Question

Which reagent is used in Bayer's process ?
A. $N{a_2}C{O_3}$
B. Carbon
C. $NaOH$
D. Silica

Answer 1

Hint: Baeyer's process is a process used for the leaching of bauxite ( an ore of aluminium ). The reagent chosen should be the one which is a strong reducing agent.

Complete answer:
Leaching is a process which consists of treating the powdered ore with a suitable reagent such as acid base or other chemicals which can selectively dissolve the ore but not the impurities. First the bayer's process is used for obtaining pure alumina from the bauxite ore . The bauxite ore contains many impurities like iron oxide , silica and titanium oxide. The powdered ore is then heated with concentrated solution of sodium hydroxide at 473 - 523K and 35-36 bar pressure. Under the above mentioned conditions, alumina dissolves forming sodium meta - aluminate and silica as sodium silicate, leaving behind the impurities.
$A{l_2}{O_3}(s) + 2NaOH(aq) + 3{H_2}O(l)\xrightarrow{{473 - 523K}}2Na[Al{(OH)_4}](aq)$
$Si{O_2}(s) + 2NaOH(aq)\xrightarrow{{473 - 523K}}N{a_2}Si{O_3}(aq) + {H_2}O(l)$
The solution obtained is then filtered , cooled and its pH adjusted downward either by dilution or by neutralization with carbon dioxide when aluminium hydroxide gets precipitated.
After this the solution is seeded with freshly prepared samples of hydrated alumina when hydrated alumina gets precipitated leaving sodium silicate in the solution.
$2Na[Al{(OH)_4}](aq) + 2C{O_2}(g) \to A{l_2}{O_3}.x{H_2}O(s) \downarrow + 2NaHC{O_3}(aq)$
Hydrated alumina obtained is then filtered , washed , and finally heated to about 1473K to get pure alumina.
$A{l_2}{O_3}.x{H_2}O(s)\xrightarrow{{1473K}}A{l_2}{O_3}(s) + x{H_2}O(g)$
Thus, in this process $NaOH$ is used as a reducing agent.

Therefore, option C is correct.

Note: It should be noted that the reagent used in baeyer's process should not be confused with baeyer's reagent as they both are different. Reagent used in baeyer's process is $NaOH$ whereas baeyer's reagent is cold, dilute, alkaline permanganate solution.