Chemical Reactions     

      A chemical reaction is a series of changes that results in the production of one or more new substances.  These chemical changes are always accompanied by a change in energy.  That means that either energy is given off during the reaction, or energy is taken in.

    Reactions that release energy are called exothermic. In this type of reaction, the products have less potential chemical energy than the reactants, because energy was given off in the form of heat. That one occurs in the combustion reaction that is taking place around the burners.  The reaction of the propane gas found in grills is shown below:

C₃H₈ + 5O₂ ---> 4H₂O + 3CO₂ + energy
propane + oxygen  yields water + carbon dioxide + energy

    Reactions, which take in energy, are called endothermic.   In this type of reaction, the products have more potential chemical energy than the reactants.  Think of the chemical reaction that takes place in "cold-packs."  A seal is broken that separates two containers with the plastic bag.  As the contents from the separate containers begin to react, energy is absorbed from the surroundings.  If you place the cold-pack on your body, your body begins to supply some of the energy that is required to get the reaction going.  What you experience as "cold" has to do with the temperature of that area of your body changing as heat flows to the cold-pack.

On the below web you can see an animation about chemical reactions.

(http://cdsearch.britannica.com/lm/animations/ochemir001d4/product.html)Science Activity - Chemical Reaction. Click on activity sheet and answer the questions.

You also have to do the Multiple-Choice Quiz, so click on quiz on the previous web.