Double-Replacement Reactions with Undissolved Reactants

In spite of the existence of printed labels, laboratory bottles with NH4OH etched on them, and wide use of the name ammonium hydroxide, no substance having the formula NH4OH exists at ordinary temperatures. The actual product is a solution of ammonia molecules, NHjCaq). The proper net ionic equation is therefore 

The reaction is reversible and yields a solution in which NH3 is the major species and NH4 and OH are minor species (Fig. 9.15). 

There is no system by which these three different molecular product reactions can be recognized. You simply must be alert to them and catch them when they appear. Once again, the predicted but unstable formulas are H2CO3, H2SO3, and NH4OH. 

Write the conventional, total ionic, and net ionic equations for the reaction between solutions of sodium carbonate and hydrochloric acid (Fig. 9.16). 

The standard double-replacement equation would predict NaCl and H2CO3 as the products of this reaction, but H2CO3 is one of the unstable ion combinations. We therefore break it down into its decomposition products, CO2 and H2O. 

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Double-Replacement Reactions with Undissolved Reactants... 

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