Enthalpies of liquefaction

The value for ethane was estimated using a derived enthalpy of liquefaction (the negative of the generally calculated enthalpy of vaporization), and numerically is ca —13 kJ moR. ... 

Figure 1. A. Differential heat of adsorption of water at 303 K on (a) A50, (b) Tri, (c) Qzpl (d) Cris and (e) Qzm outgassed at 423 K for 2 h. B. Differential heat of adsorption of water at 303 K on (a) Qzm outgassed at 1073K (b) Qzpl, (c) QmHF and (d) Qzm. Dotted line indicates the latent enthalpy of liquefaction of H20, -AHL= 44 kj mol-1.

The combined use of adsorption microcalorimetry and IR spectroscopy allows a precise evaluation of the hydrophilic or hydrophobic character of the silica powders investigated.10"12 The evolution of the enthalpy of adsorption of water with coverage (Figure 1) indicates the coverage values at which the enthalpy falls below the latent enthalpy of liquefaction which can be taken as a... 

Structure. At very low 6, i 1 may be higher because of attachment of adsorptive molecules to grain boundaries between the single crystals of graphite, whereas after completion of the monolayer, the enthalpy drops to (almost) the enthalpy of liquefaction (dashed line in the figure). In fig. 1.8b there Is no such plateau, although from the measurements with benzene It was deduced that the surface is homogeneous. The rise in I I was attributed to lateral interaction... 

The variation In lsosterlc heat of adsorption for n-nonane on T-300T fibers with surface coverage Is presented In Figure 3. Also Included are the values for qgt at T 0, obtained by adding RT to the differential heat of adsorption, q, at zero coverage, the monolayer capacity for n-nonane at 50°C and the enthalpy of liquefaction for n-nonane at 50°C, -AH. ... 

Figure 3. The isosteric heat of adsorption of n-nonane as a function of the carbon fiber (T-300T) surface coverage at 50°C. AH is the enthalpy of liquefaction. rm represents the mono-layer coverage.

Enthalpy of liquefaction of the probe vapour (kJ/mol). Surface tension or surface free energy of liquid (mN/m). 

CH3OH vapor, obtained by distilling in vacuo liquid methanol (Sigma-Aldrich), was rendered gas-free by several freeze-pump-thaw cycles. The vapor pressure of CH3OH at T = 303 K is 164 Torr, and the standard molar enthalpy of liquefaction (i.e. the latent heat of liquefaction, qi) is -AlH = 38kJ moP. ... 

Physisorption is intrinsically weak and is characterized by heats of adsorption relatively small, close to the enthalpy of liquefaction of the adsorptive, typically comprised in the 5-45 kJ mol range it is in general favored by temperatures close... 

In all cases, at increasing equilibrium pressure the adsorption approaches the adsorptive liquid phase, and the adsorption enthalpy approaches the adsorptive latent enthalpy of liquefaction AlH, as typically occurs for physical adsorption at [28, 56, 85, 104],... 

The shift in the C=C frequency, vi, for adsorbed ethylene relative to that in the gas phase is 23 cm-1. This is much greater than the 2 cm-1 shift that is observed on liquefaction (42) but is less than that found for complexes of silver salts (44) (about 40 cm-1) or platinum complexes (48) (105 cm-1). Often there is a correlation of the enthalpy of formation of complexes of ethylene to this frequency shift (44, 45). If we use the curve showing this correlation for heat of adsorption of ethylene on various molecular sieves (45), we find that a shift of 23 cm-1 should correspond to a heat of adsorption of 13.8 kcal. This value is in excellent agreement with the value of 14 kcal obtained for isosteric heats at low coverage. Thus, this comparison reinforces the conclusion that ethylene adsorbed on zinc oxide is best characterized as an olefin w-bonded to the surface, i.e., a surface w-complex. 

Enthalpies of wetting are sometimes used to obtain (integral) enthalpies of adsorption by subtracting the enthalpy of condensation. This procedure is not exact because it presupposes a model in which the interaction between the first and the second surface layer is Interpreted as purely identical to that in condensation (BET theory assumes the same). However, the heat of adsorption of the second layer Is not exactly Identical to the heat of liquefaction and the configuration of the first layer is affected by the presence of a second. In other words, entropic factors also have to be considered, and. in this connection, the packing in the first layer must be known to convert A H (in J m ) into A H (in J mol 2). Notwithstanding these reservations, a certain similarity may be expected. 

As for the standard enthalpies of adsorption for alkanes, these are normally very close to those of liquefaction in the case of low surface area graphites, and for polar molecules... 

The standard enthalpies of adsorption (Table 15) of these molecules on the thermally treated aluminosilicate support the former explanation. So the adsorption of benzene on H-473 is the most exothermic due to the specific interaction with the silanol groups. When the thermal treatment progresses, the process becomes less and less exothermic due to i) the partial dehydroxylation of the surface (H-673), ii) the change in the chemical surface groups (siloxane groups on H-873 and H-1073) and iii) the destruction of the chemical surface groups (H-1273). As a consequence of the latter process the standard enthalpy of adsorption of benzene on H-1273 is very similar to the heat of liquefaction. 

For n-hexane and cyclohexane, similar trends in AH are found as for benzene, due to the continuous destruction of the pores smaller than 3.7 nm in diameter so that when this pore volume is negligible, the standard enthalpy of adsorption is lower than the heat of liquefaction. 

Adsorption enthalpy Wide range, related to the chemical bond strength— typically 40-800 kJ/mol Related to factors like molecular mass and polarity but typically 5 0 kJ/mol (i.e. heat of liquefaction)... 

The dynamic nature of the frameworks in MIL-53 (Cr and Al) also results in very interesting CO2 and CH4 adsorption behaviours (Fig. 11) [133]. CH4 adsorption isotherms for MIL-53 are typical of those obtained with microporous materials such as zeolites and activated carbon. At 35 bar and 304 K, the amount adsorbed is 155 (Al) and 165 (Cr) v(STP)/v. The adsorption enthalpy of CH4 remains almost constant at 17 kJ moP from 0 to 10 bar, which is well above the liquefaction enthalpy of CH4 (8.2 kJ moP ). The maximum CO2 adsorption is 10 mmol g for MIL-53(A1), observed at 304 K and 30 bar. This is a moderate capacity however, the shape of the CO2 adsorption isotherm is very unusual and shows a first adsorption step below 1 bar adsorbing 2 mmol g CO2, followed by a second adsorption step at around 6 bar. Interestingly, no steps were found to be present in the CO2 desorption process. The corresponding adsorption enthalpy for CO2 on MIL-53(A1) is around 35 kJ moP during the initial adsorption process up to 6 bar, which is followed by a sharp decrease to around 17 kJ moP close to the liquefaction enthalpy of CO2 (17.2 kJ moP ). MIL-53(Cr) exhibits similar behaviour in CO2 adsorption the first step in CO2 adsorption in dehydrated MIL-53(Cr) disappears if the sample is hydrated [135]. For hydrated MIL-53(Cr) CO2... 

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