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Calculating changes in heat within a system

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Enthalpy and Handwarmers

Cold Hands? Try Enthalpy

Recall that enthalpy is the measurement of the heat content of system under constant pressure. Scientists measure the change in enthalpy when reactants are converted into products in a chemical system to find out the change in energy. Also remember that enthalpy is an extensive property, and can be influenced by different factors such as the amount of materials or state of the materials.

Credit: Milad Mosapoor
Source: http://commons.wikimedia.org/wiki/File:LG_refrigerator.jpg
License: CC BY-NC 3.0

Even your refrigerator uses enthalpy. [Figure1]

Change in enthalpy is useful for measuring many processes in the real world. Refrigerators chill food by evaporating refrigerants such as freon or R134a. The enthalpy of vaporization in the refrigerator (the energy change of liquid to gas) is equivalent to how chilled your food is!

Imagine you go to watch a football game in the middle of winter. Your hands are cold, but luckily you brought along a chemical heat pack. You shake the pack several times until it starts heating your hands. This is an example of change in enthalpy.

Credit: User: Scotto Bear
Source: http://farm5.staticflickr.com/4054/4543988492_bdde120070_o.jpg
License: CC BY-NC 3.0

Cold? Use a chemical handwarmer. [Figure2]

Chemical hand warmers are heated by the reaction of iron to oxygen, or exothermic oxidation. When you crush the pack, the reactants inside (usually cellulose, iron, water, activated carbon, vermiculite and salt) are exposed to air and begin heating up.

Creative Applications

1 .In the case of the refrigerator, are bonds breaking or forming? In the case of the hand warmer?

2. Is the reaction in the refrigerator endothermic or exothermic? The reaction in the hand warmer?

3. What kind of influences could change the outcomes of the reactions that occur in the refrigerator or heat pack?

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Image Attributions

  1. [1]^ Credit: Milad Mosapoor; Source: http://commons.wikimedia.org/wiki/File:LG_refrigerator.jpg; License: CC BY-NC 3.0
  2. [2]^ Credit: User: Scotto Bear; Source: http://farm5.staticflickr.com/4054/4543988492_bdde120070_o.jpg; License: CC BY-NC 3.0

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