A system of physical chemistry - Index of

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A system of physical chemistry - Index of

LAW OF THE PHOTOCHEMICAL EQUIVALENT 137

Bodenstein gives other instances in which the law appears to break

down badly, one quantum decomposing many molecules. Even in the

cases quoted the results are subject to considerable uncertainty. One

of the principal reasons for this is, in the writer's opinion, the fact that

the wave-length of the light employed did not correspond sufficiently

closely to the head of the absorption band of the substance decomposing.

Einstein assumes that the is decomposition brought about by a single

definite frequency. As is well known, absorption bands exhibit width,

especially when the concentration of the substance is

fairly large, and,

further, experiment goes to show that any wave-length inside the band

is capable of bringing about the photochemical effect. It is not to be

expected, however, that all wave-lengths inside a band will be equally

efficient. To test the Einstein expression in the most unequivocal

manner it would appear to be necessary to select the wave-length which

corresponds to the head of the band. In the case of other wave-lengths,

still inside the band, it is probable that a considerable fraction of the

radiation absorbed is not converted into chemical change, but is simply

into heat. This

degraded into quanta of smaller size, i.e. degradation

indeed appears to be the case in a number of experiments of Warburg

{Sitzungsber. k'on. preuss. Akad., 1912-15) on the ozonisation process,

in which the wave-length employed {20()fifi) does not correspond to the

position of maximum absorption of oxygen. The fact that an absorption

band exhibits width, and further, the fact that the width increases

with the concentration of the material is probably due to collisions

which set up forced vibrations inside the molecule thereby disturbing

the normal electronic period or frequency which an isolated molecule

would possess.

In the case of photochemical reactions in solution, effects due to

the solvent may be anticipated. This important problem has been

dealt with by Baly {Physikal. Zeitsch., 14, 893 (19 13))

in the following

manner :—

According to Baly, the chemical reactivity of an atom or molecule

is due to the existence of atomic electro-magnetic fields of force, which

differ from atom to atom in the density and location of the lines of

force. When an atomic or molecular species is activated in the chemical

sense, this is due to an " "

opening up of the atomic fields of force

which are thus capable of interacting with the corresponding fields of

other atoms, thereby producing chemical change. This " opening up "

can be brought about essentially by two agencies, (i) by the absorption

of light, and (2) by the solvent (which presumably acts in an analogous

manner in virtue of its electro-magnetic properties). Different solvents

*'

are capable of "opening the field of a molecule to different extents,

these different extents or stages being manifested by the different positions

of the absorption band or bands possessed by the dissolved substance

in various solvents. On this basis we would expect that a

substance dissolved in a given solvent, being already partially activated,

would require less energy, say of the radiational type, to complete the

activation, than the same substance would require in the absence of the

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