Balancing Chemical Equations

A chemical equation is an expression of the net composition change associated with a chemical reaction. It shows how a certain amount of reactants yields a certain amount of products. Both of these amounts are measured in moles. Chemical equations often contain information about the state of the reactants: solid, liquid, gas, or aqueous. In addition, they always adhere to the law of conservation of mass, which holds that matter can change form, but cannot be created or destroyed.

This means that the mass of a closed system of substances will remain constant, regardless of the processes acting inside the system. In other words, for any chemical equation in a closed system, the mass of the reactants must equal the mass of the products. Therefore, there must be the same number of atoms of each element on each side of a chemical equation. A properly balanced chemical equation shows this.

How to Balance Reactions

Take a look at the equation for the chemical reaction that yields table salt (NaCl, sodium chloride) from sodium metal and chlorine gas:

$Na_{(s)} + Cl_{2(g)} \rightarrow NaCl_{(s)}$

There are two chlorine atoms on the reactant's side, which is indicated by the subscript 2 next to chlorine. However, there is only one Cl atom on the product's side because the ratio of Na to Cl is one to one. Therefore, to balance this reaction, a coefficient will have to be added to the NaCl on the product's side.

Never attempt to balance a reaction by changing the subscripts on a molecule. The subscripts indicate a very specific molecule; changing the subscripts would indicate a new molecule (not the desired product).

To balance this reaction, add a 2 in front of the NaCl.

$Na_{(s)} + Cl_{2(g)} \rightarrow 2\:NaCl_{(s)}$

Now, there are two chlorine atoms on each side of the reaction. However, now there is one sodium atom on the reactant's side and two sodium atoms on the product's side. Therefore, add a 2 in front of the sodium on the reactant's side.

$2\:Na_{(s)} + Cl_{2(g)} \rightarrow 2\:NaCl_{(s)}$

Now, there are two sodium atoms on both sides and two chlorine atoms on both sides. Therefore, the reaction is balanced.

The law of conservation of mass applies in all chemical equations. This means that the number of atoms of products present is conserved in the number of atoms of reactants.