Compassvale Sec 4Exp Prelim Chem P2 2024 MS

COMPASSVALE SECONDARY SCHOOL PRELIMINARY EXAMINATION 2024 SECONDARY FOUR EXPRESS CHEMISTRY 6092 Page 1 of 8 Mark Scheme Paper 2 Theory Section A 1 (a) (i) CH4 / CO2 [1] (ii) CO2 [1] (iii) Ar [1] (iv) Cl2 [1] (v) Al3+ [1] (vi) O2‒ [1] (b) acidic oxide: any non-metal oxides (except H2O, NO, CO) It reacts with alkalis to form salt and water. / It dissolves in water to form acids. neutral oxide: H2O, NO, CO It does not react with acids or alkalis. [1] [1] 2 (a) NH3 + HCl ⇌ NH4Cl [1] (b) Universal Indicator paper colour pH 1 blue / indigo 11 / 12 2 red 1 / 2 / 3 [1] [1] (c) • Ammonia (Mr = 17) has a lower relative molecular mass than hydrogen chloride (Mr = 36.5) • hence it would diffuse faster • and the white powder forms closer to the cotton wool soaked in concentrated hydrochloric acid. [1] [1] [1] 3 (a) • similarity: good conductors of electricity / good conductors of heat / malleable • difference: sodium has a low melting point while niobium has a high melting point / sodium has a low density while niobium has a high density / sodium is soft and can be easily cut with a knife while niobium is hard / sodium forms compounds that are white in colour while niobium forms coloured compounds / sodium has a fixed oxidation state while niobium has a variable oxidation state [1] [1] (b) (i) Niobium is a transition metal hence it should react with chlorine to form an ionic compound with a giant ionic lattice structure however, it formed a covalent molecule with a simple molecular structure instead. [1] (ii) NbCl5 [1]

COMPASSVALE SECONDARY SCHOOL PRELIMINARY EXAMINATION 2024 SECONDARY FOUR EXPRESS CHEMISTRY 6092 Page 2 of 8 (c) Sodium chloride: • Sodium chloride is an ionic compound with a giant ionic lattice structure. • A lot of energy is required to overcome strong electrostatic forces of attraction between oppositely charged sodium ions and chloride ions. • Hence, sodium chloride has a high boiling point. Niobium chloride: • Niobium(V) chloride is a covalent compound with a simple molecular structure. • Little energy is required to overcome weak intermolecular forces of attraction between niobium(V) chloride molecules. • Hence, niobium(V) chloride has a low boiling point. [1] - correct structure and bonding [1] – correct amount of energy and description of bonding [1] – correct comparison of boiling point [3] 4 (a) (i) At ‒10°C, chlorine exists as a gas. • Chlorine molecules are very far apart in a random arrangement • and they move freely / in all directions at high speeds. [1] [1] (ii) • The relative molecular mass of chlorine is 71 whereas the relative molecular mass of bromine is 160 . The relative molecular mass of bromine is nearly double the relative molecular mass of chlorine. However, the density of bromine is nearly a hundred times more than the