Standard bond enthalpy

Dividing the standard enthalpy chanK between the four bonds gives an average value for the C—H bond of 416kJmol". This value is called the average standard bond enthalpy for the c — H bond. 

Tables of average standard bond enthalpies make the assumption that the standard enthalpy of a bond is independent of the molecule in which it exists. This is only roushly true. Since standard bond enthalpies vary from one compound to another, the use of avaage standara bond enthalpies gives only approximate values for standard enthalpies of reaction calculated from them. Experimental methods are used to obtain standard enthalpies of reaction whenever possible. Calculations bwd on average standard bond enthalpies are used only for reaaiona which cannot ce studied experimentally —for example, the reactions of a substance which has not been isolated in a pure state.

Mean standard bond enthalpies can be used to gne an aj estimate of the standard enthalpy diange which occurs in. ... 

During a reaaion. energy is supplied to break the bonos m me reactants, and energy is gRren out when the bonds in the products form. The difference between the sum of the standard bond enthalpies of tbe products and the standard bond enthalpies of the reactants is the staiulard enthalpy of the reaction. The value obtained is less reliable than an eaperimemal measurement. 

The energy in question is called the standard bond enthalpy. It can be tested, and the values exist in standardized tables that can be referenced in textbooks. They can be used to determine the stability of compounds that can be either covalent or ionic. However the calculation Isreaks down at times, for example if values are very low positive or negative values, the results can t always be trusted. To compensate for this, a chemist can change the conditions under which the reaction takes place to better suit crystal formation. 

Strictly, these values are bond enthalpies, but the term energies is commonly used. Other descriptions are average standard bond energies, mean bond energies . 

Procedure. Run one or more simultaneous equation programs to determine the C—C and C—H bond energies and interpret the results. The error veetor is the veetor of ealeulated values minus the veetor of bond enthalpies taken as tme from an aeeepted source. Caleulate the enor veetor using a standard souree of bond enthalpies (e.g., Laidler and Meiser, 1999 or Atkins, 1994). Expand the method for 2-butene (2-butene) = —11 kJ mol ] and so obtain the C—H, C—C,... 

In a chemical reaction, old bonds are broken and new ones formed. We can estimate reaction enthalpies if we know the enthalpy changes that accompany the breaking and making of bonds. The strength of a chemical bond is measured by the bond enthalpy, AHR, the difference between the standard molar enthalpies of a molecule, X-Y (for instance, H3C—OH), and its fragments X and Y (such as CH3 and OH) in the gas phase ... 

J 16 Use average bond enthalpies to estimate the standard enthalpy of a reaction, (Example 6.14). 

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 ... 

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

Thermochemical parameters of some unstable nitrile oxides were evaluated using corresponding data for stable molecules. Thus, for 2,4,6-trimethylbenzo-nitrile N-oxide and 2,4,6-trimethoxybenzonitrile N-oxide, the standard molar enthalpies of combustion and sublimation at 298.15 K were measured by static-bomb calorimetry and by microcalorimetry, respectively, this made it possible to derive the molar dissociation enthalpies of the N—O bonds, D(N—O) (17). 

Table 1. Standard enthalpy of formation of metal carbonyls [Mm(CO) ] in the gas phase. Bond description and bond enthalpy contributions (T, M and B) to the enthalpy of disruption, AHq...

Table 12a. Standard enthalpy of formation, A/ff(g), enthalpy of disruption, AHjy, and metal-halogen bond enthalpy contribution, (M-X), in metal carbonyl halides (kJ mol-1)...

Table 14. Standard enthalpies of sublimation, formation and disruption and bond enthalpy contributions, iT(W-N) kJ mol-1, for N-donor complexes of tungsten W(CO)6 nLn]...

Adiabatic detachment energy [7]. Abbreviations used rcoy (Em) = covalent radius of element E in a trivalent compound BE(E-E) = bond enthalpy of a single E-E bond D°298(E2) = dissociation enthalpy of the E2 molecule at standard conditions IE = ionization enthalpy EA = electron affinity AHf°(E2 g) = standard enthalpy of formation of the gaseous E2 molecule. 

