Solid sulfur reacts with oxygen gas to form sulfur trioxide.

Balancing The Chemical Equation for The Formation of Sulfur Trioxide

The purpose of balancing a chemical equation is to find the relative quantities of the molecules and atom types in the equation.  When the equation is balanced there will be the same number of each atom type on opposite sides of the equation. Consider the unbalanced equation for sulfur solid reacting with oxygen gas.

                                    S(s) + O₂(g)  ➝ SO₃(g)

                                     S=0                   S=0

                                     O=0                   O=0

The reactant side containing sulfur solid and oxygen gas and the product side containing sulfur trioxide gas do not have equal numbers of each atom type.  There appears to be eight sulfur atoms on the left of the arrow and one on the right.  This makes the sulfur unbalanced balanced.  There are two oxygen atoms on the left and only three oxygen atoms on the right.  All of the atom types must be equal before the equation is balanced. To make the number of each type of atom on opposite sides of the arrow equal, the coefficients must be adjusted until the equation is balanced.  Compare the following results. 

                                    1S(s) + 12O₂(g)  ➝ 8SO₃(g)

                                    S=8                   S=8

                                     O=24                O=24

The coefficient 1 in front of the 1S(s) results in 8 atoms of sulfur on the left side of the arrow.  The coefficient 8 in front of the SO₃(g) results in 8 atoms of sulfur and 24 atoms of oxygen on the right side of the arrow.  The coefficient 12 in front of the O₂(g) results in 24 atoms of oxygen on the left side of the arrow. The equation is balanced.  The ratio of coefficients 1:12:8 balances the reaction. The ratio of coefficients 2:24:16, 3:36:24 and 8:48:32, also, balance the reaction; however, the lowest ratio of coefficients 1:12:8 is the accepted answer.

© Pat Thayer 2014-2016