Enthalpies of condensation

The way in which these factors operate to produce Type III isotherms is best appreciated by reference to actual examples. Perhaps the most straightforward case is given by organic high polymers (e.g. polytetra-fluoroethylene, polyethylene, polymethylmethacrylate or polyacrylonitrile) which give rise to well defined Type III isotherms with water or with alkanes, in consequence of the weak dispersion interactions (Fig. S.2). In some cases the isotherms have been measured at several temperatures so that (f could be calculated in Fig. 5.2(c) the value is initially somewhat below the molar enthalpy of condensation and rises to qi as adsorption proceeds. In Fig. 5.2(d) the higher initial values of q" are ascribed to surface heterogeneity. 

The enthalpy of formation of the substance that is obtained is for the gaseous state. The enthalpy that corresponds to the gas -> liquid change of state therefore needs to be added. The enthalpy corresponding to the liquid solid change is to be ignored if the enthalpy of formation of a substance in the solid state is required, since there is not much difference between the two. For the enthalpy of formation in the liquid state the enthalpy of condensation is calculated from the equation that bonds it to the entropic factor discussed in Chapter 1, ie ... 

The AH ap sign has to be changed to obtain the enthalpy of condensation. R equals 8.31 Jmol- K-T Eb is given in Kelvin. 

O Some high-efficiency gas furnaces can heat with an efficiency of up to 97%. These gas furnaces work by allowing the water vapour produced during combustion to condense. Condensation is an exothermic reaction that releases further energy for heating. Use the information in this section to demonstrate the increased heat output, using Hess s law. The enthalpy of condensation of water is 44 kj/mol. 

Compare and contrast enthalpy of vaporization and enthalpy of condensation. 

If we wish to know the enthalpy of formation of liquid Se2Cl2, we can estimate the enthalpy of condensation (perhaps from Trouton s rule [13] or by comparison with related sulfur compounds) and can add it to the value of A// , obtained in Equation (4.49). 

Chloroethene (vinyl chloride) is a gas at the temperature considered. Hence, the enthalpy of solution (Awa//,) is given by the sum of the enthalpy of condensation (AcmiHi, which is equal to the negative enthalpy of vaporization) and the excess enthalpy in aqueous solution (Hi) (Fig. 5.1 and Eq. 5-26). Horvath et al. (1982) gives the solubilities of chloroethene at 0°C, 25°C, and 50°C and 1 bar partial pressure. Also given are the vapor pressures of the superheated liquid at these three temperatures. 

Two remarks (1) The area change is negative because the area after the phase transition is lower than before. (2) The enthalpy of condensation is of the order of the standard enthalpies of vaporization of hydrocarbons. 

This involves the heat that must be extracted from the gas to cool it to the trapping temperature and the enthalpy of condensation (or heat of adsorption) that must also be removed. This quantity increases with gas throughput. 

Adsorption desorption. Gas can stick to surfaces either by physisorption or chemisorption. Generally, in physisorption, there is a weak Van der Waals interaction between the surface and the adsorbed species. The enthalpy of adsorption is about the same as the enthalpy of condensation, e.g. the maximum observed values of the enthalpy of physisorption of H2, N2 and H20 are -84, -21 and 57kJmol-1, respectively. (In chemisorption, the adsorbing species sticks to the surface as a result of chemical bond formation. The energies involved are much greater than in physisorption.)... 

Lyklema (1995) pointed out that, in the absence of immersion calorimetry, the notion of surface hydrophilicity-hydrophobicity remains vague. Once the molar enthalpy of immersion in water is assessed it can readily be compared with the value 44 kJ molwhich corresponds to the enthalpy of condensation of water at room temperature. If it is higher, the surface is considered to be hydrophilic if lower, the surface is defined as hydrophobic. 

It is now apparent that isolated silanols have relatively low affinity for water. Thus, the hydrophobic nature of silica is manifested after dehydroxylation when only the siloxane bridges and some isolated silanols (giving an IR band at c. 3750 cm-1) remain. On the dehydroxylated surface the net adsorption enthalpy for water is negative. In this case, the enthalpy of adsorption is lower than the normal enthalpy of condensation. Application of adsorption microcalorimetry has allowed an assessment to be made of the relative extents of the hydrophilic and hydrophobic areas of the surface (Bolis et al., 1991). On the hydrophilic surface, it appears that water is adsorbed via two hydrogen bonds to two silanols - one acting as the hydrogen donor and the other as the acceptor. In the case of the weaker attachment to the isolated OH, the attachment involves one hydrogen bond. 

For gaseous samples the enrichment is determined by the free enthalpies of condensation and mixing. The free enthalpy of mixing is mainly a function of the polymer. Thus for some polymers very useful relations exists which derive the enrichment factors of various substances from their boiling points (Table 6.5-3). 

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. 

In a similar manner to that for the absorption column a linear relationship between the compositions of the two phases can be found for extraction and rectification. To illustrate this we will look at a rectification column. The basic process of rectification is when boiling a multicomponent mixture the vapour generated flows upwards countercurrent to the condensate which falls down the column. As the condensate is colder than the vapour, the components with higher boiling points, the least volatile, condense. They release their enthalpy of condensation to the components with the lower boiling points, the so called more volatile components, which are vaporized. This causes the vapour to become rich in the more volatile components while the less volatile components make up the liquid. The... 

The course of this process can be subdivided into several steps, in which a series of resistances have to be overcome. The fraction of these individual resistances in the total resistance can be very different. First, as a result of flow (convective transport) and molecular motion (diffusion transport), the vapour reaches the phase interface. In the next step the vapour condenses at the phase interface, and finally the enthalpy of condensation released at the interface is transported to the cooled wall by conduction and convection. Accordingly, three resistances in series have to be overcome the thermal resistance in the vapour phase, the thermal resistance during the conversion of the vapour into the liquid phase, and finally the resistance to heat transport in the liquid phase. 

This equation relates the vapor pressure, P, at which adsorption occurs, to the temperature of adsorption, T. AH is then the net enthalpy change (per mole) associated with the phase change from vapor to adsorbed state, and is a differential value. This enthalpy can be viewed as the sum of the enthalpies of condensation and adsorption of the vapor. If adsorption isotherms are collected at different temperatures, then a plot of In P (where P is the vapor pressure at which a particular quantity of adsorption occurs) against /T should be linear with a slope of —AH/R, Consequently, if the AH calculated in this way from equation 10.29 were just equal to the enthalpy of condensation of the vapor, then the phase change is simple condensation on surfaces or in pores, and does not involve true adsorption. 

Enthalpy of vapor enthalpy of condensed phase plus heat of vaporization. 

Decrease of the enthalpy of nuclei growth in comparison with their formation enthalpy, due to the enthalpy of condensation... 

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