Examples of Net Ionic Equations

Solutions of Bi(N03)3 and Na S are both colorless. If we mix the two solutions together, a black solid precipitates. Using solubiUty rule 7 we see that all sulfides are insoluble with only a few exceptions. The bismuth ion (BT+) is not one of the exceptions. Since we know from rule 1 that NaN03 is soluble, we conclude that Na+ and NO 3 are spectator ions in this reaction and that the black precipitate is bismuth sulfide (Bi2S3), as shown in the following net ionic equation  

Notice that the laws of conservation of mass and conservation of charge are both satisfied in this equation. There are two atoms of bismuth on each side and there are three atoms of sulfur on each side. The sum of the charges on the left-hand side is (2)(+3) + (3)(-2) = 0. The charge on the right-hand side is also zero. 

Just one more example before we move on to the next chapter. An aqueous solution of FeClj is yellow. An aqueous solution of NaOH is colorless. If we mix the two solutions together, we get a rusty brown precipitate. We know from rule 1 that NaCl is soluble. Rule 6 tells us that FeCOHJj is insoluble. Therefore, in this reaction Na+ and Cl are spectator ions and the precipitate is Fe(OH)3. This reaction is shown in the following net ionic equation  

Ferric hydroxide has a rusty brown color because, in fact, that is what rust is—either Fe(OH)j or Fe Oj. 

Throughout the remainder of this book we will usually write net ionic equations for reactions that take place in aqueous solution. For reactions occurring in the gas phase, or between liquids other than water, we will continue to write total molecular equations. 

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