Average bond enthalpies are generally defined for gas-phase molecules. Many substances are liquids in their standard state. By using appropriate thermo-chemical data from Appendix C, calculate average bond enthalpies in the liquid state for the following bonds, and compare these values to the gas-phase values given in Table 8.4:
(a) Br-Br, from Br2 (l);
(b) C-Cl, from CCl4 (l);
(c) O-O, from H2O2 (l) (assume that the O-H bond enthalpy is the same as in the gas phase).
(d) What can you conclude about the process of breaking bonds in the liquid as compared to the gas phase? Explain the difference in the ΔH values between the two phases.

TABLE 8.4 Average Bond Enthalpies (kJ/mol) Single Bonds 413 348 293 358 485 328 276 240 259 N-H N- N N-O N-F N-C 200 O-I N-Br 155 C-H - C-N C-O C-F C-Cl C-Br C-I C-S 391 O-H 163 O-O 201 -F 463 146 190 272 O-Cl 203 234 F-F Cl-F Cl-Cl 253 242 243 Br-F Br-Cl Br Br 237 218 193 H-H H-F H-Cl H-Br H-1 436 567 431 366 299 S-F S-CI S- Br S-S 339 327 253 218 266 I-CI I-Br 208 175 151 Si-H Si Si Si-C Si-O Si-Cl Multiple Bonds 323 226 301 368 464 N--N 418 02 614 839 615 891 799 1072 495 941 607 C-N C-N 523 418