21.3 Endothermic Reactions-- The Enthalpy of Solution

Chemical Concepts Demonstrated: Enthalpy, entropy,

Demonstration:

Combine 100g of NH4NO3 and 50 mL of H2O in a 250 mL beaker equipped with a thermometer.

Observations:

    The temperature lowers considerably.  In the video, the initial temperature is 27.4 degrees Celsius, and this lowers to 7.6 degrees Celsius.

Explanation:

    The enthalpy of solution of NH₄NO₃ is 25.7 kJ/mol.  Heat is therefore absorbed from the surroundings as the salt dissolves in water.  Naturally, this heat absorption lowers the temperature of the beaker and its surroundings, so the thermometer registers a lower temperature.

    Commercial ice packs that utilize this reaction contain 4.4 moles of water for each mole of NH₄NO₃.  When these packs are activated, the temperature of the ice pack drops to a minimum of -11 degrees Celsius.