The balanced equation for the complete combustion of ethanol is:
C2H5OH+3O2→2CO2+3H2O \mathrm{C_{2}H_{5}OH + 3O_{2} \rightarrow 2CO_{2} + 3H_{2}O} C2H5OH+3O2→2CO2+3H2OThe tables show the bonds in one molecule of each substance and the bond energies.
| Substance | Bonds in one molecule |
|---|---|
| C2H5OH\mathrm{C_{2}H_{5}OH}C2H5OH | five C−H\mathrm{C{-}H}C−H, one C−C\mathrm{C{-}C}C−C, one C−O\mathrm{C{-}O}C−O and one O−H\mathrm{O{-}H}O−H |
| O2\mathrm{O_{2}}O2 | one O=O\mathrm{O{=}O}O=O |
| CO2\mathrm{CO_{2}}CO2 | two C=O\mathrm{C{=}O}C=O |
| 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 | C−O\mathrm{C{-}O}C−O | O−H\mathrm{O{-}H}O−H | O=O\mathrm{O{=}O}O=O | C=O\mathrm{C{=}O}C=O |
|---|---|---|---|---|---|---|
| Bond energy (kJ/mol\mathrm{kJ/mol}kJ/mol) | 413 | 347 | 358 | 463 | 498 | 805 |
What is the energy change per mole of water produced?
−424 kJ/mol-424\,\mathrm{kJ/mol}−424kJ/mol
−636 kJ/mol-636\,\mathrm{kJ/mol}−636kJ/mol
+113 kJ/mol+113\,\mathrm{kJ/mol}+113kJ/mol
−578 kJ/mol-578\,\mathrm{kJ/mol}−578kJ/mol
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.