Chemical reactions in solution can be written in three different forms:
Gives the overall reaction stoichiometry but not necessarily the actual form of the reactants and products in solution. For example, when you mix HCl with aqueous NaOH, a reaction occurs that forms water and the salt NaCl.
HCl(aq) + NaOH(aq) → H2O(aq) + NaCl(aq)
This is a molecular equation. It doesn't give a clear picture of what actually occurs in solution. Each reactant and product, HCl(aq), NaOH(aq), H2O(aq), and NaCl(aq), actually exist in solution split into their respective cations and anions.
Complete Ionic Equation
Represents all solution phase reactants and products that are strong electrolytes as ions.
H+(aq) + Cl-(aq) + Na+(aq) + OH-(aq) → H2O(aq) + Na+(aq) + Cl-(aq)
This is a more accurate representation of what actually exists in the solution than the molecular equation shown above. In a complete ionic equation all substances that are strong electrolytes are represented as ions. From the complete ionic equation we see that Na+ and Cl- ions don't really participate in the reaction. They are examples of what we call spectator ions: Ions that appear on both sides of the equation.
Net Ionic Equation
Includes only those species undergoing a change in the reaction. That is, spectator ions are not included. Thus, for the reaction above the net ion equation would be
H+(aq) + OH-(aq) → H2O(aq)
As you might imagine, it is possible to mix substances and have no net ionic equation. Consider the following molecular equation
CaCl2(aq) + 2 HNO3(aq) → Ca(NO3)2(aq) + 2 HCl(aq)
The complete ionic equation is
Ca2+(aq) + 2 Cl-(aq) + 2 H+(aq) + 2 NO3-(aq) → Ca2+(aq) + 2 NO3-(aq) + 2 H+(aq) + 2 Cl-(aq)
Removing all the spectator ions from this equation leaves nothing! All the ions are spectator ions, so there is no net ionic reaction.
Processes that lead to a Net Ionic Equation
We will now consider the following four processes that will lead to an actual net ionic equation:
- Formation of an insoluble solid (Precipitation Reaction)
- Formation of a gas that escapes from solution
- Formation of either a weak electrolyte or non-electrolyte
- An oxidation/reduction reaction
The first three processes are generally reactions where a cation meets up with an anion and forms a bond that has enough covalent character that they can no longer be separated into individual cations and anions any more.
Chemisty, The Central Science, 10th Ed.
4.19, 4.21, 4.23, 4.25, 4.27, 4.39, 4.41, 4.43, 4.45, 4.47, 4.49, 4.51, 4.53, 4.55, 4.57