2025 ZHSS Prelim 4E5N G3 Sc(Chem) Ans

2025 4E5N Science Chemistry Prelim Paper Answers Paper 1 1 2 3 4 5 6 7 8 9 10 B C A B D C A D C B 11 12 13 14 15 16 17 18 19 20 D B A C B C C D C A Paper 3 1 (a) ZnCl2 [1] each (b) Al2O3 (c) CuCO3 (d) (NH4)2SO4 2 (a) To provide cooling surface for cooling and condensing the vapour of liquid back into distillation flask. [1] (b) hexane [1] (c) Arrangement: Changes from closely packed in a disorderly arrangement to far apart in random arrangement Movement: Changes from rolling and sliding past one another to moving randomly at high speeds [1] [1] 3 (a) Na+, Mg2+, Al3+ : 2,8 P3-, S2-, Cl- : 2,8,8 Every 3 correct: [1] [2] (b) (i) Atomic radii of non-metal atoms are smaller than metal atoms. [1] (ii) Ionic radii of metal ions are smaller than non-metal ions. [1] (iii) Metal ions have less electron shells than non-metal ions. [1] (c) Argon has a fully-filled valence shell of electrons / stable octet structure so it does not need to lose or gain electrons to form ion. [1]

2 4 (a) R : iron(II) sulfate / FeSO4 S : iron(II) carbonate / FeCO3 T : iron(II) hydroxide / Fe(OH)2 U barium sulfate / BaSO4 [4] (b) Fe2+(aq) + CO32- (aq) → FeCO3 (s) 1 mark for correct equation 0.5 mark for correctly balanced 0.5 mark for correct state symbols [2] (c) Purple acidified potassium manganate(VII) turns colourless. [1] 5 (a) (i) element oxidation state before reaction oxidation state after reaction aluminium 0 +3 iron +3 0 [1] for every 2 correct (ii) Al is oxidised when the oxidation state of Al is increases from 0 in Al to +3 in Al2O3. [1] Fe2O3 is reduced when the oxidation state of Fe decreases from +3 in Fe2O3 to 0 in Fe. [1] [2] (b) Exothermic [1] A lot of heat is released.[1] [2] (c) No. of moles of Fe = 1000/6 = 17.857 Mole ratio Fe : Al 1 : 1 Mass of Al 17.857 x 27 = 482 g (3sf) [1] [1] 6 (a) Melting and boiling point increase down the group. / Colour intensity increases down the group. / Density increases down the group. [1] (b) (i) Iodine gas/ I2 [1] (ii) Fluorine is more reactive than iodine so fluorine displaces iodine from potassium iodide, producing potassium fluoride and iodine gas. [1]

3 7 (a) (i) 1m for cation and 1m for anion. [if only valence electrons shown, 1 mark ] [2] (ii) One oxygen atom gains 2 electrons from two sodium atoms [0.5] in order to achieve a fully-filled valence shell/ stable electronic configuration of noble gas.[0.5] Two sodium atom each lose 1 electron to one oxygen atom in order to achieve a fully-filled valence shell/ stable electronic configuration of noble gas.[0.5] [1] (b) (i) [1 mark for correct number of bonding electrons ; 1 mark for correct number of non-bonding electrons] [2] (ii) NF3 has a low boiling point as it has a simple molecular