ADSS Prelim P2 Ans

Admiralty Secondary School Marking Scheme 4E Pure Chemistry PRELIMINARY EXAMINATION 2024 PAPER 2 SECTION A [70 marks] 1(a) Substance B B is made up of carbon atoms which are joined together in layers of hexagonal rings. Only 3 out of the 4 valence electrons of each carbon atom are used to form these hexagonal rings. There is one valence electron that is delocalised between the layers to conduct electricity. Hence, it can acts as an electrode. [1] [1] [1] KU Accept: mobile electrons (b) Substance C C is made up of simple molecules which has very weak intermolecular forces of attraction between molecules. Hence, little energy is needed to overcome it and its melting point is low. Hence, it is the only liquid at room temperature. [1] [1] [1] KU (c) Substance B B is made up of carbon atoms which are joined together in layers of hexagonal rings. There are weak intermolecular forces of attractions between each layer. Little energy is required to slide one layer over the other. Hence, it can be used as a lubricant. [1] [1] [1] KU 2(a) The oxygen in air may oxidise/rust iron. [1] HISP (b) The iron wool is a catalyst. [1] KU (c) The ammonia gas produced is less than 50cm3 <or a value less than 50cm3> This is due to mole ratio. However, the reaction will not be complete as it is a reversible reaction. [1] [1] [1] HISP (d) Advantage: Higher pressure leads to higher yield/ faster reaction. Disadvantage: Maintaining high pressure incurs a large cost/ more energy required. [1] [1] KU 3(a) Zinc nitrate [1] [A] any souble zinc salt HISP (b) Zn(NO3)2 (aq) + H2SO3(aq)  ZnSO3 (s) + 2HNO3 (aq) [2] [1] – balanced equation with correct chemical formula

[1] – correct state symbols ECF from 3(a) but only give [1] for balanced equation with chemical formula HISP (c) Add excess zinc nitrate to 10cm3 of 0.10mol/dm3 of sulfurous acid. Filter the precipitate using filter paper and filter funnel. Wash the precipitate with distilled water. Dry it between 2 sheets of filter papers. [2] 3 points –[2] 2 points – [1] 1/0 point – [0] KU (d) Zinc sulfate cannot be prepared using the same method as zinc sulfate is soluble in water. [1] HISP 4(a) Correct calculation for the total amount of energy absorbed during bonding breaking [i.e. (413 × 8) + (390 × 6) + (3 × 498) = 7138 kJ] Correct calculation for the total amount of energy released during bonding forming [i.e. (413 × 2) + (891 × 2) + (464 × 12) = 8176 kJ] Enthalpy change of reaction = -8176+7138 = -1038 kJ [1] [1] [1] HISP No units/incorrect units -1 marks (b) The energy absorbed for bond breaking is lesser than the energy released for bond forming. Hence, enthalpy change is negative. [1] [1] KU (c) [3] [1] - correct shape of graph [1] - correct labelling of reactant and pr