Bond enthalpies defined

These questions can be answered on a molecular level in terms of a quantity known as bond enthalpy. (More commonly but less properly it is called bond energy.) The bond enthalpy is defined as AH when one mole of bonds is broken in the gaseous state. From the... 

There is a lack of information about b.e.cs involving 4d- and 5d transition metals. The detailed variation of b.e.cs with the formal oxidation state of the metal remains to be defined beyond the generalization that there is an approximate inverse relation between the two. However, the comparison of D (W-CO) withf) (W-CH3) indicates that the bond enthalpy/oxidation state relationship is complex. 

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 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 BDEs are well-defined thermodynamic quantities that can be employed in rigorous first-law calculations, but their values strongly differ in each of the above steps. Only the average of the four BDE[CH —H] values could be taken as an estimate of bond enthalpy DH° [C—H],... 

We have already defined the terms bond enthalpy, bond energy, etc. Here we will take up important trends observed. 

The enthalpy AH of an isolated H-bond is defined as the enthalpy of the reaction ... 

Mean bond enthalpies are defined as the enthalpy changes involved in breaking bonds in molecules. They may be determined from thermochemical cycles using Hess Law, although assumptions of transferability are sometimes required. 

When a molecule contains several equal bonds, the enthalpy required to dissociate them successively is not the same. Instead of dealing with individual bond dissociation energies, it is normal to define the mean bond enthalpy. Thus B(O—H) is defined as half the enthalpy change in the process... 

Depending on whether Pauli repulsion or electrostatic attraction represents the dominant contribution to A , this term is either destabilizing or bonding in nature. Although BE values are not defined in the same way as bond dissociation enthalpies A//, they are reasonable approximations of bond enthalpy terms, which in turn provide a good description for the bond strength [9]. 

It is relatively simple to assign bond enthalpies to the bond in a diatomic molecule because in these cases the bond enthalpy is just the energy required to break the molecule into its atoms. However, many important bonds, such as the C—H bond, exist only in polyatomic molecules. For these bonds, we usually use average bond enthalpies. For example, the enthalpy change for the following process in which a methane molecule is decomposed into its five atoms (a process called atomization) can be used to define an average bond enthalpy for the C—H bond, H(C—H) ... 

Just as we can define an average bond enthalpy, we can also define an average bond length for a number of common bonds (A TABLE 8.5). Of particular interest is the relationship, in any atom pair, among bond enthalpy, bond length, and number of bonds between the atoms. For example, we can use data in Tables 8.4 and 8.5 to compare the bond lengths and bond enthalpies of carbon-carbon single, double, and triple bonds ... 

As indicated, the heat of formation (HFO) is taken as the sum of three terms. The first is the steric energy (E). The second is the bond enthalpy (BE), which is calculated from normal bond increments. The third is a partition function increment (PFC), which is itself the sum of three terms a conformational population increment (POP), a torsional contribution term (TOR), and a translation / rotation term (T / R). Since the molecular mechanics calculation has considered only one conformation, the POP and TOR terms are defined to be zero for this calculation (but see below). The T/R term is a molecular translational and rotational term that is always taken to be 2.4kcal/mol at room temperature, since even an anfi-butane molecule that is not rotating about the C2-C3 bond will still be undergoing translation and rotation of the entire molecule as a unit. Therefore, HFO is the total of the bond increment contributions (BE = —35.08) plus the steric energy (E = 2.17) plus the partition function increment (PFC = 2.4). The sum, -30.51 kcal/mol, is slightly lower than the literature value of -30.15 kcal/mol. ... 

Here is a dissociation energy of the H-bonded complex, defined as the enthalpy difference between the monomers and the complex. 

Explain why the bond enthalpy of a molecule is usually defined in terms of a gas-phase reaction. Why are bond-breaking processes always endothermic and bond-forming processes always exothermic ... 

Explain why the bond enthalpy of a molecule is always defined in terms of a gas-phase reaction. 

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