Part 17 (2/2)
Fig. 61 gives a graphic representation of the results obtained.
The melting point of the [alpha] modification is 34-35; the melting point of the unstable [beta]-modification being 130. The freezing curves AC and BC were obtained by determining the freezing points of different mixtures of known composition, and the numbers so obtained are given in the following table.
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---------------------------------------------------- Grams of the [alpha] modification
in 100 gm. of mixture.
Freezing point.
----------------------------------+----------------- 26.2
101 49.2
79 73.7
46 91.7
26.2 95.0
28.6 96.0
30.0 ----------------------------------------------------
[Ill.u.s.tration: FIG. 61.]
The eutectic point C was found to lie at 25-26, and the natural freezing point D was found to be 27.7. The equilibrium curve DE was determined by heating the liquid mixtures at different temperatures until equilibrium was attained, and then rapidly cooling the liquid. In all cases the freezing point was practically that of the point D. From this it is seen that the equilibrium curve must be a straight line parallel to the temperature axis; and, therefore, isomeric transformation in the case of the two benzaldoximes is not accompanied by any heat effect (p. 197). This behaviour has also been found in the case of acetaldoxime.[287]
The isomeric benzaldoximes are also of interest from the fact that the stable modification has the _lower_ melting point (_v._ p. 202).
_Acetaldehyde and Paraldehyde._--As a second example of the equilibria between two isomerides, we shall take the two isomeric (polymeric) forms of acetaldehyde, which have recently been exhaustively studied.[288]
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In the case of these two substances the reaction
3CH_{3}.CHO <--> (CH_{3}.CHO)_{3}
takes place at the ordinary temperature with very great slowness. For this reason it is possible to determine the freezing point curves of acetaldehyde and paraldehyde. The three chief points on these curves, represented graphically in Fig. 62, are:--
m.p. of acetaldehyde - 118.45 m.p. of paraldehyde + 12.55 eutectic point - 119.9
[Ill.u.s.tration: FIG. 62.]
In order to determine the position of the natural melting point, it was necessary, on account of the slowness of transformation, to employ a catalytic agent in order to increase the velocity with which the equilibrium was established. A drop of concentrated sulphuric acid served the purpose. In presence of a trace of this substance, isomeric transformation very speedily occurs, and leads to the condition of equilibrium. Starting in the one case with fused paraldehyde, and in the other case with acetaldehyde, the same freezing point, viz. 6.75, was obtained, the solid phase being paraldehyde. This temperature, 6.75, is therefore the natural freezing point, and paraldehyde, the solid in equilibrium with the liquid phase at this point, is the stable form.
With regard to the change of equilibrium with the temperature, it was found that whereas the liquid phase contained 11.7 molecules per cent. of acetaldehyde at the natural freezing point, the liquid at the temperature of 41.6 contains 46.6 molecules per cent. of acetaldehyde. As the temperature {206} rises, therefore, there is increased formation of acetaldehyde, or a decreasing amount of polymerisation. This is in harmony with the fact that the polymerisation of acetaldehyde is accompanied by evolution of heat.
While speaking of these isomerides, it may be mentioned that at the temperature 41.6 the equilibrium mixture has a vapour pressure equal to the atmospheric pressure. At this temperature, therefore, the equilibrium mixture (obtained quickly with the help of a trace of sulphuric acid) boils.[289]
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CHAPTER XII
SUMMARY.--APPLICATION OF THE PHASE RULE TO THE STUDY OF SYSTEMS OF TWO COMPONENTS
In this concluding chapter on two-component systems, it is proposed to indicate briefly how the Phase Rule has been applied to the elucidation of a number of problems connected with the equilibria between two components, and how it has been employed for the interpretation of the data obtained by experiment. It is hoped that the practical value of the Phase Rule may thereby become more apparent, and its application to other cases be rendered easier.
The interest and importance of investigations into the conditions of equilibrium between two substances, lie in the determination not only of the conditions for the stable existence of the partic.i.p.ating substances, but also of whether or not chemical action takes place between these two components; and if combination occurs, in the determination of the nature of the compounds formed and the range of their existence. In all such investigations, the Phase Rule becomes of conspicuous value on account of the fact that its principles afford, as it were, a touchstone by which the character of the system can be determined, and that from the form of the equilibrium curves obtained, conclusions can be drawn as to the nature of the interaction between the two substances. In order to exemplify the application of the principles of the Phase Rule more fully than has already been done, ill.u.s.trations will be drawn from investigations on the interaction of organic compounds; on the equilibria between optically active compounds; and on alloys. {208}
Summary of the Different Systems of Two Components.--Before pa.s.sing to the consideration of the application of the Phase Rule to the investigation of particular problems, it will be well to collect together the different types of equilibrium curves with which we are already acquainted; to compare them with one another, in order that we may then employ these characteristic curves for the interpretation of the curves obtained as the result of experiment.
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