Estimation from bond enthalpies

The Reaction Enthalpies Can Be Estimated from Bond Enthalpies 401... 

True or false (a) Formal charges represent actual separation of charges (b) A/7 can be estimated from bond enthalpies of reactants and products (c) all second-period elements obey the octet rule in their compounds (d) the resonance structures of a molecule can be separated from one another. 

Butene reacts with hydrogen chloride in a hydrohalogenation reaction. Estimate the standard reaction enthalpy from bond enthalpies. 

The energetics of the C—H activation process have been of concern since its discovery. Calorimetric, equilibrium constant and kinetic information have been used to obtain estimates of the M—C and M—H bond energies. In general, these bonds have been found to be stronger than originally anticipated. Nolan et al. used a combination of calorimetric and kinetic data to estimate these bond enthalpies from the reactions in the following Scheme ... 

Enthalpy of reaction, (gas-phase reaction, estimation from bond energies)... 

Because these various quantities are characteristics of the reactants and products but are independent of the reaction path, they cannot provide insight into mechanisms. Information about AG, AH, and AS does, however, indicate the feasibility of any specific reaction. The enthalpy change of a given reaction can be estimated from tabulated thermochemical data or from bond-energy data such as those in Table 1.3 (p. 14) The exan le below illustrates the use of bond-energy data for estimating the enthalpy of a reaction. 

The heat of formation of 804X4 was determined to be AH°f = +163 kcal mor. " Thus 804X4 is even more endothermic than S4X4 (AH°f = +110 kcal mok ). The mean E-X bond energies in E4X4 were estimated to be 59 kcal mol (E = Se) " and 72 kcal mol (E = S) from the enthalpies of formation. 

A note on good practice. The use of mean bond enthalpies is hazardous because actual bond enthalpies often differ considerably from mean values. The modem procedure for estimating a reaction enthalpy is to use commercial software to calculate the enthalpies of formation of the reactants and products and then to take the difference, as in Section 6.18. 

Using data from Table 9.2 and standard graphing software, determine the enthalpy and entropy of the equilibrium N204(g) — 2 N02(g) and estimate the N—N bond enthalpy in N204. How does this value compare with the mean N—N bond enthalpy in Table 6.8 ... 

They are formally isoelectronic with the (ArH)Cr(CO)3 series, and are derived from Co4(CO)12. The thermal decomposition of three representatives of the series has been studied by microcalorimetry84) and the results are shown in Table 16. Once again heats of sublimation have had to be estimated by comparison with the chromium analogues. The enthalpy disruption can be divided by taking T = 134 kJ mol-1 (Table 1) so that the b.e.c of the [Co4(CO)9] fragment in Co4(CO)i2 is 1722 kJ mol-1. The (ArHCo) bond enthalpy contribution is then obtained in the usual way the results are shown in Table 16. It is clear that as in the chromium series, the b.e.c (ArH-Co) increases along the series benzene < mesitylene < hexamethylbenzene. 

Table 4.5. Average values (kJ/mol) of the metal—metal bond enthalpies estimated from metal carbonyls (Mco) and from bulk metals (Mmet) (adapted from Mingos and Wales (1990)).

Except for the direct van t Hoff method, the enthalpies of dihydrogen bonds can be estimated from the frequency shifts, Av (HX), or the integral intensity, A (XH), determined in the IR spectra for proton-donor components, HX ... 

The most stable state of nitrogen in acidic solution is the ammonium ion, NH4(aq), which is isoelectronic with CH4 and H30+. It is a tetrahedral ion with strong N-H bonds. The mean N-H bond enthalpy in NH4(aq) is 506 kJ mol 1 (that of the O-H bonds in H30 + is 539 kJ mol" ). The enthalpy of hydration of the ammonium ion is — 345 kJ mol V This value placed into the Born equation (3.32) gives an estimate of the radius of the ammonium ion of 135 pm, a value insignificantly different from its thermochemical radius of 136 pm. The value is comparable to that estimated for the smaller H30+ ion (99 pm) from its more negative enthalpy of hydration (— 420 kJ mol -see Section 2.6.1). The proton affinity of the ammonia molecule is of interest in a comparison of its properties with those of the water molecule. The proton affinity is defined as the standard enthalpy change for the reaction ... 

Of the enthalpy of sublimation of ice, 12.20 kcal/mole, about one-fifth can be attributed to ordinary van der Waals forces (as estimated from values for other substances) the remainder, 10 kcal/mole, represents the rupture of hydrogen bonds and leads to the value 5 kcal/mole for the energy of the O—H- O hydrogen bond in ice. The small value 1.44 kcal/mole of the enthalpy of fusion of ice shows that on melting only about 15 percent of the hydrogen bonds are broken. 

Problem Estimate A//f°[C2H5OH(g)] from the bond enthalpies tabulated above. 

---