From Bond Energies to Heats of Reaction

Consider the chlorination of methane to chioromethane. The heats of formation of the reactants and products appear beneath the equation. These heats of formation for the chemical compounds are taken from published tabulations the heat of formation of chlorine, as it is for all elements. 

the chiorination of methane is caicuiated to be an exothermic reaction on the basis of heat of formation data. 

The same conciusion is reached using bond dissociation energies. The foiiowing equation shows the bond dissociation energies of the reactants and products taken from Tabie 4.3  

Because stronger bonds are formed at the expense of weaker ones, the reaction is exothermic and 

This value is in good agreement with that obtained from heat of formation data. 

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Tihe currently accepted value of the O—F bond energy (45 kcal./mole) is calculated from the standard heat of formation of OF2 (7.6 kcal./mole) which was based on an average of three values obtained in 1930 (7, 8) the precision of which was quite poor. To determine a more reliable heat of formation of OF2 and thus a better O—F bond energy, the heat of reaction of the following system was measured ... 

The substantial difference in the heats of reaction of ethane, ethene, and ethyne with bromine is reflected in a very important practical consideration in handling ethyne (acetylene), namely its thermodynamic stability relative to solid carbon and hydrogen gas. Unlike ethane, both ethene and ethyne can be shown from bond energies to be unstable with respect to formation of solid carbon and gaseous hydrogen ... 

Calculate from appropriate bond and stabilization energies the heats of reaction of chlorine with benzene to give (a) chlorobenzene and (b) 1,2-dichloro-3,5-cyclohexadiene. Your answer should indicate that substitution is energetically more favorable than addition. Assume the bond dissociation energy for a C=C it bond to be 6 5 kcal the resonance stabilization energy of benzene to be 36 kcal, and that of 1,2-dichloro-3,5-cyclohexadiene to be 3 kcal. 

We have determined the heats of ozone reaction with paraffins, according to the different mechanisms on the example of methane. The latter has been chosen since the energy of its CH-bond is the highest and thus the results from the calculations for it can be extended for other paraffins. For this purpose we have used the experimentally and calculated, marked by ( ), values of the bond energies and heats of formation [69, 70] ... 

C06-0136. The heat required to sustain animais that hibernate comes from the biochemicai combustion of fatty acids, one of which is arachidonic acid. For this acid, (a) determine its structurai formuia (b) write its baianced combustion reaction (c) use average bond energies to estimate the energy released in the combustion reaction and (d) caicuiate the mass of arachidonic acid needed to warm a 500-kg bear from 5 to 25 °C. (Assume that the average heat capacity of bear flesh is 4.18 J/g K.)... 

As seen from Fig. 12, quantum chemical calculations indeed support the fact that the activation energy for the SiHj-H bond scission reaction corresponds to the bond dissociation energy or the heat of reaction, i.e., AH, % s 90 kcal/mol. In addition, at the transition state, the Si-H bond distance is calculated to be about 2.5 A, a clear indication of the loosness and the increase in entropy associated with the formation of the transition state. 

When reactant R of an energetic material reacts to generate product P, heat is released (or absorbed). Since the chemical bond energy of R is different from that of P, the energy difference between R and P appears as heat. The rearrangement of the molecular structure of R changes the chemical potential. The heat of reaction at... 

It is probable that in general the postulate of the geometric mean leads to somewhat more satisfactory values for the energy of normal covalent bonds between unlike atoms than does the postulate of additivity. The postulate of the geometric mean is more difficult to apply than the postulate of additivity, however, since values of A can be obtained directly from heats of reaction, whereas knowledge of individual bond energies is needed for the calculation of values of A, and in the following sections of this chapter wre shall sometimes use the postulate of additivity. 

Initially, we will be concerned with the physical properties of alkanes and how these properties can be correlated by the important concept of homology. This will be followed by a brief survey of the occurrence and uses of hydrocarbons, with special reference to the petroleum industry. Chemical reactions of alkanes then will be discussed, with special emphasis on combustion and substitution reactions. These reactions are employed to illustrate how we can predict and use energy changes — particularly AH, the heat evolved or absorbed by a reacting system, which often can be estimated from bond energies. Then we consider some of the problems involved in predicting reaction rates in the context of a specific reaction, the chlorination of methane. The example is complex, but it has the virtue that we are able to break the overall reaction into quite simple steps. 

Let s look more closely at spontaneous processes and at the thermodynamic driving forces that cause them to occur. We saw in Chapter 8 that most spontaneous chemical reactions are accompanied by the conversion of potential energy to heat. For example, when methane burns in air, the potential energy stored in the chemical bonds of CH4 and 02 is partly converted to heat, which flows from the system (reactants plus products) to the surroundings ... 

The energy stored in the bonds is called the enthalpy and is given the symbol H. The change in energy going from reactants to products is called the change in enthalpy and is shown as AH (pronounced delta H ). AH is called the heat of reaction. 

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