In the Classroom edited by
Overhead Projector Demonstrations
Doris K. Kolb Bradley University Peoria, IL 61625
Demonstrating Heat Changes on the Overhead Projector with a Projecting Thermometer Chinhyu Hur, Sally Solomon,* and Christy Wetzel Department of Chemistry, Drexel University, Philadelphia, PA 19104 Abstract Demonstrating of heat changes is done on the overhead projector by illuminating the red fluid in a nonmercury thermometer using a simple andinexpensive apparatus. The setup, which consists of an alcohol thermometer inserted in a 50-mL plastic tissue culture dish, is placed on a transparency slide marked with divisions that match the readings on the thermometer. Demonstrations that provide dramatic heat changes are described, including chemical reactions, dissolving of solutes and simulation of flameless ration heaters and commercial hot and cold packs. Keywords Demonstrations Calorimetry Solutions/Solvents Thermodynamics Teaching/Learning Aids Supplementary Materials No supplementary material available.
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JChemEd.chem.wisc.edu • Vol. 75 No. 1 January 1998 • Journal of Chemical Education
Abstract
In the Classroom edited by
Overhead Projector Demonstrations
Doris K. Kolb Bradley University Peoria, IL 61625
Demonstrating Heat Changes on the Overhead Projector with a Projecting Thermometer Chinhyu Hur, Sally Solomon,* and Christy Wetzel Department of Chemistry, Drexel University, Philadelphia, PA 19104 Demonstrating heat changes in a lecture setting can be done by passing around the container in which the heating or cooling takes place. However, this is inconvenient and often dangerous because of the risk of burns or exposure to noxious reaction products. Instead, heat changes can be shown on the overhead projector using a simple and inexpensive apparatus. Heat evolution can be observed on the overhead projector by noticing vapors produced by the sublimation of iodine in contact with the reaction vessel (1). Projecting digital thermometers have been adapted for use on the overhead projector (2). Another very convenient way to observe heat changes on the overhead projector is by illuminating the red fluid in a nonmercury thermometer. Described below is a setup for doing this and a collection of demonstrations involving heat changes.
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Figure 1. A tissue culture flask with a nonmercury thermometer is used to measure heat changes on the overhead projector.
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Apparatus If the inner core of a nonmercury thermometer through which the red liquid expands and contracts is transparent, it can be projected on the overhead.1 All the experiments included here can be done with a thermometer that has a scale from { 20 to 110 °C. A convenient reaction vessel to use is a 50-mL plastic tissue culture dish, a size that requires relatively small amounts of reagents. A convenient quantity of 20 of these flasks is available from Flinn Scientific.2 To create the reaction vessel, one of the large surfaces of the plastic culture flask is removed, leaving the neck intact. An easy way to do this is to poke a hole in the top with a sharp instrument, then carefully remove the fragments. (If the neck is canted, it should be slanted up; otherwise liquid filling the flask will spill onto the overhead stage.) The thermometer, held in place by a cork inserted into the neck, is rotated to provide the best image of the red liquid. Because of the canted neck the thermometer will be at an angle to the overhead stage, a position that places the thermometer bulb near the bottom of the reaction vessel. The flask can be secured to the projector with a piece of masking or duct tape. Under the dish is placed a transparency slide marked with divisions that match the readings on the thermometer. The slide can be adjusted so that the temperature read on the thermometer is the same as the one projected. A diagram of the apparatus is shown in Figure 1. Demonstrations The demonstrations described below were chosen because they provide dramatic heat changes resulting from chemical reactions, dissolving of solutes, and simulation of commercial hot and cold packs. In each case the initial ma*Corresponding author.
terials are placed in the culture flask apparatus and the thermometer adjusted so that the projected temperature reading is the same as the actual thermometer reading. The quantities of materials used are appropriate for a 50-mL culture flask.
Chemical Reaction: Endothermic The combination of two solids, Ba(OH)2 ? 8H 2O and NH4 SCN, produces a substantial temperature drop (3). Ba(OH) 2 ? 8 H2O (s) + 2 NH4SCN(s) → Ba(SCN)2 (aq) + 2 NH3 (aq) + 10 H2O (l) About 4–5 g of ammonium thiocyanate and 8–10 g of the hydrated barium hydroxide are placed in the reaction vessel in such a way that the solid mixture is in contact with the thermometer bulb. For best results the barium hydroxide octahydrate must be fresh and “wet looking”. After a little mixing, the solids combine to form an aqueous mixture while the temperature drops as low as { 20 °C. Substituting NH4 NO3 for the thiocyanate gives a smaller drop in temperature to about {5 °C.
