146
DOC.
12 THERMODYNAMIC
DEDUCTION
Doc.
12
Thermodynamic Deduction of the
Law of Photochemical
Equivalence1
by
A.
Einstein
[Journal de physique
3
(1913):
277-282]
The
quantum hypothesis
led to
our assuming
the
following relationship
between
photochemical
processes
and the radiation that
produces
them: in
every elementary
photochemical phenomenon
involving
the
decomposition
of
a
molecule
by
radiation,
energy
hv is taken from the
latter,
where
h
is
the familiar
constant
of
Planck's
formula and
v
the
frequency
of the
acting
radiation. We
are going
to
take
up
this law
without
adopting
the
point
of view
of
the
quantum theory, starting,
on
the
contrary,
from
a more phenomenological conception
and
not
seeking
to
fashion for ourselves
a
representation
of
the mechanism of the
action
in
question.
Let
us
imagine
a
gas,
a
molecule of which shall be denoted
by
AB. In
addition,
let
us assume
that under the
influence of radiation these molecules AB
decompose
into their
components
A
and
B,
and that this
decomposition
is
linked
to
the
absorption
of
the radiation. We will also
assume
that the
frequency
interval
capable
of
producing
this reaction
is not
of
infinitely
small extension.
We
will formulate
some
hypotheses
regarding
this
photochemical
decomposition
reaction and will
draw
some
conclusions from them
by
means
of
the methods
of
classical
thermodynamics.
The first of
these
hypotheses
are:
1.
When radiation in the interval
dv,
from the
region
of the
spectrum
to
which
the reaction is
responsive,
acts
upon
the
gas,
then the number
of
molecules
decomposed per
unit time is
proportional
to
the
intensity
of the radiation and
to
the
number
of
molecules AB
present.
2.
The
light energy e
absorbed in the
decomposition
of
one
gram-molecule
of
AB
is
independent
of the
intensity
of
the radiation. But
it
may depend
on
the
frequency
of the radiation
used,
and
on
the
temperature
of
the
gas.
3. Between the radiation and the
gas
there
is
no
action other than the
photochem-
ical
phenomenon,
which
is
of such kind that the transfer of the
energy e
of
frequency
v
from the radiation
to
the
gas
is
tied in
a
univocal
manner,
as
if
by
a
rigid
mechanism, to
the
decomposition
of the
gram-molecule
of AB.
It
must
be
possible
for
black-body
radiation of
temperature
T
to
be in
thermody-
namic
equilibrium
with
a
mixture of the
gases
AB,
A,
and
B at
the
same
temperature
1Lecture
given
at the French
Physical Society,
27 March
1913.
[1]
[2]
[3]
[4]