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Chemical
Refrigerator
When
most people think of chemical reactions they think of heat. Fuels
burning, fireworks exploding or even drain cleaner being poured
down the drain. In each case, heat production is both expected
and desirable. However, endothermic reactions, reactions which
consume heat, should also be expected and can be just as exciting.
The most exciting example of this that I have found is when solid
barium hydroxide and solid ammonium thiocyanate are mixed together
in a small flask.
Caution: This activity requires the use of hazardous substances
and has the potential for hazardous reactions. Carefully review
the Safety Precautions section and relevant Materials Safety Data
Sheets.
Materials:
- 10
g, Ammonium thiocyanate, NH4SCN
- 20
g, Barium hydroxide octahydrate, Ba(OH)2
· 8H20
- Laboratory
weighing paper
- 125
mL Erlenmeyer flask
- Thermometer,
if you have one graduated to at least -30oC
- Stirring
rod
Procedure:
- Weigh out 10 g of ammonium thiocyanate and 20 g of Barium
hydroxide using laboratory weighing paper.
- Transfer both to the 125 mL Erlenmeyer flask and mix with
a glass or plastic stirring rod.
- Within two or three minutes, the solids become liquid. A thermometer
placed in the mixture reveals the temperature falling far below
freezing. (Don't allow the temperature to fall below the last
graduation on your thermometer.) The irritating odor of ammonia
will be apparent but not overwhelming. Loosely stopper the flask
with a cork stopper to keep the odor down.
- Place a small six inch by six inch square of cardboard or
light wood on the demonstration desk. Pour one or two milliliters
of water on the square and sit the flask in the puddle. Now
the students will see just how "cool" this demonstration is;
the water will freeze the flask to the square.
Discussion:
This demonstration is a good way to introduce the concept of heat
of reaction. Chemical reactions may be either endothermic or exothermic.
This reaction is strongly endothermic absorbing so much heat that
the water freezes.
The
reaction between the solids is:
NH4SCN(s)
+ Ba(OH)2 · 8H2O(s)
+ heat → Ba(SCN)2(aq) + 2NH3(aq)
+ 10H2O(l)
Safety
Precautions:
Barium salts are toxic by ingestion. Ammonium thiocyanate is also
toxic by ingestion. Wear chemical splash goggles, chemical-resistant
gloves and a chemical-resistant apron when working with these
chemicals.
Use
caution when handling the cold flask. Use tongs if available.
The temperatures involved are cold enough to freeze skin. Ammonia
vapor is very irritating to the eyes and respiratory tract. Do
not inhale the gas.
Disposal:
Consult a reliable reference manual for appropriate disposal methods.
Reference:
Shakhashiri, B. Z., Chemical Demonstrations: A Handbook for
Teachers of Chemistry; University of Wisconsin: Madison,
WI. 1983; Vol. 1, pp. 10-12
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