Hydrogen reacts with the halogens to form hydrogen halides.
H2(g)+X2(g)→2HX(g)\mathrm{H_{2}(g)} + \mathrm{X_{2}(g)} \rightarrow \mathrm{2HX(g)}H2(g)+X2(g)→2HX(g)
X is the symbol for a halogen atom. The bond energy of H–H is 436 kJ mol−1\mathrm{kJ\,mol^{-1}}kJmol−1.
The table shows some bond energies and energy changes for these reactions.
| Halogen | X–X bond energy / kJ mol−1\mathrm{kJ\,mol^{-1}}kJmol−1 | H–X bond energy / kJ mol−1\mathrm{kJ\,mol^{-1}}kJmol−1 | Energy change / kJ mol−1\mathrm{kJ\,mol^{-1}}kJmol−1 |
|---|---|---|---|
| fluorine | 158 | 568 | −542 |
| chlorine | 243 | 432 | −185 |
| bromine | 193 | 366 | |
| iodine | 151 | 298 | −9 |
Calculate the energy change for the reaction of hydrogen with bromine.
Describe the trend in the energy change as you go down group 7.
Explain the trend in the energy change. Use data from the table in your answer.
Astatine is below iodine in group 7.
Predict whether the reaction of hydrogen with astatine is exothermic or endothermic. Justify your prediction.
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.