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16.19: Predicting Precipitates Using Solubility Rules

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    Predicting the weather is tricky business. A thorough examination of a large amount of data is needed to make the daily forecast. Wind patterns, historical data, barometric pressure—these and many other data are fed into computers that then use a set of rules to predict what will happen based on past history.

    Solubility Rules

    Some combinations of aqueous reactants result in the formation of a solid precipitate as a product. However, some combinations will not produce such a product. If solutions of sodium nitrate and ammonium chloride are mixed, no reaction occurs. One could write a molecular equation showing a double-replacement reaction, but both products, sodium chloride and ammonium nitrate, are soluble and would remain in the solution as ions. Every ion is a spectator ion and there is no net ionic equation at all.

    It is useful to be able to predict when a precipitate will occur in a reaction. To do so, you can use a set of guidelines called the solubility rules (shown in Table \(\PageIndex{1}\)).

    Table \(\PageIndex{1}\): Solubility Rules for Ionic Compounds in Water
    Soluble Compounds containing the alkali metal ions \(\left( \ce{Li^+}, \: \ce{Na^+}, \: \ce{K^+}, \: \ce{Rb^+}, \: \ce{Cs^+} \right)\) and ammonium ion \(\left( \ce{NH_4^+} \right)\).
    Soluble Compounds containing the nitrate ion \(\left( \ce{NO_3^-} \right)\), acetate ion \(\left( \ce{CH_3COO^-} \right)\), chlorate ion \(\left( \ce{ClO_3^-} \right)\), and bicarbonate ion \(\left( \ce{HCO_3^-} \right)\).
    Mostly Soluble Compounds containing the chloride ion \(\left( \ce{Cl^-} \right)\), bromide ion \(\left( \ce{Br^-} \right)\), and iodide ion \(\left( \ce{I^-} \right)\). Exceptions are those of silver \(\left( \ce{Ag^+} \right)\), mercury (I) \(\left( \ce{Hg_2^{2+}} \right)\), and lead (II) \(\left( \ce{Pb^{2+}} \right)\).
    Mostly Soluble Compounds containing the sulfate ion \(\left( \ce{SO_4^{2-}} \right)\). Exceptions are those of silver \(\left( \ce{Ag^+} \right)\), calcium \(\left( \ce{Ca^{2+}} \right)\), strontium \(\left( \ce{Sr^{2+}} \right)\), barium \(\left( \ce{Ba^{2+}} \right)\), mercury (I) \(\left( \ce{Hg_2^{2+}} \right)\), and lead (II) \(\left( \ce{Pb^{2+}} \right)\).
    Mostly Insoluble Compounds containing the carbonate ion \(\left( \ce{CO_3^{2-}} \right)\), phosphate ion \(\left( \ce{PO_4^{3-}} \right)\), chromate ion \(\left( \ce{CrO_4^{2-}} \right)\), sulfide ion \(\left( \ce{S^{2-}} \right)\), and silicate ion \(\left( \ce{SiO_3^{2-}} \right)\). Exceptions are those of the alkali metals and ammonium.
    Mostly Insoluble Compounds containing the hydroxide ion \(\left( \ce{OH^-} \right)\). Exceptions are those of the alkali metals and the barium ion \(\left( \ce{Ba^{2+}} \right)\).

    For practice using the solubility rules, predict if a precipitate will form when solutions of cesium bromide and lead (II) nitrate are mixed.

    \[\ce{Cs^+} \left( aq \right) + \ce{Br^-} \left( aq \right) + \ce{Pb^{2+}} \left( aq \right) + 2 \ce{NO_3^-} \left( aq \right) \rightarrow ?\nonumber \]

    The potential precipitates from a double-replacement reaction are cesium nitrate and lead (II) bromide. According to the solubility rules table, cesium nitrate is soluble because all compounds containing the nitrate ion, as well as all compounds containing the alkali metal ions, are soluble. Most compounds containing the bromide ion are soluble, but lead (II) is an exception. Therefore, the cesium and nitrate ions are spectator ions and the lead (II) bromide is a precipitate. The balanced net ionic reaction is:

    \[\ce{Pb^{2+}} \left( aq \right) + 2 \ce{Br^-} \left( aq \right) \rightarrow \ce{PbBr_2} \left( s \right)\nonumber \]

    Summary


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