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# Exothermic and Endothermic Processes

## Describes process of heat exchange and the units of heat

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Practice Exothermic and Endothermic Processes

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Credit: Pete Birkinshaw
Source: http://www.flickr.com/photos/93001633@N00/1458555513/

Squirrels are probably well equipped to handle the cold of a snowy winter.  Luckily for humans, we’ve learned to take advantage of chemistry to heat the way.

#### News You Can Use

• Nearly every chemical process is accompanied by the flow of heat, which itself generally leads to a change in temperature.  Some transformations are endothermic and cause the surroundings to decrease in temperature.  That is clearly a reaction you would want to avoid if you were cold during the winter.
• Alternatively, there are processes that are exothermic and thus increase the temperature of the surroundings.  A useful example of this is the crystallization of sodium acetate from a supersaturated solution, which has been exploited commercially to produce hand warmers.
• Credit: Wikipedia
Source: http://en.wikipedia.org/wiki/Hand_warmer

A sodium acetate hand warmer with a metallic disc [Figure2]

• To see the dramatic crystallization, watch this video: http://www.youtube.com/watch?v=K7iM0lu61zg

#### Can You Apply It?

1. What mass of KCl (potassium chloride) can dissolve in 100g of water at 25 °C?
2. KCl doesn’t readily form supersaturated solutions.  What happens to prevent the formation of supersaturated solutions?
3. Name some ways to cause crystallization to occur in a supersaturated solution.
4. Describe a procedure for preparing a supersaturated solution.

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