11.2 The chemical properties of the halogen elements and the hydrogen halides

A halogen acts as an oxidising agent when X₂ gains electrons to form halide ions: X₂ + 2e⁻ → 2X⁻. Oxidising power decreases from fluorine to iodine.

Down the group, the incoming electron is attracted less strongly because the atom is larger and more shielded. This makes reduction of X₂ less favourable, even though the molecules have more electrons.

Chlorine can oxidise bromide ions: Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂. Bromine cannot similarly displace chloride under the same conditions.

Oxidising strength is not the same as volatility or colour. Decide it from electron-transfer evidence or displacement reactions.

Hydrogen reacts with a halogen to form a hydrogen halide: H₂ + X₂ → 2HX. The HX molecule is covalent in the gas phase, while its aqueous behaviour depends on how readily the H–X bond ionises.

Hydrogen halides dissolve in water to form acidic solutions. Down the group the H–X bond becomes longer and generally weaker, so bond cleavage and ion formation become easier; hydrogen fluoride is the important weak-acid exception in water.

Hydrogen chloride is a strong acid in aqueous solution, whereas hydrogen fluoride is weak and can form hydrogen bonds. Keep gas-phase molecules and aqueous acid strength as separate descriptions.

“Covalent” does not mean “non-acidic”; acid strength is about ionisation in the solvent.

Thermal stability describes resistance to decomposition on heating. Hydrogen halides become less thermally stable down Group 17 as the H–X bond lengthens and weakens.

A larger halogen atom gives poorer orbital overlap with hydrogen, so less energy is needed to break the bond. This bond-strength explanation is distinct from the aqueous acid-strength question, even though both involve H–X cleavage.

HI decomposes more readily than HCl when heated. A comparison should mention bond length/overlap, not merely say iodine is “more reactive”.

Thermal stability is not the same as boiling point, volatility or acid strength; identify the process being measured.