Average bond enthalpies

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

To calculate the average bond enthalpy of the (C-H) bond in methane, E (C-H), the average of the four bond dissociation enthalpies is calculated ... 

Because average bond enthalpies refer to the endothermic process of bond breaking, they always have a positive sign. 

In a diatomic molecule, e.g. chlorine (CI2), the bond dissociation enthalpy and the average bond enthalpy will have the same value. This is because both enthalpy changes refer to the process Cl2(g)--> 2Cl(g). 

Example Use the average bond enthalpy data below ... 

Each mole of NEbig) contains three moles of (N-H) bonds. Therefore AHat (NH3(g) = 3 x E(N-H) where E(N-H) is the average bond enthalpy for the (N-H) bond. Therefore 3E(N-H) =+1173.2... 

The variations in AHBE are clearly huge, so we usually work with an average bond enthalpy, which is sometimes written as BE or A//2L The average bond enthalpy for... 

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. 

Table 2.2 Average Bond Lengths (A) and Average Bond Enthalpies (kJ mol 1) of Some Important Covalent Bonds"...

An average bond enthalpy is the average enthalpy change accompanying the dissociation of a given type of bond. 

STRATEGY Decide which bonds are broken and which bonds are formed. Use the average bond enthalpies in Table 6.7 to calculate the decrease in... 

Bond Average bond enthalpy Bond Average bond enthalpy... 

The bond enthalpy in NO is 632 kj-mol-1 and that of each N-O bond in N02 is 469 kj-mol-1. Using Lewis structures and the average bond enthalpies given in Table 6.7, explain (a) the difference in bond enthalpies between the two molecules (b) the fact that the bond enthalpies of the two bonds in N02 are the same. 

It must be remembered that mathematically bonds formed give a negative value and bonds broken give a positive value. To make use of this approach, bond enthalpies are needed, and Table 4.1 shows average bond enthalpies for numerous kinds of bonds. 

Bond enthalpy is the enthalpy change associated with making or breaking a chemical bond. It is based on the idea that bond enthalpy, like bond energy (see Chapter 3), is approximately independent of the rest of the molecule. Average bond enthalpies are tabulated from measurements over a broad range of molecules in which the same bond appears. 

A second CFC used as a refrigerant and in aerosols (besides that discussed in Example 12.8) is CCI3F. Use the atomization enthalpies and average bond enthalpies from Table 12.3 to estimate the standard enthalpy of formation (AH°) of this compound in the gas phase. 

Propane has the structure IT3C—CH2—CH3. Use average bond enthalpies from Table 12.3 to estimate the change in enthalpy AH° for the reaction... 

---