Dissociation tension

The dissociation tension of carbonate of calcium at a temperature t varies with this temperature and increases with it H. Debray found the following values for this tension ... 

The decomposition of calcium carbonate is not the only reaction showing a dissociation tension, fixed at each temperature and similar in all points to the tension of saturated vapors H. Debray found the same law in studying the decomposition of a certain number of hydrated salts into water vapor and anhydrides. 

We may evidently construct, for each of the cases of which we have spoken, a curve of dissociation tensionsj which possesses ail the properties of the vapor-pressure curve for saturated vapors. In each of the reactions studied by Debray the determinations of this investigator give us a certain number of points on the curve of dissociation tensions but these points are too few and too widely separated from each other to allow us to draw the curve. 

At a given temperature there is emission or absorption of ammonia gas according as the pressure of this gas is less or greater than a certain dissociation tension. The dissociation tension at a given temperature depends exclusively upon this... 

Since the time when Isambert published this work several chemists have made known a great number of dissociation tension curves. These curves have exactly the appearance and properties of those for the tension of saturated vapors of solids or liquids. 

A tem containing calcium carbonate, lime, and carbonic acid gas consists of two independent components (o 2) divided into three phases =S) it is a monovariant i stem at a givax temperature the i stem is in equilibrium for a definite value of the pressure, called Ihe dissociation tension of calcium carbonate at the given temperature this tension depends exclusively upon the temperature it depends in no wise upon the masses of the independent components, lime, and carbonic anhydride, which make up the intern this is the celebrated law predicted by Henri Sainte-Claire Deville, demonstrated by Debray for the case that we have just taken as example, and confirmed by Debray and by G. Wiedemann vdien studying the dissociation of hydrated salts, and by Isambert from a study of the dissociation of compounds that ammonia gas forms with certain metallic chlorides. 

R6le of monovariant systems in the history of chemical mechanics.— The rdle that the monovariant systems have played in the history of chemical mechanics is well known it is because they appealed to monovariant systems that Debray, Isambert, Troost, and Hautefeuille found, in the study of chemical decompositions, in the study of allotropic modifications, a dissociation tension, a transformation tension, analogous to the tension of saturated vapors it is in showing the analogy between the dissociation tension, the transformation tension, and the tension of saturated vapor that th have made ev the most skeptic accept the far-reaching thought of Henri Sainte-Claire Deville Ihere is no chemical mechanics distinct from physical mechanics aU... 

Dissociation tensions.— The equilibrium of a system in which a definite solid component exists with two independent components, one of which is solid, the other gaseous, necessitates... 

Put with certain metallic chlorides ammonia gas is absorbed by these chlorides and forms with them definite solid compounds the dissociation of an ammonia chloride into a metallic chloride and ammonia gas corresponds to a curve of transformation tensions which is here called the curve of dissociation tensions, Isam-bert has determined a certain number of these curves and has shown their analogy to the curve fo. vapor tension of saturated vapors from liquids his work has since been completed by Joan-nis and Croizier. ... 

Other curves of dissociation tension have been determined by chemists we only note here the curves, determined by Joannis, of the tensions of dissociation of potassammonium into potassium and ammonia gas, and of sodammonium into sodium and ammonia. 

The two curves have been determined by H. P labon they are in nowise identical the curve C is much higher than the curve C for instance, at the temperature of 520 C. the transformation tension which we have denoted by P is measured by 417.6 centimetres of mercury, and the transformation tension denoted by P by 844.0 centimetres of mercury. From this we see how unsafe it would be to speak, without further limitations, of the dissociation tension of mercuric oxide at a given temperature,... 

Cupric oxide dissociates into cuprous oxide and oxygen. Two independent components, cuprous oxide and oxygen, form the system, which] bdow a certain temperature is divided among three phases, solid cupric oxide, solid cuprous oxide, oxygen gas. The system is monovariant, admitting a curve of dissociation tensions. 

These phenomena are very sharply produced when the temperature does not exceed a certain limit, near the fusing-point of gold beyond this limit the two oxides exist in contact as a fused mass instead of forming two solid phases, th form but one liquid phase from mono variant the system becomes bivariant at a given temperature there can be no longer any definite dissociation tension. 

