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Similar to a molecular equation, which expresses compounds as molecules, an ionic equation is a chemical equation where the electrolytes in aqueous solution are expressed as dissociated ions. Usually, this is a salt dissolved in water, where the ionic species are followed by (aq) in the equation to indicate they are in aqueous solution. The ions in aqueous solution are stabilized by ion-dipole interactions with water molecules. However, an ionic equation may be written for any electrolyte that dissociates and reacts in a polar solvent. In a balanced ionic equation, the number and type of atoms are the same on both sides of the reaction arrow. Additionally, the net charge is the same on both sides of the equation.
Strong acids, strong bases, and soluble ionic compounds (usually salts) exist as dissociated ions in aqueous solution, so they are written as ions in the ionic equation. Weak acids and bases and insoluble salts are usually written using their molecular formulas because only a small amount of them dissociates into ions. There are exceptions, especially with acid-base reactions.
Examples of Ionic Equations
Ag+(aq) + NO3-(aq) + Na+(aq) + Cl-(aq) → AgCl(s) + Na+(aq) + NO3-(aq) is an ionic equation of the chemical reaction:
AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq)
Complete Versus Net Ionic Equation
The two most common forms of ionic equations are complete ionic equations and net ionic equations. The complete ionic equation indicates all of the dissociated ions in a chemical reaction. The net ionic equation cancels out ions that appear on both sides of the reaction arrow because they essentially don't participate in the reaction of interest. The ions that are canceled out are called spectator ions.
For example, in the reaction between silver nitrate (AgNO3) and sodium chloride (NaCl) in water, the complete ionic equation is:
Ag+(aq) + NO3-(aq) + Na+(aq) + Cl-(aq) → AgCl(s) + Na+(aq) + NO3-(aq)
Notice the sodium cation Na+ and nitrate anion NO3- appear on both the reactants and products side of the arrow. If they are canceled out, the net ionic equation may be written as:
Ag+(aq) + Cl-(aq) → AgCl(s)
In this example, the coefficient for each species was 1 (which is not written). If every species had started with a 2, for example, each coefficient would be divided by a common divisor to write the net ionic equation using the smallest integer values.
Both the complete ionic equation and the net ionic equation should be written as balanced equations.
Brady, James E. "Chemistry: Matter and its Changes. John Wiley & Sons." Frederick A. Senese, 5th Edition, Wiley, December 2007.