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Calculating an unknown C—H bond energy
Figure 9 shows the displayed formula equation for the reaction between propane and oxygen.
The overall energy change of this exothermic reaction is 2219 kJ/mol.
| Bond | C — C | C — H | O=O | C=O | O — H |
|---|---|---|---|---|---|
| Energy in kJ/mol | 347 | X | 498 | 805 | 464 |
Calculate the bond energy of the C — H bond (X).
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Marking points
- 1 Counts broken bonds to obtain 3184 + 8X.
- 2 Counts made bonds to obtain 8542.
- 3 Uses the 2219 kJ/mol exothermic energy change with the two bond totals.
- 4 Solves for 8X as 3139 kJ/mol.
- 5 Final C—H bond energy is 392 kJ/mol or an accepted correctly rounded equivalent.
Full-mark answer
Bonds broken = 2(347) + 8X + 5(498) = 3184 + 8X. Bonds made = 6(805) + 8(464) = 8542. For the 2219 kJ/mol exothermic change: 2219 = 8542 − (3184 + 8X), so 8X = 3139 and X = 392.375 ≈ 392 kJ/mol.
Why this answer loses marks
I added the energies of all bonds together.
Separate broken and made totals, apply the exothermic sign, and then solve the unknown term.