Using Bond Energies to Calculate

We can think of a reaction as a two-step process in which heat is absorbed (AH° is positive) to break reactant bonds and form separate atoms and then is released (AH° is negative) when the atoms rearrange to form product bonds. The sum (symbolized by Z) of these enthalpy changes is the heat of reaction, A//°xr,  

An equivalent form of Equation 9.2 uses bond energies  

A/ rxn l Ereactant bonds broken Eproducl bonds formed 

The minus sign is needed because all bond energies are positive values (see Table 9.2). 

We use bond energies to calculate A/y°xn by assuming that all the reactant bonds break to give individual atoms, from which all the product bonds form. Even though the actual reaction may not occur this way—typically, only certain bonds break and form—Hess s law (see Section 6.5) allows us to sum the bond energies (with their appropriate signs) to arrive at the overall heat of reaction. (This method assumes that all reactants and products are in the same physical state. When phase changes occur, additional heat must be taken into account. We address this topic in Chapter 12.) 

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Figure 9.16 Using bond energies to calculate AH xn- Any chemical reaction can be divided conceptually into two hypothetical steps (1) reactant bonds break to yield separate atoms in a step that absorbs heat (+ sum of BE), and (2) the atoms combine to form product bonds in a step that releases heat (- sum of BE). When the total bond energy of the products is greater than that of the reactants, more energy is released than is absorbed, and the reaction is exothermic (as shown) AH xn is negative. When the total bond energy of the products is less than that of the reactants, the reaction is endothermic AH°xn is positive.

Let s use bond energies to calculate A/y°xn for the combustion of methane. Figure 9.17 shows that all the bonds in CH4 and O2 break, and the atoms form the bonds in CO2 and H2O. We find the bond energy values in Table 9.2, and use a positive sign for bonds broken and a negative sign for bonds formed ... 

Figure 9.17 Using bond energies to calculate AH°,n of methane.

SAMPLE PROBLEM 9.3 Using Bond Energies to Calculate AHrxn Problem Calculate AH°xn for the chlorination of methane to form chloroform ... 

Figure 9.16 Using bond energies to calculate A// for HF formation.

Changes in Bond Energy Where Does A/frxn Come From 290 Using Bond Energies to Calculate A/frxn 290... 

Another potential future fuel is methanol (CH3OH). Write a balanced equation for the combustion of gaseous methanol and use bond energies to calculate the enthalpy of combustion of methanol in kJ/mol. 

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