5.1 Endothermic Reactions--Ba(OH)₂ * H₂O + NH₄SCN

Chemical Concepts Demonstrated: Spontaneity, entropy

Demonstration:

Combine 16g of NH4SCN or NH4NO3 and 32 g of Ba(OH)2 * 8 H2O in a 150 mL beaker. Place the beaker on top of a wet wooden board.

Observations:

    The reaction causes the water around the beaker to freeze.  One can pick up the board by lifting the beaker because they are frozen together.

Explanation (include important chemical equation):

    The reaction is as follows:

    Ba(OH)₂ * 8 H₂O + 2 NH₄SCN -> Ba(SCN)₂ + 2 NH₃ + 10 H₂O

    The large increase in the entropy of the system (495 J/mol K) is a result of the large number of molecules present in the sum of the products.  This increase in entropy overcomes the endothermicity (102.2 kJ) of the system, so the reaction is spontaneous.  The endothermicity of the reaction takes thermal energy away from its surroundings, so much so that the water around the beaker will freeze as a result.