Iodine vapour, dissociation

Let us consider a particular reaction. Iodine vapour dissociates into atoms, to a progressively greater extent as the temperature is raised. Is this fact amenable to thermodynamic treatment This equation shows us that it is. Looking at it qualitatively, we see that the sign of the right-hand side is the same as that of AH. For the reaction ... 

The vapour pressure ratio of actinides to noble metals is also the basis of the actinide metal preparation by thermal dissociation of intermetallic compounds. Such intermetallic compounds of An and noble metals can be prepared by hydrogen reduction of a mixture of an An oxide and a finely divided noble metal (Pt, Ir.. in the absence of noble metals, hydrogen reduction of An oxides is impossible. Am and Cm metals have been obtained by thermal dissociation of their intermetallic compounds with Pt and Ir High purity Th and Pa, the least volatile actinide metals, can be prepared by thermal dissociation of their iodides, which form readily by reaction of iodine vapour with car-... 

The iodine vapours act as transporting agent in this modified version of the van Arkel procedure ". By replacing the resistance heated dissociation wire by an induction heated metal sphere (W or Th, respectively) large samples of Th or Pa crystals can be... 

Similar results have been observed with iodine, but the dissociation 9 is much more marked at even lower temp. The theoretical density for the two-atom molecule is 8758, air unity or 126-92 oxygen=16 and for the one-atom molecule the theoretical density is 4 379, air unity. When iodine vapour is heated above 700° its density diminishes steadily up to about 1700°, when it becomes constant at half its value at the lower temp. 

Without doubt, the iodine molecule, I2, dissociates into atoms I2=I+I. The state of the system in equilibrium will be represented by kCi2=k Ciz. If x denotes the proportion of iodine dissociated, and v the volume of the iodine vapour, then, Bince v volumes of iodine vapour become 2v volumes of dissociated iodine vapour, it follows that the concentration of the dissociated iodine will be xjv, and of the iindissociated iodine (1—x)/v. Hence for equilibrium... 

In every mol. of iodine (I2) at 1043°, 0-25 mol. will be dissociated hence, a 2=0-0625 1— =0 75 and K=0 0833/v. To evaluate v, remember that one mol. of iodine vapour at 0° and 760 mm. occupies 22 3 litres and at 1043°, 107-5 litres. This quantity of gas contains 0 25 more molecules of iodine because of dissociation, and hence its volume is 107 5+J of 107-5=134-4 litres. Hence A=0 0833 -134-4 =0-00062 oi k A =0-00062 1 or 1 1600 (nearly). Otherwise expressed, Ct2=1600 Ci2, that is, the atoms of iodine will unite 1600 times as fast as the molecules dissociate under such conditions that unit concentration of each is present. The dissociation of iodine molecules is a unimolecular reaction because one molecule is concerned in the reaction and the formation of the two-atom molecule by the union of two one-atom molecules is a bimolecular reaction because two molecules are concerned in the process. 

S. Landau and E. Stenz examined the effect of low temp, and dissociation on the fluorescence of iodine vapour at low press. Fluorescence decreases as the temp, is raised, but does not cease at 800°. Dissociation destroys both fluorescence and the resonance spectra. It is therefore inferred that the complex vibrating system is not inherent in the atom, but in the molecule that the structure of the atom is relatively simple and that, in all probability, the absorption lines which are so characteristic of diatomic iodine and so sensitive to the action of monochromatic light, do not belong to the absorption spectrum of monatomic iodine. 

As a third example of the utility of the approximate equation (11a), let us calculate the dissociation of iodine vapour (l2=2I).f In this case the heat of reaction is unknown, but the observations are nevertheless in agreement with an equation of the form (11a). We have here... 

Iodine monobromide /77S9-55-57 M 206.8, m 42 . The brown-black crystals are purified by iqjeated fractional crystallisation from its melt. The vapour dissociates on heating [Yost et al. J Amer Chem Soc 5 5 5521933, Schmeisser in Handbook of Preparative Inorganic Chemistry (Ed. Brauer) Academic Press Vol I pp 291-292 1963]. 

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. 

The authors studied the thermal dissociation of thorium iodide mass-spectrometrically, and suggested that substantial decomposition to lower iodides and iodine vapour occurs when both the solid and liquid are heated. Coarse ciystals of Thl4(cr), prepared from the elements, were used to minimise the oxygen and water contents of the samples used in the (unspecified) Knudsen cell. The variation of the intensities of the TI1I4, TI1I3, TI1I2,12 and I ions observed in the mass-spectrometer was studied from ca. 625 to 800 K. 

Iodine is a dark-coloured solid which has a glittering crystalline appearance. It is easily sublimed to form a bluish vapour in vacuo. but in air, the vapour is brownish-violet. Since it has a small vapour pressure at ordinary temperatures, iodine slowly sublimes if left in an open vessel for the same reason, iodine is best weighed in a stoppered bottle containing some potassium iodide solution, in which the iodine dissolves to form potassium tri-iodide. The vapour of iodine is composed of I2 molecules up to about 1000 K above this temperature, dissociation into iodine atoms becomes appreciable. 

T. E. Thorpe s formula for the specific volume i> at the temp. 0 is u=l +00009158960 +0 0000008329602 +0 00000000275O03. The vapour density is that theoretically required for IC1 with oxygen 32, the value for IC1 is 162 38 at 120°, the vapour density is 1606, and at 512°. 156 4. Conclusions as to the degree of dissociation at different temp, cannot be derived from the vapour density determinations since it proceeds without changing the number of molecules—2ICI—>I2+C12- Iodine trichloride vapour is almost completely dissociated into the monochloride and chlorine. K. Beck s value for the viscosity is 7 029 at 15°, 5 069 at 28 4°—benzene at 5° unity. 

J. J. van Laar has shown how the form of the vap. press, curves of a liquid mixture can furnish an indication, not a precise computation, of the degree of dissociation of any compound which maybe formed, on the assumption that the different kind of molecules in the liquid—12, Br2, and IBr—possess partial press, each of which is equal to the product of the vap. press, of a given component in the unmixed state and its fractional molecular concentration in the liquid. It is assumed that in the liquid, there is a balanced reaction 2IBr I2-)-Br2, to which the law of mass action applies, where K is the equilibrium constant, and Clt C2, and C respectively denote the concentration of the free iodine, free bromine, and iodine bromide. From this, P. C. E. M. Terwogt infers that at 50 2°, K for the liquid is 7j and that for iodine monobromide about 20 per cent, of the liquid and about 80 per cent, of the vapour is dissociated. That the vapour of iodine monobromide is not quite dissociated into its elements is evident from its absorption spectrum, which shows some fine red orange and yellow lines in addition to those which characterize iodine and bromine. In thin layers, the colour of the vapour is copper red. 0. Ruff29 could uot prove the formation of a compound by the measurements of the light absorption of soln. of iodine and bromine in carbon tetrachloride. 

Now trichloride of iodine in the fused state is partially dissociated into chlorine and iodine monochloridc, as is shown by the fact that the vapour in equilibrium with the liquid contains chlorine and a little of the monochlorid(i. The pressure curve, of which is a point, corresponds exactly to the pressure curve for hydrated calcium chloride given in Fig. 31 by there is a vertical tangent... 

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