Enthalpy effects

This fundamental relationship points out that the temperature at which crystal and liquid are in equilibrium is determined by the balancing of entropy and enthalpy effects. Remember, it is the difference between the crystal and... 

While the substituent effect in the gas phase is assumed to be nearly entirely an enthalpy effect, it can be shown that in solution the substituent effect is largely the result of changes in AS. 

Equation 3.6.10. given above shows that intrinsic defect concentrations will increase with increasing temperature and that they will be low for high Enthalpy of defect formation. This arises because the entropy effect is a positive exponential while the enthalpy effect is a negative exponential. Consider the following examples of various types of compounds and the natural defects which may occur (depending upon how the compounds were originally formed) ... 

Figure 5.41 Pressure and local enthalpy effects on CHF. (From Macbeth, 1963a. Reprinted with permission of UK AEA Technology, Didcot, Oxfordshire, UK.)...

Channel Length and Inlet Enthalpy Effects and Orientation Effects... 

Figure 5.69 Correlating function of inlet enthalpy effect on CHF, F(Hm), where F(HJ = [0.8258 + 0.000794(//sat - //in)]. (From Tong, 1968a. Copyright 1968 by American Nuclear Society, La-Grange Park, IL. Reprinted with permission.)... 

Enthalpy effect Virtually always exothermic, similar in magnitude to heats of reaction Always exothermic, similar in magnitude to heats of condensation... 

An increase in the number of methylene units in alkyl benzene did not significantly affect the 7t-energy effect on their retention, but the enthalpy effect increased dramatically [80]. This means that a hydro-phobic compound can be adsorbed directly onto an octadecyl-bonded silica gel. The value of enthalpy effect of a methylene unit in alkyl benzene was calculated to be 500 cal/mol. 

In the competition between the two pathways leading to the R and S enantiomers, the enthalpy effect dominated, which resulted in the formation of an excess of the S enantiomer [33]. 

It is a purely entropic mechanism and makes no provision for enthalpy effects. ... 

Knowledge of the factors that determine the sign and magnitude of ATG° for reaction (15.33) contributes to an understanding of the range of stability of ternary oxides relative to binary oxides. In some cases, favorable enthalpy effects may tend to stabilize a material, while in other instances, a favorable entropy effect may dominate stability considerations. ... 

T2-18C6 complex with Ag+, the entropy effect works against the enthalpy effect so that ArG° is smaller and the complex is weaker than for the corresponding 1,10-A2-18C6 complex. 

Heberger K, Lopata A (1998) Assessment of nudeophilicity and electrophilicity of radicals, and of polar and enthalpy effects on radical addition reactions. J Org Chem 63 8646-8653 HerakJN, Behrens G (1986) Formation and structure of radicals from D-riboseand 2-deoxy-D-ribose by reactions with SO4 radicals in aqueous solution. An "in-situ" electron spin resonance study. Z Naturforsch 41c 1062-1068... 

In the words of Humphreys and Hammett, there is therefore considerable basis for the generalization that the conversion of an electrically neutral acetic acid derivative to an electrically charged entity, no matter whether this be positively charged or negatively charged, involves a considerably greater decrease in entropy than does the analogous process in the case of a formic acid derivative, whereas the enthalpy effects are likely to be nearly the same for the acetic and formic acid derivatives . 

The enthalpy effect of rod packing influences the microstructures at the air- water interface. The copolymer with the long rods, forms a cylindrical structure in the monolayer due to the bigger enthalpy decrease. Copolymer, with the middle-sized rods, will form micellar structure in the monolayer by self - assembly. Copolymer... 

If apolar hydration is characterized by the conditions that AG° > 0, TAS < 0 and AH < 0, then a process which minimizes exposure of apolar groups to water should be a thermodynamically favoured process. Then if two apolar groups of either the same or different molecules come together in water, AS for this process will be positive because some of the structured water is released into the bulk solvent. Such association is called hydrophobic, hydrophobic bonding or hydrophobic interaction (Kauzmann, 1959). The term bond is probably inappropriate because the association is due to entropy rather than to enthalpy effects, a consequence of the disruption of the clathrate structure around the apolar solute (Jolicoeur and Friedman, 1974). Despite the general acceptance of the concept of hydrophobic association, there are different approaches to the problem of understanding this phenomenon. 

The mathematical description of simultaneous heat and mass transfer and chemical reaction is based on the general conservation laws valid for the mass of each species involved in the reacting system and the enthalpy effects related to the chemical transformation. The basic equations may be derived by balancing the amount of mass or heat transported per unit of time into and out of a given differential volume element (the control volume) together with the generation or consumption of the respective quantity within the control volume over the same period of time. The sum of these terms is equivalent to the rate of accumulation within the control volume ... 

Enthalpy effect As high as possible for high storage density... 

Any molecular factors that tend to stabilize the transition state decrease AG and thus increase the rate of the reaction. Thus, this rate enhancement can result from either entropy or enthalpy effects, or from both. 

Steric effects are usually unimportant however, in extreme cases as in 2,6-di-/-butylpyridine (p Ta = 3.6) the pKu does fall significantly below that of pyridine. This is attributed to entropy rather than enthalpy effects as it is the entropy change for the transfer of the protonated 2,6-di-/-butylpyridine from the gas phase to the aqueous phase that is abnormal. Aqueous protonated 2,6-di-/-butylpyridine is hydrogen bonded to a water molecule via the NH bond and the observed loss of entropy is due to the substantial restrictions in the internal rotations in the solution complex of protonated 2,6-di-/-butylpyridine <1984JA4341>. 

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