3:
__ C₂H₄ + __ O₂ --> __ CO₂ + __ H₂O
Balance with respect to C₂H₄ first, then finish balancing (see item 4 above).

C₂H₄ + 3 O₂ --> 2 CO₂ + 2 H₂O

Example 4:
__ C₂H₆ + __ O₂ --> __ CO₂ + __ H₂O

Balance with respect to C₂H₆ first, (see item 4 above).
C₂H₆ + O₂ --> 2 CO₂ + 3 H₂O

Now have an even-odd relationship in O's so multiply everything except O₂ by 2 and then finish balancing

2C₂H₆ + 7 O₂ --> 4 CO₂ + 6 H₂O

Example 5:
__ Al₂(CO₃)₃ + __ H₂SO₄ --> Al₂(SO₄)₃ + __H₂O + __ CO₂
Balance for Al₂(CO₃)₃ first (item 4 and item 5)

__ Al₂(CO₃)₃ + __ H₂SO₄ --> Al₂(SO₄)₃ + 3H₂O + 3 CO₂
then finish balancing

Al₂(CO₃)₃ + 3 H₂SO₄ --> Al₂(SO₄)₃ + 3 H₂O + 3 CO₂

Example 6:
__B + __ H₂O --> __ B(OH)₃ + __ H₂
Rewrite H₂O as HOH

__B + __ HOH --> __ B(OH)₃ + __ H₂

Then balance the H's and OH's

__B + 6 HOH --> 2 B(OH)₃ + 3 H₂
Then finish the balancing

2 B + 6 HOH --> 2 B(OH)₃ + 3 H₂
or

2 B + 6 H₂O --> 2 B(OH)₃ + 3 H₂