-,
JAMES 0.SCHRECK Univeffiily of Nonhern Colorado
filtrates e residues Heat of Solution: Hot Packs Henry 0. Daley, Jr. Bridgewater State College Bridgewater, MA 02325
An experiment done in many general chemistry laboratories is the measurement of heats of solution (1).The heat of solution of sodium hydroxide often is determined with students trying simply to reproduce the value found in the literature. A more interesting variation of this experiment relates heat of solution to commercial hot packs (2a)or cold packs (26). A commercial hot pack has a small inner bag containing water (117 mL). This is put into another bag with a f ~ e d amount of anhydrous calcium chloride (CaC12; 8 mesh). To activate the hot pack, the inner bag is crushed, and the water and CaClz are shaken together. The exothermic reaction occurring when the CaClz dissolves releases the heat needed to reach the desired tem~erature.Hot packs are available in either of two temperature ranges,120-130 "F (49-54") and 150-160°F ( 6 6 7 1 T 1 .Cold packs have a similar design with ammonium nltrate (NH,NO, used in place of the CaCll and a temperature ranxe of 37kil'F.
Greeley, CO 80639
Typical Data for Hot Packs Containing 117 mL Hz0
grams of CaCb
initial temperature (" C)
final temperature (" C)
AT
C)
7. Plot the maximum temperature of the system versus the
mass of solid used. (This is not a straight line.) From the maph, determine the number of grams of solid tc be used Fn
