There is energy in a piece of wood. Left quietly to itself, it seems
completely incapable of bringing about any kind of work. Set it on fire,
however, and the wood plus the oxygen in the air will give off heat and
light that are clearly forms of energy. The heat could help boil water
and run a steam engine.
The amount of energy in burning wood could be measured if it were mixed
with air and allowed to burn in a closed container that was immersed in
a known quantity of water. From the rise in temperature of the water,
the quantity of energy produced could be measured in units called
“calories” (from a Latin word for “heat”). The instrument was therefore
called a “calorimeter”.
In the 1860s the French chemist Pierre Eugène Marcelin Berthelot
(1827-1907) carried through hundreds of such determinations. His work
and similar work by others made it clear that such “chemical energy”—the
energy derived from chemical changes in matter—fit the law of
conservation of energy.
Here’s how it looked in the last decades of the 19th century.
Molecules are composed of combinations of atoms. Within the molecules,
the atoms stick together more or less tightly. It takes a certain amount
of energy to pull a molecule apart into separate atoms against the
resistance of the forces holding them together.
If, after being pulled apart, the atoms are allowed to come together
again, they give off energy. The amount of energy they give off in
coming together is exactly equal to the amount of energy they had to
gain before they could separate.
This is true of all substances. For instance, hydrogen gas, as it is
found on earth, is made up of molecules containing 2 hydrogen atoms each
(H₂). Add a certain amount of energy and you pull the atoms apart; allow
the atoms to come back together into paired molecules, and the added
energy is given back again. The same is true for the oxygen molecule,
which is made up of 2 oxygen atoms (O₂) and of the water molecule (H₂O).
Always the amount of energy absorbed in one change is given off in the
opposite change. The amount absorbed and the amount given off are always
exactly equal.
However, the amount of energy involved differs from molecule to
molecule. It is quite hard to pull hydrogen molecules apart, and it is
even harder to pull oxygen molecules apart. You have to supply about 12%
more energy to pull an oxygen molecule apart than to pull a hydrogen
molecule apart. Naturally, if you let 2 oxygen atoms come together to
form an oxygen molecule, you get back 12% more energy than if you allow
2 hydrogen atoms to come together to form a hydrogen molecule.
It takes a considerably larger amount of energy to pull apart a water
molecule into separate atoms than to pull apart either hydrogen or
oxygen molecules. Naturally, that greater energy is also returned once
the hydrogen and oxygen atoms are allowed to come back together into
water molecules.
Next, imagine pulling apart hydrogen and oxygen molecules into hydrogen
and oxygen atoms and then having those atoms come together to form
_water_ molecules. A certain amount of energy is put into the system to
break up the hydrogen and oxygen molecules, but then a much greater
amount of energy is given off when the water molecules form.
It is for that reason that a great deal of energy (mostly in the form of
heat) is given off if a jet of hydrogen gas and a jet of oxygen gas are
allowed to mix in such a way as to form water.
Just mixing the hydrogen and oxygen isn’t enough. The molecules of
hydrogen and oxygen must be separated and that takes a little energy.
The energy in a match flame is enough to raise the temperature of the
mixture and to make the hydrogen and oxygen molecules move about more
rapidly and more energetically. This increases the chance that some
molecules will be broken up into separate atoms (though the actual
process is rather complicated). An oxygen atom might then strike a
hydrogen molecule to form water (O + H₂ → H₂O) and more energy is given
off than was absorbed from the match flame. The temperature goes up
still higher so that further breakup among the oxygen and hydrogen
molecules is encouraged.
[Illustration: _The formation of a sodium chloride molecule._]
This happens over and over again so that in very little time, the
temperature is very high and the hydrogen and oxygen are combining to
form water at an enormous rate. If a great deal of hydrogen and oxygen
are well-mixed to begin with, the rate of reaction is so great that an
explosion occurs.
Such a situation, in which each reacting bit of the system adds energy
to the system by its reaction and brings about more reactions like
itself, is called a “chain reaction”. Thus, a match flame put to one
corner of a large sheet of paper will set that corner burning. The heat
of the burning will ignite a neighboring portion of the sheet and so on
till the entire sheet is burned. For that matter a single smoldering
cigarette end can serve to burn down an entire forest in a vastly
destructive chain reaction.