T515: Prep Notes

Introduction

Two dry solids --- barium hydroxide and ammonium nitrate --- are mixed to produce an endothermic reaction.  The temperature of the mixture drops approximately 45oC and is cold enough to freeze the reaction beaker to water on a block of wood

Equations

Ba(OH)2.8H2O(s) + 2NH4NO3(s)   →   Ba(NO3)2(aq) +  2NH3(aq) + 10H2O(l)

 

Supplies

1.         Prepare the following chemicals in labeled bottles:

    • 32g Ba(OH)2.8H2O (barium hydroxide octahydrate)
    • 17g NH4NO3 (ammonium nitrate) or 11g NH4Cl (ammonium chloride) or 16g NH4SCN (ammonium thiocyanate)

Note: If solids are not free-flowing, grind with a clean mortar and pestle, and pour back into the labeled bottle.

2.         Assemble the following equipment:

    • 250mL beaker
    • stirring rod
    • -38oC to +50oC thermometer
    • DI water
    • small wooden block
    • googles
    • gloves 

 

To Conduct Demonstration

    • Mix the two solids together in a 250mL beaker.  Stir for a few seconds with a stirring rod.  (Do not stir with the thermometer.)  The mixture should become moist from the water released by the Ba(OH)2.8H2O.  DO NOT ADD ANY WATER TO THE BEAKER.
    • Place a drop or two of water on the wooden block.  Place the container on top of the block of wood, covering the water.
    • Place the thermometer in the beaker.
    • The mixture will become extremely cold and freeze the reaction beaker to the damp block of wood.  The entire block can be lifted up by lifting the beaker.
    • This reaction releases NH3 gas.  Students may be able to detect the odor if they are close enough.
    • Read the final temperature from the thermometer.

 Demo Time: 10 – 15  minutes

Safety and Disposal

Barium salts are toxic.  Avoid inhaling ammonia vapors.

Dispose of waste in the barium waste carboy or the heavy metals waste carboy.