Will A Precipitate Form? Will It Dissolve? The Role of Lecture Demonstrations in Facilitating the Introduction to Solubility Product Equilibria
Demonstration 2: Will a Precipitate Dissolve? (NiCOs) Principle
A precipitate will dissolve if the concentration (strictly speaking, activity) of either or both of its ions can be lowered to the point a t which
Submined by
Richard A. Pacer Indiana University-Purdue University Fort Wayne Campus, Fort Wayne, IN 46805 Checked by
Kathryn Williams University of Florida Gainesville, FL32611 The reauirement for precipitate formatiou (Q > K,,) is an example &a principle ihat dan he brought to life wlih suitable lecture demonstrations. As O'Hri~n( 1 1 has pointed out, students still at a concrete operational stage, whether in high school or college, find words alone to be insufficient for promoting conceptual learning. Over the past several decades, a number of solubility or solubility product demonstrations have appeared in this Journal. Several of these ( 2 4 )focus on the polarlnonpolar nature of solute and solvent (the "like dissolves like" rule). The advantages of precipitation from homogeneous solution. oRen notine the effect of relative su~ersaturationon the size and rhn~actcrist~cs of the crystall;ne solid formed. eauiis the focus of others 15-9,. The eftkt of'rom~etitive libria, primarily complex ion formation, in &lubility considerations is demonstrated frequently with species of silver(1) (9-13);other transition metals are used, such as cadmium (14) and iron (15), hut less frequently. The effect of oxidation on solubility considerations may be demonstrated, as in the precipitation of Fe(OH)z(15).Or the goal of a demonstration may be to challenge a solubility rule, such as "all nitrates are soluble", as in the precipitation of Ba(NO3I2(16). Relatively few demonstrations actually set out to calculate a K,, value. One particularly interesting one, taking advantage of the Tyndall effect using a Heme laser, is described by Nogar and Jalenak (17). For an extensive compilation of sources of science demonstrations, one should consult the article by Katz (18)that lists journals, books, and other sources suitable for preschool through college use. The demonstrations presented here complement the above. They are simple and safe to carry out, require little prelecture preparation time, and are particularly suitable for introductory courses. Each demonstration focuses on a certain principle. Following the demonstration, calculations are carried out and questions are posed to the class to enhance discussion. Demonstration 1: Will a Precipitate Form? (Pblz) Principle A precipitate will form if the reaction quotient (ion product), Q, exceeds the solubility product, K,,. Procedure Mix 10 mL of 2 x lo3 M Pb(NO& with 10 mL of 2 x lo3 M KI. Stir. Observe that no precipitate forms. Check from time to time and note that there is no evidence of any precipitate. Repeat, using 2 x lo-' M solutions. Obsewe that a yellow precipitate forms immediately. 'Presented before the Division of Chemical Education in a poster session at the ACS National Meeting in San Francisco, April 9, 1992 (PaperW85).
Q
