A technician passes water vapour through a tube in an electric furnace. At a very high temperature, some water decomposes to form hydrogen and oxygen.
2H2O→2H2+O2 \mathrm{2H_{2}O \rightarrow 2H_{2} + O_{2}} 2H2O→2H2+O2The bond arrangements for the equation as written are shown below.
| Reactant bonds | Product bonds |
|---|---|
| H–O–H H–O–H | H–H H–H O=O |
Bond energy data are provided below.
| Bond | Bond energy (kJ/mol) |
|---|---|
| O–H | 463 |
| H–H | 436 |
| O=O | 498 |
No more hydrogen or oxygen is produced after the furnace is switched off and the gases cool.
Calculate the total energy needed to break the bonds shown on the reactant side.
Calculate the energy change for the decomposition of water.
Explain why hydrogen and oxygen are produced only while the furnace is kept at a very high temperature.
Your answer should refer to activation energy, the answer to (b) and the relative energy levels of the 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.