Molecular, Complete Ionic, and Net Ionic Equations
Chemists use three types of equations to describe single and double displacement
reactions. These different types of chemical equations are called molecular equations,
complete ionic equations, and net ionic equations.
To distinguish among these types, let’s consider a double displacement reaction
between aqueous solutions of sodium chloride and silver nitrate. The equation for the
double displacement reaction would be:
NaCl(aq) + AgNO3(aq)  NaNO3(aq) + AgCl(s)
We have learned from the solubility rules that the mixing of these two solutions will
produce an insoluble white salt, silver chloride. The equation written above is called the
molecular equation for this reaction because at shows all the species as molecules,
although we actually know that the compounds with (aq) after them are in fact ions.
To represent the fact that some of the species in the above equation are indeed ions, let us
write them that way. Thus, another way to represent the above equation is to write all the
compounds with (aq) after them as ions.
Na+(aq) + Cl-(aq) +Ag+(aq) + NO3-(aq)  AgCl(s) + Na+(aq) + NO3-(aq)
This equation is called the complete ionic equation because it explicitly shows all
the ions in the solution. Note that this equation gives you the exact same information that
the molecular equation does. However, it actually shows what (aq) means.
Note that Na+ and NO3- exist on both sides of the complete ionic equation. These ions
do not react in the solution. They just stand around and watch while the silver and the
chloride ions react to form solid silver chloride. Hence they, as are any ion that exists on
both the reactant and product sides of a chemical equation, are called spectator ions.
Indeed, with all of these extra ions on both sides of the equation, it tends to obscure what
is really going on. So we don’t we cancel them out from either side? If we do this we
would get:
Ag+(aq) + Cl-(aq)  AgCl(s)
This is called the net ionic equation. It gives a concise statement of the reaction that
occurred in solution, showing only the ions that reacted. This is the most important of the
three equations because it describes the chemistry that has occurred.
For any single replacement on double replacement reaction, you should be able to
write the molecular, complete ionic, and net ionic equations. Here is the procedure I go
through to write these equations:
1) Determine the possible products for the reaction and see if one is a solid, gas, or
water. If not, then no reaction has occurred and we are done. If so, balance them and
put the proper states on each reactant and product. This is the molecular equation.
2) Write any ionic compound that has (aq) after it as ions. This is the complete ionic
equation.
3) Cancel out any spectator ions that exist on both sides of the complete ionic equation.
What remains is the net ionic equation. You are then done!