Energy in Chemical Reactions
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. When you
stand next to a barbecue grill, you feel the heat being released by 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.
Exothermic reactions require some energy to get them started, but then
they release more energy than they originally took in. Think of the fact
that a match requires initial energy, provided by the friction between it
and the sandpaper on the matchbook, to start burning. Once the match
starts burning, it releases more energy than it took in, so the reaction is
still exothermic. The products still have less potential chemical energy
than the reactants. The initial energy that is required to get the reaction
to begin is called activation energy.