1. What effect does the startine of the hae -tem~erature . - have on the final temperature of the system? (Because we are measuring T, a lower startingtemperature means a lower final temperature.) 2. If the CaCI2 is allowed to sit in an open jar in the room before being used, what effectwill that have on the fmal temoerature of the svstem?(The CaClois hvmoscodc and absarbs a lot of wate; The more water"it pi& up, ;he less heat is released as it goes into solution.) 3. What effectwould using 20 mesh CaC12have on the number of grams needed for a hot pack? (This form of CaC12 dissolves more slowly than 8 mesh and thus less heat is released at one time so the maximum temperature reached is lower.) 4. Haw does the working curve change if a different amount of water (e.g., 200 mL) is used? (Less CaC12 is needed to reach the desired maximum temperature, because more CaClJunit of water dissolves immediately. Some experimental results: 10 g calcium chloride in 100 mL of water e w e a AT of 13.5O C and in 200 mL of water gave a AT of 7 "C; 25 1:gave .\Tvalurs nf3;i.S 'and IS", r&pertivrly., 5 How does the extent uf'mixing nNect the final temperaturr? rThe hater the mixing, rhe more CaCI? dlssulves, and the larger the AT.) 6. How does the heat loss from the plastic compare to that of the glass? (The glass is much thicker and is thus a better insulator. Less heat is lost from the glass per unit of time.) 7. The heat of solution of CaCI2is -82.93 kJlmol. Assuming no loss of heat, what would he the final temperature of a system containing 10 g of the salt and 100 mL of water? (The molar mass of calcium chloride is 111. The beat released per gram is thus 82,930 (Jlmol)l111(glmol) = 747 Jlg of salt. The specific heat of water is 4.18 Jlg deg. One hundred milliliters of water weighs about 100 g; thus, the heat capacity of the water is 418 Jldeg. Ten grams of CaC12 will release about 7470 J; thus, a temperature C could be observed.) change of 74701418 = 17.9O For a discussion of hot and cold packs see Shakhashiri
(2). Acknowledgment I would like to thank the high school teachers who tested this experiment, especially those who carried it out with their students. I also would like to thank the NSF (TPE #90-55358) for its support in developing this experiment. Literature 1. Slorinaki, E. J.; Wolsey, W C.;Msaterton.W.L.CkmimlPnncipks in Ihelabomfory: Saundera: New York.1989. 2. ShaLhashin,B. 2. Chsmim1Demonstmfions:A Handbook for Zbeckrs of Chemistry; University of Wisconsin: Madison. W1;Vol. 1,pp (a) 3631SbI &9. 3. Consolidated Pmduds & Service Inc, Quiney, MA02169.
A Salty Experience in Experimental Design Thomas H. Bindel, Charles Mueldener, and Timothy C. Smiley Pomona Senior High School 8101 West Pomona Drive Arvada. CO 80005 We describe a laboratory experiment developed and used for several years in our first-year high school chemistry classes. The strength of the laboratory experiment is that it is a "minds-on" approach, as opposed to a "cookbook approach." Students are required to determine experimentally the molar solubility of sodium chloride.' No procedure and minimal help is provided. Thus, students are allowed to demonstrate their own creativity and independence. The use of sodium chloride lends itself extremely well to the desired situation. I t is inexpensive, readily available, nontoxic. nonhazardous. and the high water solubility allows for the experiment'al design t o k e uncomplicatedand not too difficult.
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Experiment Students work in pairs providing a n interactive situation. The students are provided with any reasonable equipment or glassware. Three 50-min periods are sufficient for most students to complete the experiment. In general, students have a saturated solution of the sodium chloride in their possession by the second period. By the end of the second period, they have calculated the molar solubility. The third period is used to conduct any final weighings that need to be done because chemicals must be dry prior to final weighings. Materials Water-softener salt is used instead of table salt, because table salt contains water-insoluble sodium silicoaluminate. Having the following equipment on hand will reduce the time the instructor must use in tracking down equipment:' Bunsen burner funnel ring stand and ring filter paper wire screen salt mortar and pestle watch glasses balance labeling tape beaker graduated cylinder spatula distilled water beaker tong stirring rod matches Results Several different exoerimental desiens were observed in solving the proposed'problem. students make conscious . thev mind and unconscious decisions. For e x a m ~ l eshould up the salt, stir the solution, weigh tLe salt, determine the volume of water to be used, use distilled water, or allow extra time to reach satufation. Students have difficulty in coming UD with some thin^ a s simple a s a saturated soiution. some groups add salt,&e gram a t a time, to a known volume of water until they "feel" no more will dissolve. Others heat the water (the quantity is known in some cases and not in others) to boiling and then add salt until it no longer dissolves. Still others take water (some measure it and others do not) and add to it a large quantity of salt (again, some measure the amount and others do not) and allow it to stand overnight. Caution. Goggles should be worn because of spattering problems with solutions. Afterwards, the saturated solutions or what are believed to be saturated. are filtered or decanted. Then some m o u ~ s weigh the dryresidue from the filtration, while others evaporate solutions to dryness and weigh the residue.
Discussion The high solubility of sodium chloride causes the volume of the aaueous solution to be quite different from the volume of water used. In other words, the molarity (MI and molality (m) differ significantly. The student must he able to recoenize and deal with this in the ~alculation.~ The kported molar solubility of sodium chloride is 5.326 M a t 20 "C (7)and 5.42 M a t 25 "C? We find the solubilitv to be 5.50 M (36.0 g1112 mL solution) or 6.16 m (36.0 g / l ~ b 'One could make the problem more challenging by having the students determine an approximate value for the 6,of sodium chloride (1-6). We calculate the thermodynamic K,, of sod~umchloride to be 39 - - 17b). , -, 2Some teachers may nor w sn lo have these items on vlew, so as not to in1 Lence tne st~dentsas to what to Lse 3We I no sbdents rea y earn how to mane mo ar so LI ons aher committing the solution volume mistake. 4Molarsolubility (8)= (317 g/L)/(58.44glmol)
mL water) at 25 "C. in close ameement with the reported value. We also ca~citatethe avirage molar solubili& (from 29 student pairs of data) to be 5.05 M.5 We feel this experiment is the most worthwhile exp&ment we do all . year,. and we would like to see more experiments of this type included in the science curriculum. %ome groups may not have saturated solutions.
Literature Cited 1.Koubek. E. J. Chem. Educ. 1993,70.155. 2. Carpenter, J . H J Chem.Educ. 1363,66,184. 3. ~ i t e s , ~pods, . ; J. S. ~ ; ~ h o m s a , ~J. . cch. m . ~ d u x988,48.66~. r 4. Martin, R. B.J Cham. E d u c 1988.63.411. 5. Russo, S. 0.:Hanania, G. I. H.J. Chem E d u c 1989.66.149. 6 . soeneer . ~~.J.N. J cham. E ~ U CI ~ Z69.182. 7. weat, R. C. Xandbmk of c h e m i s t j dnd physics. 61st ed.; chemical ~"bbercornpany: Cleveland, OH. 1978,(a1 D-262, (b) D75, D85. 8. Budauari,S. The M~rekInder,llthed.;MerckandCo.,lnc.: Rahway,NJ. 1989,MISC107.
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