Water can be split into hydrogen and oxygen.
2H2O→2H2+O2 \mathrm{2H_{2}O \rightarrow 2H_{2} + O_{2}} 2H2O→2H2+O2In this reaction, four O–H bonds are broken. Two H–H bonds and one O=O bond are made.
| Bond | Bond energy (kJ/mol) |
|---|---|
| O–H | 463 |
| H–H | 436 |
| O=O | 498 |
State whether breaking covalent bonds takes in energy or releases energy.
Use the table to calculate the sum of the bond energies for the four O–H bonds.
Calculate the energy released when the two H–H bonds form.
Use your answer to (c) to calculate the total energy released when all the bonds in the products form.
Use your answers to (b) and (d) to calculate the energy change for the reaction.
Explain what the sign of your answer to (e) shows about the reaction.
State why the value calculated using bond energies is only an estimate.
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.