Q4. Arrange B, Al and Ga in order of increasing covalent radius, and explain the place of gallium.
B (85 pm) < Ga (124 pm) < Al (126 pm). Ten of gallium’s 18 extra protons come with ten 3d electrons, which shield the nucleus poorly, so its valence shell is pulled in, and it ends up no bigger than aluminium (radii from the Wikipedia data page; reasoned).
Q5. The second and third ionisation enthalpies are 1820.7 and 2704 kJ mol−1 for indium and 1971 and 2878 kJ mol−1 for thallium. Find the energy needed to remove the ns2 pair from each M+ ion, and say what it suggests.
In: 1820.7 + 2704 = 4524.7 kJ mol−1. Tl: 1971 + 2878 = 4849 kJ mol−1, about 324 kJ mol−1 more, although thallium is the bigger atom. The 6s pair of thallium is held unusually tightly, which contributes to the inert pair effect. Actual compound stability also includes bonding, lattice and solvation energies (Wikipedia, “Inert-pair effect”).
Q6. What volume of hydrogen, at 273.15 K and 1 bar, does 8.1 g of aluminium give with excess dilute hydrochloric acid? (Al = 27 g mol−1; molar volume 22.7 L mol−1.)
8.1/27 = 0.30 mol Al. From 2Al + 6HCl → 2AlCl3 + 3H2, 0.30 mol Al gives 0.45 mol H2, and 0.45 × 22.7 ≈ 10.2 L (reasoned).
Q7. Why can cold concentrated nitric acid passivate aluminium while aqueous sodium hydroxide attacks it?
Under suitable cold concentrated conditions, nitric acid leaves aluminium passive, protected by its oxide layer (Wikipedia, “Aluminium”). Sodium hydroxide dissolves the amphoteric oxide, Al2O3 + 2NaOH → 2NaAlO2 + H2O (NCERT Class 10 Science, page 41), and then attacks the metal, giving an aluminate and hydrogen.
Q8. Assertion: ordinary boron compounds do not contain simple B3+ cations. Reason: the sum of the first three ionisation enthalpies of boron is very high. Are both true, and does the reason explain the assertion?
Both are true, and the reason explains the assertion: removing three electrons would need about 6887 kJ mol−1, so boron shares its electrons and favours covalent bonding instead (data from Wikipedia, “Molar ionization energies of the elements”; reasoned).