Bond dissociation enthalpy diatomic molecules

In a diatomic molecule, e.g. chlorine (CI2), the bond dissociation enthalpy and the average bond enthalpy will have the same value. This is because both enthalpy changes refer to the process Cl2(g)--> 2Cl(g). 

The bond dissociation enthalpy (BDE) for a diatomic molecule AB is the amount of energy necessary to bring about homolytic cleavage, that is, the enthalpy of the dissociation reaction... 

The enthalpy change for the dissociation of a diatomic molecule such as H2 into its atoms in the gas phase can be termed the bond dissociation energy (or more strictly the bond dissociation enthalpy). If we consider the reaction... 

Bond dissociation enthalpies for many diatomic neutral molecules including, for example, metal hydrides, have been obtained by the Birge-Sponer extrapolation method.This method is based on the analysis of the vibrational energy levels in a molecule obtained from spectroscopic studies. 

Strengths of Chemical Bonds, In CRC Handbook of Chemistry and Physics-, CRC Press Boca Raton. Tables with bond dissociation energies of diatomic molecules, bond dissociation enthalpies of polyatomic molecules, and standard enthalpies of formation of (mainly organic) radicals (at 298.15 K). The CRC Handbook is regularly updated, so the tables may vary according to the edition number. 

The strength of the chemical bond in a diatomic molecule can be expressed by the bond dissociation enthalpy, D. This is equal to the standard enthalpy change, A/f, for a reaction in which the bond is broken. For the dissociation of hydrogen chloride ... 

A more useful quantity for comparison with experiment is the heat of formation, which is defined as the enthalpy change when one mole of a compound is formed from its constituent elements in their standard states. The heat of formation can thus be calculated by subtracting the heats of atomisation of the elements and the atomic ionisation energies from the total energy. Unfortunately, ab initio calculations that do not include electron correlation (which we will discuss in Chapter 3) provide uniformly poor estimates of heats of formation w ith errors in bond dissociation energies of 25-40 kcal/mol, even at the Hartree-Fock limit for diatomic molecules. 

The terms AHAm and AHAy are the enthalpies of atomization of the metal and the nonmetal, respectively. For gaseous diatomic nonmetals, AHA is the enthalpy of dissociation (bond energy plus RT) of the diatomic molecule. For metals which vaporize to form monatomic gases, AHA is identical to the enthalpy of sublimation. If sublimation occurs to a diatomic molecule, M2> then the dissociation enthalpy of the reaction must also he included ... 

The strength of a bond, thermochemically speaking, must be measured by the energy required to break it. If we are dealing with a diatomic molecule, this should be given directly by the dissociation enthalpy... 

Values of average dissociation energies in text Table 2.4 are actually those for the property of bond enthalpy (see Chapter 6), measured at 298.15 K. The bond dissociation energies of diatomic molecules given in Table 2.3 apply at 0 K (absolute zero). 

In the unique case of diatomic molecules, however, bond energies and dissociation energies are, in principle, identical and exact (although, there may be disagreement in the literature as to what the exact values are). The enthalpy change for the reaction... 

Table 14.3 Some covalent bond enthalpy terms (klmoP ) the values for single bonds refer to the group 15 elements in 3-coordinate environments, and values for triple bonds are for dissociation of the appropriate diatomic molecule.

For diatomic molecules, bond enthalpies differ slightly from bond dissociation energies, and this difference can be determined using Equation 7.16 from Section 7.2 ... 

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