Balancing Equations and Reactions with Polyatomic Ions

General Chemistry Index

When working with chemical notation, it has been established that the subscripts (or numbers FOLLOWING the elements) represent the number of those elements present. The coefficient tells how many of the molecule is present.

Thus,

3 H20 or sometimes 3 H2O

means 3 molecules of water in which are 2 atoms of hydrogen and 1 atom of oxygen.

Understanding Polyatomic Nomenclature

There is another variation of this notation that is applied in certain cases with some compounds. Because of how the compounds form, there is sometimes a value in keeping some of the elements as a unit and subscripting the whole unit to show how many of that unit are present.

Look at the reaction below:

CaC2   +   2H2O   --->   Ca(OH)2   +   C2H2

Notice on the product side, the OH is inside parenthesis. This represents that that is a unit of molecules that are being kept together based on how the compound is formed. The subscript indicates that there are two of these units present.

Examples:

3Ca(OH)2 has in it, 3 Ca, 6 O, and 6 H. (The subscripted 2 applies to both atoms inside the parenthesis, and the coefficient of 3 applies to the whole molecule.)

Ca3(PO4)2 has in it, 3 Ca, 2 P, and 8 O. (The subscripted 2 applies to the PO4, so there are 2 P and 8 0)

2Cu(NO3)2 has—to begin with, there are 2 molecules of Cu(NO3)2 as indicated by the coefficient.

• EACH molecule has 1 Cu and 2 (NO3). Since there are 2 (NO3) (The subscript 2 applies to everything inside the parenthesis.), that means there are 2 N and 6 O in each Cu(NO3)2 molecule.
• Since there are 2 molecules of Cu(NO3)2, there are in TOTAL:
• 2 Cu
• 4 N
• 12 O

Summary:

• Coefficients, the numbers in front, apply to the whole molecule and tell how many molecules or "sets" of molecules are present.
• Subscripts (or number FOLLOWING the atom symbols) tell how many of that atom are in the molecule.
• If a group of atoms are inside parenthesis:
• They are to be kept together as a unit.
• Any subscripts to the closing parenthesis means that there are that many units of the atoms inside the parenthesis are present.

Balancing Polyatomic Reactions

Keeping in mind that the process of balancing ANY equation means finding coefficients that result in the same numbers of the same types of atoms appearing on both sides of the reaction…

…then to do so with polyatomic reactions means doing a little more work.

The first thing you need to do is get the reaction written out unbalanced…

Fe(NO3)3 + (NH4)2CO3 → Fe2(CO3)3 + NH4NO3   <<<--- Unbalanced reaction

The second step is to do a little inspection. This is vital. And tedious. The goal is to identify the polyatomic "chunks" that move across the reaction unchanged. The parenthesis, if present, will help!

The chunk has to move across UNCHANGED. If for instance a chunk of PO4 becomes PO3, then it changed. If it does move unchanged, then we can balance the equation looking at the "chunks."

In the above example, there are three polyatomic chunks (ions) that move across:

NO3

CO3

NH4

Since they move unchanged, they can be treated chunk by chunk. Optionally, if it makes it easier on the eyes and brain, you can even use an abbreviation for or color code the chunk.

Once you have identified the chunks you can treat in whole, the third step is to do the balancing:

Fe(NO3)3 + (NH4)2CO3 → Fe2(CO3)3 + NH4NO3   <<<--- Unbalanced reaction

            Reactant Side                    Product Side

Fe                 1                                           2

NO3              3                                            1

NH4              2                                            1

CO3              1                                            3

2Fe(NO3)3 + 3(NH4)2CO3 → Fe2(CO3)3 + 6NH4NO3   <<<--- Balanced reaction

            Reactant Side                    Product Side

Fe                 2                                           2

NO3              6                                           6

NH4               6                                           6

CO3              3                                           3