Trends in physical and chemical properties of hydrogen halides:
Direct combination is a useful means of preparing hydrogen chloride. The reaction between hydrogen and fluorine is violent while the reaction between hydrogen and bromine or hydrogen and iodine are reversible and don’t produce pure forms.
Concentrated sulphuric acid displaces hydrogen chloride from ionic chlorides. At higher temperatures the hydrogen sulphate formed react with further ionic chloride. Displacement can be used for the preparation of hydrogen fluorides from ionic fluorides. Hydrogen bromide and hydrogen iodide are oxidised by concentrated sulphuric acid and can’t be prepared in this method.
Gaseous hydrogen halides are produced when water is added in drops to phosphorus tri halides except phosphorus trifluoride.
PX3 + 3H2O → H3PO3 + 3HX
Hydrogen bromide may be obtained by adding bromine dropwise to a paste of red phosphorous and water while hydrogen iodide is conveniently produced by adding water dropwise to a mixture of red phosphorous and iodine.
2P + 3X2 → 2PX3
2PX3 + 3H2O → H3PO3 + 3HX
(where X=Br or I)
Any halogen vapours which escapes with the hydrogen halide is removed by passing the gases through a column of moist red phosphorous.
Halogens are reduced to hydrogen halides by hydrogen sulphide.
H2S + X2 → 2HX + S
Hydrogen chloride is obtained as a by-product of the reactions between hydrocarbon of halogens.
In line with the decreasing bond dissociation enthalpy, the thermal stability of hydrogen halides decreases from fluoride to iodide.
For example, Hydrogen iodide decomposes at 400° C while hydrogen fluoride and hydrogen chloride are stable at this temperature.
At room temperature, hydrogen halides are gases but hydrogen fluoride can be readily liquefied. The gases are colourless but, with moist air gives white fumes due to the production of droplets of hydrohalic acid. In HF, due to the presence of strong hydrogen bond it has high melting and boiling points. This effect is absent in other hydrogen halides.
HX + H2O → H3O+ + X−
(X – F, Cl, Br, or I)
Solutions of hydrogen halides are therefore acidic and known as hydrohalic acids. Hydrochloric, hydrobromic and hydroiodic acids are almost completely ionised and are therefore strong acids but HF is a weak acid i.e. 0.1mM solution is only 10% ionised, but in 5M and 15M solution HF is stronger acid due to the equilibrium.
HF + H2O ↔ H3O+ + F-
HF + F ↔ HF2-
At high concentration, the equilibrium involves the removal of fluoride ions is important. Since it affects the dissociation of hydrogen fluoride and increases and hydrogen ion concentration Several stable salts NaHF2, KHF2 and NH4HF2 are known. The other hydrogen halides do not form hydrogen dihalides.
Hydrohalic acid shows typical acidic properties. They form salts with acids, bases and reacts with metals to give hydrogen. Moist hydrofluoric acid (not dry) rapidly react with silica and glass.
SiO2 + 4HF → SiF4 + 2H2O
Na2SiO3 + 6HF → Na2SiF6 + 3H2O
Oxidation: Hydrogen iodide is readily oxidised to iodine hence it is a reducing agent.
2HI ↔ H+ + I2 + 2e-
Acidic solution of iodides is readily oxidised. A positive result is shown by liberation of iodine which gives a blue-black colouration with starch.
Hydrogen bromide is more difficult to oxidise than HI. HBr reduces slowly H2SO4 into SO2
2HBr + H2 SO4 → 2H2O + Br2 + SO2
But hydrogen iodide and ionic iodides are rapidly reduced by H2SO4 into H2S and not into SO2.
8HI + H2SO4 → 4H2O + 4I2 + H2S
Reducing property of hydrogen iodide can be also explained by using its reaction with alcohols into ethane. It converts nitric acid into nitrous acid and dinitrogen dioxide into ammonium.
Hydrogen chloride is unaffected by concentrated sulphuric aid by only strong oxidising agents like MnO2, potassium permanganate or potassium chloride.
Copyright © 2018-2021 BrainKart.com; All Rights Reserved. (BS) Developed by Therithal info, Chennai.