Enthalpy stabilisation

This law is particularly useful as it allows the total entropy of a substance to be obtained if sufficiently low-temperature enthalpy or heat-capacity measurements are available. However, questions remain as to its validity when considering metastable crystalline forms and the law, as stated, would not apply to defect-stabilised structures and amorphous phases. 

Merenyi R, Janousek Z, Viehe HG (1986) Studies on the captodative effect. Entropy/enthalpy compensation as solvent effect in radical forming reactions. A relative radical stabilisation scale. In Viehe HG (ed) Substituent effects in radical chemistry. Reidel, Dordrecht, pp 301-324 Merga G, Schuchmann H-P, Rao BSM, von Sonntag C (1996) The oxidation of benzyl radicals by Fe(CN)63. J Chem Soc Perkin Trans 2 551-556... 

Ab initio calculations [195] have been carried out to model the effect of some substituents substitution on the C2 carbon by a ti donor group stabilises the product more than the starting material and thus will be favourable for the reaction enthalpy. An NH2 group will provide more stabilisation than SH. This correlates well with the experimental results. 

The increasing use of nonionic macromolecules as stabilisers, which has occurred since the development of the DLVO theory, has led to the awareness of other stabilising forces. The approach of particles with hydrated macromolecules adsorbed to their surfaces leads, on the interaction of these layers, to repulsion (Fig. 7.7), because of the consequent positive enthalpy change +AH which ensues. In more general terms, the approach of two particles with adsorbed stabilising chains leads... 

Figure 7.7 Enthalpic stabilisation representation of en-thalp ic stabilisation of particles with adsorbed hydrophilic chains. The hydrated chains of the polyoxyethylene molecules —(0CH2CH2) 0H protrude into the aqueous dispersing medium. On close approach of the particles to within 26 (twice the length of the stabilising chains), hydrating water is released, resulting in a positive enthalpy change which is energetically unfavourable.

C, and the cation 2.111 into the W-shaped cation 2.110 with the same half-life at 35 °C. These correspond to enthalpies of activation of 74 and 101 kJ mol 1 (18 and 24 kcal mol-1), respectively. This measurement only sets lower limits to the rotation barrier of an allyl cation, because it is not known whether rotation takes place in the cations themselves or in the corresponding allyl chlorides with which they could be in equilibrium.141 The barrier in cations is also much affected by solvation and by the degree of substitution at the termini, since the transition structure for rotation draws on such stabilisation more strongly than the delocalised allyl cation does. 

As a consequence, the crystal field stabilisation energy (CFSE) for grafted metal complexes will be less negative than for the original, homogeneous-phase complex, if one starts with aqua complexes (which is most usual in catalyst preparation) or with ammine complexes. Since the change in CFSE is a major component of the adsorption enthalpy AHads- transition metal adsorption is not expected to be strongly exothermic. 

The standard Gibbs energy of formation of NiO has been experimentally determined over the temperature range from 900 to 1400 K using a galvanic cell with the solid electrolyte made of 15% calcia-stabilised zirconia. The measured value of AfG (NiO) at 1300 K was - 123.555 kJmol with a precision of+0.057 kJ-mof and an estimated accuracy not worse than 0.200 kJ-mol. This precision is equivalent to an error of only 0.2 - 0.3 mV in the cell potential (emf). In comparison, most previous studies have reported a precision of 1 - 2 mV. Using the third law analysis, the authors obtained for the enthalpy of formation of NiO, Af//° (NiO, 298.15 K) = - 240.110 kJ-mol. ... 

Let us now discuss mixtures of proteins with low-molecular weight surfactants. These mixtures are of great practical importance for the stabilisation of emulsions and foams, and play an important role in biological systems. Let us consider equilibrium ideal (with respect to the enthalpy) solutions of proteins and surfactants. If such a solution contains i different surfactants (or proteins) which exist in j different states at the interface, the following system of equations from (2.26) and (2.27) may be formulated [24,25] ... 

---