Ethanol burns in oxygen.
C2H5OH+3O2→2CO2+3H2O \mathrm{C_{2}H_{5}OH + 3O_{2} \rightarrow 2CO_{2} + 3H_{2}O} C2H5OH+3O2→2CO2+3H2OUse these structures when counting bonds: ethanol is H3C−CH2−OH\mathrm{H_{3}C{-}CH_{2}{-}OH}H3C−CH2−OH, carbon dioxide is O=C=O\mathrm{O{=}C{=}O}O=C=O and water is H−O−H\mathrm{H{-}O{-}H}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 |
Which value is the energy change for this reaction?
−1629 kJ/mol-1629\,\mathrm{kJ/mol}−1629kJ/mol
+339 kJ/mol+339\,\mathrm{kJ/mol}+339kJ/mol
−1271 kJ/mol-1271\,\mathrm{kJ/mol}−1271kJ/mol
+1271 kJ/mol+1271\,\mathrm{kJ/mol}+1271kJ/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.