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17-5: Precipitation Reactions

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    Precipitation reactions

    \[\underbrace{ Ag^+(aq) + Cl^-(aq) \leftrightharpoons AgCl(s)}_{\text{Precipitation of silver chloride}}\]

    are the opposite of solubility expressions.

    \[\underbrace{AgCl(s) \leftrightharpoons Ag^+(aq) + Cl^-(aq)}_{\text{Dissolution of silver chloride}} , \; \; K_{sp} =[Ag^+][Cl^-]\]

    Therefore solubility constants can be used to determine if a precipitate will form. Although we could write these as precipitation reactions (in which case Kppt = 1/Ksp ), we will use solubility expressions and state that a precipitate will form for a "Product Loaded System," (when reactants are formed), that is, when the back reaction occurs. Review section 15.2.5, the Reaction Quotient.

    Ksp and Q

    Q, the reaction quotient is the equilibrium constant expression at any condition, and a review of 15.2.5, shows there are three possibilities.

    • Q=K, No reaction, system is at equilibrium
    • Q>K, Product loaded, reactants are produced until Q=K
    • Q<K, Reactant loaded, products are produced until Q=K

    A precipitate forms if Q>Ksp .

    Exercise \(\PageIndex{1}\)

    The solubility product of calcium fluoride (CaF2) is 3.45 × 10−11. If 2.0 mL of a 0.10 M solution of NaF is added to 128 mL of a 2.0 × 10−5M solution of Ca(NO3)2, will CaF2 precipitate?


    \(Q=[Ca^{+2}][F^-]^2 \)

    \[[Ca^{+2}]=\frac{n_{Ca^{+2}}}{V_T}=\frac{0.128L\left ( 2.0x10^{-5}M \right )}{0.128L+.002L}=1.97x10^{-5}M\]

    \[[^{F-}]=\frac{n_{F^{-}}}{V_T}=\frac{0.002L\left ( 0.10M \right )}{0.128L+.002L}=1.54x10^{-3}M\]


    Yes because Q>Ksp , but not much, as it is very close.



    Robert E. Belford (University of Arkansas Little Rock; Department of Chemistry). The breadth, depth and veracity of this work is the responsibility of Robert E. Belford, You should contact him if you have any concerns. This material has both original contributions, and content built upon prior contributions of the LibreTexts Community and other resources, including but not limited to: