Copy of SKSS Chem Prelim Paper 1 & 2 Answers

MARK SCHEME for SKSS 2024 4E Chemistry Prelim Paper 1 & 2 Setter: Ms Kwok Honey 1 PAPER 1 [40 marks] 1 2 3 4 5 6 7 8 9 10 D C B B A B B D C B 11 12 13 14 15 16 17 18 19 20 D C C A C C C C C B 21 22 23 24 25 26 27 28 29 30 A D A D D D B A D D 31 32 33 34 35 36 37 38 39 40 C D D B C B C C D B PAPER 2 Section A [70 marks] 1 [This question mainly assesses students’ memory work.] [accept if correct chemical formula is written each time] (a) ethanol/water [1] [accept if both are written] (b) calcium hydroxide [1] (c) sulfur dioxide [1] (d) aluminium nitrate [1] (e) ammonia [1] (f) methane [1] 2 [This question is similar to the Specimen Paper Q4] (a) [1m for each correct answer; max. of 2m] Any TWO of the following answers: • forms/gives coloured compounds • higher density • higher melting and boiling point [reject: good catalyst, variable oxidation states as these are not physical properties] (b) [1m for all correct number of electrons and protons; 1m for all correct number of neutrons; max. of 2m] Cr24 52 Cr24 53 number of electrons 24 24 number of neutrons 28 29 number of protons 24 24 (c) (i) 2Cr2O3(s) + 3C(s) → 4Cr(s) + 3CO2(g) [1] [reject if the coefficients are not in the simplest form]

MARK SCHEME for SKSS 2024 4E Chemistry Prelim Paper 1 & 2 Setter: Ms Kwok Honey 2 [Note to marker: All state symbols must be written correctly to be awarded 1m if students choose to include in the balanced chemical equation.] (ii) Amphoteric oxide can react with both acids and bases while acidic oxide can only react with bases. [1] Any ONE of the following equations: • Cr2O3 + 6HCl → 2CrCl3 + 3H2O OR with any other acids • CO2 + Ca(OH)2 → CaCO3 + H2O OR with any other bases [1] [Note to marker: It is not within the syllabus for students to write the chemical equation of chromium(III) oxide with a base as complex ions are formed.] (d) • crystal dissolves [1] • (idea of collision) particles collide / particles bounce off each other [1] • (idea of diffusion) particles move further apart / particles move/diffuse from higher concentration to lower concentration / movement of particles down a concentration gradient [reject the word ‘spread’ to describe diffusion as this word is already seen in the question] [1] (e) [1m for every 2 correct order of arrangement; max. of 2m] (most reactive) sodium, lanthanum, nickel, mercury (least reactive) 3 (a) [1] (b) (i) H H │ │ I―C―C―Cl │ │ H H [1] (ii) ∆Hbond breaking = 614 + 4(413) + ? = (2266 + ?) kJ ∆Hbond forming = 240 + 4(413) + 348 + 328 = 2568 kJ ∆Hbond breaking − ∆Hbond forming = −94 2266 + ? – 2568 = −94 [1m with correct working] ? = 208 kJ/mol [1m with correct unit] (iii) [This part of the question is similar to Specimen Pa