Question

i) Gradual addition of Kl solution to Bi(NO₃)₃ solution initially produces a dark brown precipitate which dissolves in excess of KI to give a clear yellow solution. Write chemical equations for the above reactions.

ii) A colour less inorganic salt (A) decomposes completely at about 250°C to give only two products (B) and (C), leaving no residue. The oxide (C) is a liquid at room temperature and neutral to moist litmus paper while the gas (B) is a neutral oxide. White phosphorus burns in excess of (B) to produce a strong white dehydrating agent. Write balanced equations for the reactions involved in the above process.

Answer 1

(i) Gradual addition of KI solution to Bi(NO₃)₃ solution, initially dark brown precipitates of I₂ are obtained which are dissolved in excess KI and give clear yellow colour solution due to formation of KI₃. In Bi(NO₃)₃, Bi³⁺ ions hydrolysed and to give free H⁺ ion in solution.

The liberated H⁺ ion along with – NO^–₃ ion of Bi(NO₃)₂ oxidises I^–  to I2 2NO^-₃  + 8H⁺ 6I^-  → 3I + 4H₂ O + 2NO

This I₂ (dark brown ppt.) dissolves in excess KI and to give Kl₃ 

ii) Given that

P + B → Strong white dehydrating agent phosphorus

If `A’ is taken to be  NH₄NO₃

Thus, P₄O₁₀ is strong dehydrating agent.