According to the first hypothesis the system is divided into three phases, which are ammonia gas, the solid C, the solid am-moniacal compound the system is therefore monovariant for each temperature T should correspond a definite dissociation tension P if the ammonia gas is removed, a certain quantity of the soliri ammoniacal compound will be dissociated so as to restore the value P to the tension of the gaseous atmosphere, and this operation may be repeated several times up to the moment when the whole of the solid ammoniacal compoimd is destroyed if ammonia gas is now added, the excess of gas will be absorbed by the solid C and the tension brought back to the value P at the moment when the whole of the substance C has changed over to the state of ammoniacal compound. 

Nature furnishes us with a certain number of hydrated silicates which mineralogists call zeolites the dehydration of certain hydrates offers curious peculiarities analcime, for instance, may be completely dehydrated without any sudden variation in form or optical properties of the crystals being observed Georges Friedel has shown that analcime had not, at a given temperature, an invariable dissociation tension let us suppose the temperature constant in a first equilibrium state the tension of the water vapor which exists in equilibrium above the crystals has the value P remove a portion of this water vapor the analcime will undergo a certain dehydration and the tension of the water vapor will increase, but only to a value P, less than P and BO on analcime is therefore not a definite hydrate, but only a solid solution in which water is mixed with an anhydrous silicate. 

The existence of a dissociation tension does not always prove the existence of a ilefinite compound. Dissociation of palladium hydride.— The absence of a fixed dissociation allows also, in certain cases, to demonstrate that a body is not a definite compound prudence must be exercised in concluding from the existence of a fixed dissociation tension the existence of a definite compound the study of the absorption of hydrogen by palladium will show us that such a conclusion must sometimes be accepted with caution. 

Troost and Hautefeuille interpreted these observations by admitting that there is first formed a hydride of palladium, of definite composition, to which they assign the formula Pd,H it ia only when all the palladium ivould have passed over to the hydride that the hydride in its turn would absorb hydrogen, forming a solid hydrogen solution to the first form of reaction corresponded a fixed dissociation tension, while the second would be characterized by the absence of such a tension. 

From the existence of a fixed dissociation tension, the only consequence which we may deduce with certainty is that the system is monovariant and as the system is certainly formed of two independent components, palladium and hydrogen, this conclusion is equivalent to the following the system is divided into three phases. 

As long as the pressure tt is less than the dissociation tension P of the hydrogenized palladium at the temperature T, the palladium docs not absorb hydrogen the representative point describes a segment OP of the line Ott. 

While m is less than s, the hydrogenized palladium is composed of a single solid solution Hie system, which includes only two phases, is bivariant it does not have a fixed dissociation tension the tension of the gaseous hydrogen for the system in equilibrium increases with the amount of hydrogen held in the solid solutions the representative point describes a curve Oa which rises from left to right. 

Let us take as a second example a monovariant system formed of calcium carbonate, carbonic acid gas, and lime denote by P the dissociation tension of calcium carbonate for the temperature T if, at the same temperature, the pressure is greater than P, the system will be the seat of a reaction accompanied by a decrease in volume, that is, of a combination of carbonic acid gas with lime if, on the contrary, the pressure is less than P, the system will be the seat of a reaction accompanied by an increase in volume, or of dissociation of calcium carbonate. 

At a given temperature and under a pressure equal to the dissociation tension for this temperature, the combination of carbonic acid gas with lime is accompanied by a decrease in volume of the system and by a liberation of heat the curve of dissociation tensions of calcium carbonate should rise from left to right. 

Let us suppose the compound exothermic taking place at the temperature 0, under a pressure equal to the dissociation tension Py the formation of a gramme of the compound sets free L calories the formation of w grammes of the compound liberates Q—wL calories. 

Much more satisfactory applications of Gapeyron s formula to dissociation phenomena have been made by Bonnefoi. They treat of the combinations of lithium chloride and lithium bromide with ammonia and the amines the following reactions liberate a quantity of heat Q which has been both measured by thermochemical methods (Qobs.) And calculated by Clape nx)n s equation from the dissociation tensions (Qcaio.) ... 

