Temperature equilibrium dissociation pressure

Since a higher desorption temperature is usually related to a higher equilibrium pressure via Van t Hoff equation, there is a tendency to attribute the kinetic effects to the difference in temperatures when desorption temperature is being changed from a relatively low value to a high value. However, one must remember that the underlying factor is still the difference between the experimental desorption pressure and the equilibrium dissociation pressure. 

Figure 2. Equilibrium dissociation pressure vs. the reciprocal temperature for TiCuH -x...

Figure 2 shows a plot of equilibrium dissociation pressure vs. the reciprocal temperature for TiCuH x. The plot was obtained by selecting a composition near the middle of the plateau in Figure 1 and measuring the dissociation pressure as a function of temperature. Since the volume of the system was very small and a large sample was used, the sample composition was nearly constant during the measurements. The enthalpy of Reaction 7 determined by this method is —75 kj/mol H2, and the entropy change is —113 J/deg mol H2. 

This paper present new clathrate equilibrium dissociation pressure data for the binary phenol-carbon dioxide, p-cresol-methane and ternary water-phenol-carbon dioxide over a range of temperatures above the normal melting temperature of phenol and p-cresol. 

The experiment began by charging the equilibrium cell with about 30 cm3 of either phenoPp-cresol or phenol-water solution mixture. The cell was then pressurized with either methane or carbon dioxide until the phenol clathrate formed under sufficient pressure. The systems were cooled to about 5 K below the anticipated clathrate-forming temperature. Clathrate nucleation was then induced by agitating the magnetic spin bar. After the clathrates formed, the cell temperature was slowly increased until the clathrate phase coexisted with the liquid and vapor phases. The nucleation and dissociation steps were repeated at least twice in order to diminish hysteresis phenomenon. The clathrates, however, exhibited minimal hysteresis and the excellent reproducibility of dissociation pressures was attained for all the temperatures and found to be within 0.1 K and 1.0 bar at each time. When a minute amount of phenol or p-cresol clathrate crystals remains and the system temperature was kept constant for at least 8 hours after attaining pressure stabilization, the pressure was considered as an equilibrium dissociation pressure at that specified temperature. 

Figure 3 shows the clathrate equilibrium measurements for p-eresol-methane and the results are presented in Table 2. Contrary to phenol-carbon dioxide, the clathrate equilibrium dissociation pressure line for p-cresol-methane monotonously increases with temperature. This dependence is similar to that observed with gas hydrates and clathrates. 

This expression shows that the rate of gas evolution should increase until steady state concentrations of all oxidizing species are reached. After this time, tiy the concentrations of oxidizing intermediates remain constant and depend on the equilibrium dissociation pressure of HP at the experimental temperature. The rate then remains constant until the HP becomes depleted. 

Crystals of the heptahydrate possess the same vapour tension at 44 01° C. as magnesium sulphate, MgS04.7H20. Below this temperature their dissociation pressure is greater, and above it is less, than that of the magnesium salt.5 In the case of zinc sulphate, ZnS04.7H20, the equilibrium temperature between the two salts is 16 4° C.6... 

The equilibrium dissociation pressures in the H/Zr composition range of 1.55-1.7 at temperatures up to 1300°C agree closely with the values obtained from extrapolation of the reported data, which extend to 950°C However, for H/Zr of 1.4-1.5, hydrogen dissociation pressures are lower than the values extrapolated from below 950°C as a result of phase changes at the elevated temperatures. ... 

The hydride bodies should be elad to prevent loss of H2 at elevated temperatures. The equilibrium dissociation pressure of H/Zr = 1.6 is about 1 Pa at 300°C. An oxide film or cladding may act as barrier to H2 loss ° . 

Iron oxides The equilibrium dissociation pressure of oxygen over iron(III) oxide has been measured at temperatures up to 1300 K. Values are lower [64] when Fc304 is present in the FCjOj (900 to 1200 K). Braun and Gallagher [65] prepared P-FcjOj by dehydration of tetragonal P-FeOOH in vacuum at 440 K. This modification was more stable than the y-form and transformed to a-FcjOj on heating in air at 670 K. 

The relationship between the equilibrium dissociation pressure (P) of a hydride and the absolute temperature (T) is expressed by the van t Hoff equation, i.e.. 

The invariant plateau pressure is the equilibrium dissociation pressure of the hydride at the temperature of the isotherm and is a measure of the stability of the hydride. After complete conversion to the hydride phase, further dissolution of dihydrogen in the non-stoichiometric hydride takes place as the pressure increases, see eq. (4). 

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... 

The system, therefore, is at equilibrium at a given temperature when the partial pressure of carbon dioxide present has the required fixed value. This result is confirmed by experiment which shows that there is a certain fixed dissociation pressure of carbon dioxide for each temperature. The same conclusion can be deduced from the application of phase rule. In this case, there are two components occurring in three phases hence F=2-3 + 2 = l, or the system has one degree of freedom. It may thus legitimately be concluded that the assumption made in applying the law of mass action to a heterogeneous system is justified, and hence that in such systems the active mass of a solid is constant. 

At a fixed temperature, AGr is constant, and so the equilibrium oxygen partial pressure, po2, will also be constant. This oxygen pressure is called the decomposition pressure or dissociation pressure of the oxide and depends only upon the temperature of the system. 

This is another example of the application of thermogravimetry for determination of equilibrium temperatures in dissociation studies. This also enables one to calculate the heat of dissociation from the linear relation between log of dissociation pressure and 1/T. Determination of the specific heat by means of DTA was used afterwards for conversion of the heat of dissociation into the standard values of formation at 298 °K. Ba02 was chosen for these investigation56 because it has been investigated in the past by calorimetric methods and therefore gives a possibility for comparing those values obtained from static methods with those obtained from values from dynamic methods. 

We define the number of components in a system as N R, which is also the minimum number of chemical species from which all phases in the system can be prepared. Each equilibrium relationship decreases by one the number of species required to prepare a phase. Thus, the quantity (N — R) in Equation (13.12) is equivalent to C in Equation (13.9). For example, water in equilibrium with its vapor at room temperature and atmospheric pressure is a one-component system. Water in equilibrium with H2 and O2 at a temperamre and pressure at which dissociation... 

Later, the energy aspects of combustion were studied, beginning with determination of the heat of combustion of various compounds and the pressures and temperatures which develop in explosions, and ending with the application of chemical thermodynamics to questions of equilibrium, dissociation and completeness of combustion in flames (1820-1900). 

In Figure 7.34 the following initial points are used (with C,D,E,F corresponding to letters on Well No. 109 in the reservoir diagram of Figure 7.30) AB = hydrate equilibrium line C = temperature at the top of the pay zone D = temperature at a level of gas and water contact E average gas-hydrate temperature F = temperature at boundary surface between gas and gas-hydrate reserves H = beginning dissociation pressure for gas hydrates. 

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