Dissociation pressures

The dissociation pressure of pure carbamate has been iavestigated extensively (10—12) and the average values are shown ia Table 5. 

Uthium Mydride. Lithium hydride [7580-67-8] is very stable thermally and melts without decomposition. In the temperature range 600—800°C, the dissociation pressure for hydrogen, Pp, in units of kPa is expressed by... 

Sodium Hydride. Sodium hydride [7646-69-7] decomposes to its elements without melting, starting at ca 300°C. Decomposition is rapid at 420°C. The dissociation pressure in kPa between 100 and 600°C for the decomposition range 15—90% NaH can be found from... 

Magnesium titanium alloys form the hydrides Mg2TiHg [74811-18-0] and MgTi2H [58244-88-5] (17). Traces of a third metal are often added to adjust dissociation pressures and/or temperatures to convenient ranges. 

It can be readily calculated that pyrite will exert a sulphur dissociation pressure of 1 atmos only at 1512K. However, when the sulphide reacts with air the main gaseous product is SO2, and the reaction is then... 

The sulphur dissociation pressure of Fe/FeS at the matte blowing temperature of about 1573 K can be calculated from the following data... 

Figure 8.19 F.llingham diagram for the free energy of formation of metallic oxides. (After F. D. Richardson and J. H. F. Jeffes, J. Iron Steel Inst. 160, 261 (1948).) The oxygen dissociation pressure of a given M - MO system at a given temperature is obtained by joining on the lop left hand to the appropriate point on the M-MO frec-energy line, and extrapolating to the scale on the right hand ordinate for POi (atm).

Gaseous SO2 is readily soluble in water (3927 cm SO2 in lOOg H2O at 20°). Numerous species are present in this aqueous. solution of sulfurous acid" (p. 717). At 0° a cubic clathrate hydrate also forms with a composition S02.6H20 it.s dissociation pressure reaches I atm at 7.1°. The ideal composition would be SO2.55H2O (p. 627). 

Molten I2CI6 has been much less studied as an ionizing solvent because of the high dissociation pressure of CI2 above the melt. The appreciable electrical conductivity may well indicate an ionic self-dissociation equilibrium such as... 

Consider Ni exposed to Oj/HjO vapour mixtures. Possible oxidation products are NiO and Ni (OH)2, but the large molar volume of Ni (OH)2, (24 cm compared with that of Ni, 6.6 cm ) means that the hydroxide is not likely to form as a continuous film. From thermodynamic data, Ni (OH)2 is the stable species in pure water vapour, and in all Oj/HjO vapour mixtures in which O2 is present in measurable quantities, and certainly if the partial pressure of O2 is greater than the dissociation pressure of NiO. But the actual reaction product is determined by kinetics, not by thermodynamics, and because the mechanism of hydroxide formation is more complex than oxide formation, Ni (OH)2 is only expected to form in the later stages of the oxidation at the NiO/gas interface. As it does so, cation vacancies are formed in the oxide according to... 

With copper alloys containing more noble metals the oxide will be substantially pure copper oxide since the oxides of the noble metals have higher dissociation pressures than the copper oxides. With alloys containing baser metals, however, the alloying element will appear as an oxide in the scale, often in greater concentration than in the alloy itself, and sometimes to the exclusion of copper oxides. The dissociation pressures of many oxides have been calculated by Lustman... 

Variation in the pressure of the reacting gas can affect corrosion processes in two ways. In the cases more usually met with in practice, in which the corrosion rate is controlled by diffusion processes in the surface film of corrosion product, the influence of gas pressure on corrosion rate is slight. If, however, the dissociation pressure of the oxide or of a constituent of the scale lies within the range involved, the stability of the corrosion product will be critically dependent on the pressure. The effect of temperature is, however, far more critical and thus, in practical cases, pressure variations rarely decide the stability of corrosion products. 

The pressure of oxygen developed by an intimate mixture of a pure metal and its oxide is conventionally referred to as the dissociation pressure (Po ) and it can be seen that at a given temperature T... 

TABLE V. Dissociation Pressures and Degrees of Occupation of Gas Hydrates at 273°K30... 

The equilibria between clathrate and gas, and Qa, clathrate, and gas could be determined by using w-propanol as the auxiliary solvent.53 In the latter equilibrium, the composition of the clathrate is found from the amount of gas required for the conversion of a given amount of solid a-hydroquinone suspended in the propanol solution into clathrate at constant temperature and pressure. The dissociation pressure of the clathrate is given by the total pressure of the four-phase equilibrium -clathrate-solution-gas, corrected for the vapor pressure of w-propanol saturated with a-hydroquinone. Using this technique it was found that the equilibrium clathrates of hydroquinone and argon have yA = 0.34 at 25°C63 and 0.28 at 60°C.28... 

Methane hydrate and propane hydrate crystallize in Structures I and II respectively, their dissociation pressures at — 3°C have been determined and were found to be 23.1 and 1.48 atm. Above a... 

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