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Chemwiki

Endothermic Reactions-- The Enthalpy of Solution

Chemical Concepts Demonstrated

Demonstration

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

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