# How can I balance decomposition reactions?

May 8, 2015

You balance decomposition reactions just like any other reaction. Add coefficients in front of each formula so that the number of atoms of each element are the same on both sides, being careful not to change formulas by changing subscripts.

For example, let's look at the decomposition of magnesium oxide into magnesium and oxygen.

$\text{MgO(s)}$$\rightarrow$$\text{Mg(s)"+"O"_2("g")}$

Notice that the number of oxygen atoms is not the same on both sides. If we write a coefficient of 2 in front of the $\text{MgO}$, the $\text{O}$ atoms will balance.

$\textcolor{red}{2} \text{MgO(s)}$$\rightarrow$$\text{Mg(s)"+"O"_2("g")}$

However, now the $\text{Mg}$ atoms are not balanced. We now have 2 $\text{Mg}$ atoms on the left, but only 1 $\text{Mg}$ atom on the right. So we add a coefficient of 2 in front of the $\text{Mg}$ on the right.

$\textcolor{red}{2} \text{MgO(s)}$$\rightarrow$$\textcolor{red}{2} \text{Mg(s)"+"O"_2("g")}$

We now have 2 $\text{Mg}$ atoms on both sides and 2 $\text{O}$ atoms on both sides, and the equation is balanced.