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6.3: Precipitation Reactions

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    Skills to Develop

    • Define precipitation reactions
    • Classify chemical reactions as one of these three types given appropriate descriptions or chemical equations
    • Write and balance precipitation equations in molecular, total ionic, and net ionic formats.

    Faced with a wide range of varied interactions between chemical substances, scientists have found it convenient (or even necessary) to classify chemical interactions by identifying common patterns of reactivity. The following three modules will provide an introduction to three of the most prevalent types of chemical reactions: precipitation, acid-base, and oxidation-reduction.

     

    Precipitation Reactions and Solubility Rules

    A precipitation reaction is one in which dissolved substances react to form one (or more) solid products. Many reactions of this type involve the exchange of ions between ionic compounds in aqueous solution and are sometimes referred to as double displacement, double replacement, or metathesis reactions. These reactions are common in nature and are responsible for the formation of coral reefs in ocean waters and kidney stones in animals. They are used widely in industry for production of a number of commodity and specialty chemicals. Precipitation reactions also play a central role in many chemical analysis techniques, including spot tests used to identify metal ions and gravimetric methods for determining the composition of matter (see the last module of this chapter).

    The extent to which a substance may be dissolved in water, or any solvent, is quantitatively expressed as its solubility, defined as the maximum concentration of a substance that can be achieved under specified conditions. Substances with relatively large solubilities are said to be soluble. A substance will precipitate when solution conditions are such that its concentration exceeds its solubility. Substances with relatively low solubilities are said to be insoluble, and these are the substances that readily precipitate from solution. More information on these important concepts is provided in the text chapter on solutions. For purposes of predicting the identities of solids formed by precipitation reactions, one may simply refer to patterns of solubility that have been observed for many ionic compounds (Table \(\PageIndex{1}\)).

    Table \(\PageIndex{1}\): Solubilities of Common Ionic Compounds in Water
    Soluble compounds contain Exceptions to these solubility rules include
    • group 1 metal cations (Li+, Na+, K+, Rb+, and Cs+) and ammonium ion \(\left(\ce{NH4+}\right)\)
    • the halide ions (Cl−, Br−, and I−)
    • the acetate \(\ce{(C2H3O2- )}\), bicarbonate \(\ce{(HCO3- )}\), nitrate \(\ce{(NO3- )}\), and chlorate \(\ce{(ClO3- )}\) ions
    • the sulfate \(\ce{(SO4- )}\) ion
    • halides of Ag+, \(\ce{Hg2^2+}\), and Pb2+
    • sulfates of Ag+, Ba2+, Ca2+, \(\ce{Hg2^2+}\), Pb2+, and Sr2+
    Insoluble compounds contain Exceptions to these insolubility rules include
    • carbonate \(\ce{(CO3^2- )}\), chromate \(\ce{(CrO4^2- )}\), phosphate \(\ce{(PO4^3- )}\), and sulfide (S2−) ions
    • hydroxide ion (OH−)
    • compounds of these anions with group 1 metal cations and ammonium ion
    • hydroxides of group 1 metal cations and Ba2+

    A vivid example of precipitation is observed when solutions of potassium iodide and lead nitrate are mixed, resulting in the formation of solid lead iodide:

    \[\ce{2KI}(aq)+\ce{Pb(NO3)2}(aq)\rightarrow \ce{PbI2}(s)+\ce{2KNO3}(aq)\]

    This observation is consistent with the solubility guidelines: The only insoluble compound among all those involved is lead iodide, one of the exceptions to the general solubility of iodide salts.

    The net ionic equation representing this reaction is:

    \[\ce{Pb^2+}(aq)+\ce{2I-}(aq)\rightarrow \ce{PbI2}(s)\]

    Lead iodide is a bright yellow solid that was formerly used as an artist’s pigment known as iodine yellow (Figure \(\PageIndex{1}\)). The properties of pure PbI2 crystals make them useful for fabrication of X-ray and gamma ray detectors.

    Video \(\PageIndex{1}\):  A precipitate of PbI2 forms when solutions containing Pb2+ and I− are mixed. Credit: MrLundScience

    The solubility guidelines in Table \(\PageIndex{1}\) may be used to predict whether a precipitation reaction will occur when solutions of soluble ionic compounds are mixed together. One merely needs to identify all the ions present in the solution and then consider if possible cation/anion pairing could result in an insoluble compound. For example, mixing solutions of silver nitrate and sodium fluoride will yield a solution containing Ag+, \(\ce{NO3-}\), Na+, and F− ions. Aside from the two ionic compounds originally present in the solutions, AgNO3 and NaF, two additional ionic compounds may be derived from this collection of ions: NaNO3 and AgF. The solubility guidelines indicate all nitrate salts are soluble but that AgF is one of the exceptions to the general solubility of fluoride salts. A precipitation reaction, therefore, is predicted to occur, as described by the following equations:

    \[\ce{NaF}(aq)+\ce{AgNO3}(aq)\rightarrow \ce{AgF}(s)+\ce{NaNO3}(aq)\hspace{20px}\ce{(molecular)}\]

    \[\ce{Ag+}(aq)+\ce{F-}(aq)\rightarrow \ce{AgF}(s)\hspace{20px}\ce{(net\: ionic)}\]
     

