Perhaps the most convenient form in which to store energy is chemical energy. The
foods we eat, combined with the oxygen we breathe, store energy that our bodies
extract and convert into mechanical and thermal energy. The fuels that we burn
in our automobiles, furnaces, and campfires also store energy in chemical form.
Batteries store chemical energy as well, for later retrieval in the form of electrical
Achemical reaction is simply a rearrangement of the way that atoms are con-
nected together to form molecules. Asimple example is
H2 + 12O2 −→ H2O,
the combination of hydrogen gas with oxygen gas to form water.
hydrogen atoms are connected together in pairs to form H2 molecules, and the
oxygen atoms are similarly connected in pairs. To form a water molecule (H2O),
we need two hydrogen atoms but only one oxygen atom, or equivalently, an entire
H2 molecule but only half an O2 molecule. (In practice, the other half of the O2
will usually combine with another H2, to form a second water molecule.)
Many chemical reactions, especially those that consume oxygen, give off energy.
For example, each time reaction 4.1 occurs, a very small amount of energy, roughly
half a billionth of a billionth of a joule, is given off. If the reaction occurs by ordinary
combustion (burning), then the energy is given off as thermal energy, but sometimes
the energy released can take on other forms, such as electrical or even mechanical
Because the amount of energy released by a single molecular reaction is so tiny,
we often refer to the energy given off when a large number of identical molecular
reactions take place. The standard number used by scientists for this purpose is
called Avogadro’s number,
NA = 602, 000, 000, 000, 000, 000, 000, 000 = 6.02 × 1023.
This number was chosen for convenience, because it happens to equal the number of
hydrogen atoms in one gram of hydrogen. When we have this many of something,
we say that we have one mole of that substance.
Amole of hyd