Dissociation tensions.— Dupr4 s formula must apply likewise, as is evident, to dissociation phenomena, when a sin e gas is concerned in the reaction. Bertrand showed, in fact, that formulae of this type could represent in a satisfactory manner the... 

He found that the dissociation tensions of sodammonium, eTTpressed in centimetres of mercury, were very exactly represented between the temperatures of —78 C. and +26. 21 C. by the formiila... 

Between the temperatures of —20 C. and +35 .15 C. the dissociation tensions of potassammonium, measured in the same unit, are very exactly represented by... 

Five barometer tubes, graduated in tenths of a cubic centimetre, and containing ammonia carbonate, were placed side by side in a heating-chamber the first, containing no excess of carbonic acid gas nor of ammonia, gave directly the dissociation tension 7c of ammonia carbonate in vacuo at the temperature of the oven these results are found in column I of the following table. 

T being the dissociation tension of the solid, at the temperature con-sidered, in an enclosure containing no gas at the start. 

The measurement of , joined to the knowledge of (o, allows deriving from equation (12) a first value of Pji the measurement of the dissociation tension x of ammonium cyanide in an enclosure empty at first allows finding from equation (13) another vale of P29 which we shall denote by p/ if the law we are concerned with is true, the two pressures pj, p, should be equal to each other. 

If p, the vapour pressure of solid iodine, is known at any given temperature, we can hence calculate it, the dissociation tension of silver iodide at this temperature. We know the equilibrium between solid silver, silver iodide, and gaseous iodine for all temperatures if the vapour-pressure curve of iodine is given according to classical thermodynamics this is the case if, in addition to the specific heat of solid iodine already required for the calculation of A, we know that of iodine vapour as well, and also either the heat of evaporation and the vapour pressure at one temperature, or the vapour pressures at two temperatures. 

Thermodynamic Potential.—The energy of transformation of condensed systems one into the other ist of course, accessible to calculation if we know their vapour tensions or dissociation tensions in place of these the solubilities may also be employed. 

DER/YNG] Derby, I. H., Yngve, V., The dissociation tensions of certain hydrated chlorides and the vapor pressures of their saturated solutions, J. Am. Chem. Soc., 38, (1916), 1439-1451. Cited on pages 128,130,131,257,269. 

H Sb,Oj is obtained by dissolving antimony trisulphide in an alk uline liquor, which is subsequently sulphurised by the addition of copper sulphate Fresetiius and ohaffner) until a portion of the liquid, filtered off, gives, with an acid, a white precipitate. Aided by heat, it changes gradually to the anhydride form, even in the presence of water. The dissociation tension (Lissoziationsspannung) of this hydrate is greater than the vapour-pressure of water. Serono doubts the existence of this hydrate. 

As chemists, we are so accustomed to this ontological ambiguity that we read passages such as the following with no surprise "Muller-Erzbach bias shown that according to the dissociation tensions of the si ates of magnesium, nickel, and cobalt the seventh water is bound more weakly than the six others" (KT 35). As usual, a macroscopic measure is interpreted at the microscopic level, but the seventh water (siebente Wasser WB 288), with its six stronger sisters, is still somewhat absurd. 

The experimental difficulty of obtaining the isotherms generally lies in the realization of the perfectly controlled low gas pressures that carmot be obtained directly but can be established indirectly starting from a chemical equilibrium with fixed gas pressure (e.g. very low oxygen pressures with the help of dissociation tension of an oxide). 

The obtained solid ABO2 forms a continuous layer, including point defects, ensuring the transport of matter. We assume, in our example, that the initial oxides present a very weak vapor tension that prohibits the transport of cationic species by a gas phase. On the other hand, these oxides can present a dissociation tension so that the transport of ojgrgen can also be ensiued by the gas phase. Moreover, it is legitimate to assitme that these oxides can present anion defects in the presence of a gas phase (oxygen, for example) and thus the transport of oxygen can also be carried out by the sohd phase via anion defects. 

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