Covalent bonds enthalpy terms

Table 13.2 Some experimental covalent bond enthalpy terms (kJmoP ) the values for single bonds refer to the group 14 elements in tetrahedral environments.

Table 13.2 lists some experimentally determined values for covalent bond enthalpy terms. When we try to interpret the chemistry of the group 14 elements on the basis of such bond energies, caution is necessary for two reasons ... 

Analogies between groups 14 and 15 are seen if we consider certain bonding aspects. Table 14.3 lists some covalent bond enthalpy terms for group 15 elements. Data for most single bonds follow trends reminiscent of those in group 14... 

Table 15.2 Some covalent bond enthalpy terms (kJmol ) for bonds involving oxygen, sulfur, selenium and tellurium.

The standard enthalpy change for the conversion of a gaseous covalent molecule into gaseous atoms is the sum of the bond enthalpy terms for the bonds in that molecule. Bond enthalpy terms do not vary greatly from one molecule to another. ... 

Understand the relationship between bond enthalpy terms and the enthalpies of atomization of gaseous covalent molecules, and calculate one from the other. (Question 4.3)... 

The enthalpy change during donor-acceptor adduct formation has been related by Drago to the sum of two terms (1) the product of the electrostatic properties of the acid and the base, Ef. and E and (2) the product of their tendency toward covalent bonding, Ca and Cb [10]. For the particular case, where the acceptor is specified to be SbCb (and the inert solvent is 1,2-dichloroethane), the negative of this enthalpy change (in kcal mol", 1 cal = 4.184 J) is the Gutmann donor number, DN [2,11]. These concepts are further discussed in Chapter 3. 

The maximum stability for vanadium as the V3 4 ion in acidic solution can be understood in terms of the maximization of the enthalpy of hydration for the +3 ion, above which hydrolysis alters the form and stability of the higher states. The + 4 and + 5 states are considerably electronegative compared to the lower oxidation states, and are able to engage in covalent bonding to ligand oxide ions to form V=0 bonds. 

The strength of a covalent bond is measured in terms of its bond dissociation energy. Bond energies can be used to estimate the enthalpy of reactions. 

The preceding considerations are the result of the respective bond strengths from 25 to 2(X)kcalmol for the covalent bond (the enthalpy term is largely prevailing AH TAS) and, from 1 to 10kcalmol for the irreversible binding (the enthalpy and entropy are of similar importance AH TAS). ... 

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