No symbol has been approved by the IUPAC for dissociation energy in the chemical thermodynamics section [13]. Under Atoms and Molecules, either El or D is indicated. The latter is more common, and IUPAC recommends Do and De for the dissociation energy from the ground state and from the potential minimum, respectively. Because the bond energy concept will be omnipresent in this book and can be explored in a variety of ways, some extra names and symbols are required. This matter will be handled whenever needed, but for now we agree to use DUP for a standard bond dissociation internal energy and DHj for a standard bond dissociation enthalpy, both at a temperature T. In cases where it is clear that the temperature refers to 298.15 K, a subscript T will be omitted. 

Experimental values of bond dissociation enthalpies are scarce compared with the data available for standard enthalpies of formation. This is not surprising because most chemical reactions that have been studied thermochemically involve the cleavage and the formation of several bonds. The measured standard reaction enthalpies are thus enthalpy balances of various bond dissociation enthalpies, whose individual values are often unknown. Consider, for example, reaction 5.10, where the arene ring in (q6-bcnzene)chromium tricarbonyl is replaced by three carbonyl ligands. The enthalpy of this reaction at 298.15 K,... 

Let us concentrate on the thermochemical cycle of figure 5.6 that involves the disruption of the complex Cr(CO)3(C6H6). The enthalpy of this reaction, previously calculated as 497.9 10.3 kJ mol-1 from standard enthalpy of formation data, can be related (equation 5.24) to the bond enthalpy contributions EsfCr-CO ) andE s(Cr (V.He) through the reorganization energies ER(C() ) and ER(C(tHf )- Two asterisks indicate that the fragment has the same structure as... 

The so-called Laidler scheme was developed as a tool to estimate standard enthalpies of formation of organic compounds [90], It relies on the bond-additivity concept, that is, it assumes that the standard enthalpy of atomization of a given molecule in the gas phase (Aat//°, defined as the standard enthalpy of the reaction where all the chemical bonds are cleaved, yielding the gaseous ground-state atoms) can be evaluated by adding the relevant bond enthalpy terms. For instance, in the case of phenol, its standard enthalpy of atomization, or simply its enthalpy of atomization, refers to reaction 5.28 at 298.15 K ... 

The most reliable values of Laidler terms that can applied to a wide variety of compounds are those recommended by Cox and Pilcher [89]. They were derived from a consistent database that includes experimentally determined standard enthalpies of formation for hundreds of organic compounds. This lengthy but simple exercise involves the choice of a set of bond enthalpy terms that affords the best agreement between experimental and calculated standard enthalpies of... 

In the preceding sections we discussed methods of obtaining enthalpies of reaction at a fixed temperature (generally 298.15 K). In particular, we pointed out that it is possible to tabulate enthalpies of formation and bond enthalpies and to use these to calculate enthalpies of reaction. Such tables of enthalpies of formation are available for only a few standard temperatures. Frequently, however, it is necessary to know the enthalpy of a reaction at a temperature different from those available in a reference table. Therefore, we consider now the procedures that can be used to calculate the enthalpy of reaction (at constant pressure) at one temperature, from data at another temperature. 

Table V. Standard Bond Dissociation Enthalpies DH° (X—Y) of Some Oxide and Poly oxide Molecules and Free Radicalsa...

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 ... 

The sum of bond enthalpies is therefore related (with change of sign) to AHf of the gaseous compound, but with an altered reference state of gaseous atoms El(g) [rather than the standard form El(std)] for each element El, ... 

The standard enthalpies of formation of BH3(g) and diborane are +100 kj-mol 1 and +36 kj-mol, respectively, and the enthalpies of formation of B(g) and H(g) are +563 kj-mol-1 and +218 kj-mol"1, respectively, (a) Use these values to calculate the mean bond enthalpies of the B—H bonds in each case (b) Assume that terminal B—H bonds have the same strengths in each compound and estimate the bond enthalpy of the three-center B—H—B bonds in diborane. (c) Which bonds would you expect to be longer, the terminal B—H bonds or the three-center bonds Explain your answer. 

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