A ceramic kiln uses a propane burner. Propane and oxygen enter the burner at 20 ∘C20\,^{\circ}\text{C}20∘C without reacting. A brief electrical igniter starts a flame. The flame remains after the igniter is switched off, provided that propane and oxygen continue to enter.
C3H8+5O2→3CO2+4H2O \mathrm{C_{3}H_{8} + 5O_{2} \rightarrow 3CO_{2} + 4H_{2}O} C3H8+5O2→3CO2+4H2OA propane molecule contains eight C–H bonds and two C–C bonds. Each oxygen molecule contains one O=O bond. Each carbon dioxide molecule contains two C=O bonds. Each water molecule contains two O–H bonds.
In a separate process, oxygen gas passes through a very hot electric arc. Some oxygen molecules separate into individual oxygen atoms. Production of oxygen atoms stops when the arc is switched off and the gas cools.
O2→2O \mathrm{O_{2} \rightarrow 2O} O2→2OThe table shows average bond energies.
| Bond | Bond energy (kJ/mol) |
|---|---|
| C–H | 413 |
| C–C | 347 |
| O=O | 498 |
| C=O | 805 |
| O–H | 463 |
Explain why the two systems behave differently.
Use the bond-energy data and refer to activation energy and the relative energy levels of reactants and products.
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.