A chemist uses average bond energies to estimate energy changes for the complete combustion of propane and butane.
C3H8+5O2→3CO2+4H2O \mathrm{C_{3}H_{8}+5O_{2}\rightarrow3CO_{2}+4H_{2}O} C3H8+5O2→3CO2+4H2O 2C4H10+13O2→8CO2+10H2O \mathrm{2C_{4}H_{10}+13O_{2}\rightarrow8CO_{2}+10H_{2}O} 2C4H10+13O2→8CO2+10H2OThe number of each type of bond in one molecule is shown below.
| Molecule | Bonds in one molecule |
|---|---|
| propane, C3H8\mathrm{C_{3}H_{8}}C3H8 | eight C−H\mathrm{C{-}H}C−H and two C−C\mathrm{C{-}C}C−C |
| butane, C4H10\mathrm{C_{4}H_{10}}C4H10 | ten C−H\mathrm{C{-}H}C−H and three C−C\mathrm{C{-}C}C−C |
| oxygen, O2\mathrm{O_{2}}O2 | one O=O\mathrm{O{=}O}O=O |
| carbon dioxide, CO2\mathrm{CO_{2}}CO2 | two C=O\mathrm{C{=}O}C=O |
| water, H2O\mathrm{H_{2}O}H2O | two O−H\mathrm{O{-}H}O−H |
| Bond | C−H\mathrm{C{-}H}C−H | C−C\mathrm{C{-}C}C−C | O−H\mathrm{O{-}H}O−H | O=O\mathrm{O{=}O}O=O | C=O\mathrm{C{=}O}C=O |
|---|---|---|---|---|---|
| Average bond energy (kJ/mol\mathrm{kJ/mol}kJ/mol) | 413 | xxx | 464 | 498 | 805 |
For the propane equation as written, the estimated energy change is -2054 kJ.
Calculate the average bond energy, xxx, of a C−C\mathrm{C{-}C}C−C bond.
Use your answer to (a) to calculate the estimated energy change when one mole of butane burns.
106 exam-style questions on OCR GCSE Chemistry 3.2 Energetics, covering 3.2.1 Exothermic and endothermic reactions and reaction profiles and 3.2.2 Calculating energy change from bond energies. Each one has a worked solution and a mark scheme showing where the marks go.