Man's physical universe



in cases where at least one of the molecules has been activated. Ions

may be regarded as activated molecules which combine immediately

upon contact. As we have pointed out above, the conditions determine

whether these ions will remain combined or separate again at

once. The activation of covalent molecules is thought to consist of

the addition of an unusual amount of energy to the molecule. This

may be accomplished by collision with photons of radiant energy or

with swiftly moving electrons, atoms, molecules, or ions. The energy

thus added produces an increase in

kinetic energy, a displacement of

electrons from relatively stable orbits to less stable orbits, an increased

energy of vibration of the atoms within the molecules, or an increased

energy of rotation of the whole molecules.

This added energy may be emitted in the form of radiant energy as

fluorescence or phosphorescence as the electrons return to their more

stable orbits. In other cases this energy of activation is removed by

collision with other particles, thus increasing their kinetic energy;

this represents an increase in temperature. The energy represented by

the vibration of the atoms may be transmitted from atom to atom

within the molecule until it reaches a weak or ruptured bond, when the

molecule is either decomposed or internally rearranged. Sometimes

activated molecules lose their extra energy by direct union with other

molecules; in such cases energy is released. This released energy then

activates other molecules, producing what is known as a chain reaction.

In case the amount of energy released is greater than that required for

activation, the reaction will be exothermic. On the other hand, the

amount of energy evolved may be less than that required for activation,

and the reaction can be made to continue only by adding sufficient

energy to activate the reacting molecules. Such reactions are endothermic.

In the explosion of an explosive mixture of gases, an electric spark

furnishes all of the energy necessary to activate a few molecules. These

activated molecules react to produce sufficient energy to activate many

more molecules, and so the reaction proceeds. Explosions are typical

chain reactions. In the starting of a fire, the heat of a burning match

is sufficient to activate a few molecules; these, in turn, activate many

more molecules; soon a whole city or forest may burn up unless the

reaction can be stopped. The usual method of stopping such reactions

is by pouring on water, which uses up the energy evolved by the reactions

as the water is heated and finally vaporized.

On the other hand, it is necessary to keep on adding energy when

one chars some sugar or bakes a cake because the changes in this case

are endothermic.

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