"Seeing" an Endothermic Reaction

Infrared photography allows us to see a chemical reaction pull heat from its environment.

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Teachable Topics

• endothermic reactions
• heat transfer
• chemical cryogenics

 Theory

Chemical reactions that involve heat transfer fall into two categories, endothermic and exothermic.  Exothermic means that thermal energy is given off the reaction, causing its environment to warm up.  Endothermic means that thermal energy is absorbed, causing the local environment to cool down, sometimes by several degrees!

This demonstration of endothermic reactions uses the science-fair classic baking soda and vinegar.  Baking soda is known chemically as sodium bicarbonate and has the formula NaHCO₃.  Regular household white vinegar is typically a 5% solution of acetic acid with the formula CH₃COOH.  When introduced to each other, the following reaction occurs:

NaHCO₃ + CH₃COOH -> CH₃COONa + H₂O + CO₂

The reaction produces sodium acetate, water, and carbon dioxide gas.  It is the gas that creates the bubbles in the liquid.  For the new chemical products to be formed, heat is needed from the environment.  This spontaneous action causes the temperature of the liquid and the surrounding environment to cool down.  Looking at the reaction with an infrared camera, you can see the cooling occurs immediately.

Procedure 

1. Fill a small container with vinegar.  Be sure to fill it at least half-way.
2. Place the container on a tray or other suitable surface that will catch the inevitable spillage.
3. Turn on the infrared camera and aim it at the top of the container so that the surface of the vinegar is fully visible. 
4. Dump in a tablespoon or so of baking soda and then watch the temperature drop!

Safety Warnings 

 This experiment uses household kitchen products and no significant health hazards are present.  Be sure to clean up the residue completely afterwards using warm water.