Endothermic Reaction
Solid barium hydroxide octahydrate (Ba(OH)2.8H2O) and ammonium thiocyanate (NH4SCN) are mixed in a beaker, which is then placed on a few drops of water on a small board. The reaction that occurs is highly endothermic, and it causes the beaker to freeze to the board. The low temperature (below 0 oC) is read by a digital thermometer. The gas produced by the reaction is shown to be basic by the use of pH paper. The equation for the reaction is given.
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Keywords
acid-base reaction, endothermic process
Multimedia
Cold Enough to Freeze Water
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(QuickTime 3.0 Sorenson, duration 42 seconds, size 3.3 MB)
Ammonium thiocyanate is mixed with barium hydroxide. The mixture is stirred and the two solids react. A slush is formed because water is one of the products of the reaction. A drop of water is placed on a block of wood and the beaker is placed on the water drop. The reaction is endothermic, that is, it takes in heat. As a result, the bottom of the beaker becomes cold enough to freeze the water and stick to the wood.
 Ammonium thiocyanate is mixed with barium hydroxide. |
 A drop of water is placed on a block of wood. |
 The bottom of the beaker becomes cold enough to freeze the water and stick to the wood. |
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Temperature is Measured
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(QuickTime 3.0 Sorenson, duration 52 seconds, size 4 MB)
An electronic thermometer is used to determine that the reactants in this experiment are at room temperature. Ammonium thiocyanate is mixed with barium hydroxide. The mixture is stirred and the two solids react. A slush is formed because water is one of the products of the reaction. After the reaction, the temperature is again measured. The reaction is endothermic, that is, it takes in heat. The temperature is now negative 25 degrees Celsius. (When the temperature is measured it has dropped to between negative 20 and negative 30 degrees Celsius so the temperature is well below the freezing point of water. The equation for the reaction shows that barium thiocyanate, ammonia gas and water are formed by the reaction of the solid barium hydroxide and the solid ammonium thiocyanate.)
 Ammonium thiocyanate is mixed with barium hydroxide. |
 The mixture is stirred. Water is one of the products of the reaction. |
 The reaction is endothermic. The temperature has dropped. |
Additional still images for this movie
Resulting Gas is Basic
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(QuickTime 3.0 Sorenson, duration 31 seconds, size 2.1 MB)
Ammonium thiocyanate is mixed with barium hydroxide. The mixture is stirred and the two solids react. A slush is formed because water is one of the products of the reaction. pH paper is moistened and held over the reaction products. The color indicates that a basic gas has been formed.
 pH paper is moistened... |
 ...and held over the reaction products. |
 The color indicates that a basic gas has been formed. |
 pH paper color scale |
Additional still images for this movie
Discussion
When the temperature is measured it has dropped to between negative 20 and negative 30 degrees Celsius so the temperature is well below the freezing point of water. The equation for the reaction shows that barium thiocyanate, ammonia gas, and liquid water are formed by the reaction of the solid barium hydroxide and the solid ammonium thiocyanate.
The equation for the reaction is:
Ba(OH)2.8H2O(s ) + 2 NH4SCN(s ) --> Ba(SCN)2(s ) + 10 H2O(l ) + 2 NH3(g )
Demonstration Notes, Warnings, Safety Information, etc.
Exam and Quiz Questions
1. Based on the videos, predict products for the reaction of Ba(OH)2.8H2O + NH4SCN
2. Write a balanced equation for the reaction of Ba(OH)2.8H2O + NH4SCN
3. What observable evidence indicates that a chemical reaction has taken place?