Atomization enthalpies

Derivation of bond enthalpies from themioehemieal data involves a system of simultaneous equations in which the sum of unknown bond enthalpies, each multiplied by the number of times the bond appears in a given moleeule, is set equal to the enthalpy of atomization of that moleeule (Atkins, 1998). Taking a number of moleeules equal to the number of bond enthalpies to be determined, one ean generate an n x n set of equations in whieh the matrix of eoeffieients is populated by the (integral) number of bonds in the moleeule and the set of n atomization enthalpies in the b veetor. (Obviously, eaeh bond must appear at least onee in the set.)... 

The bond matrix expresses 2 C—C bonds plus 8 C—H bonds for propane and 3 C—C bonds plus 10 C—H bonds for n-butane. Eaeh enthalpy of atomization is obtained by subtraeting the enthalpy of formation of the alkane from the sum of atomie atomization enthalpies (C 716 H 218 kJ mol ) for that moleeule. For example, the moleeular atomization enthalpy of propane is 3(716) +8(218) — (—104) = 3996 kJ mol . Enthalpies of formation are available from Pedley et al. (1986) or on-line at www.webbook.nist.gov. 

Figure 7.4 Thermodynamic data needed in evaluation of the enthalpy of formation of MX(s). (a) Lattice enthalpy of sodium halides (b) lattice enthalpy of alkali iodides (c) electron gain and dissociation enthalpies of halides (d) ionization and atomization enthalpies of alkali metals.

Figure 7.9 Thermodynamic data (b)-(d) needed in analysis of the enthalpy of formation of the binary transition metal compounds given in (a), (b) Atomization enthalpy of first series transition metals (c) sum of first and second ionization enthalpies of first series transition metals (d) derived lattice enthalpy of transition metal dihalides.

The atomization enthalpies show sharper distinctions between the nine approaches. The Hartree-Fock are very poor, as expected given the non-inclusion of electronic correlation. The MP2-FC are much better, but do not match the B3LYP, which actually achieve essentially 0% error in the two best cases. (Such perfection should not be expected regularly ) The atomization enthalpies do show a steady improvement as the basis set is increased. 

Figure 6.2 Plots of atomization enthalpy, ionization energy and the negative value of the enthalpy of hydration for the Group 1 elements and ions...

A The dill erenee between the ioni/ation energy sums and the negative values ol the enthalpies of hydration increases in the order Na < Mg < Al. and that this dill erenee is the main determinant of the trend in the reduction potentials. The trend in atomization enthalpies is also in the order Na < Mg < Af and the greater the value ol the alomi/alion enthalpy, the less negative is the value of the reduction potential lor the couple. 

Lynch and Truhlar (2003a) and Zhao et al. (2004) 6-31+G(d,p) basis set the Reaction column refers to the atomization enthalpies for six molecules chosen to be representative of a larger set in a fashion analogous to the H-atom transfer reactions, namely, SiO, S2, silane, propyne, glyoxal, and cyclobutane. 

Temperature corrections for the atomic enthalpies are also needed in the thermodynamic cycle in Figure 1 to obtain the enthalpy of formation at 298 K or some other temperature. Theoretical enthalpies of formation at 298 K are calculated by correction to AH (0 K) as follows ... 

The electronic energy calculated by the MINDO/3, MNDO, AMI, and PM3 methods is normally converted automatically in the computer program (Table 2) to an enthalpy of formation by subtracting the electronic energy of the isolated atoms and adding the experimental atomic enthalpies of formation. The zero-point energies and temperature corrections (0 to 298 K) are assumed to be included implicitly by the parameterization. For a molecule ABH, the AHf is defined in these methods as... 

Formation Enthalpies AH Atomization Enthalpies A//°t", Atomization Enthalpies Referred to the Number of Atoms AH°Jna, Heat Capacities Clb, and Entropies Sob of Gaseous Cyclic Selenium Molecules Se (n = 5-12)... 

The standard reduction potentials (see Redox Potential) of the elements and their compounds have many important applied implications for chemists, not the least of which is being aware when a compound or mixture of compounds they are handling has the potential for exploding. This should be considered as a possibility when the appropriate potentials differ by more than about one volt and appropriate kinetics considerations apply. A simply predictable case is the sometimes-violent reaction of metals with acids, as illustrated in a recently produced discovery video. Redox activities of elements are most commonly (and most precisely) analyzed via thermo chemical cycles such as the familiar Bom-Haber cycle for the production of NaCl from Na and CI2. A similar analysis of the activities of different metals in their reactions with acids shows that the standard reduction potential for the metal (the quantitative measure of the activity of the metal) can be expressed in terms of the appropriate ionization energies of the metal, the atomization energies of the metal (see Atomization Enthalpy of Metals), and the hydration energies... 

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