Q7(b)(ii). Order of reactivity of five interhalogens

1. Why interhalogens are reactive: the polar X–Y bond is attacked more easily than the X–X bond of a halogen, so interhalogens are more reactive than the parent halogens (fluorine excepted): strong oxidising and fluorinating agents; their reactivity is mostly a question of how easily they give up F.
2. ClF₃, the most reactive: a small Cl(III) centre carries three F atoms and two lone pairs (T-shaped); the axial Cl–F bonds are long and weak (three-centre bonding). It fluorinates faster than F₂ itself, reacts explosively with organic matter, sets asbestos alight and converts metal oxides to fluorides.
3. BrF₅ and IF₇: the larger central atoms form stronger X–F bonds and are less crowded, so they are milder; IF₇, although I is in the +7 state, is less aggressive than BrF₅.
4. ClF: only one F per molecule to transfer, so it is less vigorous than the higher fluorides; still a strong fluorinating agent and the most stable of the diatomic interhalogens.
5. IF, the least: very unstable; it disproportionates as soon as it forms (5IF → 2I₂ + IF₅), so it is the poorest fluorinating agent of the set.
6. General trend: for the same central atom, more F atoms means more reactivity; for the same formula type, reactivity falls as the central halogen becomes larger (Cl > Br > I).

Application. ClF₃ is used to passivate nickel apparatus for fluorine chemistry and to make UF₆ from uranium for isotope enrichment; mild IF₅ can be handled in glass and is used for selective fluorination.