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    Some Like it Hot — Some Like it Cold

    Kids, would you believe that when something rusts, heat is produced?  Or that when some compounds are mixed the temperature can go down?  A chemical reaction that produces heat is called “exothermic” and one that needs heat, where the temperature decreases, is “endothermic”. 

    Please note:  All chemicals and experiments can entail an element of risk, and no experiments should be performed without proper adult supervision.

    In our test exothermic reaction, vinegar is used to remove the protective coating from steel wool, allowing it to rust.  When the iron combines with oxygen, heat is released.  For supplies you will need a thermometer, a clear jar with a lid, steel wool and vinegar.  First, put the thermometer in the jar and close the lid.  Wait about 5 minutes, remove the thermometer and record the temperature.  Now soak a piece of steel wool in vinegar for 1 minute.  Squeeze the excess vinegar out of the steel wool. Wrap the wool around the thermometer bulb and place the wool/thermometer in the jar, sealing the lid.  Wait for another 5 minutes, then read the temperature and compare it with the first reading.  What happened?

    In the rusting of iron, four atoms of solid iron react with three molecules of oxygen gas (O2) to form two molecules of solid rust (iron oxide, Fe2O3). The reaction looks like this:

        4 Fe + 3 O2 —> 2 Fe2O3

    Rust is the common name for a very common compound, iron oxide. Iron combines quickly with oxygen, so quickly that pure iron is rarely found in nature. Iron (or steel) rusting is an example of corrosion, which is an electrochemical process. Did you know that liquids like acid rain (and our vinegar), seawater and the salt-loaded spray from snowy roads make them better electrolytes than pure water, allowing their presence to speed the process of rusting on iron objects?

    An endothermic reaction requires energy to proceed, which may be observed as a decrease in temperature during reaction. Once the reaction is complete, the temperature will return to normal. For our “endothermic” reaction example you will need: 25 ml citric acid solution (dissolved in water), 15 grams baking soda, a Styrofoam cup, thermometer, and something to stir with. First, pour the citric acid solution into the cup.  Record the initial temperature.  Stir in the baking soda (sodium bicarbonate, NaHCO3).  Track the change in temperature as a function of time.  The reaction is:

        H3C6H5O7 + 3 NaHCO3 —> 3 CO2 + 3 H2O + NaC6H5O7 (sodium citrate)

    When done, simply wash the cup out in a sink. The exact amounts of citric acid in water and baking soda are not critical, and you can vary them to test this. 

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    Kathleen Carrado Gregar, PhD, Argonne National Labs 
    kcarrado@anl.gov
    October 2006

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    Reference:
    Dr. Anne Marie Helmenstine at http://chemistry.about.com/cs/howtos/ht/exothermic.htm and http://chemistry.about.com/cs/howtos/ht/endothermic.htm . Most large grocery/drug stores sell citric acid in capsule or tablet form; also, this link offers some commercial chemicals in small quantities at reasonable prices: www.chemistrystore.com/citric_acid.htm