Explain4 marksStandard
A student investigates the reactions of chlorine, bromine and iodine with hydrogen. Chlorine reacts explosively in sunlight, bromine reacts slowly when heated, and iodine reacts only at high temperatures and the reaction is reversible. The student also heats samples of hydrogen chloride, hydrogen bromide and hydrogen iodide. Hydrogen iodide decomposes to a much greater extent than hydrogen chloride. The bond dissociation energies of the hydrogen halides are given below.
| Hydrogen halide | Bond dissociation energy / kJ mol |
|---|---|
| HCl | 431 |
| HBr | 364 |
| HI | 297 |
Explain these observations in terms of the relative reactivity of the halogens and the bond strengths in the hydrogen halides.