Page 2
Procedure:
1. Pour a small amount of water (enough to ensure good contact with the base of the Erlenmeyer flask) on
a square wooden board.
2. Place 50 g of barium hydroxide octahydrate and 25 g ammonium thiocyanate into the Erlenmeyer flask.
Place a rubber stopper in the flask. Swirl to mix.
3. Place the flask in the puddle of water on the board. (You can touch the outside of the flask but don't hold
it in your hand while performing the reaction.)
4. Lift the flask after a few minutes.
Expected Results:
As the two compounds in the flask are mixed, an endothermic reaction occurs, creating a freezing slush that will make the flask
bottom stick to the piece of moistened wood. The reaction vessel gets cold enough to freeze the water between the board and
the flask. Energy absorbed by the reaction freezes the water so that the board will be attached to the flask.
The solid barium hydroxide reacted with solid ammonium thiocyanate produces barium thiocyanate, ammonia gas, and liquid
water. This reaction gets down to -20°C or -30°C, which is more than cold enough to freeze water. The reaction proceeds according
to the following equation:
Ba(OH)2 • 8H2O(s) + 2 NH4SCN(
s
) —>Ba(SCN)2(
aq
) + 2NH3(
g
) +10 H2O(
l
)
Follow up/Extension:
• Ammonium nitrate can be a substitute for ammonium thiocyanate.
• Enthalpy and heat of reaction, the reaction between two solids.
• Demonstration of an endothermic reaction: dissolve ammonium chloride in water.
• Demonstration of an exothermic reaction: dissolve calcium chloride in water.
Have your students write the equations for these reactions.
Disposal/Clean-Up:
Open the flask in a fume hood and allow the ammonia to evaporate overnight.
The remaining contents can be placed in the science department's heavy metal waste container for proper disposal.
Freezer Flask Demonstration (continued)
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'
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science
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