Explanation: Ionic compounds are different from those containing only covalent bonds. The latter exist as molecules whether in the pure state (crystals, liquids) or in solution. The former are a combination of two different kinds of particles. One kind (cations) carry a positive charge, and the second kind (anions) carry a negative charge. Sodium chloride is a typical ionic compound. It exists not as sodium chloride units but as sodium ions and chloride ions. In the solid state, these ions are arranged in a three-dimensional lattice, one kind of ion alternating with the other. In solution, the sodium ions and the chloride ions behave independently of each other.
Consider a particular example of this independent behavior. Suppose you have two solutions. One contains silver ions and an anion whose silver salt is soluble. The second contains chloride ions and a cation whose chloride is soluble. When the two solutions are combined, a white precipitate of silver chloride forms. We can write an equation for the reaction:
Ag+ + Cl- AgCl ()
The other ions are still present in the solution surrounding the precipitate (the supernatant liquid), but they do not participate in the reaction (see Figure 11.5a).
Earlier we would have written the equation differently. Assuming that nitrate was the anion with silver and sodium the cation with chloride, the equation would have been
AgNO3 + NaCl AgCl () + NaNO3
Another way to write the equation shows the ionic compounds in solution as separate ions:
Ag+ + NO3- + Na+ + Cl- AgCl()+ Na+ + NO3-
This last equation shows clearly that only the silver and the chloride ions take part in the reaction; the sodium and nitrate ions do nothing. These nonparticipating ions are called spectator ions, meaning that they do not participate in the reaction - just as people in the stands at a football game are spectators, meaning that they do not participate in the game. The best equation for a reaction is one that shows only the participating ions; such an equation is called the net ionic equation. The net ionic equation for the reaction of silver ion with chloride is
Ag+ + Cl- AgCl ()
Consider another situation. When an acid reacts with a hydroxide, water and a salt are formed. All acids release hydrogen ions in solution. Hydroxides furnish hydroxide ions. Hydrogen ions and hydroxide ions react to form water, a covalent un-ionized molecule (see Figure 11.5b). The equation for this reaction is
H+ + OH- H2O
Previously we would have written this neutralization reaction using complete formulas. For example, the equation for the reaction of hydrochloric acid with sodium hydroxide would be
HCl + NaOH NaCl + H2O
Rewriting this equation to show the acid, the hydroxide, and the salt as ions - which is the way these compounds exist in solution - would give the equation
H+ + Cl- + Na+ + OH- Na+ + Cl- + H2O
This equation shows clearly that the hydrogen and the hydroxide ions react and that the sodium and chloride ions are spectator ions. They would not appear in the net ionic equation.