AH values

AH values relate to defined conditions, usually to the standard state of the substance at 298 K and 1 atm pressure, indicated by AH%i,. That is. 

In this section you have seen how heats of com bustion can be used to determine relative stabilities of isomeric alkanes In later sections we shall expand our scope to include the experimentally determined heats of certain other reactions such as bond dissociation energies (Section 4 16) and heats of hydrogenation (Section 6 2) to see how AH° values from various sources can aid our understanding of structure and reactivity... 

The heats of formation of most organic com pounds are derived from heats of reaction by arith metic manipulations similar to that shown Chemists find a table of AH values to be convenient because it replaces many separate tables of AH° values for indi vidual reaction types and permits AH° to be calcu lated for any reaction real or imaginary for which the heats of formation of reactants and products are available It is more appropriate for our purposes however to connect thermochemical data to chemi cal processes as directly as possible and therefore we will cite heats of particular reactions such as heats of combustion and heats of hydrogenation rather than heats of formation... 

Converting the probability function described above into an excluded volume is accomplished by integrating the probability 1 - 0(d) of exclusion over a spherical volume encompassing ah values of d ... 

AH values are maximum unless otherwise indicated. Value is minimum. 

Table 1.11. Calculated and Experimental AH Values for Some Isodemic Reactions...

When a Mollier chart is available for the gas involved the first method, which is illustrated by Figure 12-12A is the most convenient. On the abscissa of Figure 12-12A four enthalpy differences are illustrated. (Hg — Hj) is the enthalpy difference for the isentropic path. (Hg — Hi°) is the ideal gas state enthalpy difference for the terminal temperatures of the isentropic path. The other AH values are the isothermal pressure corrections to the enthalpy at the terminal temperatures. A generalized chart for evaluating these pressure corrections was presented previously. 

Very frequently AH data are available for inorganic substances but not for organic compounds for which AHj values are more readily available. Because AH of hydrocarbons are not easily measurable, they are often deduced by Hess s law from known AH of the hydrocarbon and known AH values of the products of combustion. 

We have now written several thermochemical equations. In each case, we have cited the corresponding value of AH. Literally thousands of such equations would be needed to list the AH values for all the reactions that have been studied. Clearly, there has to be some more concise way of recording data of this sort. These data should be in a form that can easily be used to calculate AH for any reaction. It turns out that there is a simple way to do this, using quantities known as enthalpies of formation. 

The slope of the Arrhenius plot has units (temperature) 1 but activation energies are usually expressed as an energy (kJ mol 1), since the measured slope is divided by the gas constant. There is a difficulty, however, in assigning a meaning to the term mole in solid state reactions. In certain reversible reactions, the enthalpy (AH) = E, since E for the reverse reaction is small or approaching zero. Therefore, if an independently measured AH value is available (from DSC or DTA data), and is referred to a mole of reactant, an estimation of the mole of activated complex can be made. 

In equations 5 and 6, AH values were obtained by a calorimetric method.)... 

Then the AH values of sulphoxides and amides were correlated with Taft s a affording the following equation ... 

The gas-phase AH values are based on analysis of gas-phase experiments, which are given in Ref. 6. 

The heat of dissociation in hexane solution of lithium polyisoprene, erroneously assumed to be dimeric, was reported in a 1984 review 71) to be 154.7 KJ/mole. This value, taken from the paperl05> published in 1964 by one of its authors, was based on a viscometric study. The reported viscometric data were shown i06) to yield greatly divergent AH values, depending on what value of a, the exponent relating the viscosity p of a concentrated polymer solution to DPW of the polymer (q DP ), is used in calculation. As shown by a recent compilation 1071 the experimental a values vary from 3.3 to 3.5, and another recent paper 108) reports its variation from 3.14 to 4. Even a minute variation of oe results in an enormous change of the computed AH, namely from 104.5 KJ/mole for oe = 3.38 to 209 KJ/mole for oe = 3.42. Hence, the AH = 154.7 KJ/mole, computed for a = 3.40, is meaningless. For the same reasons the value of 99.5 KJ/mole for the dissociation of the dimeric lithium polystyrene reported in the same review and obtained by the viscometric procedure is without foundation. 

Considerations fully discussed elsewhere 106) place an upper limit for the questionable AH as 63-67 KJ/mole. The reviewers 7I) quote the pertinent paper, but incorrectly state that the AH values quoted in Ref. 106) were given without citing or providing experimental or theoretical evidence for their justification . 

The order of reactivity of the halogens can be explained by energy considerations. For the substrate methane, AH values for the two principal propagation steps are... 

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