Vapour pressure variation with temperature

Plotting, therefore, the difference in rate at two fixed pressures of hydrogen against temperature gives the variation with temperature of the homogeneous part of the reaction. This variation is partly due to the change with temperature in the concentration of the sulphur vapour... 

Thus, if we find how the electromotive force changes when the temperature of the cell is altered on open circuit, i.e., when no current is passing, we can at once calculate A, the latent heat, just as we can calculate the latent heat of evaporation of a liquid when we know the variation of its vapour pressure with temperature. Since E changes only slightly with T, we can evaluate dE... 

The vapour pressure of Zn as a function of temperature, which implicitly also shows the variation of the boiling temperature with pressure, is shown in Figure 2.5. 

It is a well known fact that the boiling point of a liquid is the temperature at which its vapour pressure becomes equal to the external pressure. So, equation (6) represents the variation of boiling point of a liquid with pressure P. Thus equation (6) can also be written as ... 

The variation of vapour pressure with temperature is given by ... 

From eqn.(3.6) we conclude that there are two solute-dependent factors that affect retention. In the first place, this is the vapour pressure of the pure solute. p° is a strong function of the temperature (see below) and therefore, temperature may be used as a parameter to influence retention. However, the vapour pressure is a pure component property and it cannot be changed at will. Differences in the vapour pressure of two solutes (or differences in variation of vapour pressure with temperature) may or may not provide us with a means to achieve separation. When the vapour pressure is not sufficiently different, we need to create differences in the second solute-dependent factor. 

Eqn.(3.6) provides a good insight into the variation of retention with temperature in GLC. Both the activity coefficient and the vapour pressure of the solute vary with temperature in an exponential way. For the activity coefficient we can write... 

The variation of surface-vapour pressure with temperature has not yet been measured the difficulties of sufficiently accurate measurement except at room... 

The Clausius-Clapeyron equation gives the variation of the vapour pressure p of a liquid with absolute temperature T. To derive the relationship involves integration of the expression... 

Uranium hexafluoride yflelds jglistening, colourless or pale yellow, monoclinie crystals, which fume in the air and sublime under reduced pressure at ordinary temperature. It boils at 56-2° C., and the calculated mean latent heat of evaporation between 42° and 57° C. is 29 4 calories per gram ( = 10360 calories per gram molecule). The variation of the boiling-point with the vapour pressure is as follows ... 

The most diverse empirical formulae for the variation of vapour pressure with temperature have been proposed by various investigators (see Winkelmann, Handb. d. Physik, vol. iii. p. 949). 

The differential heat of solution is of theoretical importance in the calculation of the variation of solubility with pressure. If a saturated solution containing solid solute is subjected to a pressure greater than the vapour pressure of the solution, the gaseous phase disappears and the system becomes divariant (two phases and two components). The concentration of the saturated solution (i.e. the solubility) is then a function of the pressure as well as of the temperature. When a condensed system of this kind is subjected to a further change in pressure the solid solute and the solution will not remain in equilibrium unless the temperature is changed simultaneously. As in the analogous case of the variation with pressure of the melting point of a pure substance (p. 221), the Clausius equation assumes the form — L/ ... 

Dilute solutions. As has already been stated (p. 266), the relationship between the osmotic pressure of a solution and the concentration and chemical character of solvent and solute cannot be derived from purely thermodynamical considerations. There are several ways of attaining this end. In the first instance, the variation of the osmotic pressure with the concentration can be determined experimentally, and the results embodied in an empirical equation of the form p=/(c). Or we may deduce relationships from kinetic conceptions of the nature of solutions, in much the same way as the gas laws were deduced. Or, finally, we may deduce the osmotic pressure laws, with the aid of the thermodynamical equations of the previous paragraph, from empirical or theoretical researches on the vapour pressure of solutions. These methods all lead to the same result, that the osmotic pressure of dilute solutions obeys the same laws as the pressure of a perfect gas. In other words, the osmotic pressure of a substance in solution is equal to the pressure which the substance would exert in the form of a perfect gas occupying, at the same temperature, the volume of the solution. 

VARIATION OF THE WATER VAPOUR PRESSURE WITH TEMPERATURE... 

The increased motion of the molecules of the liquid following an increase of temperature leads to a greater tendency for escape of molecules into the vapour phase with a consequent increase of vapour pressure. The variation of vapour pressure with temperature may be expressed in terms of the molar enthalpy of... 

Solutions of macromolecules in a low molecular weight solvent show this phenomenon to a very much greater degree. Fig. 25.5 shows the activity of toluene determined from the vapour pressure as a function of concentration for the toluene-rubber system.t The experimental points correspond to Yiosmotic pressure of these solutions as a function of temperature the variation of In y with T, and hence the heat of solution (c/. 24.14) of toluene in the mixture can be evaluated. The value found in this way is very small and is quite incapable by itself of accounting for the observed deviations. 

Vaues of a may be very much less than unity and be temperature dependent. Somoijai and Lester [40] comment that "all the kinetic information is contained in the evaporation coefficient and its variation with conditions of vaporization", and they recommend the avoidance of the use of ot, in describing the rates of evaporation of solids under non-equilibrium conditions. The rate of sublimation is dependent on the attaimnent of sufficient energy by suitably disposed siuface molecules (possibly accompanied by electron or proton transfer in ionic solids). The overall rate of reactant removal is sensitive to the presence of impurities at the surface. The reverse reaction may be significant if the volatile material is not immediately removed from the vicinity of the reactant particles. Arrhenius parameters measured for sublimation processes may include a term which represents a temperature dependent concentration of surface intermediates [42]. The observation that measured evaporation rates are lower than those estimated from equilibrium vapour pressures suggests that the kinetics may be determined by a surface dissociation that precedes evaporation. This view is supported by evidence that, in selected systems, specific additives can considerably promote evaporation rates. For example [40], the evaporation rate of red phosphorous between 550 and 675 K was found to be increased by three orders of magnitude by the presence of thallium. 

The enthalpy changes obtained from the variation of the vapour pressure with temperature are (Se02, cr, 600 K) = (109.90 + 1.50) kJ-mol and (Se02,... 

Bakeeva, Pashinkin, Bakeev, and Buketov [73BAK/PAS] measured the selenium dioxide pressure over gold selenite in the interval 489 to 599 K by the dew point method. The pressure was calculated from the dew point temperature by the relationship for the saturated vapour pressure in [69SON/NOV]. The data in the deposited VlNITl document (No. 4959-72) have been recalculated with the relationship selected by the review. The enthalpy and entropy changes obtained from the temperature variation of the equilibrium constant are A //° ((V.123), 544 K) = (576.8 13.0) kJ-mol and A,S° ((V.123), 544 K) = (899.4 + 24.0) J-K -mor. The uncertainties are entered here as twice the standard deviations from the least-squares calculation. 

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