Question

What happens when,
a.) borax is heated strongly,
b.) boric acid is added to water,
c.) aluminum is treated with dilute NaOH,
d.) \[B{{F}_{3}}\] is reacted with ammonia?

Answer 1

Hint: Borax and boric acid are two different compound formulations of the same element boron. In all actuality, there wouldn’t be any borax without boric acid as it a sodium salt of boric acid.

Complete step by step solution:
a.) When borax (\[N{{a}_{2}}{{B}_{4}}{{O}_{7}}.10{{H}_{2}}O\,\]) is strongly heated, it goes through many transformations. As a first step, it loses its water molecules and grows in volume and forms Sodium metaborate (\[N{{a}_{2}}{{B}_{4}}{{O}_{7}}\]). If we continue heating, it becomes anhydride and starts solidifying into a transparent liquid having an appearance similar to glass called borax bead (\[NaB{{O}_{2}}\]).

\[N{{a}_{2}}{{B}_{4}}{{O}_{7}}.10{{H}_{2}}O\,\xrightarrow{heat}\,N{{a}_{2}}{{B}_{4}}{{O}_{7}}\,+\,10{{H}_{2}}O\]
\[N{{a}_{2}}{{B}_{4}}{{O}_{7}}\,\xrightarrow{heat}\,2NaB{{O}_{2}}+\,{{B}_{2}}{{O}_{3}}\]

b.) Boric acid (\[B{{(OH)}_{3}}\]) when added to water, behaves as a weak monobasic acid by accepting a pair electron from hydroxyl ion \[O{{H}^{-}}\] of water due to the small size of boron and presence of six electrons in its valence shell to form a hydrated species and releases a proton \[{{H}^{+}}\]in the solution. Hence, acting as a Lewis acid.
\[B{{(OH)}_{3}}\,+\,2{{H}_{2}}O\,\to \,{{[B{{(OH)}_{4}}]}^{-}}\,+\,{{H}_{3}}{{O}^{+}}\,\]

c.) Aluminium acts as an acid when treated with dilute NaOH and forms sodium tetrahydroxoaluminate (III), (\[Na[Al{{(OH)}_{4}}]\]). Hydrogen gas is liberated in the process.
\[2Al\,+\,2NaOH\,+\,6{{H}_{2}}O\,\to \,2N{{a}^{+}}{{[Al{{(OH)}_{4}}]}^{-}}\,+\,3{{H}_{2}}\]

d.) The reaction forms an adduct and results in a complete octet around boron. An additional complex (adduct) is formed as ammonia \[N{{H}_{3}}\] acts as a Lewis base and \[B{{F}_{3}}\] as a Lewis acid. This reaction results in formation of complete octet around B in \[B{{F}_{3}}\] by sharing with lone pairs of \[N{{H}_{3}}\].
\[{{F}_{3}}B\,+\,:\,N{{H}_{3}}\,\to \,{{F}_{3}}B\,\leftarrow \,:N{{H}_{3}}\]


Note: This sodium aluminate salt (\[Na[Al{{(OH)}_{4}}]\]) produced is often used in water treatment plants for softening systems and to improve flocculation. It is also used in construction for accelerating the solidification of concrete.