Chemical reactions bond enthalpy

Enthalpies are often used to describe the energetics of bond formations. For example, when an amide forms through the condensation reaction between an ester and an amine, the new C-N bond, has an enthalpy of formation of -293 kj/mole. The higher the negative value for the bond enthalpy of formation, the stronger the bond. An even more useful concept is the enthalpy of a reaction. For any reaction, we can use the fact that enthalpy is a state function. A state function is one whose value is independent of the path traveled. So, no matter how we approach a chemical reaction, the enthalpy of the reaction is always the same. The enthalpy of... 

Chemistry can be divided (somewhat arbitrarily) into the study of structures, equilibria, and rates. Chemical structure is ultimately described by the methods of quantum mechanics equilibrium phenomena are studied by statistical mechanics and thermodynamics and the study of rates constitutes the subject of kinetics. Kinetics can be subdivided into physical kinetics, dealing with physical phenomena such as diffusion and viscosity, and chemical kinetics, which deals with the rates of chemical reactions (including both covalent and noncovalent bond changes). Students of thermodynamics learn that quantities such as changes in enthalpy and entropy depend only upon the initial and hnal states of a system consequently thermodynamics cannot yield any information about intervening states of the system. It is precisely these intermediate states that constitute the subject matter of chemical kinetics. A thorough study of any chemical reaction must therefore include structural, equilibrium, and kinetic investigations. 

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 ... 

Write the balanced chemical equation for the complete fluorination of methane to tetrafluoromethane. Using bond enthalpies, estimate the enthalpy of this reaction. The corresponding reaction using chlorine is much less exothermic. To what can this difference be attributed ... 

The steric environment of the atoms in the vicinity of the reaction centre will change in the course of a chemical reaction, and consequently the potential energy due to non-bonded interactions will in general also change and contribute to the free energy of activation. The effect is mainly on the vibrational energy levels, and since they are usually widely spaced, the contribution is to the enthalpy rather than the entropy. When low vibrational frequencies or internal rotations are involved, however, effects on entropy might of course also be expected. In any case, the rather universal non-bonded effects will affect the rates of essentially all chemical reactions, and not only the rates of reactions that are subject to obvious steric effects in the classical sense. 

In this section we deal with the first of the physical effects which impinge on reactivity — the influences which heats of reaction and bond dissociation energies have on the course of chemical reactions. Both heats of reaction and bond dissociation energies are enthalpy values that are experimentally determined by thermochemical methods, in the first case usually by direct calorimetric methods, in the second by more indirect techniques 22). 

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,... 

There are alternative ways of viewing the previous problem that are closer to the idealized concept of chemical bond strength. Consider reaction 5.20, where all the chromium-ligand bonds are cleaved simultaneously. The enthalpy of this disruption reaction at 298.15 K, calculated as 497.9 10.3 kJ mol-1 by using enthalpy of formation data [15-17,31], can be given as a sum of three chromium-carbonyl and one chromium-benzene bond enthalpy contributions (equation 5.21). 

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 calculation of the enthalpy of formation of a given compound depends on the determination of the enthalpy of at least one reaction of this substance. Frequently, it is desirable to estimate the enthalpy of a chemical reaction involving a hitherto unsynthesized compound, or one that has been synthesized but has not been characterized calorimetrically. For the solution of problems of this type, a system of average bond enthalpies has been established such that, if the molecular structure of the compound is known, it is possible to approximate the enthalpy of formation by adding the appropriate average bond enthalpies. 

It should also be mentioned here that many of the chemical reactions which have been "explained with the HSAB model (2) occur in polar solvents and many involve the formation of ionic species. Thermodynamic cycles can be constructed for these reactions which show how many different kinds of effects are operative. When one considers that much of the data involve rate constant and equilibrium constant measurements, the explanation of this data becomes even more complex for there are entropy terms as well as enthalpy terms for all the steps in any cycle that is constructed. Even less information is available concerning these steps than we had above for the coordination model yet explanations are offered based solely on one step (4) — the strength of the bonding. 

Enthalpy, H, is the heat content of the reacting system. It reflects the number and kinds of chemical bonds in the reactants and products. When a chemical reaction releases heat, it is said to be exothermic the heat content of the products is less than that of the reactants and AH has, by convention, a negative value. Reacting systems that take up heat from their surroundings are endothermic and have positive values of AH. 

All chemical reactions are influenced by two forces the tendency to achieve the most stable bonding state (for which enthalpy, H, is a useful expression) and the tendency to achieve the highest degree of randomness, expressed as entropy, S. The net driving force in a reaction is AG, the free-energy change, which represents the net effect of these two factors AG = AH- TAS. 

In a chemical reaction, old bonds are broken and new ones formed. We ought to be able to estimate reaction enthalpies if we know the enthalpy... 

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