SIMPLE COMPOUNDS. 55
v, or, in other words, as Nernst predicted from the theory and
Haber has found in practice, the yield increases as the pressure
increases. This is shown very clearly by the values given for
the yields of ammonia obtained by Haber in experiments with the
equilibrium mixture of 3 vol. H 2 to 1 vol. N 2.(.
400°
600
1 Atm.0.44%
0.049
30 Atm.10.7%
1.43
100 Atm.25.1%
4.47
200 Atm.36.3%
8.25
Further applications of the mass-action law are illustrated in the prepara-
tion of nitrogen peroxide (No. 29), hydrobromic and hydriodic acids from the
elements (No. 35), and phosphorus pentachloride and trichloride (No. 46);
cf. also, Dissociation of Electrolytes, p. 66.
DEPENDENCE OF EQUILIBRIUM CONSTANTS UPON THE TEMPERATURE. It is
also apparent from the above expression that the yield in the contact process
is dependent on the value of the constant, K; if K can be made greater, the
proportion of SO 3 is increased. The value of K depends upon the temperature,
and its variation can be predicted with the aid of thermochemical data and
the principles of thermodynamics.
Chemical reactions are, from a thermochemical standpoint, divided into
two classes: those in which heat is evolved, or set free (exothermic reactions),
and those in which heat is absorbed, or used up (endothermic reactions).
Endothermic compounds, or, in other words, those in the formation of which
from their elements heat is absorbed, are far less common (cf. cyanogen,
No. 59, and hydrogen peroxide, No. 67). Experience has shown that the
conditions for the formation of endothermic compounds are more favorable
at high temperatures, while for exothermic compounds the reverse is true.
Since in the formation of a substance the size of the constant, K, is, under
otherwise equal conditions, a measure of the yield, it seems plausible that in
the case of exothermic reactions the value of K diminishes with rise of tem-
perature, whereas in endothermio reactions it increases.
According to van't Hoff, there exists between the heat of reaction Q, the
absolute temperatures Ti and T 2 , at which the reaction takes place, and the
corresponding equilibrium-constants Ki and K 2 , together with the gas con-
stant R, the exact relation:from which it follows that when T 2 > 7\, and the value of Q is positive (exo-
thermic reactions), Ki becomes smaller than K\, and for negative values of Q
(endothermic reactions) K% is greater than Ki. This is based upon the assump-
tion that Q is independent of the temperature at which the reaction takes
place. Whether this is true or not must be ascertained in the case of each