DOC.
2
LAW OF PHOTOCHEMICAL
EQUIVALENCE
89
Doc.
2
Thermodynamic
Proof of the Law of Photochemical
Equivalence
by
A.
Einstein
[Annalen
der
Physik
37 (1912):
832-838]
In what
follows,
the Wien radiation law and the law of
photochemical equivalence
will be derived
simultaneously
in
an
essentially thermodynamic way. By
the
photochemical equivalence
law
I
understand the
proposition
that the
decomposition
of
a gram-equivalent by
a
photochemical process requires
the absorbed radiation
energy Nhv,
if N
denotes the number of molecules in
a
gram-molecule,
h
the familiar
constant
in Planck's radiation
formula,
and
v
the
frequency
of the
acting
radiation.1
The law
appears essentially
to
be
a
consequence
of the
assumption
that the number
of molecules
decomposed per
unit time is
proportional
to
the
density
of
the
acting
radiation;
however,
it
should be
emphasized
that the
thermodynamic relationships
and
the radiation law do
not
permit
the
replacement
of this
assumption
by any
other
arbitrary
assumption, as
will
be shown in brief
at
the end of this
paper.
Furthermore,
the
following
shows
clearly
that the
equivalence
law
and
the
assumptions leading
to
it
are
valid
only
as
long
as
the
acting
radiation
is
within the
region
of
validity
of the Wien law.
But
now
for such radiation the
validity
of the law
can hardly
be doubted
any longer.
§1.
On the
Thermodynamic Equilibrium
between Radiation
and
a
Partially
Dissociated Gas
from
the
Standpoint
of
the Mass Action Law
Suppose
that
a
mixture of three
chemically
different
gases
with molecular
weights
m1,
m2, m3
is
present
in
a
volume
V.
Let
n1
be the number of moles
of
the first
gas,
n2
of the
second,
n3
of the
third.2
Suppose
that
a
reaction
is
possible
between these three
kinds of
molecules
in
which
a
molecule of the first kind
decomposes
into
a
molecule
of the second
kind and
a
molecule of the third kind. In
a
state
of
thermodynamic
equilibrium,
the reactions
1Cf.
A.
Einstein,
Ann. d.
Phys.
17
(1905): 132.
2Naturally,
one
of
the
gases
with the indices
2 and
3
can
consist
of
electrons.
[1]
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