    Example \(\PageIndex{1}\): Predicting Precipitation Reactions

    Predict the result of mixing reasonably concentrated solutions of the following ionic compounds. If precipitation is expected, write a balanced net ionic equation for the reaction.

    1. potassium sulfate and barium nitrate
    2. lithium chloride and silver acetate
    3. lead nitrate and ammonium carbonate

    Solution

    (a) The two possible products for this combination are KNO3 and BaSO4. The solubility guidelines indicate BaSO4 is insoluble, and so a precipitation reaction is expected. The net ionic equation for this reaction, derived in the manner detailed in the previous module, is

    \[\ce{Ba^2+}(aq)+\ce{SO4^2-}(aq)\rightarrow \ce{BaSO4}(s) \nonumber\]

    (b) The two possible products for this combination are LiC2H3O2 and AgCl. The solubility guidelines indicate AgCl is insoluble, and so a precipitation reaction is expected. The net ionic equation for this reaction, derived in the manner detailed in the previous module, is

    \[\ce{Ag+}(aq)+\ce{Cl-}(aq)\rightarrow \ce{AgCl}(s) \nonumber\]

    (c) The two possible products for this combination are PbCO3 and NH4NO3. The solubility guidelines indicate PbCO3 is insoluble, and so a precipitation reaction is expected. The net ionic equation for this reaction, derived in the manner detailed in the previous module, is

    \[\ce{Pb^2+}(aq)+\ce{CO3^2-}(aq)\rightarrow \ce{PbCO3}(s) \nonumber\]

    Exercise \(\PageIndex{1}\)

    Which solution could be used to precipitate the barium ion, Ba2+, in a water sample: sodium chloride, sodium hydroxide, or sodium sulfate? What is the formula for the expected precipitate?

    Answer

    sodium sulfate, BaSO4

     

    Double Displacement Reactions

    Precipitation reactions are an example of a double-displacement reaction. 

    In a double replacement reaction, which can also be referred to as a double displacement reaction, either the cationic or the anionic portions of two compounds exchange their relative positions.  The unique characteristic of a double replacement reaction is that the components that are repositioned must both exist within compounds.

    A double replacement reaction can be represented symbolically, as shown below.  

    AD+QZ→QD+AZAD+QZ→QD+AZ

    The following pattern also accurately reflects the description that is provided above.

    AD+QZ→AZ+QDAD+QZ→AZ+QD

    The inclusion of two letters within each of the symbolic representations that are shown above indicates that the reactants and products for both reactions are all compounds.  Furthermore, these equations share a common pair of reactants and products.  The only discernable difference between these patterns is the order in which the products, "QD" and "AZ," are written.  However, the order in which chemicals are written has no impact on the classification of a reaction, as long as the position of those substances is consistent, relative to the reaction arrow.

    A characteristic of a double-replacement equation is that there are two compounds as reactants and two different compounds as products. An example is

    \[\ce{CuCl2(aq) + 2AgNO3(aq) → Cu(NO3)2(aq) + 2AgCl(s)}\]

    There are two equivalent ways of considering a double-replacement equation: either the cations are swapped, or the anions are swapped. (You cannot swap both; you would end up with the same substances you started with.) Either perspective should allow you to predict the proper products, as long as you pair a cation with an anion and not a cation with a cation or an anion with an anion.

    Summary

    Video \(\PageIndex{2}\): A review of precipitation reactions.

    Chemical reactions are classified according to similar patterns of behavior. A large number of important reactions are included in three categories: precipitation, acid-base, and oxidation-reduction (redox). Precipitation reactions involve the formation of one or more insoluble products.

    Glossary

     
    insoluble
    of relatively low solubility; dissolving only to a slight extent
    precipitate
    insoluble product that forms from reaction of soluble reactants
    precipitation reaction
    reaction that produces one or more insoluble products; when reactants are ionic compounds, sometimes called double-displacement or metathesis
    double-displacement reaction
    a type of reaction that occurs when the cations and anions switch between two reactants to form new products
    soluble
    of relatively high solubility; dissolving to a relatively large extent
    solubility
    the extent to which a substance may be dissolved in water, or any solvent
     

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