Problem 147 - A Complete Resource Book in Chemistry for JEE Main

Question

Borax \(\left[\mathrm{Na}_{2} \mathrm{~B}_{4} \mathrm{O}_{7} .10 \mathrm{H}_{2} \mathrm{O}\right]\) when heated on platinum loop it gives a dark transparent glass like bead. The hot bead is dipped in the salt till it reacts with transition metal oxide. It produces characteristic bead of meta borate. $$ \begin{array}{ll} \text { Colour of the bead } & \text { Ion } \\ \text { Blue green or light blue } & \mathrm{Cu}^{2+} \\ \text { Yellow } & \mathrm{Fe}^{2+} \text { or } \mathrm{Fe}^{3+} \\ \text { Green } & \mathrm{Cr}^{3+} \\ \text { Violet } & \mathrm{Mn}^{2+} \\ \text { Dark blue } & \mathrm{Co}^{2+} \\ \text { Brown } & \mathrm{Ni}^{2+} \end{array} $$ The hybridisation of \(\mathrm{B}\) in borax is (a) sp (b) \(\mathrm{sp}^{2}\) (c) \(\mathrm{sp}^{3}\) (d) Both (B) and (C)

Step-by-Step Solution

Answer

The hybridization of \(\mathrm{B}\) in borax is both \(\mathrm{sp}^{2}\) and \(\mathrm{sp}^{3}\), so the answer is (d) Both (B) and (C).

Step 1: Understand the Composition of Borax

Borax is a complex compound represented by the chemical formula \(\text{Na}_2\text{B}_4\text{O}_7 \, 10\text{H}_2\text{O}\). It contains boron (B) atoms, which are the focus of determining the hybridization.

Step 2: Identify Key Boron Component in Borax

The boron atoms in borax form negatively charged polyatomic ions called borates, which are derived from \(\text{B}_4\text{O}_7^{2-}\). In this arrangement, boron typically forms part of a tetrahedral geometry due to covalent bonding with oxygen atoms.

Step 3: Determine Hybridization of Boron

In a borate ion, each boron atom is typically bonded to three oxygen atoms in a trigonal planar manner, which implies the use of \(\mathrm{sp}^{2}\) hybridization for planar formations. However, considering the overall polyatomic structure and the hydration coordination forming tetrahedrals, it could also exhibit \(\mathrm{sp}^{3}\) hybridization.

Step 4: Evaluate Hybridization Options

From the borate ion structure in borax, we find that boron displays both \(\mathrm{sp}^{2}\) and \(\mathrm{sp}^{3}\) hybridizations depending on its interaction within the polyatomic structure of borax.

Key Concepts

Transition metal ions

Transition metal ions have a unique ability to produce different colored beads when involved in the borax bead test. This fascinating test helps identify metal ions based on the specific color they impart to the bead when heated. When borax is melted on a platinum loop and coated with a metal salt containing these ions, it can give us a hint of the ion's identity. In this process, each metal ion displays a distinct color due to the transition of electrons between different energy levels, often referred to as d-orbital transitions. For example:- Copper ions \(\text{Cu}^{2+}\) yield a blue-green or light blue color.- Iron ions \(\text{Fe}^{2+} \, \text{or} \, \text{Fe}^{3+}\) create a yellow hue.- Chromium ions \(\text{Cr}^{3+}\) exhibit a green color.- Manganese ions \(\text{Mn}^{2+}\) produce a violet tint.- Cobalt ions \(\text{Co}^{2+}\) result in a dark blue shade.- Nickel ions \(\text{Ni}^{2+}\) show a brown color.These color changes occur because transition metals have partially filled d-orbitals, which allow for these electronic transitions that are important in fields like analytical chemistry and mineralogy.

Hybridization in borax

The concept of hybridization helps us understand the bonding and structure within borax. Borax contains boron, which is a key component in determining how atoms are arranged and bonded. Within the borax compound \((\text{Na}_2\text{B}_4\text{O}_7 \cdot 10\text{H}_2\text{O})\), each boron atom bonds differently based on its placement.

• In a borate ion, typically, each boron connects with three oxygen atoms, forming a trigonal planar structure. This specific arrangement necessitates \(\text{sp}^{2}\) hybridization.
• However, boron can also develop tetrahedral configurations, especially when coordinating with four other atoms, going for \(\text{sp}^{3}\) hybridization.

This dual hybridization is key to understanding the versatility of boron within the borax structure. The coexistence of both \(\text{sp}^{2}\) and \(\text{sp}^{3}\) configurations in borax's structural matrix highlights the complexity and adaptability of boron atoms.

Borate ion structure

The borate ion is integral in understanding borax's functionality and properties. It arises from the composition \(\text{B}_4\text{O}_7^{2-}\), which is created through bonding among boron and oxygen.A borate ion generally contains:

• Tetrahedral and trigonal planar geometries, based on the hybridization of boron.
• Covalent bonds formed with oxygen, allowing borate to exhibit stable, negatively charged polyatomic ion characteristics.

These structures help solidify borax's function in processes like crystal growth and as a flux in metallurgy. The borate ion's ability to bond in multiple ways, resulting in intricate geometrical shapes, reflects borax's wider adaptability and usefulness in chemical reactions. Consequently, understanding its structure paves the way for appreciating its catalytic and cleaning application across numerous industries.