NEET Practice Questions (MCQs) with Answers & Solutions

In the Hall-Héroult process, the primary reagent used is aluminum oxide (Al_2O_3) which is dissolved in molten cryolite (Na_3AlF_6). Cryolite acts as a flux to lower the melting point of the mixture and increase its conductivity, making the electrolysis process more efficient. Therefore, the main reagent is mixed with Na_3AlF_6.

$ 2Al(s) + 2 NaOH(aq) + 6H_2 O _{l} \rightarrow 2 Na^+ [ Al (OH)_4 ] ^ - _{(aq)} + 3 H_{2(g) } $ sodium tetra hydroxo aluminate (III)

Among the given options, boron (B) is the non-metal. Boron is a metalloid, which means it has properties intermediate between metals and nonmetals, but it is classified as a non-metal due to its chemical behavior and bonding characteristics.

In diborane ($B_2H_6$), there are two types of B-H bonds: terminal B-H bonds and bridging B-H bonds. The terminal B-H bonds are shorter (about 1.19 Ã…) compared to the bridging B-H bonds (about 1.33 Ã…). This difference is due to the presence of two 3-center-2-electron bonds in the bridging B-H bonds, which makes all B-H bond lengths not equal.

Boron shows a single oxidation state of +3 primarily due to the absence of the inert pair effect. The inert pair effect is more significant in heavier elements, where the s-electrons of the outermost shell are less available for bonding. Since boron is a light element, it does not exhibit this effect, leading to its consistent +3 oxidation state.

$BF_6^{2-}$ does not exist because boron cannot expand its octet to accommodate more than 8 electrons. In the $BF_6^{2-}$ ion, boron would have to accommodate 12 electrons, which is not possible due to its electron configuration and the lack of available d-orbitals.

Thermite welding is a process that uses the exothermic reaction between aluminum powder and a metal oxide, typically iron(III) oxide (Fe2O3). The reaction: $ 2Al + Fe_2O_3 ightarrow Al_2O_3 + 2Fe $ produces molten iron and aluminum oxide, which is used to weld railway tracks and other large metal parts. The intense heat generated by this reaction melts the metals, allowing them to fuse together when cooled.

Hydrogen bonding occurs when hydrogen is covalently bonded to highly electronegative atoms such as nitrogen, oxygen, or fluorine. In water (H2O) and methanol (CH3OH), hydrogen bonds are present due to the hydrogen-oxygen bonds. In inorganic benzene (B3N3H6), hydrogen can form bonds with nitrogen. However, in diborane (B2H6), the hydrogen atoms are bonded to boron, which is not highly electronegative enough to form hydrogen bonds. Therefore, hydrogen bonding is absent in diborane.

Boron hydrides like $B_2H_6$ (diborane) act as Lewis acids because they have electron-deficient boron atoms. A Lewis acid is a species that can accept an electron pair. In $B_2H_6$, the boron atoms are in need of electrons to complete their octet, making it an effective Lewis acid.