11.2.2—Reactions of the elements with hydrogen
- Syllabus
- 9701–2028–2029
- Objective
- 11.2.2
- Level
- AS
Hydrogen halides form covalent molecules but give different aqueous acids
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.
ConceptA-Level CAIE Chemistry AS