Enthalpy / enthalpic

The abscissa of the Mollier diagram is humidity x. The axis is provided with a pitched scale. A straight line is drawn with a 45° angle to the abscissa, and it is provided with an enthalpy scale hj ) according to the equation hj. - = X = 2501.6 X, kj/kg (Fig. 4.8). Consequently the enthalpic scale... 

Thermodynamically, intermixing as above arises from achieving a negative free energy from the process in mm, this arises by balancing a contribution from enthalpy (heat content) with another from entropy (involving material stmctural aspects). The solubility parameter reflects the enthalpic term. 

The small and positive values of enthalpy of solution of water in AOT-reversed micelles indicate that its energetic state is only slightly changed and that water solubilization (unfavorable from an enthalpic point of view) is driven mainly by a favorable change in entropy (the destructuration of the water at the interface and its dispersion as nanodroplets could be prominent contributions) [87],... 

Calorimetry investigations of zinc ions with functionalized pyridines have been carried out in both dimethylformamide and acetonitrile. The pyridines used were pyridine, 3-methylpyridine, and 4-methylpyridine. In DMF, for all three pyridines, four- and six-coordinate species formed and their formation constants, reaction enthalpies and entropies were determined. The stability increases linearly with increasing basicity of the pyridine derivative. The formation of the 3-methylpyridine complex is enthalpically less favorable and entropically more favorable than... 

Tris(2-aminoethyl)amine (tren) and hexamethyl tren form pentacoordinate complexes with a bound water. The enthalpies of ionization have been determined.242 These results dispute the data from the systems that led to the view that solvent structure mediates the pAia of zinc bound water in zinc hydrolytic enzymes. The effect was shown to be entirely enthalpic. 

Once we have determined the entropy and enthalpy of polymerization, we can calculate the free energy of the process at a variety of temperatures. The only time this is problematic is when we are working near the temperatures of transition as there are additional entropic and enthalpic effects due to crystallization. From the free energy of polymerization, we can predict the equilibrium constant of the reaction and then use this and Le Chatelier s principle to design our polymerization vessels to maximize the percent yield of our process. 

In Equation (10), Vc represents a hard-core repulsion that is entropic in nature since it is linearly dependent on temperature in the expression for energy. Repulsion is generally associated with enthalpic interactions and we can consider the effect of an enthalpic interaction. Since Vc is associated with a single Kuhn unit we consider the average enthalpy of interaction per pair-wise interaction and the number of pair-wise interactions per Kuhn unit,... 

The binary systems we have discussed so far have mainly included phases that are solid or liquid solutions of the two components or end members constituting the binary system. Intermediate phases, which generally have a chemical composition corresponding to stoichiometric combinations of the end members of the system, are evidently formed in a large number of real systems. Intermediate phases are in most cases formed due to an enthalpic stabilization with respect to the end members. Here the chemical and physical properties of the components are different, and the new intermediate phases are formed due to the more optimal conditions for bonding found for some specific ratios of the components. The stability of a ternary compound like BaCC>3 from the binary ones (BaO and CC>2(g)) may for example be interpreted in terms of factors related to electron transfer between the two binary oxides see Chapter 7. Entropy-stabilized intermediate phases are also frequently reported, although they are far less common than enthalpy-stabilized phases. Entropy-stabilized phases are only stable above a certain temperature,... 

Taken from NIST Chemistry Webbook. b Estimated from CH2(CN)2 adding -7 kcal/mol (exchange of a CH2 group by a CHMe group). c Enthalpy difference indicates a destabilization effect for gem-disubstitution with respect to alkanes. d Enthalpy difference shows a stabilization effect for gem-dialkoxy-disubstitution with respect to alkanes. c Enthalpy difference shows increases of enthalpic anomeric effect for tertiary acetals compared to secondary acetals. 

The double-scale four-parameter enthalpic equation proposed in 1965 (54) and successfully developed by Drago et al seems to be the best tool so far available for correlating and predicting the formation enthalpies of Lewis adducts in the gas phase or, if really necessary, in solution. 

The effective molarity (EM) is formally the concentration of the catalytic group (RCOO- in [5]) required to make the intermolecular reaction go at the observed rate of the intramolecular process. In practice many measured EM s represent physically unattainable concentrations, and the formal definition is probably relevant only in reactions (which will generally involve very large cyclic transition states) where the formation of the ring or cyclic transition state per se is enthalpically neutral, or in diffusion-controlled processes. For the formation of small and medium-sized rings and cyclic transition states the EM as defined above contains, and may indeed be dominated by, the enthalpy of formation of the cyclic form. This topic has been discussed briefly by Illuminati et al. (1977) and will be treated at greater length in a future volume in this series. 

It is obvious from the definition of standard enthalpy of formation that these quantities do not represent the absolute enthalpic stability of compounds. They merely reflect their enthalpic stability relative to that of the chemical elements in standard reference states (to which AfH° = 0 has been arbitrarily assigned). It is thus unreasonable to state that a given substance is more stable than another just because it has a lower standard enthalpy of formation. We can only use AfH° values to make such direct comparisons when we are assessing the relative stability of isomers. 

Let us suppose that the acetic acid content of the final aqueous solution is 5%, corresponding to a ratio of approximately 1 mol of CH3COOH to 60 mol of H2O. As the yield of reaction 2.1 will be near 100% (recall that reaction 2.2 is rather exothermic, implying a very high equilibrium constant see section 2.9), the same value will be used for the molar ratio (H2 O) / n (C 2115OII), despite the increased total amount of substance of water in the reaction products. In the present case, the difference of 1 mol of water between the product and the reactant mixtures has a negligible enthalpic effect. The enthalpies associated with the solution of ethanol and acetic acid in 60 mol of water are derived from literature data [17] as Asin//(1) = -10.0 0.1 kJ mol-1 and Asin//(3) = —1.0 0.1 kJ mol-1. This calculation will be detailed in section 2.5. 

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