4.4: Other Common Reactions

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Learning Objectives

Predict the formation of a weak electrolyte from its constituent ions.
Write the net ionic equation for the formation of a weak electrolyte.
Predict the formation of carbon dioxide gas from the reaction of carbonate or bicarbonate ions with acid.

Constituent Ions will Associate to form Weak Electrolytes

Recall that weak electrolytes are solutes where only a small percentage of the molecules present dissociate into ions when dissolved in water. We call this partial dissociation. Alternatively, because most of the ions remain associated, we could describe the solution of a weak electrolyte as mostly molecular. Either way we describe it, the behavior is consistent. The constituent ions of a weak electrolyte tend are strongly attracted to one another. Therefore, if you mix solutions containing the constituent ions of a weak electrolyte, the ions will associate (the opposite of dissociate) and the major product will be the weak electrolyte.

For example, weak acids--weak electrolytes, by definition--will be formed whenever their constituent ions are mixed. Here are two examples:

Mixing solutions of HCl and KC₂H₃O₂, the major product will be acetic acid, HC₂H₃O₂.

\[\ce{H^+}(aq)+\ce{C2H3O2^-}(aq)\rightarrow \ce{HC2H3O2}(aq)\]

Mixing solutions of HNO₃ and NaF, the major product will be hydrofluoric acid, HF.

\[\ce{H^+}(aq)+\ce{F^-}(aq)\rightarrow \ce{HF}(aq)\]

You have been given one example of a weak base, ammonia. Recognize that combining the ions ammonium (NH₄⁺) and hydroxide (OH^-) will result in the formation of the weak electrolyte, ammonia (NH₃). Here is an example:

Mixing solutions of NaNH4 and KOH, the major product will be ammonia, NH₃.

\[\ce{NH4^+}(aq)+\ce{OH^-}(aq)\rightarrow \ce{NH3}(aq) + \ce{H2O}(l)\]

Special Reactions of Carbonate and Bicarbonate Ions

Carbonate ion and bicarbonate ion react with excess H⁺ to form H₂CO₃ (carbonic acid, a weak electrolye). However, carbonic acid can only exist at very low concentrations. Under normal circumstances, carbonic acid decomposes into CO₂ and H₂O. Therefore, carbon dioxide and water are the normal products whenever carbonate or bicarbonate react with excess acid. Here are some examples:

Mixing solutions of NaHCO₃ and HCl:

step 1: \[\ce{HCO3^-}(aq)+\ce{H^+}(aq)\rightarrow \ce{H2CO3}(aq)\]

step 2: \[\ce{H2CO3}(aq)\rightarrow \ce{H2O}(l)+\ce{CO2}(g)\]

Or, overall: \[\ce{HCO3^-}(aq)+\ce{H^+}(aq)\rightarrow \ce{H2O}(l)+\ce{CO2}(g)\]

Adding a solution of HCl to solid CaCO₃ (with the acid in excess):

step 1: \[\ce{CaCO3}(s)+\ce{H^+}(aq)\rightarrow \ce{Ca^2+}(aq)+\ce{HCO3^-}(aq)\]

step 2: \[\ce{HCO3^-}(aq)+\ce{H^+}(aq)\rightarrow \ce{H2CO3}(aq)\]

step 3: \[\ce{H2CO3}(aq)\rightarrow \ce{H2O}(l)+\ce{CO2}(g)\]

Overall: \[\ce{CaCO3}(s)+\ce{2H^+}(aq)\rightarrow \ce{Ca^2+}(aq)+\ce{H2O}(l)+\ce{CO2}(g)\]

Mixing solutions of HC₂H₃O₂ and K₂CO₃ (with the acid in excess):

step 1: \[\ce{CO3^2-}(aq)+\ce{HC2H3O2}(aq)\rightarrow \ce{C2H3O2^-}(aq) + \ce{HCO3^-}(aq)\]

step 2: \[\ce{HCO3^-}(aq)+\ce{HC2H3O2}(aq) + \rightarrow \ce{C2H3O2^-}(aq)+\ce{H2CO3}(aq)\]

step 3: \[\ce{H2CO3}(aq)\rightarrow \ce{H2O}(l)+\ce{CO2}(g)\]

Overall: \[\ce{CO3^2-}(aq)+\ce{2HC2H3O2}(aq)\rightarrow\ce{2C2H3O2^-}(aq)+\ce{H2O}(l)+\ce{CO2}(g)\]

You can see an example of this kind of reaction for yourself by mixing baking soda (NaHCO₃) and vinegar (a solution of acetic acid) in your kitchen. The mixture will bubble vigorously as gaseous carbon dioxide forms.