Balancing Chemical Equations

 

The Law of Conservation of mass.

This means the atoms that exist today are essentially the same ones that existed thousands and millions of years ago. The atoms in use today have been recycled time after time. Everyone has been recycling with chemists since the beginning of time. Pretty cool, huh?

 

Balancing equations.

The letter symbols that represent atoms and molecules in equations are treated like objects. Balancing chemical equations literally means counting the number of times atom symbols appear in the reactants and products to make sure the counts are the same on both sides. Conservation of mass is linked to "conservation " of element symbols.

The law of conservation of mass is met when the count of element symbols on reactants side is equal to the count of element symbols on the product side. This rests on the additional assumption that the symbols also represent the masses of the elements.

A balanced equation has equal counts (number) of atoms of each element in both reactants and products.

Equations are balanced by adjusting the multipliers (coefficients) in front of formula symbols so the counts of atoms are the same in reactants and products. The subscripts are not changed.

Subscripts in chemical formulas are NEVER changed in the balancing process. Changing the subscripts changes the identity of compounds and the sense of the equation.

Example:

The reaction between solid sulfur and oxygen is summarized in the unbalanced equation below.

reactant	combine	reactant	condition	product
sulfur solid	+	oxygen gas	heat	sulfur dioxide gas
S6 (s)	+	O2 (g)		SO2 (g)

Changes of coefficients are done so multiplying coefficients and subscripts gives the same number for both sides of the equation for an element. Picking coefficients is done by "inspection" followed by an organized trial and error process.

Step 1 : Identify the substance that has the most influence on the equation and insert a coefficient for that formula. Here insert a "1" "1"in front of the S6(s) . This means there are 1 x 6 sulfur atoms in reactants.

sulfur solid	+	oxygen gas	heat	sulfur dioxide gas
1 S6 (s)	+	O2 (g)		??? SO2 (g)

1 x 6 sulfur

------

?? x 2 oxygen

--------------------

??? x 1 sulfur

?? x 2 oxygen

Step 2: The number of sulfurs must be equal to six in the products. The subscript on "S" is a one in sulfur dioxide. You have to decide how many SO2(g) molecules are needed to give a count of 6 sulfur atoms. ??? x 1 = 6

sulfur atom in reactants

=

sulfur atom in products

coefficient x subscript	=	coefficient x subscript
1 x 6 "S" atoms	=	??? x 1 "S" atoms
1 x 6 "S" atoms	=	??? x 1 "S" atoms
6 "S" atoms	=	??? x 1 "S" atoms

1 S6 (s)

+

?? O2 (g)

6 SO2 (g)

1 x 6 sulfur

?? x 2 oxygen

6 x 1 sulfur

?? x 2 oxygen

Step 3: Check to see if subscripts on an atom are the same in formulas in reactants and products. If the subscripts are the same the coefficient for the formulas must be the same. The "O" atoms have a subscript of "2" in both reactants and products. The coefficient must be "6 " for both O2 and SO2.

1 S6 (s)

+

3 O2 (g)

6 SO2 (g)

1 x 6 sulfur

6 x 2 oxygen

6 x 1 sulfur

6 x 2 oxygen

When the unbalanced equation is illustrated with ball and stick models we can see the numbers of atoms of various elements.

S6 (s)	O2 (g)		SO2 (g)
	two oxygen atoms -		two oxygen atoms
six sulfur atoms -----------	--------------------------		one sulfur atom

 

The ball and stick formulas showing the correct counts and a balanced equation are shown below.

1 S6 (s)	6 O2 (g)		6 SO2 (g)
	two "O" atoms in a molecule		two "O" atoms in a molecule
	6 molecules x 2 "O" atoms		6 molecules x 2 "O" atoms
6 "S" atoms per molecule			one "S" atom in a molecule
1 x 6 "S" atoms---------------------	----------------------------------------	-	6 x 1 "S" atoms