Chemistry · Ch 11 — The p-Block Elements
Orthoboric Acid
Orthoboric Acid
Orthoboric Acid,
Orthoboric acid is a white, crystalline solid with a distinctly soapy feel. It is only sparingly soluble in cold water but dissolves much more readily in hot water.
Preparation. It is made by acidifying an aqueous solution of borax with hydrochloric acid:
It also forms whenever most boron compounds — halides, hydrides, and so on — undergo hydrolysis (react with water or dilute acid).
Structure. Boric acid has a layered structure: planar triangular units are linked to one another through hydrogen bonds, building up sheets (see Fig. 11.1 — a diagram of stacked triangles, each boron bonded to three oxygens that each carry an O–H, with dotted lines showing the hydrogen bonds joining one triangle's H atoms to the next triangle's oxygens).
Acidic behaviour. Boric acid is a weak, monobasic acid — but it is NOT a Brønsted (proton-donating) acid. Instead it behaves as a Lewis acid: it accepts a pair of electrons from the hydroxide ion supplied by a water molecule, releasing a proton in the process: …
What this figure shows. A schematic chemical structure diagram of boric acid showing several planar BO3 (boron bonded to three oxygens) triangular units linked together into an extended layer. Each boron atom (labelled B) sits at the centre of a triangle of oxygen atoms (labelled O), and each oxygen carries an attached hydrogen (H) via a solid O-H bond. The B and O atoms are connected by solid lines forming the covalent BO3 triangles, while dotted lines connect the H atoms of one BO3 unit to the O atoms of a neighbouring unit, representing the hydrogen bonds that link the planar units into a she …