Handwarmers (Demonstration)
(Reference: Chemfax No. 1933, Flinn Scientific, Inc., Batavia, IL, 1990.)
 

Thermodynamics : Exothermic : Endothermic : Heat : Cold
 

Purpose:
Illustration of reactions which release and absorb heat in an ordinary consumer product
 

Safety:
Warn students that hand warmer may get hot.  Instructor should wear goggles when preparing solutions.
 

Materials:
Pocket hand warmer (If you donít have a hot pack use NaC2H3O2 and water.)
Cold pack (If you donít have a cold pack use NH4NO3 or NH4Cl and water.)
 

Procedure:
About 50 g of sodium acetate can be dissolved in 5 mL water, then cooled and stimulated with a quick shake to recrystallize.  Similarly the ammonium nitrate can be mixed with equal parts water and temperature change may be observed.  Or if you have the hot and cold packs pass them around the room at room temperature, then activate them and pass around and explain
 

Explanation:
When chemical bonds are broken heat is released, which is what is occurring in the hot pack.  This is an example of an exothermic reaction- one in which heat is given off.  When a chemical change absorbs energy there is an endothermic reaction.  In the case of a cold pack ammonium nitrate is dissolving in water and in order to do so requires 26.2 kJ/mol of energy.  The drop in temp will be about 20° C when equal volumes of ammonium nitrate and water are mixed.
 

Clean-up:
Sodium acetate solution can be reused or flushed down drain with excess water.  Ammonium nitrate solution may be flushed.  Hot and cold packs may be disposed of in trash.
 
 

Ginger Chateauneuf, 2000.
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