Heat of formation values

Table 7-2 also contains, besides the values for heats of formation, values for... 

For values of heats of combustion of large numbers of organic compounds hydrocarbons and others, see Cox, J.D. Pilcher, G. Thermochemistry of Organic and Organometallic Compounds, Academic Press NY, 1970 Domalski, E.S. J. Phys. Chem. Ref. Data, 1972,1, 221. For large numbers of heats of formation values (from which heats of combustion are easily calculated) see Stull, D.R. Westrum Jr., E.F. Sinke, G.C. The Chemical Thermodynamics of Organic Compounds, Wiley NY, 1969. 

Table 1 Standard heat of formation values for some compounds at 25°C 12 Heat, Chemical Kinetics and Electrochemistry...

Computation of the heat of any process. To compute the heat of any process involving the disappearance of a substance or substances in the states given in the table and the appearance of other or the same substances in states given in the table Add together the heat of formation (values for Qf) of the products of the process in the final states and subtract therefrom the sum of the heats of formation (values for Qf) of the reactants in their initial states. The value so obtained represents the heat evolved when the given process takes place at a constant pressure (or a fugacity) of one atmosphere and at a temperature of 18°. The following are examples ... 

We can calculate just how much heat is released when plaster is mixed with water using a concept called heat of reaction. Symbolized AH, it is derived from the heat of formation, AHf, of the reactants and products in a chemical change. Heats of formation values (AHf), are compiled on special thermodynamic tables and expressed using the unit kilojoules per mole (kj/mole). The heats of formation of plaster of Paris, gypsum, and water are -1575.2 kj/mole, -2021.1 kj/mole, and -285.8 kj/mole, respectively. To find the heat of the reaction, it is necessary to subtract the heat of formation of the reactants from the heat of formation of the products ... 

This is very close to our atomization heat of formation value above (—195.7 kJ mol x). 

The heat of formation value, combined with the most recent heats of atomization of fluorine (18.860 keal.) and oxygen (59.559 keal.) (7, 8) yields a value of 50.7 kcal./mole for the O—F bond energy in OF2. 

Values in normal text are derived from experimental heats of formation. Values in italics are from G2MP2-computed energies. 

To sum up, a formation reaction is understood by convention to be a reaction that forms 1 mole of compound from the elements that make it up. Standard heats of reaction at 25°C and 1 atm for any reaction can be calculated from the tabulated (or experimental) standard heats of formation values by using Eq. (4.34), because the standard heat of formation is a state (point) function, as illustrated in the examples below. 

Based on the heat of formation values of CFC-113 and the major products (obtained by AM-1 calculations) [15] which were found in the product mixture, the heats of reaction of various component steps leading to the major products have been calculated. 

This is very close to our atomization heat of formation value above (-195.9 kJ mol" ), and a little more negative than the experimental value (-190.7 or -189.8kJmol" [128]). 

For comparison, from Reference 3 we find that the spread of the heats of formation values from the complete all nine possible solid ethyl and dimethyl substituted benzoic acids is 6.7 kcalmol- L In this case, a few this means 3.4. 

Table A-17 Auxiliary data used in the recalculation of NaBr heat of formation values with the data from [78STU/FER].

Heat of formation. Values of 151,900 cal/mole and 143,800cal/ mole have been quoted for calcium oxide and magnesium oxide, respectively [13.1]. 

For details on the calculation of heats of formation from molecular mechanics, molecular orbital theory, or bond/group increments, see the discussion in Allinger, N. L. Schmitz, L. R. Motoc, 1. Bender, C. Labanowski, J. K. /. Am. Chem. Soc. 1992,114,2880. The strain energy of a conformation can be calculated from values of heats of formation from these sources also. The relationships in Figure 3.39 among heat of formation, T/R, strainless increments, and strain energy hold no matter what the source of the heat of formation value. 

The strain energy at 25° of the substance butane should therefore be taken to be the difference between the strainless heat of formation and the heat of formation value that includes estimates of POP and TOR, and that difference is 0.67 kcal/mol. Notice that this value is not shown on the molecular mechanics output. Again, it must be emphasized that the strain energy shown on the molecular mechanics output above reflects only the inherent strain of the particular conformer calculated, not the strain in the substance represented by the computer model. A value of zero for pop and tor on the output is an indication that appropriate values must be entered if the calculated value of AHf is expected to reproduce an experimental value and if a more accurate value for the strain energy is desired. 

This leads to an estimated heat of formation value for ethanol of —56.20 kcal/mol, which is essentially identical with the value found in the NIST webbook [44]. The estimations of entropies and heat capacities... 

First, note that this compound exists in cis and Irans forms and that the latter is much more highly strained. (It may not be evident from the structures shown, but while the internal and external C-C-C bond angles at the bridgehead carbon are lOb-O and 115.4°, respectively, in the cis isomer, those in the Irans are 102.3° and 125.8°.) If we use two cyclopentane increments, we can calculate approximately correctly the heat of formation for the cis isomer, where the total strain is approximately the same as that of two cyclopentanes. But the trans isomer is much more strained than that (about 6kcal/mol more), due to very distorted bond angles, and the calculated heat of formation value would be very wrong. 

The pressure of Th over ThC2 + C has been studied by emission spectrometric techniques (Lofgren and Krikorian, 1964). Although some difficulty was experienced in obtaining consistent data, a heat of formation value near — 23 kcal/mole (third law) was reported. 

Many heats of formation values are available, and it is worthwhile to spend a few moments to consider some differences among them. For example, the heat of formation of cyclohexane is-29.9 kcal/mol. What is the meaning of this number The fact that it is negative means that Eq. 2.9 is exothermic. In other words, a mole of cyclohexane is more enthalpically stable than the equivalent amount of graphite plus H2. A further insight arises when we compare cyclohexane s heat of formation to another molecule with the same molecular formula, such as methylcyclopentane (AH° = -25.5 kcal/mol). Clearly, methylcyclopentane is less stable than cyclohexane. 

Use the group increments of Chapter 2 to derive the heat of formation values for various structures considered in the discussion of the Cope rearrangement (Figure 15.24). 

Measurements in the gas phase show that vinyl carbocations are very imstable. Remember that the gas-phase heat of formation values do not include the solvent, which is very important to any cationic species. Remember also that direct comparisons can be made only between isomeric species. Nonetheless, one can get an approximate idea of the great instability of vinyl carbocations from Table 10.2. The absolute values of the energy differences are not important, but the relative stability order of the various carbocations is. 

AUenes are high-energy molecules, just as are alkynes. The sp hybridization of the central carbon of this functional group is not an efficient way of using orbitals to make bonds, because 7t bonds are much weaker than o bonds. The 7t bond energy of about 66 kcal/mol is substantially lower than most o bond energies. This inefficiency is reflected in relatively positive heats of formation values). Table 12.1... 

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