ANALYTICAL QUESTIONS IN p-BLOCK ELEMENTS

1) When conc. H2SO4 was added to an unkNown salt presence in a test tube, a brown gas(A) was evolved. This gas intensified when copper turnings were added into test tube . On cooling gas (A) changed into colourless gas(B) Identify the gas ‘A’ & ‘B’. Write the equation for the reaction evolved

1 ANS The gas ‘A’ is NO2 whereas ‘B’ is N2O4 XNO3+H2So4XHSO4+HNO3 salt (conc.) Cu+4HNO3(conc.)Cu(No3)2+2NO2+2H2O blue brown(A) 2NO2(on cooling)N2O4 colourless (B)

Gradual addition of KI to Bi(NO3)3 solution initially 2. A colourless 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. Gradual addition of KI 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 a

2 ANS A  B + C Colourless Neutral oxide liquid inorganic salt P(Phosphorus) +B  strong white dehydrating agent the reactions are explainable if A is taken to be NH4NO3 NH4NO3(s)N2O+2H2O; P4+10N2OP4O10+10N2

3) A translucent white waxy solid ‘A’ on heating in an inert atmosphere is converted in to its allotropic form (B). Allotrope ‘A’ on reaction with very dilute aqueous KOH liberates a highly poisonous gas ‘C’ having rotten fish smell. With excess of chlorine ‘A’ forms ‘D’ which hydrolysis to compound ‘E’. Identify compounds ‘A’ to ‘E’?

3 ANS The white waxy solid (A) is white phosphorus. When white phosphorus is heated in an inert atmosphere at 573K,it changes to red phosphorus (B) is red Phosphorus (A) on heating with KOH liberates phosphine (c) which is poisonous gas with rotten fish smell. P4+3KOH+3H2O PH3+3KH2PO2 phosphine (C) White phosphorus (P4) burns with excess of Cl2 to form phosphorus pentachloride (D). P4+10Cl2 heating 4PCl5 (D) Hydrolysis of (D) gives phosphoric acid (E) PCl5+4H2OH3PO4+5HCL (E)

4. When conc. sulphuric acid was added to an unknown salt present in a test tube, a brown gas (A) was evolved. This gas intensified when copper turnings were also added into this tube. On cooling, the gas ‘A’ changed into a colourless gas ‘B’. (a) Identify the gases A and B. (b) Write the equations for the reactions involved.

4 ANS A=NO2(g) B=N2O4(g) MNO3+H2SO4 (heat) MHSO4+HNO3 4HNO3 (heat) 4NO2+2H2O+O2 Nitrogen dioxide(Brown gas) Cu+4HNO3 (heat) Cu(NO3)2+2H2O+2NO2 copper turnings 2NO2 N2O4 (Brown gas) (colourless)

5. An element ‘A’ exists as a yellow solid in standard state 5. An element ‘A’ exists as a yellow solid in standard state. It forms a volatile hydride ‘B’ which is a foul smelling gas and is extensively used in qualitative analysis of salts. When treated with oxygen, ‘B’ forms an oxide ‘C’ which is colourless, pungent smelling gas. This gas when passed through acidified KMnO4 solution, decolourizes it. ‘C’ gets oxidized to another oxide ‘D’ in the presence of a Heterogeneous catalyst.Identify A, B, C, D and also give the chemical equation of reaction of ‘C’ with acidified KMnO4 solution and for conversion of ‘C’ to ‘D’

5 ANS ‘A’ is sulphur which exists as yellow solid in standard state. it forms ‘B’ i.e., H2S(g) which is volatile hydride and is used in group II and IV in qualitative analysis of salts. 2H2S+3O2  2SO2(g)+2H2O ‘B’ ‘C’ (colourless and pungent smelling) 2KMnO4+5SO2+2H2Ok2SO4+2MnSO4+2H2SO4 ‘C’ SO2 decolourises acidified KMnO4 because it is reducing agent and KMnO4 is an oxidising agent. 2SO2(g) +O2(g)2SO3(g) ‘D’