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Bond Energies

The bond energy also known as bond dissociation energy is the energy required to break a covalent bond.  The bond energy is a measure of the strength of a bond and is the enthalpy change for a gas  phase reaction in which the bond breaks.  These energies are usually expressed in kilojoules per mole of bonds and are compiled in tables like the one below.

The bond energy between a given pair of atoms varies slightly from one compound to another.  For example the bond dissociation energy for the C-H bond in methane, CH4 is 435 kJ while in ethane, C2H6 it is 410 kJ.  Because these variations in strength are not very great, the average bond energy is a good guide to the strength of a bond in any molecule. It is these average bond energies that are given in tables like the one below.

#### Bond Energies

Single Bonds
 H C N O S F Cl Br I
 H 432 C 411 346 N 386 305 167 O 459 358 201 142 S 363 272 --- --- 226 F 565 485 283 190 284 155 Cl 428 327 313 218 255 249 240 Br 362 285 --- 201 217 249 216 190 I 295 213 --- 201 --- 278 208 175 149

Multiple Bonds

 C=C 602 C=N 615 C=O 745(799 in CO2 C C 835 C N 887 C O 1072 N=N 418 N=O 607 S=O (in SO2) 532 N N 942 O2 494 S=O (in SO3) 469

When possible it is preferable to use thermochemical data calculate the enthalpy of a reaction.  If the thermochemical data are not known, however, bond energies can be used to give you an estimate of the heat of reaction.  To illustrate this consider the following example.

Use bond energies (Table above) to estimate H (kJ) for the following gas-phase reaction:
N2 + 2 F2 ==> NF2-NF2
Solution
In the reaction, a N N triple bond and two F-F bonds are broken. One N-N single bond and four N-F bonds are formed. H = [BE(N N) + 2BE(F-F)] - [BE(N-N) + 4BE(N-F)] H = [942 + 2(155)] kJ - [167 + 4(283)] kJ = -47 kJ

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