Tuesday, April 2, 2013

Hot and cold reactions

As part of our recent Chemical Reactions Workshop, we explored endo- and exothermic reactions. Many chemical reactions create light, heat or other forms of energy. An exothermic reaction is one in which heat is made and released by the reaction. The term is obvious if you look at the roots: ex = out/outside, therm = heat. Therefore, endothermic (en = in/inside) is a reaction where energy (in the form of heat) must be pulled in for the reaction to occur.

Both are fun and easy to demonstrate. Grab the following supplies:

  • 2 Styrofoam cups (reusuable insulted cups are even better)
  • 1 thermometer
  • baking soda
  • citric acid (you can find it with the canning supplies in the grocery store)
  • hydrogen peroxide
  • a packet of yeast
For the first reaction, create a baking soda solution by adding baking soda to warm water one teaspoon at a time, stirring after each addition. Stop when no more will dissolve. Let the solution cool to room temperature, then place it in your cup. Use the thermometer to check the temperature. Add a teaspoon of citric acid. The solution will fizz and the temperature will drop. Keep adding citric acid and see how low you can make the temperature go. This is an endothermic reaction. Kids can put their fingers into the solution to feel the cold directly. Just wash your hands afterwards.

For the second reaction, we're going to generate heat through a decomposition reaction. Fill your cup with hydrogen peroxide. Check the temperature. Slowly add the yeast, a little at a time. Soon you'll have a big bubbling mess and spike in the solution temperature. Keep feeding the reaction. How high will the temperature go? Again, kids can touch the solution without fear. 

Older student may enjoy exploring the concepts of enthalpy and entropy as well. Have fun playing! Let me know what you discover!

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