AES Prelim Ans

1 Assumption English School Science Department – Preliminary Examination 2024 4E Chemistry 6092 Marking Scheme Paper 1: Multiple-Choice Questions [40 m] 1 2 3 4 5 6 7 8 9 10 B C C D C C A D B A 11 12 13 14 15 16 17 18 19 20 D D C B D B D D A B 21 22 23 24 25 26 27 28 29 30 C D D C B C D A B D 31 32 33 34 35 36 37 38 39 40 A B C C A C C D B B Paper 2: Section A: Structured Questions [70 m] 1 (a) Silicon dioxide [1] (b) Lead(II) oxide [1] (c) Zinc bromide [1] (d) Potassium iodide [1] (e) Copper(II) oxide [1] 2 (a) 2 H2O2 → 2 H2O + O2 (state symbols not required) [1] (b) Bonding electrons-[1]; Non-bonding electrons of Oxygen- [1] [2] (c) When it is warmed, particles gain energy and move faster. More particles have the minimum activation energy to react and collide [1], which leads to a higher frequency of effective collisions between the reacting molecules. [1] [2] (d) Oxygen in H2O2 is oxidised as the oxidation state of oxygen increases from -1 in H2O2 to 0 in O2. [1] [2]

2 Oxygen in H2O2 is also reduced as the oxidation state of oxygen decreases from -1 in H2O2 to -2 in H2O. [1] Hence, since oxygen simultaneously undergoes oxidation and reduction, decomposition of hydrogen peroxide is a disproportionation reaction. 3 (a)  Soft, can be cut easily with a knife  Low melting point/ low boiling point  Low density Any 1- [1] [1] (b) As the atomic radius increases increases down the group from 152 pm in Lithium to 244 pm in Rubidium, the valence electrons are further away from the nucleus [1] The weaker attraction between the nucleus and the valence electron makes it easier for the electron to be lost, increasing the metal's reactivity. [1] [2] (c) Upon losing one electron to form a positively charged ion, lithium ion has one less electron shell (2) compared to lithium atom (3), hence it has a smaller radii. [1] (d)(i) The rapid production of hydrogen gas from the reaction creates bubbles, thereby pushing the sodium around the water surface and causing it to skid. [1] (d)(ii) Potassium will react more vigorously with water and catch fire [1] as it is more reactive than sodium, hence causing a faster reaction. [1] [2] 4 (a) Copper conducts electricity through mobile electrons whereas copper(II) sulfate conducts electricity through mobile ions. [1] Copper conducts electricity in solid (and molten) whereas copper(II) sulfate conducts electricity in aqueous and molten states. [1] [2] (b)(i) Cu2+(aq) + 2e → Cu(s) [1] Copper is less reactive than hydrogen/ Cu2+ ions are below H+ ions in the reactivity series, hence Cu2+ ions are selectively discharged over H+ ions to form a coating/ copper atoms on the cathode, hence increasing its mass. [1] [1]- Half equation, [1]- Explanation [2] (b)(ii) 0.92g [1] (c)(i) 4OH-(aq) → O2(g) + 2H2O(l) + 4e- Need to include state symbols [1] (c)(ii) Copper solid is deposited at the cathode in both experiments. [1] (c)(iii) Universal