Chemical Reaction: Exothermic When aqueous solutions of hydroxylamine hydrochloride and sodium nitrite are mixed there is considerable evolution of heat (4). NaNO3 (s) + H2NOH ? HCl(s) → NaCl(s) + 2H2O(g) + N 2O(g) ∆H = {136.5 kJ/mol Two aqueous solutions, H2 NOH ? HCl (1 g /5 mL) and NaNO3 (1 g / 5 mL), are mixed by pouring them over the bulb of the thermometer. The flask need not be filled; using more
JChemEd.chem.wisc.edu • Vol. 75 No. 1 January 1998 • Journal of Chemical Education
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In the Classroom reactants will cause the mixture to overflow the container. Bubbling begins at once and becomes increasingly vigorous as the temperature soars more than 40° in less than 30 s. Another heat-producing process is the reaction of magnesium with water, the basis of a product known as a Flameless Ration Heater or FRH (5). Mg (s) + 2 H2O (l) → Mg (OH) 2 (s) + H 2 (g) ∆H = {351 kJ/ mol Mg The addition of iron and chloride ion increases the rate of this reaction significantly. One way to simulate what happens in an FRH is by embedding iron into a magnesium ribbon (6). Wearing gloves, expose the shiny metal surface of a 10-cm piece of magnesium ribbon by holding the ribbon in one hand and scraping (and curling) with a knife held in the other. The magnesium strip is then placed in a plastic bag with fine (50 mesh) iron filings and pounded until the mass of the strip nearly doubles. The Mg/Fe ribbon is coiled and wrapped around the bulb of the thermometer. Once the culture dish apparatus is in place, a pinch of salt and a few milliliters of water (no more) will initiate the reaction, which produces bubbles of hydrogen and a temperature increase within a minute or two of anywhere from 20 °C to 40 °C, depending upon how successfully the iron and magnesium have been combined.
Heat of Solution Heats of solution are measured for various compounds. About 0.04 mol of each solute is dissolved in 30 mL of water at room temperature, then stirred to help dissolution. The most dramatic heat of solution observed is for anhydrous calcium chloride, which has a ∆H of {81.3 kJ/mol. (The CaCl2 ?2H2O can be dehydrated by heating on a hotplate until about 20% of the mass is removed.) When 4 g of anhydrous CaCl2 is added to 30 mL of water a temperature increase of about 20 °C results.
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An equivalent amount of crushed pellets of sodium hydroxide (1.5 g), with a heat of solution of { 44 kJ/mol, produces an increase of about 10 °C. Dissolving ammonium nitrate in water is endothermic, with a ∆H of + 25.3 kJ/mol. For 3 g of NH4NO 3 in 30 mL of water the decrease in temperature is about 5 °C. The dissolving of calcium chloride is used in some brands of commercial hot packs, and dissolving of ammonium nitrate is used in some cold packs.
Freezing Point Depression The addition of solutes to cubed or crushed ice lowers the freezing point. The addition of about 1 part NaCl (15 g) to 2 parts ice (25–30 g) causes the temperature to decrease rapidly to { 5 °C and eventually as low as { 10 °C. Adding 10 mL of methanol to 25–30 g of ice produces a temperature drop of about 10 °C within a minute or two. Temperatures as low as { 78 °C can be reached using isopropyl alcohol (acetone will dissolve the flask) and dry ice. Dropping a few small pieces of dry ice into 25 mL of isopropyl alcohol will drop the temperature below {20 °C, the lowest measurable with common alcohol thermometers. Notes 1. Fisher brand Red Spirit thermometers, catalog number 14997, work very well. 2. Flinn Scientific Inc., P.O. Box 219, 131 Flinn St., Batavia, IL 060510; the catalog number is AP1449.
Literature Cited 1. 2. 3. 4. 5.
Boschmann, E. J. Chem. Educ. 1970, 47, A206. DuPre D. B.; Just, W. E. J. Chem. Educ. 1994, 71, 691. Hambley, E. J. Chem. Educ. 1969, 46, A55. Scoot, E. S. J. Chem. Educ. 1992, 69, 1028. Kuhn, W. E. Flexible Electrochemical Heater. U.S. Patent 4,522,190, June 11, 1985. 6. Scott, D.; Meadows, R. Chem Matters Feb. 1992, Feb, 12.
Journal of Chemical Education • Vol. 75 No. 1 January 1998 • JChemEd.chem.wisc.edu
