Q4. The first ionisation enthalpies are 1402 kJ mol−1 for N and 1314 kJ mol−1 for O. Why is nitrogen’s higher, although oxygen has the greater nuclear charge?
Nitrogen’s three 2p electrons are in separate orbitals, a half-filled shell. In oxygen two of the four 2p electrons share an orbital and repel each other, so one of them is easier to remove (NCERT Class 11 Chemistry, page 89; values from Wikipedia, “Molar ionization energies of the elements”).
Q5. BiF5 is an extremely powerful fluorinating agent, while SbF5 and PF5 are much more stable. Explain.
For bismuth the 6s pair is reluctant to bond (the inert pair effect), so Bi(V) readily gives up fluorine to return to the stable +3 state. Phosphorus and antimony hold +5 more easily (Wikipedia, “Pnictogen”).
Q6. Assertion: NF3 is stable but NCl3 is explosive. Reason: nitrogen has a maximum covalency of 4. Are both true, and does the reason explain the assertion?
Both are true (Wikipedia, “Pnictogen”; NCERT Class 11 Chemistry, page 93), but the reason does not explain the assertion: both molecules have only three bonds to nitrogen, so the covalency limit plays no part.
Q7. What is the oxidation state of phosphorus in P4O10, PH3 and PCl3?
P4O10: 4x − 20 = 0, so +5. PH3: with H as +1, P is −3, as NCERT marks it in its Eq. (7.46) (NCERT Class 11 Chemistry, page 244). PCl3: +3 (reasoned; NCERT Class 11 Chemistry, Unit 7).
Q8. Show that As2O3 is amphoteric.
It dissolves readily in alkali, giving arsenites, As2O3 + 6NaOH → 2Na3AsO3 + 3H2O, and it also dissolves in hydrochloric acid: it reacts as an acid and as a base (Wikipedia, “Arsenic trioxide”; NCERT Class 11 Chemistry, page 94, names it amphoteric).