Boron Allotropes and Compounds Borax Boric Acid Diborane - UNSOLVED PRACTICE SET

Q7. Describe the structure of diborane (B₂H₆). What are "banana bonds" or "3-centre-2-electron bonds," and where are they located in the molecule?

Q8. Write the chemical reactions for:

(a) Preparation of borax from colemanite (Ca₂B₆O₁₁)

(b) Preparation of boric acid from borax

Q9. Why is boric acid used as an antiseptic and eye wash? What property makes it suitable for these applications?

Q10. Explain why diborane is called an "electron-deficient compound." How does this electron deficiency influence its chemical reactivity?

Q11. Your grandmother uses borax (suhaga) as a cleaning agent and for preserving food in some traditional Indian recipes. Explain the chemical basis of borax's use as a cleaning agent.

Q12. Why does boric acid act as a Lewis acid rather than a Brønsted acid? Write the reaction showing how it accepts OH⁻ ions from water.

Q13. Describe the preparation, properties, and uses of borax (Na₂B₄O₇·10H₂O). Include:

(a) Preparation from colemanite or from other boron minerals

(b) Action of heat on borax — what happens when it is heated strongly?

(c) The borax bead test — how is it used to identify metal ions? Give examples with cobalt and chromium.

(d) Uses of borax in laundry, glass and ceramic industries, and as a food preservative

Q14. Discuss the structure and bonding in diborane (B₂H₆):

(a) Draw the structure showing terminal and bridging hydrogen atoms

(b) Explain the concept of 3-centre-2-electron bonds with orbital overlap diagram

(c) Why can't diborane be explained by conventional 2-centre-2-electron bonds?

(d) Compare the bonding in diborane with that in ethane (C₂H₆)

Q15. Boron compounds have both traditional and cutting-edge applications. Discuss:

(a) The use of borax in Indian households for cleaning and in the glass industry

(b) Why boric acid is used in nuclear reactors as a neutron absorber

(c) The role of boron fibres in composite materials for aerospace applications

(d) The emerging use of boron nitride nanotubes as alternatives to carbon nanotubes

Q16. Borax (Na₂B₄O₇·10H₂O) is heated to obtain anhydrous borax, which on further heating gives boron trioxide (B₂O₃).

(a) Write the balanced chemical equations for both steps.

(b) Calculate the mass of anhydrous borax obtained from 381 g of borax decahydrate.

(c) Calculate the mass of B₂O₃ obtained from this anhydrous borax.

[Given: Atomic masses: Na = 23, B = 11, O = 16, H = 1]

Q17. Diborane (B₂H₆) reacts with water according to the equation: B₂H₆ + 6H₂O → 2H₃BO₃ + 6H₂

(a) Calculate the mass of boric acid (H₃BO₃) produced when 27.6 g of diborane reacts with excess water.

(b) Calculate the volume of hydrogen gas evolved at STP.

(c) If this hydrogen is used to fill balloons, and each balloon requires 2.24 L of H₂ at STP, how many balloons can be filled?

[Given: Atomic masses: B = 11, H = 1, O = 16; Molar volume at STP = 22.4 L]

Q18. In the borax bead test, a borax bead is formed by heating borax on a platinum wire loop. The bead weighs 0.01 g and contains cobalt oxide (CoO).

(a) Write the reaction showing the formation of cobalt metaborate (Co(BO₂)₂) in the bead.

(b) If the bead contains 0.002 g of CoO, calculate the mass of cobalt metaborate formed.

(c) What colour would this bead appear, and why is platinum wire preferred over copper wire for this test?

[Given: Atomic masses: Co = 59, B = 11, O = 16]