Halides are all gases with the following properties:
- HF is a colourless fuming gas - b.pt = 20oC
- HCl is a colourless gas - b.pt = -85oC
- HBr is a colourless gas - b.pt = -69oC
- HI is a colourless gas - b.pt = -35oC
The abnormality of HF is due to the presence of hydrogen bonding.
They are covalent substances soluble in polar solvents.
The bond strength of HX decreases down the group as the bonds become less polar. This property governs the reactivity of the halides.
Thermal reactivity decreases down the group. HF is the most stable to heat.
Reactions with water - this determines their acidity i.e. their ease at losing H+.
All the hydrogen halides are very soluble in water. Their solutions are acidic due to the formation of H+ ions.
HX(g) + (aq) → H+(aq) + X-(aq)
The acid strength depends upon the ease at which the HX bond can be broken. Hence, the acid strength increases down the group as bond energies decrease.
HF is a weak acid due to its high bond energy.
In solution the halide ions act as reducing agents, the strongest ability increases down the group. HI is the strongest reducing agent.
HI in will reduce H2SO4 to H2S.
HBr will only reduce H2SO4 to SO2.