Lecture Notes from CHM 1341
25 June 1996

Precipitation Reactions

Solubilities (Atkins/Beran)

CLASSES OF SOLUBLE COMPOUNDS CLASSES OF INSOLUBLE COMPOUNDS
compounds of Group I elements. carbonates (CO32- ), phosphates (PO43- ), oxalates (C2O42- ), and chromates (CrO42- ) EXCEPT Group I elements and NH4+.
ammonium (NH4+) compounds. sulfides (S2-) EXCEPT Group I, II, and NH4+.
chlorides (Cl-), bromides (Br-), and iodides (I-), EXCEPT Ag+, Hg22+ , and Pb2+. hydroxides (OH-) and oxides (O2-) EXCEPT Group I and II and NH4+.§
nitrates (NO3-), chlorates (ClO4-), perchlorates (ClO4-), and acetates (CH3CO22- ).
sulfates (SO42- ) EXCEPT Sr2+, Ba2+, and Pb2+. § Ca(OH)2 and Sr(OH)2 are slightly soluble. Mg(OH)2 is very slightly soluble.

Precipitates are solids which separate themselves from solution because their concentration in the aqueous phase exceeds their solubility limit. When they do so, it's not like making rocks; the precipitates grow from dispersed molecules, so they don't just lump up and drop out. As a consequence, there's fairly wide variety of morphologies of precipitates from granular ones (e.g., sulfides) easily filtrated to flocculant ones (e.g., aluminum trihydroxide) so dispersed that their particle size is comparable to the wavelengths of visible light which they then scatter in the same mechanism which renders the sky blue.

Gillespie et al. make an arbitrary assignment of solubility limits, calling those in excess of 0.1 moles/L "soluble," those below 0.01 moles/L "insoluble," and the rest "partially soluble." Actually any chemist's judgement of solubility sort of depends upon whether the precipitate is an interesting product or a contaminant. Hoped for reaction yields and critical purities will then determine if a precipitate is insufficiently or too soluble.

Often, however, precipitate is taken as a substance which separates fairly quantitatively from its reaction mixture. Indeed, it is that very separation which gives impetus to the reaction, "driving" the equilibrium which, via the equivalence of forward and reverse reaction rate expressions, is dependent upon the (solution) concentration of the product above its precipitate. So "precipitation" as a reaction category is another in the vein of "redox" or "acid/base" which refers to the motive force impelling the reaction to proceed.

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Chris Parr
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Last modified 25 June 1996.