Halons are fluorocarbons that contain bromine, such as CBrF3. They are used extensively as foaming agents for fighting fires. Like CFCs, halons are very unreactive and ultimately can diffuse into the stratosphere.
(a) Based on the data in Table 8.4, would you expect photodissociation of Br atoms to occur in the stratosphere?
Table 8.4 Average Bond Enthalpies(kJ/mol)
Single Bonds | |||||||
C—H | 413 | N—H | 391 | 463 | O—H | 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 |
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| Br—F | 237 |
C—Br | 276 |
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| S—H | 339 | Br—Cl | 218 |
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 |
Si—H | 323 | H—Br | 366 | S—S | 266 | I—Br | 175 |
Si—Si | 226 | H—l | 299 |
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| I—l | 151 |
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 |
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| S═S | 418 |
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C═O | 799 |
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C≡O | 1072 |
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(b) Propose a mechanism by which the presence of halons in the stratosphere could lead to the depletion of stratospheric ozone.
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