From Bond Enthalpies to Heats of Reaction

From Bond Enthalpies to Heats of Reaction 161 Chapter 5 Ethylene 179 Chapter 6... 

The energy stored in the bonds is called the enthalpy and is given the symbol H. The change in energy going from reactants to products is called the change in enthalpy and is shown as AH (pronounced delta H ). AH is called the heat of reaction. 

An alternative reference state from which to tabulate thermodynamic functions is that of isolated atoms. The use of this reference state is shown in Fig. 4, where Abd//, is a bond dissociation enthalpy in a reactant or product molecule. (Remember that the v, s are negative for reactants.) The heat of reaction is then... 

When reactant R of an energetic material reacts to generate product P, heat is released (or absorbed). Since the chemical bond energy of R is different from that of P, the energy difference between R and P appears as heat. The rearrangement of the molecular structure of R changes the chemical potential. The heat of reaction at constant pressure, represented by Qp, is equal to the enthalpy change of the chemical reaction ... 

Solution The heat of reaction is the enthalpy, AH. In a reaction, if more stable bonds are fonned, AH is negative and the reaction is exothermic. A reaction with positive AH is endothermic. To solve for AH, we substract the sum of the bond dissociation energies of the products from the sum of the bond dissociation energies of the reactants. In mathematical symbols, if "D" is the bond dissociation energy of the bond formed or broken, then ... 

Heats of reactions can be measured by the calorimetric and kinetic methods, using photo- and mass-spectrometry. Bond dissociation enthalpy calculated from the thermal effect of the reaction at ambient pressure is close to the bond energy because PV is small, for example for the hydrogen molecule PV 2.5 kJ/mol. Finally, the difference between the dissociation energy at 0 K and that at room temperature is also very small for the hydrogen molecule the difference is AE 1 kj/mol. 

The relative bond enthalpies from the photoacoustic calorimetry studies can be placed on an absolute scale by assuming that the value for D//(Et3Si—H) is similar to D/f(Me3Si—H). In Table 2.2 we have converted the D/frei values to absolute T>H values (third column). On the basis of thermodynamic data, an approximate value of D//(Me3SiSiMc2—H) = 378 kJ/mol can be calculated that it is identical to that in Table 2.2 [1]. A recent advancement of photoacoustic calorimetry provides the solvent correction factor for a particular solvent and allows the revision of bond dissociation enthalpies and conversion to an absolute scale, by taking into consideration reaction volume effects and heat of solvation [8]. In the last colunm of Table 2.2 these values are reported and it is gratifying to see the similarities of the two sets of data. 

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