Enthalpy of surface

If the dependence on temperature as well as on composition is known for a solution, enthalpies and entropies of adsorption may be calculated from the appropriate thermodynamic relationships [82]. Neam and Spaull [147] have, for example, calculated the enthalpies of surface adsorption for a series of straight-chain alcohols. They find an increment in enthalpy of about 1.96 kJ/mol per CH2 group. 

The ratio of the enthalpy of surface formation to the enthalpy of vaporization, hs hvap, for various substances is given in Table 2.2. Substances with nearly sphericalshaped molecules have ratios near 1/2, while substances with a polar group on one end have a much smaller ratio. This difference indicates that the latter molecules are oriented with the nonpolar end toward the gas phase and the polar end toward... 

Enthalpy of Surface Formation, hs [10 erg/molecule], and Ratio to Enthalpies of Vaporization, hvap... 

Sverjensky, D.A. and Sahai, N., Theoretical prediction of single-site enthalpies of surface protonation for oxides and silicates in water, Geochim. Cosmochim. Acta, 62, 3703, 1998. 

In order to evaluate the standard enthalpy of surface reactions involving potential determining ions by calorimetry, one should first perform a theoretical analysis to find the appropriate experimental conditions. It is possible to avoid electrostatic effects by performing symmetric experiments, i.e. experiments in which the point of zero charge is exactly the mean value of the initial and final pH [22,55,56]. 

A more advanced task would be to use calorimetry for examination of the electrostatic effect on the enthalpies of surface reactions. In doing so it is necessary to account for two different contributions to the enthalpy, i.e. the chemical , ArH°, and electrostatic terms... 

Preocanin, T., Krehula, S.. and Kallay, N., Enthalpy of surface reactions Temperature dependency of pH of acidic or basic concentrated hematite suspension, Appl. Surf. Sci., 196, 392, 2002. 

Ratio of Enthalpy of Surface Formation, and the Enthalpies of Vaporization, hy3p (See Text for Details)... 

Abstract Enthalpy of charging of hematite surface was determined by calorimetry and also by measuring the dependence of the point of zero charge (p.z.c.) on temperature. The calorimetric experiment was designed in such a way that the point of zero charge was in the middle between the initial and final pH. In that case, the reaction heat corresponds to the difference in standard reaction enthalpies of surface deprotonation andprotonation reactions, without... 

Flere is the volume of gas required to saturate the monolayer, V the total volume of gas adsorbed, P the sample pressure, P the saturation vapour pressure and C a constant related to the enthalpy of adsorption. The resulting shape of the isothemi is shown plotted in figure Bl.26.6 for C = 500. A plot of P/V(P - Pq) against P/Pq should give a straight line having a slope (C - )/y C and an intercept The BET surface area is... 

It should be stressed that although these symmetry considerations may allow one to anticipate barriers on reaction potential energy surfaces, they have nothing to do with the thermodynamic energy differences of such reactions. Symmetry says whether there will be symmetry-imposed barriers above and beyond any thermodynamic energy differences. The enthalpies of formation of reactants and products contain the information about the reaction s overall energy balance. 

Fig. 2.11 Curves of the differential enthalpy of adsorption of nitrogen against surface coverage 0 (= for samples of Sterling carbon black heated at the following temperatures (a) 1500°C (fc) 1700°C (c) 2200 C (d) 2700°C. The curve for 2000°C was similar to (c). but with a lower peak. The calorimetric temperature was 77-5, 77-7, 77-4, 77-4 K in (a), (fc), (c) and (d) respectively.

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. 

An overview of some basic mathematical techniques for data correlation is to be found herein together with background on several types of physical property correlating techniques and a road map for the use of selected methods. Methods are presented for the correlation of observed experimental data to physical properties such as critical properties, normal boiling point, molar volume, vapor pressure, heats of vaporization and fusion, heat capacity, surface tension, viscosity, thermal conductivity, acentric factor, flammability limits, enthalpy of formation, Gibbs energy, entropy, activity coefficients, Henry s constant, octanol—water partition coefficients, diffusion coefficients, virial coefficients, chemical reactivity, and toxicological parameters. 

The approximate calculation of the surface energies of metals as a function of crystal structure described earlier uses the enthalpy of sublimation, s, and the co-ordination number to calculate the energy as a function of the atomic concentration on the surface. The atomic areas of the principal configurations are as follows ... 

The enthalpy of interaction of the grafted polymer with the filler surface is given by... 

Basu and Searcy [736] have applied the torsion—effusion and torsion— Langmuir techniques, referred to above for calcite decomposition [121], to the comparable reaction of BaC03, which had not been studied previously. The reaction rate at the (001) faces of single crystals was constant up to a product layer thickness of 1 mm. The magnitude of E (225.9 kJ mole-1) was appreciably less than the enthalpy of the reaction (252.1 kJ mole-1). This observation, unique for carbonates, led to the conclusion that the slowest step in BaC03 vacuum decomposition at 1160—1210 K is diffusion of one of the reaction components in a condensed phase or a surface reaction of C02 prior to desorption. 

It was concluded [734] from visual inspection and chemical analysis of partially decomposed dolomite, that reaction was initiated at the outer surfaces of the crystallites and the interface established advanced thereafter into the bulk. The deceleratory a—time curves obeyed the contracting volume equation [eqn. (7), n = 3] and the values of E determined were between 206 and 232 kJ mole-1. These values of E were generally greater than those reported for other studies ( 190 kJ mole-1) which are in the range mentioned [121] for CaC03 dissociation and slightly larger than the enthalpy of that reaction. On exposure of the residue from vacuum decomposition of dolomite to C02, the gas uptake at 1070 K was... 

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