Solubility and Association of Ions

PURPOSE OP EXPERIMENT Perform a number of chemical reactions involving salts in aqueous solutions to study the effect of formation of a sparingly soluble solid, a sparingly soluble gas, or a weak electrolyte as driving forces for chemical reactions. 

When several kinds of positive ions (cations) and several kinds of negative ions (anions) are mixed in aqueous solution, there are a number of different ways in which the cations can combine with the anions to produce different crystalline ionic compounds. The actual association of ions that takes place when a solid separates from a solution at a particular temperature will depend upon the concentration of the ions and the solubilities of the various ionic compounds that can be formed. The first salt for which the concentrations of its ions exceed its solubility limit at the given temperature will normally be the first one to crystallize from solution. These considerations are important when we wish to prepare ionic compounds by crystallization from solutions containing the proper ions. One objective of this experiment is to investigate the factors involved in the association of ions by studying a reaction of two water-soluble salts that place four different ions in solution and produce two other water-soluble salts. 

A large proportion of one or more product ions is removed from solution in one of three ways, each of which is illustrated below by a complete equation, a complete ionic equation, and a net ionic equation. 

Formation of a sparingly soluble solid CuS04(aq) + 2 NaOH(aq) ---- Cu(OH)2(s) + Na2S04(aq) 

In this experiment you will start with two ionic solids, nickel(II) nitrate hexahydrate, Ni(NO3)2 6H2O, and potassium chloride, KCl. You will dissolve these solids in water and use either a temperature change or a concentration change by evaporation to crystallize relatively soluble salts from the aqueous solution. The solubilities at different temperatures of the four possible compounds involved are given in TABLE 19.1. 

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