Question 10.10: Use bond enthalpies from Table 10.4 to estimate the enthalpy...
Use bond enthalpies from Table 10.4 to estimate the enthalpy of reaction for the combustion of methane:
\text{CH}_4(\text{g}) + 2\text{O}_2(\text{g}) → \text{CO}_2(\text{g}) + 2\text{H}_2\text{O}(l)
Strategy Draw Lewis structures^* to determine what bonds are to be broken and what bonds are to be formed.
Setup
Bonds to break: 4 \text{C—H} and 2 \text{O=O}.
Bonds to form: 2 \text{C=O} and 4 \text{H—O}.
Bond enthalpies from Table 10.4: 414 \text{kJ/mol (C—H)}, 498.7 \text{kJ/mol (O=O)}, 799 \text{kJ/mol} (\text{C=O} in \text{CO}_2), and 460 \text{kJ/mol (H—O)}.
| TA B L E 1 0 . 4 Bond Enthalpies | |||
| Bond | Bond enthalpy (kJ/mol) | Bond | Bond enthalpy (kJ/mol) |
| \text{H—H}^* | 436.4 | \text{N—N} | 193 |
| \text{H—N} | 393 | \text{N=N} | 418 |
| \text{H—O} | 460 | \text{N}\equiv\text{N} | 941.4 |
| \text{H—S} | 368 | \text{N—O} | 176 |
| \text{H—P} | 326 | \text{N=O} | 607 |
| \text{H—F} | 568.2 | \text{N—F} | 272 |
| \text{H—Cl} | 431.9 | \text{N—Cl} | 200 |
| \text{H—Br} | 366.1 | \text{N—Br} | 243 |
| \text{H—I} | 298.3 | \text{N—I} | 159 |
| \text{C—H} | 414 | \text{O—O} | 142 |
| \text{C—C} | 347 | \text{O=O} | 498.7 |
| \text{C=C} | 620 | \text{O—P} | 502 |
| \text{C}\equiv\text{C} | 812 | \text{O=S} | 469 |
| \text{C—N} | 276 | \text{O—F} | 190 |
| \text{C=N} | 615 | \text{O—CI} | 203 |
| \text{C}\equiv\text{N} | 891 | \text{O—Br} | 234 |
| \text{C—O} | 351 | \text{O—I} | 234 |
| \text{C=O}^† | 745 | \text{P—P} | 197 |
| \text{C}\equiv\text{O} | 1070 | \text{P=P} | 489 |
| \text{C—P} | 263 | \text{S—S} | 268 |
| \text{C—S} | 255 | \text{S=S} | 352 |
| \text{C=S} | 477 | \text{F—F} | 156.9 |
| \text{C—F} | 453 | \text{Cl—Cl} | 242.7 |
| \text{C—Cl} | 339 | \text{Cl—Cl} | 193 |
| \text{C—Br} | 276 | \text{Br—Br} | 192.5 |
| \text{C—I} | 216 | \text{I—I} | 151.0 |
| ^*Bond enthalpies shown in red are for diatomic molecules. | |||
| ^†The C O bond enthalpy in CO2 is 799 kJ/mol. | |||
^*Student Annotation: Don’t skip the step of drawing Lewis structures [≪ Section 6.3]. This is the only way to know for certain what types and numbers of bonds must be broken and formed.
^*Student Annotation: Remember that heats of reaction are expressed in kJ/mol, where the “per mole” refers to per mole of reaction as written [≪ Section 10.3].