Multiple choiceQuestion 1 · Recall
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By the end you can describe the relative thermal stabilities of the hydrogen halides and explain them in terms of H–X bond strengths.
Thermal stability
How well a substance resists breaking down when it is heated.
A thermally stable compound needs a very high temperature before it decomposes. A compound with low thermal stability decomposes easily on heating.
Heat a hydrogen halide and it can fall apart into its elements:
The four hydrogen halides behave very differently:
| hydrogen halide | on strong heating |
|---|---|
| stable; does not decompose at normal heating temperatures | |
| only slightly decomposes, even at high temperature | |
| decomposes noticeably, forming bromine | |
| decomposes readily, forming a purple vapour of iodine |
So the order of thermal stability is:
To decompose a hydrogen halide, you must break the H–X bond. The stronger that bond, the more energy is needed, and the more stable the compound.
| bond | bond length / nm | bond enthalpy / kJ mol⁻¹ |
|---|---|---|
| – | 0.092 | 562 |
| – | 0.127 | 431 |
| – | 0.141 | 366 |
| – | 0.161 | 299 |
Down the group the halogen atom gets larger, so the H–X bond gets longer. The shared pair is further from the hydrogen nucleus and is shielded by more inner electrons, so the bond is weaker.
A weaker bond is easier to break, so thermal stability decreases from HF to HI.
Imagine four ropes of increasing length but otherwise identical. The short one is tightly bound and hard to snap; the long one is loose and gives way easily.
The H–X bonds behave like that: the short, strong H–F bond survives heating, while the long, weak H–I bond breaks apart.
Where the analogy stops: bond strength does not come from length alone. The number of protons in the halogen nucleus and the shielding by inner electrons both matter, which is why bond enthalpy falls smoothly down the group.
Worked example: Energy needed to break H–I bonds
The H–I bond enthalpy is . How much energy is needed to break all the H–I bonds in 2.0 mol of hydrogen iodide?