Balancing chemical equations

Atoms can neither be destroyed nor created during a simple chemical reaction. Therefore, in a chemical reaction

The sum of atoms before reaction = the sum of atoms after reaction

General Rules for balancing chemical equations – Rule 5

Balancing chemical equations may require some trial and error. There are some general rules that could be helpful, but they may not work all the time.

• Rule 1: Balancing chemical equations using the one’s and two’s technique
• Rule 2: Balancing chemical equations using the two’s and three’s technique
• Rule 3: Balancing chemical equations using the CHO technique
• Rule 4: Balancing chemical equations using the even technique
• Rule 5: Balancing chemical equations containing polyatomic ions

In this page, we will look at some examples of applying

Rule 5: Balancing chemical equations containing polyatomic ions

Case 1: If the polyatomic ion remains the same before and after the reaction, then treat it as “a single element” for ease of calculation. Try to start with the most complicated-looking group.

Example:

Balance the equation

Ca + HNO₃ → H₂ + Ca(NO₃)₂

Solution:

• The nitrate ion NO₃ is unchanged before and after the reaction.
• Start with Ca(NO₃)₂ since it looks most complicated.
• There is one NO₃ on the left and two on the left.
• Using the one’s and two’s technique, the equation is balanced by placing the coefficient of 2 for HNO₃

  Ca + 2HNO₃ → H₂ + Ca(NO₃)₂

• We test the other atoms and we find that we already have a balanced equation.

• Check to make sure that all coefficients are in the lowest-possible ratio

The following videos show some examples of balancing chemical equations containing polyatomic ions.

Case 2: If the polyatomic ion is changed after the reaction then it would be necessary to consider each atom individually.

Example:

Balance the chemical equation

Ba(OH)₂ + H₃PO₄ → BaHPO₄ + H₂O

Solution:

• Use the CHO technique. Since we don't have carbon, we could try to balance the hydrogen first.
• We have five hydrogen atoms on the left and three hydrogen atoms on the right.
• To balance the hydrogen, we can place the coefficient of 2 at H₂O

  Ba(OH)₂ + H₃PO₄ → BaHPO₄ + 2H₂O

• We test the other atoms and we find that we already have a balanced equation.

• Check to make sure that all coefficients are in the lowest-possible ratio.

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