Liquid hydrogen and liquid oxygen are used as fuel in some space rockets. In the rocket engine the hydrogen burns to form steam.
2H2(g)+O2(g)→2H2O(g)\mathrm{2H_{2}(g)} + \mathrm{O_{2}(g)} \rightarrow \mathrm{2H_{2}O(g)}2H2(g)+O2(g)→2H2O(g)
The table shows some bond energies.
| Bond | Bond energy / kJ mol−1\mathrm{kJ\,mol^{-1}}kJmol−1 |
|---|---|
| H–H | 436 |
| O=O | 498 |
| O–H | 464 |
State the type of energy change that happens when bonds are made.
Calculate the energy change for the reaction.
Explain what the sign of your answer to (b) shows about the reaction.
Hydrogen and oxygen can be mixed at room temperature without reacting. The mixture reacts when it is ignited.
Explain why the mixture does not react until it is ignited.
53 exam-style questions on Edexcel GCSE Chemistry 8.2 Heat energy changes in chemical reactions, covering 8.2.1 Exothermic and endothermic changes, 8.2.2 Bond breaking, bond making and overall energy change, 8.2.3 Calculating energy change from bond energies, and 8.2.4 Activation energy and reaction profiles. Each one has a worked solution and a mark scheme showing where the marks go.