Pure Chem CHS Sec 3 Mark Scheme / Answers

1 2024 Sec 3 (O-level Programme) Term 4 End-Of-Year Examination Section A Section B Qn Answers Mark Guidance 1a oxygen 1 1b sulfur dioxide 1 1c sulfur dioxide 1 1d nitrogen monoxide 1 1e argon 1 2a 80 1 2b energy is absorbed to overcome attractive forces between molecules 1 2c vibrate faster; (1) about their fixed positions;(1) 2 3a true false P has a giant molecular structure, but Q has a simple molecular structure. ✔ P is used in drills while Q is used as a lubricant. ✔ Atoms in both P and Q bond to achieve the electronic configuration of a noble gas. ✔ Both P and Q are elements. ✔ Both P and Q have high melting points. ✔ 3 3b element B C mass /g 78.6 21.4 Ar 11 12 no. of moles /mol 78.6 / 11 = 7.1455 21.4 / 12 = 1.7833 mole ratio 7.1455 / 1.7833 = 4.01 ≈ 4 1.7833 / 1.7833 = 1 empirical formula is B4C 4a PdCl2(s) + H2O (g) + CO(g) → Pd(s) + 2HCl(g) + CO2(g) 1 2 3 4 5 6 7 8 9 10 B B D D C A C C A D 11 12 13 14 15 16 17 18 19 20 B C A C D B B A D C 21 22 23 24 25 26 27 28 29 30 A A D D C B D C C C

2 4bi element oxidation state before the reaction oxidation state after the reaction palladium +2 0 carbon +2 +4 4bii C (in CO) was oxidised + oxidation state of C increased from +2 (in CO) to +4 (in CO2); Pd (in PdCl2) was reduced + oxidation state of Pd decreased from +2 (in PdCl2) to 0 (in Pd); 4c hydrogen chloride 4d [1] – shared electrons [1] – lone pair 5a 4 1 5bi atoms of the same element + same number of protons + different number of neutrons; 1 5bii isotope number of protons number of neutrons number of electrons 286Fl 114 172 114 289Fl 114 175 114 2 5c flerovium has giant metallic structure + oxygen has simple molecular structure; flerovium has higher melting point than oxygen + as more energy needed; to overcome the strong electrostatic forces of attraction between metal ions / cations / positive ions and ‘sea of (delocalised/mobile) electrons’ (in flerovium) than the weak intermolecular forces of attraction between molecules (in oxygen); 3 6a potassium chloride: reagent 2: (aqueous) potassium hydroxide / potassium carbonate; titration; lead(II) sulfate: reagent 2: aqueous lead(II) nitrate; filtration; 4 6b 40 cm3 / doubled; 2

3 twice the number of moles of H+ in the sulfuric acid compared to hydrochloric acid (as sulfuric acid is dibasic and hydrochloric 6c 1 7ai Mg(OH)2 + 2HCl → MgCl2 + 2H2O; 1 7aii No. of moles of HCl = 27.60 1000 x 0.220 = 0.006072 mol; Mg(OH)2: HCl 1 : 2 No. of moles of Mg(OH)2 = 0.006072 ÷ 2 = 0.003036 mol + Concentration of Mg(OH)2 = 0.003036 ÷ 25.0 1000 = 0.12144 mol/dm3 = 0.121 mol/dm3; 2 7b mass of Mg(OH)2 = 0.12144 x (24 + 16x2 + 1x2) = 7.0435 g = 7.04 g 1 7c 1 dm3 of water can dissolve 7.04 g of Mg(OH)2 volume of water needed to dissolve 10.0 g of Mg(OH)2 = 10.0 ÷ 7.0435 = 1.42 dm3 1 8a A: calcium carbonate / CaCO3 C: calcium chloride / CaCl2 D: carbon dioxide / CO2 E: calcium hydroxide / Ca(OH)2 4 8b Ca2+(aq) + 2OH–(aq) → Ca(OH)2(s) 1 8c aqueous barium nitrate: no observable change; aqueous silver nitrate: white precipitate formed; 2 8di copper 1 8dii metal B / copper + causes oxygen to be reduced + oxygen gains electrons (to form O2-) A: metal B / copper + as metal B / copper is itself oxidised + by losing electrons 1 9a SCl2 1 9bi SiCl4 1 9bii PCl3 + 3H2O → H3PO3 + 3HCl 1 9c student 2 is correct + student 1 is wrong/not completely correct; 3

4 all covalent chlorides hydrolyse + all ionic chlorides dissolve/do not hydrolyse; all non-metal chlorides hydrolyse / all metal chlorides dissolve/do not hydrolyse + except for AlCl3 which hydrolyses; 9d pH less than 7 + chloride has hydrolysed + as acidic HCl is produced; pH at 7 + chloride has not hydrolysed + as the chloride is neutral; A: response using Universal Indicator instead 2 Section C 10a seawater boils + (pure) water vapour/steam rises; (at the top which is cooler), water vapour condenses (and is collected); salt left behind OWTTE; Stage 1 should take place before distillation + Stage 2, after; 4 10bi arrow pointing into lower aperture only 1 10bii condenser 1 10biii stopper shown in diagram; gases or vapours escape; A: thermometer + stopper shown in diagram; cannot identify distillate/substance collected (in conical flask) based on boiling point; 2 10biv water bath cannot exceed 100°C (as water has a boiling point of 100°C); C and D have a boiling point above 100°C; 2 11ai conical flask + gas jar A: measuring cylinder 1 11aii no (rubber) bung in second flask + gas jar should not be inverted A: stopper / flask not sealed / should be downward delivery 1 11aiii to dry the gas / remove water 1 11aiv no + only products of neutralisation are salt and water + chlorine is produced 1 11bi shows collection over water; in a measuring cylinder/burette + graduations shown on collection vessel; 2 11bii filter (the mixture); wash the residue/magnesium with distilled water + dry with filter paper; evaporate filtrate to dryness; weigh (separately) using (electronic) balance; R: crystallisation 4