Midas’ Magic

WARNING NOTICE The experiments described in these materials are potentially hazardous. Among other things, the experiments should include the following safety measures: a high level of safety training, special facilities and equipment, the use of proper personal protective equipment, and supervision by appropriate individuals. You bear the sole responsibility, liability, and risk for the implementation of such safety procedures and measures. MIT and Dow shall have no responsibility, liability, or risk for the content or implementation of any of the material presented. Midas’ Magic Abstract Two clear colorless solutions are mixed together resulting in the formation of a thick yellow precipitate. Materials Lead Nitrate Pb(NO3)2 Potassium Iodide KI 1000 mL beaker 2- 500 mL containers glass stir rod Safety Lead nitrate and potassium iodide should both be considered hazardous. Both chemicals can cause skin, eye and respiratory irritation. Prolonged exposure to lead nitrate can result in burns and can be toxic to the central nervous system and the peripheral nervous system. When working with these compounds work in a vented hood and avoid breathing in any dust particles. Procedure Preparation of Stock Solutions: 0.1M lead nitrate solution—dissolve 16.6 g Pb(NO3)2 in distilled water and dilute to 500 mL. 0.1M potassium iodide solution—dissolve 8.3 g KI in distilled water and dilute to 500 mL. 1 Presentation: To an empty 1000 mL beaker add about 100 mL of the first colorless solution Pb(NO3)2, and then about 100 mL of the second colorless solution KI and a dense yellow precipitate forms instantly. Discussion This experiment is dramatic in that it shows the creation of a colorful product from two colorless solutions. The reaction is a classic example of a precipitation reaction, which results in the formation of an insoluble product. In this example, when an aqueous solution of lead nitrate Pb(NO3)2 is added to an aqueous solution of potassium iodide KI, a yellow precipitate of lead iodide PbI2 is formed as well as some potassium nitrate KNO3 which stays in solution: Pb(NO3)2 (aq) + 2KI (aq) ------------- PbI2 (s) + KNO3 (aq) The reaction is an example of a metathesis reaction, which involves the exchange of ions between the Pb(NO3)2 and KI. The Pb+2 ends up going after the I- resulting in the formation of PbI2, and the K+ ends up combining with the NO3- forming KNO3. Ion NO3C2H3O2Cl-,Br-, I SO42CO32- and PO43OHS2- Na+, K+, and NH4+ General Solubility Rule All nitrates are soluble. All acetates are soluble (AgC2H3O2 only moderately.) All chlorides, bromides, and iodides are soluble except Ag+, Pb+, and Hg22+. (PbCl2 is slightly soluble in cold water and moderately soluble in hot water.) All sulfates are soluble except those of Ba2+, Pb2+, Ca2+ and Sr2+. All carbonates and phosphates are insoluble except those of Na+, K+, and NH4+. (Many acid phosphates are soluble.) All hydroxides are insoluble except those of Na+ and K+. Hydroxides of Ba2+ and Ca2+ are slightly soluble. All sulfides are insoluble except those of NA+, K+, NH4+, and those of the alkaline earths: Mg2+, Ca2+, Sr2+, and Ba2+. (Sulfides of Al3+ and Cr3+ hydrolyze and precipitate as the corresponding hydroxides.) All salts of sodium ion, potassium ion and ammonium ion are soluble except several uncommon ones. 2 A quick glance at a solubility table shows that all nitrates are always soluble and all iodides are soluble except those formed with Pb+2, Ag+ and Hg22+ Disposal The products of the reaction should be poured out into a properly labeled waste container for proper disposal. References Shakhashiri, B. Z. Chemical Demonstrations: A Handbook for Teachers of Chemistry; University of Wisconsin: Madison, WI, 1983; Vol. 1, pp. 286-292 3