Part 6 (2/2)
Reference has already been made to such systems in the case of ammonium chloride. On being heated, ammonium chloride dissociates into ammonia and hydrogen chloride. Since, however, in that case the vapour phase has the same total composition as the solid phase, viz. NH_{3} + HCl = NH_{4}Cl, the system consists of only one component existing in two phases; it is therefore univariant, and to each temperature there will correspond a definite vapour pressure (dissociation pressure).[146]
If, however, excess of one of the products of dissociation be added, the system becomes one of two components.
In the first place, a.n.a.lysis of each of the two phases yields as the composition of each, solid: NH_{4}Cl (= NH_{3} + HCl); vapour: _m_NH_{3} + _n_HCl. Obviously the smallest number of substances by which the composition of the two phases can be expressed is two; that is, the number of components is two. What, then, are the components? The choice lies between NH_{3} + HCl, NH_{4}Cl + NH_{3}, and NH_{4}Cl + HCl; for the three substances, ammonium chloride, ammonia, hydrogen chloride, are the only ones taking part in the equilibrium of the system.
Of these three pairs of components, we should obviously choose as the most simple NH_{3} and HCl, for we can then represent the composition of the two phases as the _sum_ of the two components. If one of the other two possible pairs of components be chosen, we should have to introduce negative quant.i.ties of one of the components, in order to represent the composition of the vapour phase. Although it must be allowed that the introduction of negative quant.i.ties of a component in such cases is quite permissible, still it will be {80} better to adopt the simpler and more direct choice, whereby the composition of each of the phases is represented as a sum of two components in varying proportions (p. 12).
If, therefore, we have a solid substance, such as ammonium chloride, which dissociates on volatilization, and if the products of dissociation are added in varying amounts to the system, we shall have, in the sense of the Phase Rule, a _two-component system existing in two phases_. Such a system will possess two degrees of freedom. At any given temperature, not only the pressure, but also the composition, of the vapour-phase, _i.e._ the concentration of the components, can vary. Only after one of these independent variables, pressure or composition, has been arbitrarily fixed does the system become univariant, and exhibit a definite, constant pressure at a given temperature.
Now, although the Phase Rule informs us that at a given temperature change of composition of the vapour phase will be accompanied by change of pressure, it does not cast any light on the relation between these two variables. This relations.h.i.+p, however, can be calculated theoretically by means of the Law of Ma.s.s Action.[147] From this we learn that in the case of a substance which dissociates into equivalent quant.i.ties of two gases, the product of the partial pressures of the gases is constant at a given temperature.
This has been proved experimentally in the case of ammonium hydrosulphide, ammonium cyanide, phosphonium bromide, and other substances.[148]
Univariant Systems.--In order that a system of two components shall possess only one degree of freedom, three phases must be present. Of such systems, there are seven possible, viz. S-S-S, S-S-L, S-S-V, L-L-L, S-L-L, L-L-V, S-L-V; S denoting solid, L liquid, and V vapour. In the present chapter we shall consider only the systems S-S-V, _i.e._ those systems in which there are two solid phases and a vapour phase present.
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As an example of this, we may first consider the well-known case of the dissociation of calcium carbonate. This substance on being heated dissociates into calcium oxide, or quick-lime, and carbon dioxide, as shown by the equation CaCO_{3} <--> CaO + CO_{2}. In accordance with our definition (p. 9), we have here two solid phases, the carbonate and the quick-lime, and one vapour phase; the system is therefore univariant. To each temperature, therefore, there will correspond a certain, definite maximum pressure of carbon dioxide (dissociation pressure), and this will follow the same law as the vapour pressure of a pure liquid (p. 21). More particularly, it will be independent of the relative or absolute amounts of the two solid phases, and of the volume of the vapour phase. If the temperature is maintained constant, increase of volume will cause the dissociation of a further amount of the carbonate until the pressure again reaches its maximum value corresponding to the given temperature.
Diminution of volume, on the other hand, will bring about the combination of a certain quant.i.ty of the carbon dioxide with the calcium oxide until the pressure again reaches its original value.
The dissociation pressure of calcium carbonate was first studied by Debray,[149] but more exact measurements have been made by Le Chatelier,[150] who found the following corresponding values of temperature and pressure:--
-------------+-------------------------
Temperature.
Pressure in cm. mercury.
-------------+-------------------------
547
2.7 610
4.6 625
5.6 740
25.5 745
28.9 810
67.8 812
76.3 865
133.3 -------------+-------------------------
From this table we see that it is only at a temperature of about 812 that the pressure of the carbon dioxide becomes equal to atmospheric pressure.
In a vessel open to {82} the air, therefore, the complete decomposition of the calcium carbonate would not take place below this temperature by the mere heating of the carbonate. If, however, the carbon dioxide is removed as quickly as it is formed, say by a current of air, then the entire decomposition can be made to take place at a much lower temperature. For the dissociation equilibrium of the carbonate depends only on the partial pressure of the carbon dioxide, and if this is kept small, then the decomposition can proceed, even at a temperature below that at which the pressure of the carbon dioxide is less than atmospheric pressure.
Ammonia Compounds of Metal Chlorides.--Ammonia possesses the property of combining with various substances, chiefly the halides of metals, to form compounds which again yield up the ammonia on being heated. Thus, for example, on pa.s.sing ammonia over silver chloride, absorption of the gas takes place with formation of the substances AgCl,3NH_{3} and 2AgCl,3NH_{3}, according to the conditions of the experiment. These were the first known substances belonging to this cla.s.s, and were employed by Faraday in his experiments on the liquefaction of ammonia. Similar compounds have also been obtained by the action of ammonia on silver bromide, iodide, cyanide, and nitrate; and with the halogen compounds of calcium, zinc, and magnesium, as well as with other salts. The behaviour of the ammonia compounds of silver chloride is typical for the compounds of this cla.s.s, and may be briefly considered here.
It was found by Isambert[151] that at temperatures below 15, silver chloride combined with ammonia to form the compound AgCl,3NH_{3}, while at temperatures above 20 the compound 2AgCl,3NH_{3} was produced. On heating these substances, ammonia was evolved, and the pressure of this gas was found in the case of both compounds to be constant at a given temperature, but was greater in the case of the former than in the case of the latter substance; the pressure, further, was independent of the amount decomposed.
The behaviour of these two substances is, therefore, exactly a.n.a.logous to that shown by calcium carbonate, and the explanation is also similar.
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