Applications of the simplified approximation formula

Applications of the Simplified Approximation Formula. —Let us consider first the case when Ev — o, i.e. when there are just as many gaseous molecules disappearing as being formed in the reaction then 

as in the transition from one optical isomer into its antipode, there are only two molecular species concerned, 

Here we have to expect, from the smallness of the heat change, that an equilibrium should be obtained even at low temperatures, and that, though it must be displaced in the direction of the formation of hydriodic acid, yet all the molecular species take part in the equilibrium with appreciable partial pressures. Experience confirms this fully. In addition to the examples mentioned, we may recall the formation of an ester, the classical case of chemical equilibrium since the formation of gaseous ethyl acetate and water from alcohol and acetic acid vapours takes place without any considerable heat change, an equilibrium must be produced just as with hydriodic acid, in which all the components participate in considerable concentrations since the vapour pressures of the four substances are not very different, this equilibrium must also be found for the liquid mixture. 

It is well known how important a r6le has been played in the theory of chemical affinity by investigations on the ester equilibrium (Berthelot and P an St. Gilles) and on the dissociation of hydriodic acid (Lemoine and Bodenstein). As we now know, this is no accident, for in both cases reactions are concerned which must, on thermodynamic grounds, show marked evidence of the equilibrium state at quite low temperatures. 

Neglecting the specific heats, i.c. the variability of Q which they cause, the Second Law gives for the above cases 

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