D O C . 3 9 P R O P A G A T I O N O F S O U N D 1 8 3
39. “Propagation of Sound in Partly Dissociated Gases”
[Einstein 1920c]
Submitted 8 April 1920
Published 29 April 1920
In: Königlich Preußische Akademie der Wissenschaften (Berlin). Sitzungsberichte (1920):
380–385.
Although our knowledge of the chemical equilibrium of gases is well advanced, we
have only inadequate knowledge about the reaction velocity of gas reactions. A
particular difficulty for the experimental investigation of reaction velocities lies in
the fact that the latter are influenced catalytically by rigid walls. High temperature,
too, associated with most gas reactions, and certainly the high values of reaction
velocities that one expects constitute difficulties. It seems to me now that all these
difficulties could be circumvented by investigating reaction velocities indirectly via
sound propagation in partially dissociated gases.
That such investigations can serve to determine the reaction velocity can be seen
from the following consideration. If the volume of a partially dissociated gas is
changed adiabatically so fast that practically no appreciable chemical reaction can
occur during the time of the change in volume, then the gas behaves just like an
ordinary mixture. However, if the volume is changed so slowly that the process is
practically a sequence of pure chemical equilibria, then the density dependence of
the pressure is such that the compressibility of the mixture is lower than in the pre-
vious case. Consequently, the sound velocity will have to increase with the
frequency from an initial value to some upper limit. For frequencies between these
two extremes, the reaction will lag behind the compression such that—under
change of mechanical work into heat—a kind of temporal delay of the pressure
curve behind the density curve takes place. In the following we shall, preliminarily
only, present a theoretical investigation of sound propagation in partially dissociat-
ed gases, whereby we assume a reaction of the simplest type imaginable, i.e.,
[p. 380]
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[p. 381]