### Enthalpy

Bond Enthalpy
is the energy required to break a chemical bond. It is usually expressed in units of kJ mol-1, measured at 25C. The enthalpy of a particular chemical bond depends upon the molecule in which the bond exists. For this reason, values are typical given as averaged values. This can be expressed mathematically as shown below. H =  H(bonds broken) –  H(bonds formed)

(reactants)                 (products) This basically means that you add up all the energies of the broken bonds; add up all the energies of the bonds that are reformed and subtract one from the other. Let’s take a look at one simple example below. See the spreadsheet table below to see the enthalpy calculations and net result. Bonds broken No. of bonds Energy Total Energy Bonds formed No. of bonds Energy Total Energy C – H 4 413 1652 C = O 2 -805 -1610 O = O 2 498 996 O – H 4 -464 -1856 Total 2648 -3466 Net energy -818

### Bond Energies

 Bonds Ho (kCal/mole) Ho (kJoule/mole) B-F 150 628 B-O 125 523 Br-Br 46 193 C=C 146 611 C=N 147 615 C=O (CO2) 192 804 C=O (aldehyde) 177 741 C=O (amide) 179 749 C=O (ester) 179 749 C=O (halide) 177 741 C=O (ketone) 178 745 C=S (CS2) 138 578 C C 200 837 C N 213 892 C O 258 1080 C-B 90 377 C-Br 68 285 C-C 83 348 C-Cl 81 339 C-F 116 486 C-H 99 414 C-I 51 214 Cl-Cl 58 243 C-N 73 306 C-O 86 358 C-P 70 293 C-S 65 272 C-Si 76 318 F-F 37 153 H-B 90 377 H-Br 88 366 H-Cl 103 431 H-F 135 565 H-H 104 436 H-I 71 297 H-N 93 389 H-O 111 465 H-P 77 322 H-S 81 339 H-Si 75 314 I-I 36 151 N=N 109 456 N=O (HONO) 143 599 N N 226 946 N-CO 86 360 N-N 38 161 N-O 55 230 O=O 119 498 O-CO 110 461 O-O 35 147 P=O (POCl3) 110 461 P=P 84 352 P=S (PSCl3) 70 293 P P 117 490 P-Br 65 272 P-Cl 79 331 P-O 90 377 P-P 50 209 S=O (DMSO) 93 389 S=O (SO2) 128 536 Si-Cl 90 377 Si-F 135 565 Si-O 110 461 Si-Si 52 218 S-O 87 364 S-S 54 226 H-C 99 414