Thermochemical equations table

Using Table 8.2, write thermochemical equations for the following. 

Standard enthalpies of combustion are listed in Table 6.4 and Appendix 2A. We have seen in Toolbox 6.1 how to use enthalpies of combustion to obtain the standard enthalpies of reactions. Here we consider another practical application— the choice of a fuel. For example, suppose we want to know the heat output from the combustion of 150. g of methane. The thermochemical equation allows us to write the following relation... 

Table 8.3 Thermochemical equations and data used for the derivation of the enthalpy of reaction 8.9. Data from [143].

Write the thermochemical equation for the combustion of methane. Refer to Table 15.3. 

A value calculated from one of the thermochemical equations above, taking enthalpy data fro atoms from Table 2 and for radicals from Table 4. 

From Table 6.2 you find the enthalpies of formation for CH4(g), CCl4(/), and HCl(g). You can then write the following thermochemical equations ... 

Thermochemical equations can be rearranged and added to give enthalpy changes for reactions not included in the data tables. The basis for calculating enthalpies of reaction is known as Hess s law The overall enthalpy change in a reaction is equal to the sum of enthalpy changes for the individual steps in the process. The energy difference between reactants and products is independent of the route taken to get from one to the other. In fact, measured enthalpies of reaction can be combined to calculate enthalpies of reaction that are difficult or impossible to actually measure. 

Use the enthalpy of formation data in Appendix Table B-14. Solve by combining the known thermochemical equations. Verify the result by using the general equation for finding enthaipies of reaction from enthaipies of formation. 

The problem with eqn 1.22 is that we have no way of knowing the absolute enthalpies of the substances. To avoid this problem, we can imagine the reaction as taking place by an indirect route, in which the reactants are first broken down into the elements and then the products are formed from the elements (Fig. 1.24). Specifically, the standard enthalpy of formation, AfH, of a substance is the standard enthalpy (per mole of the substance) for its formation from its elements in their reference states. The reference state of an element is its most stable form under the prevailing conditions (Table 1.7). Don t confuse reference state with standard state the reference state of carbon at 25 C is graphite (not diamond) the standard state of carbon is any specified phase of the element at 1 bar. For example, the standard enthalpy of formation of liquid water (at 25 C, as always in this text) is obtained from the thermochemical equation... 

Heat Capacity, C° Heat capacity is defined as the amount of energy required to change the temperature of a unit mass or mole one degree typical units are J/kg-K or J/kmol-K. There are many sources of ideal gas heat capacities in the hterature e.g., Daubert et al.,"" Daubert and Danner,JANAF thermochemical tables,TRC thermodynamic tables,and Stull et al. If C" values are not in the preceding sources, there are several estimation techniques that require only the molecular structure. The methods of Thinh et al. and Benson et al. " are the most accurate but are also somewhat complicated to use. The equation of Harrison and Seaton " for C" between 300 and 1500 K is almost as accurate and easy to use ... 

There is another use of the Kapustinskii equation that is perhaps even more important. For many crystals, it is possible to determine a value for the lattice energy from other thermodynamic data or the Bom-Lande equation. When that is done, it is possible to solve the Kapustinskii equation for the sum of the ionic radii, ra + rc. When the radius of one ion is known, carrying out the calculations for a series of compounds that contain that ion enables the radii of the counterions to be determined. In other words, if we know the radius of Na+ from other measurements or calculations, it is possible to determine the radii of F, Cl, and Br if the lattice energies of NaF, NaCl, and NaBr are known. In fact, a radius could be determined for the N( )3 ion if the lattice energy of NaNOa were known. Using this approach, which is based on thermochemical data, to determine ionic radii yields values that are known as thermochemical radii. For a planar ion such as N03 or C032, it is a sort of average or effective radius, but it is still a very useful quantity. For many of the ions shown in Table 7.4, the radii were obtained by precisely this approach. 

The above analysis reveals that some of the thermochemical data for organotin compounds may not be as accurate as one could hope. Although the information is in general of much better quality than in the case of germanium and lead analogues, we believe that some values in Table 3 should be redetermined. Other examples could have been used to illustrate this point (see also the next section), but once again we wish to resist the temptation of recommending data that in some cases conflict with the available experimental results. By a judicious use of the Laidler terms in Table 4 and/or correlations similar to those in equation 2, it is rather simple to assess other values from Table 3 and predict new data. 

Table 3. Standard enthalpy of formation of the gaseous metal atom, A/ff (M, g), coordina-tion number n of the atom in the bulk metal at room temperature and values of M and T calculated by equations 1 and 3. Thermochemical values in kj mol-1...

On the basis of this equation, systematically considering all the available thermochemical data and arbitrarily assigning to the most electronegative element, fluorine, a value of about 4, Pauling was able to prepare a complete electronegativity scale. This is shown in Table 2.1. 

The tabulated values of E hf for olivine end-members (table 1.12) correspond to the summation of the first four terms on the right side of equation 1.86. Table 1.14 lists the various energy terms of the Born-Haber-Fayans thermochemical... 

Additional BDEno-h data for a few other R2NOH compounds were calculated in compliance with the thermochemical cycle reported in equation l 3,69 on the basis of available p fa (RiNOH) and E° (R2NO /R2NO ) data (cf. Scheme 4), and are listed in Table 2. The thermochemical calculation reproduces the experimental BDEno-h value (i.e. 88.1 kcalmoG ) of Af-hydroxyphthalimide (HPI) exactly, so that these calculated values can be confidently compared with the experimental ones in Table 1. DFT calculations of BDEno-h also provide reliable results ... 

The expectation that the k rate constants correlate with thermochemical bond-energy data in this radical process has indeed found quantitative support through the determination of the activation parameters, on running the H-abstraction experiments by BTNO from selected substrates at various temperatures. From the Arrhenius equation (logfe = log A — Ei /RT), log A and were obtained (Table 7). 

As we end this section, let us reconsider ionic radii briefly. Many ionic compounds contain complex or polyatomic ions. Clearly, it is going to be extremely difficult to measure the radii of ions such as ammonium, NH4, or carbonate, COs, for instance. However, Yatsimirskii has devised a method which determines a value of the radius of a polyatomic ion by applying the Kapustinskii equation to lattice energies determined from thermochemical cycles. Such values are called thermochemical radii, and Table 1.17 lists some values. 

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