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Boron trihydride, B2H6

Boron trihydride or boro-ethane, B2H6 has been mentioned that the compound B4H10 is very unstable. When kept over mercury at the ordinary temperature it decomposes with the formation of a gaseous hydride B2H6 and hydrogen. The rate of decomposition is greatly augmented by ultraviolet light, but not appreciably by sunlight. The change is practically complete at 100° after one hour.

Boron trihydride is a colourless gas possessing a repulsive odour which recalls that of B4H10, and also that of hydrogen sulphide. Its vapour density corresponds with the molecular formula B2H6. The hydride melts at -169° and boils at -87° C. The vapour pressure of the liquid at various temperatures is as follows: -

Temp. °C-130°-120°-112°-110°-100°-90°-87°
Vap. press, (in cms.)41022.425406576


Boron trihydride is fairly stable, but it slowly decomposes even at the ordinary temperature. When heated with a free flame, solid, but not liquid, hydrides are produced; when sparked, boron is deposited and non-volatile hydrides of unpleasant odour are produced. In the presence of air it gives rise to solids containing both boron and oxygen, and when exploded with oxygen it reacts thus: -

B2H6 + 3O2 = B2O3 + 3H2O.

It dissolves in carbon disulphide, and the solution decolorises bromine; it does not react with dry hydrogen chloride, but combines readily with dry ammonia.

Boron trihydride reacts with water to give boric acid and hydrogen: -

B2H6 + 6H2O = 2H3BO3 + 6H2.

Like the hydride B4H10, it gives a hypoborate with alkali hydroxide, but some hydrogen is evolved immediately the gas and. the hydroxide come into contact. The hypoborate slowly decomposes, and the final result may be expressed thus: -

B2H6 + 6NaOH = 2Na3BO3 + 6H2.

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