Average bond enthalpies are generally defined for gas-phase molecules. Many substances are liquids in their standard state. (Section) By using appropriate thermochemical 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:
Table Average Bond Enthalpies(kJ/mol)
Single Bonds | |||||||
C–H | 413 | N–H | 391 | O–H | 463 | F–F | 155 |
C–C | 348 | N–N | 163 | O—O | 146 |
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C–N | 293 | N–O | 201 | O–F | 190 | Cl–F | 253 |
C–O | 358 | N–F | 272 | O–Cl | 203 | Cl–Cl | 242 |
C–F | 485 | N–Cl | 200 | O–l | 234 |
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C–Cl | 328 | N–Br | 243 | Br–F | 237 |
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C–Br | 276 | S–H | 339 | Br–Cl | 218 |
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C–l | 240 | H–H | 436 | S–F | 327 | Br–Br | 193 |
C–S | 259 | H–F | 567 | S–Cl | 253 |
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H–Cl | 431 | S—Br | 218 | I–Cl | 208 |
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Si–H | 323 | H–Br | 366 | S—S | 266 | I–Br | 175 |
Si–Si | 226 | H–l | 299 | I–l | 151 |
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Si–C | 301 |
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Si–O | 368 |
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Si–Cl | 464 |
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Multiple Bonds | |||||||
C=C | 614 | N=N | 418 | O=O | 495 |
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C≡C | 839 | N≡N | 941 |
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C=N | 615 | N=O | 607 | S=O | 523 |
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C≡N | 891 | S=S | 418 |
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C=O | 799 |
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C≡O | 1072 |
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(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 that in the gas phase).
(d) Does the process of breaking bonds in the liquid as compared to the gas phase cost more energy? Explain the difference in the ΔH values between the two phases.
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