Physical Chemistry Third Edition

2.8 Calculation of Energy Changes of Chemical Reactions 99

Exercise 2.31

What will be the rise in temperature of the calorimeter in the previous example if a pellet of

anthracene of mass 1.345 g is burned? The enthalpy change of combustion of anthracene is

–7114.5 kJ mol−^1.

The calorimetry of substances reacting in solutions can also be carried out at constant

pressure. In this caseqis equal to∆H.

Average Bond Energies

If it were possible to determine exactly the energy required to break every type of

chemical bond, it would be possible to calculate energy changes of a gaseous chem-

ical reaction from bond energies. However, breaking chemical bonds between the

same pair of elements in different compounds requires slightly different amounts of

energy, which makes exact tables impractical. Tables of average values have been

constructed, from which estimates of energy changes can be made. Table A.9 in

Appendix A is a table of such values, given as the energy required to break 1 mol

of the given bond. To estimate the energy change for a gas-phase reaction, you can

write

∆U(sum of all bond energies in reactants)

−(sum of all bond energies in products) (2.8-8)

The calculation can be simplified if one omits from both sums those bonds that occur

in both reactants and products, because their contributions cancel:

∆U(sum of all bond energies of bonds that are broken)

−(sum of all bond energies of bonds that are formed) (2.8-9)

EXAMPLE2.36

Estimate the energy change of the reaction

C 2 H 4 (g)+H 2 (g)−→C 2 H 6 (g)

Compare with the value obtained from enthalpy changes of formation.

Solution

For 1 mol of reaction, 1 mol of CC bonds and 1 mol of H–H bonds must be broken, while

1 mol of C–C bonds and 2 mol of C–H bonds must be formed.

∆U613 kJ mol−^1 +436 kJ mol−^1 −348 kJ mol−^1 −2(413 kJ mol−^1 )

−125 kJ mol−^1