Hydrogen Halides as Acids

Hydrogen halides readily react with water to form acidic solutions.Hydrogen fluoride is a weak acid so the reaction is reversible.HF + H₂O ⇌ H₃O⁺ + F^-HCl + H₂O → H₃O⁺ + Cl^-The hydrogen halides are colourless gases that are very soluble in water. In moist air they form misty fumes as they dissolve in tiny water droplets.When a hydrogen halide dissolves, it donates a proton (H⁺) to a water molecule, so it acts as an acid. The water molecule accepts the proton and forms the oxonium ion, H₃O⁺, which makes the solution acidic.HCl, HBr and HI are strong acids: they dissociate completely in water, so a single arrow is used. Their solutions are called hydrochloric acid, hydrobromic acid and hydroiodic acid.HBr + H₂O → H₃O⁺ + Br^-HI + H₂O → H₃O⁺ + I^-Acid strength increases down the group: HF < HCl < HBr < HI. Down the group the halogen atom gets larger, so the H-X bond gets longer and its bond enthalpy decreases. A weaker bond breaks more easily, so H⁺ ions are released more readily.The H-F bond is very strong, which is why hydrogen fluoride only partly dissociates and is a weak acid.