Bond enthalpy change

Figure 9.8 Bond enthalpy changes In (a) an endothermic reaction and p) an exothermic reaction.

VB valence bond enthalpy change associated with the gain of an electron... 

Enthalpy changes of atomization are always positive, because energy must be absorbed to pull the atoms apart and break the chemical bonds. Enthalpy changes of atomization are usually found indirectly by calculation from other enthalpy changes, using Hess s law. Enthalpy changes of atomization are used in Born-Haber cycle calculations. 

Experimental measurements will actually give the bond enthalpy change AH for this process, which is related to the change of internal energy AU for an experiment taking place at constant pressure P by... 

Electron affinity and hydration energy decrease with increasing atomic number of the halogen and in spite of the slight fall in bond dissociation enthalpy from chlorine to iodine the enthalpy changes in the reactions... 

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. 

We assess the relative stability of alkyl radicals by measuring the enthalpy change (AH°) for the homolytic cleavage of a C—H bond m an alkane... 

The bond dissociation energy (enthalpy change) for a bond A—B which is broken through the reaction... 

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. 

Air Enthalpy change T he heat of reaction, or difference in strength between the bonds broken in a reaction and tire bonds formed. When All is negative, the reaction releases heat and is exothermic. When A IT is positive, the reaction absorbs heat and is endothermic. 

Enthalpy change, AH (Section 5.7) The heat of reaction. The enthalpy change that occurs during a reaction is a measure of the difference in total bond energy between reactants and products. 

Bond enthalpy The enthalpy change (kj/mol) when a particular type of bond is broken in the gas state, 212-214 Boyle, Robert, 359... 

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 ... 

A bond dissociation energy (Section 2.14) is strictly the energy required to break the bond at T = 0 a bond enthalpy is the change in enthalpy at the temperature of dissociation (typically 298 K). The two quantities differ by a few kilojoules per mole. 

Reaction enthalpies can be estimated by using mean bond enthalpies to determine the total energy required to break the reactant bonds and form the product bonds. In practice, only the bonds that change are treated. Because bond enthalpies refer to gaseous substances, to use the tabulated values, all substances must be gases or converted into the gas phase. 

STRATEGY Decide which bonds are broken and which bonds are formed. Use the mean bond enthalpies in Table 6.8 to estimate the change in enthalpy when the reactant bonds break and the change in enthalpy when the new product bonds form. For diatomic molecules, use the information in Table 6.7 for the specific molecule. Finally, add the enthalpy change required to break the reactant bonds (a positive value) to the enthalpy change that occurs when the product bonds form (a negative value). 

A mean bond enthalpy is the average molar enthalpy change accompanying the dissociation of a given type of bond. 

In series with a constant entropy, reactivity is controlled by enthalpy changes. The interpretation is usually based on electronic effects (12-14), which do not affect the form of the transition state, but only bond strengths (116). 

The decomposition of N2 O4 requires a bond to break. This is the reason why the decomposition has a positive A 77 °. At the same time, the number of molecules doubles during decomposition, which is the reason AS° has a positive value. The positive enthalpy change means that energy Is removed from the surroundings and constrained, whereas the positive entropy change means that matter is dispersed. At temperatures below 315 K, the enthalpy term dominates and decomposition is not spontaneous, but at temperatures above 315 K, the entropy term dominates and decomposition is spontaneous. 

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