2022 TMJC H2 Chem Prelim P2 (Ans)

2022 H2 Chem Prelim P2 (Ans) 1 (a) The elements of Period 3 form different oxides when reacted with oxygen. (i) State and explain the variation in bonding within the oxides across Period 3. ……………………………………………………………………………………………………... ……………………………………………………………………………………………………... ……………………………………………………………………………………………………... ……………………………………………………………………………………………………... ………………………………………………………………………………………………..... [2] Compounds A and B are oxides of Period 3 elements which exist in the solid state at room temperature. When excess water is added to a sample of A, the solid dissolves completely and an acidic solution is obtained. However, when excess water is added to B, the solid remains insoluble and a neutral solution is obtained. (ii) Identify compound A. ………………………………………………………………………………………………….. [1] (iii) Suggest two possible identities of compound B and describe how the identity of compound B can be confirmed using the acid-base behaviour of Period 3 oxides. Include all relevant observations and an equation in your answer. …………………………………………………………………………………………………...... …………………………………………………………………………………………………...... …………………………………………………………………………………………………...... ……………………………………………………………………………………………….......... …………………………………………………………………………………………………...... ……………………………………………………………………………………………….......... The bonding within the oxide changes from ionic to covalent across the Period. [1] Large difference in electronegativity between metallic atoms (e.g. Na/Mg/Al) and oxygen results in the transfer of electrons and hence formation of ionic bonds. Small difference in electronegativity between non-metallic atoms (e.g. Si/P/S) and oxygen results in sharing of electrons and hence formation of covalent bonds. [1] P4O10 [1] Possible identities of B: aluminium oxide / Al2O3 and silicon dioxide / SiO2. [1] Add HCl(aq) (or H2SO4(aq)) to a sample of compound B. If it is soluble / dissolves in HCl(aq), B is Al2O3. If it is insoluble, B is SiO2. [1] for test, observation Al2O3 + 6HCl → 2AlCl3 + 3H2O OR Al2O3 + 3H2SO4 → Al2(SO4)3 + 3H2O [1] for equation Markers’ Comments • Many candidates misinterpreted the question and gave the structures (or even the acid-base nature) of the Period 3 oxides instead of the bonding. Even so, the structures of the oxides were also incorrect e.g. giant metallic structure for the metallic oxides or simple molecular structure for Al2O3. This is a book work question and candidates are advised to revise and distinguish accurately between the different types of structures and chemical bonding. • Many candidates were also unable to explain why the type of bonding changes across the Period. Markers’ Comments • SO3 is a colourless liquid that fumes in air at r.t.p. and hence is not accepted as the identity of compound A. • Many candidates struggled to give the correct formula of phosphorus (V) oxide e.g. PO4, or suggested the identity of the