2l6 
FREDERICK BARRY 
many cases, and successfully, to calculate from known values of the 
equilibrium constant of a reaction, at different temperatures, the heat 
of reaction or total energy change. If, conversely, it be used to show 
the influence of temperature change in shifting chemical equilibrium, 
most illuminating implications of a general character are brought to 
notice. 
For instance, we see from the formula that if U, the heat of re- 
action, is greater than zero, then with increase in temperature, K will 
become smaller; while if U is less than zero, it will become greater. 
Inasmuch as R is always made to indicate by its magnitude the 
tendency of a reaction in the direction indicated also by the exothermal 
chemical equation, this means that a rise of temperature will displace 
a chemical equilibrium in that direction in which the reaction takes 
place with an absorption of heat.^° Thus exothermal tendencies will 
be weakened, and endothermal tendencies strengthened by rise of tem- 
perature. This principle, which van t'Hoff called the Law of Mobile 
Equilibrium, accounts at once for the comparatively greater stability 
at low temperature of compounds formed by exothermal reaction, and 
at high temperature of those formed endothermally. By explaining 
the prevalence of exothermal reactions at ordinary temperatures 
(which are low even in the experimental temperature range) it gives 
real significance to the erroneous and much criticized "Principle of 
Maximal Work" advanced by Thomsen and defended by Berthellot, 
to the effect that the only spontaneous chemical reactions were those 
in which heat was evolved in greatest possible quantity. Theoretically, 
this principle would be strictly true at the ai)solute zero of tempera- 
ture. An examination of the fundamental equation, A — U = T 
(dA/dT), shows that if T vanishes, then A equals U: that is, the change 
in the free energy of the reaction, which we now know to be the factor 
which determines whether or not a given change will occur, then 
equals the total energy change. At this temperature, then, all re- 
actions will be exothermal; while at higher temperatures, such re- 
actions will at first predominate. Moreover, inasmuch as an increase 
in temperature always increases a reaction velocity, these exothermal 
1° This principle was shown by Le Chatelier to be one particular case of a still 
wider generalization which has been thus expressed: "If some stress (for example 
by change of temperature, pressure or concentration) is brought to bear on any system 
in equilibrium, by which the equilibrium is displaced, the equilibrium is displaced 
in that direction which tends to undo the effect of the stress (8)." 
" A very few apparent exceptions to this rule have been noted, explicable on the 
assumption that negative catalytic effects are involved (9). 
