`
`
`
`Chemical
`Demon
`
`ions
`
`A Handbook for
`
`Teachers of Chemistry
`
`
`
`Bassam Z. Shakhashiri
`
`auras‘
`
`THE UNIVERSITY OF WISCONSIN PRESS
`
`
`
`002
`
`002
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`004
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`
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`004
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`
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`1.2
`
`Chemical cold Pack
`
`
`
`First-aid cold packs demonstrate spontaneous endothermic reactions.
`
`MATERIALS
`
`Instant cold packs can be obtained from medical supply outlets. We use a Kwik
`Kold Instant Ice Pack [I] which contains ammonium nitrate and water.
`
`PROCEDURE
`
`The Kwik Kold Instant Ice Pack consists of an outer pouch containing solid
`ammonium nitrate and an inner pouch containing water with a blue dye. Firmly
`squeeze the inner pouch to release the water and mix it with the ammonium nitrate.
`Since the mixture is well sealed in the outer pouch. the pack can be passed around the
`class.
`
`HAZARDS
`
`Since the cold pack reaches temperatures significantly lower than body tempera-
`ture. prolonged application to the body should be avoided.
`This mixture of chemicals is not harmful. Ammonium nitrate is a very strong
`oxidant in dry form and can initiate combustion in flammable materials. If the outer
`pouch breaks. its contents should be flushed down the drain with water. Spills should
`be absorbed with a cloth or sponge. which should then be thoroughly rinsed with
`water.
`
`DISPOSAL
`
`The sealed pack should be discarded in a waste container. If the pack breaks. its
`contents should be flushed down the drain with water.
`
`DISCUSSION
`
`When the inner pouch containing the water is broken. the ammonium nitrate
`dissolves. This process absorbs heat from the materials present and causes the temper-
`ature of the system to drop. The reaction is
`
`NI-l.NO,(s)—-> Nll.‘(aq) + NO,'(aq)
`
`
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`
`
`005
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`005
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`1.2 Chemical cold Pack
`
`This reaction absorbs 26.2 Id/mole or approximately 326 J/g of ammonium nitrate
`[2]. The standard heat of formation values. AH?. for NH.NO,(s) and NH.NO,(aq) are
`— 365.6 Id/mole and -339.4 Id/mole. respectively [2]. Recorded values for the sol-
`ubility of ammonium nitrate vary sharply. ranging from ll8.3 g/I00 ml [3, 4] to 50
`g/lm ml [5] in water at 0“C. Our observations are consistent with the lower value. In
`the commercial cold pack system used. 2l4 g of NH.N0, were present in the outer
`pouch and 220 ml of water in the inner pouch. We mixed these in a Dewar flask with
`a heat capacity of 42 J/deg and obtained a temperature of - 7°C. Significant amounts
`of undissolved solid remained.
`
`REFERENCES
`
`I. Kwik Kold Division of Kay Laboratories. Inc.. Moberly. Missouri 65270; patent numbers
`2 925 7l9 and 3 I75 558.
`
`2. "Selected Values of Chemical Therrnodynarnic
`Technical Note 270-I; p 67.
`3. Weast. R. C.. Ed. “CRC Handbook of Chemistry and Physics." 49th ed.: CRC Press:
`Cleveland. Ohio. 1968: pp D-38. B-I76.
`4. Kaufman. J. A. 1. Client. Educ. I970. 47. 518.
`
`Natl. Bur. Stand. (U.S.) I965:
`
`of Inorganic and Metal Organic Compounds." 4th ed.; American
`5. Linke. W. F.
`Chemical Society: Washington. D.C.. 1965: Vol. II. pp 708-9.
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`006
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`006