Benzene vapor pressure, variation with temperature

FIGURE 8.4 The variation of the vapor pressure of liquids with temperature, for diethyl ether (orange), benzene (red), ethanol (green), and water (blue). The normal boiling point is the temperature at which the vapor pressure is 1 atm (760 Torr). 

Trifluoromethyl)phenylcopper was found to be an octamer by consideration of the kinetics of its decomposition, and by cryoscopy and vapor pressure osmometry in benzene solution 36). Its F NMR spectrum in ether at room temperature is a sharp singlet. Consequently, the suggested structure is a central copper cube with equivalent bridging benzotri-fluoride groups. The initial decomposition product, Cu8( n-CF3CgH4)e, is considered to be a Cu(0)—Cu(I) octanuclear cluster compound 36). For the octameric m-(trifluoromethyl)phenylcopper, the tetrameric ortho isomer, and pentafluorophenylcopper tetramer, the F NMR spectra were found to vary with temperature. The changes are not considered to involve important structural alterations, but only variations in solvent complexes and rotamer populations 32, 37). The spectra also... 

Emission lifetimes for benzene with vapor phase are subject to similar variations with pressure and excitation wavelength as are fluorescence yields. Data are collected in Table 6 and can be seen to show considerable variation with source and experimental technique. Recent measurements, using a single-proton counting technique (114) have shown the emission lifetimes of high pressure CgHg and CgDg to be 77 and 92 ns, respectively, at 25°C. The temperature dependence of the fluorescence lifetime is also shown in Fig. 7. 

Below its boiling point the variation of the vapor pressure of benzene with temperature is given by... 

At 70 C, the vapor pressures of carbon tetrachloride and benzene are 617.43 and 551.03 mm, respectively at 50 C, the values are 312.04 and 271.34 mm., respectively [Scatchard, Mochel and Wood, J. Am, Chem, Soc., 62, 712 (1940)U. Assuming ideal behavior, since the actual deviations are small, plot the curve giving the variation of the (mole fraction) composition of the vapor with that of the liquid at each temperature. Would the separation of the two components by fractional distillation be more efficient at high or at low temperature ... 

To determine the overall effect on pressure error of normally distributed variation in temperature, the Antoine equation must be employed. Thus, the pressure error is a function of the compound of interest. To focus on typical situations, three compounds representative of the range of application of the isoteniscope were used. These were benzene with a relatively high vapor pressure ("true value" = 95 torr at 25°C) toluene for a medium vapor pressure ("true value" = 28 torr at 25°C) and chlorobenzene for a low vapor pressure ("true value" = 12 torr at 25°C). 

A variation on this technique is twin ebulliometers. In this technique, two matched ebulliometers are connected to the same external pressure at the top of the condenser. A standard substance with accurately known vapor pressure is placed in one ebulliometer and the test sample in the other. When steady boiling is attained in both sides, they are at the same pressure. Pressure is not measured directly rather the two boiling temperatures are measured. Pressure is established by converting the boihng point of the standard to pressure using a previously determined relationship. For organic liquids, water, benzene, or decane are often used as standards. 

Because these constants differ for various compounds, the vapor pressures for different components usually do not change at the same rate with temperature variations. For an ideal system, the relative volatility between the two components, as expressed by Equation 7-6, varies with the boiling temperature. As an example, consider the separation of a system of toluene and ethyl benzene. Assume that the separation produces substantially pure toluene as distillate and equally pure ethyl benzene as bottoms. If the distillation column is operated at atmospheric pressure, the relative volatility between these two components is 2.1. If the distillation pressure is raised to 5 atmospheres absolute, the relative volatility is reduced to 1.8. However, if the pressure of distillation is lowered to 200 mm Hg absolute, the relative volatility increases to 2.3. Thus, separation becomes easier as the column pressure is reduced. 

---